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PHOTOPOLYMERIZATION OF 2 HYDROXYETHYL

METHACRYLATE IN THE DEVELOPMENT OF

GLASS-IONOMER CEMENTS

SYED MUHAMMAD KEFI IQBAL

B.D.S. (KU), M. Sc. (London)

A thesis submitted to the Baqai Medical University for the degree of

Doctor of Philosophy

Department of Dental Material Sciences

Faculty of Medicine and Dentistry

Baqai Medical University

Karachi, Pakistan

June, 2011

iii

CERTIFICATE

This is to certify that the work presented in this thesis entitled “Photopolymerization

of 2-Hydroxyethyl Methacrylate in the Development of Glass-Ionomer Cements” is

original and has been conducted by Mr. Kefi Iqbal under my supervision as fulfillment

of the requirement of Ph.D. degree from the Faculty of Medicine and Dentistry, Baqai

Medical University, Karachi.

Professor Dr. Iqbal Ahmad

Faculty of Pharmaceutical Sciences,

Baqai Medical University, Karachi.

iv

ABSTRACT

The present investigation is based on a comparative study of the

polymerization reactions of 2-hydroxyethyl methacrylate (HEMA) using riboflavin

(RF), camphorquinone (CQ) and safranin T (ST) as photoinitiators and

triethanolamine (TEOHA) as a co-initiator in aqueous and organic solvents. HEMA is

involved in photoinitiated polymerization reactions occurring in glass-ionomer

cements (GICs). The above photoinitiators are stable on exposure to a 15 W lamp

emitting in the visible region during the irradiation period. A UV spectrophotometric

method has been developed to determine the concentration of HEMA in polymerized

solutions at low conversion, i.e., about 5% concentration change in the monomer. The

effect of pH, solvent characteristics and HEMA / TEOHA concentration on the rate of

polymerization reactions has been evaluated. The rates of the reactions have been

found to increase with pH probably as a result of the deprotonation of TEOHA and

the presence of a labile proton on the hydroxyl group of HEMA. The steady-state

assumption of the rate of initiation being equal to the rate of termination is considered

valid only at a low conversion of the monomer. Under these conditions the

polymerization reactions have been found to follow pseudo first-order kinetics

(within about 5 % change in HEMA concentration) and the determined apparent first-

order rate constants (kobs) range from 5.44–7.63×10–4

s–1

at pH 6.0–9.0 using 0.01M

TEOHA. The polymerization reactions of HEMA are affected by the dielectric

constant and viscosity of the medium. There is a linear relation between the rate

constants and the solvent dielectric constant indicating the involvement of a polar

intermediate in the reaction. The observation of a linear relation between the rate

v

constants and the inverse of solvent viscosity indicates the effect of viscosity on the

diffusional processes of reacting species and hence the decrease in rates with an

increase in the viscosity of the medium. For all the photoinitiators used the

polymerization reaction is dependent on the TEOHA concentration and the second-

order rate constants for the interaction of HEMA (1–3M) and TEOHA 0.0025–

0.01M) range from 1.79–8.87×10–2

M–1

s–1

at pH 6.0–9.0. The reactivity of the

photoinitiators appears to depend on the energy of the excited triplet state, and its

degree of interaction with the amine to form a radical ion pair, its dissociation and

further interaction of amine radicals with HEMA to undergo polymerization. In the

present work, the reactivity of the photoinitiators has been found to be in the order:

RF > CQ > ST. The irradiation wavelength of the photoinitiators also increases in the

same order resulting in decreased energy and, therefore, it may influence the efficacy

of the photoinitiator. The polymerization of HEMA photoinitiated by the three

compounds is faster in aqueous solution compared with the organic solvent under

identical conditions. The results indicate that the reactivity of the photoinitiators

depends upon their structural features, ionization behavior, spectral characteristics,

triplet energy and solvent characteristics. It appears that the photoinitiators absorbing

in the lower visible region are more efficient (RF 445 nm, CQ 468 nm) in causing

polymerization of HEMA compared to those absorbing in the relatively higher visible

region (ST 520 nm), as indicated by their absorption maxima. Therefore, the use of

RF as a photoinitiator in glass ionomer cements would require a relatively less curing

time for the setting of resin restorative materials and it appears to be the most

efficient photoinitiator in the polymerization process under the conditions studied.

vi

ACKNOWLEDGEMENTS

All praise to Allah Who is the most Beneficent and the most Merciful.

I am extremely grateful to my supervisor, Prof. Dr. Iqbal Ahmad,

Department of Pharmaceutical Chemistry, Faculty of Pharmaceutical Sciences, for his

guidance, keen interest, cheerful support and healthy criticism throughout this study.

Without his help and encouragement through the various phases of this work, the

project would have never reached fruition.

I would also like to thank my co-supervisor Prof. Dr. Aminuddin,

Department of Pharmaceutical Chemistry, for his advice, inspiration and support

during this study.

I pay gratitude to Prof. Lt. Gen. (R) Dr. Syed Azhar Ahmed, Vice

Chancellor, Baqai Medical University, for his interest, encouragement and support.

His keen interest in the promotion of research activities in the University is highly

appreciated.

I would like to express my grateful thanks to Dr. Muhammad Ali Sheraz and

Dr. Sofia Ahmed of the Department of Pharmaceutics for their continued support,

and help during the study.

I consider myself lucky to have met and worked with the staff of the Faculty

of Pharmaceutical Sciences.

My appreciation and thanks go to all my colleagues in the Department of

Dental Material Sciences. I would particularly like to thank Dr. Maria, Dr. Adel, Dr.

Sana, Dr. Afreen and Mr. Altaf for their help and support throughout this study.

vii

I would also like to thank my students, colleagues and friends for their

kindness and support during the period of this study.

I don’t have words to thank Mrs. Prof. Dr. Iqbal Ahmad for her kindness,

love and support.

Finally I am highly indebted to my family who provided me enormous

support, love and understanding at every step of my path to this achievement.

K. I.

viii

Dedicated

to my sweet mother for her endless love and

care

ix

ABBREVIATIONS

AA Acrylic acid

ATR-FT-IR Attenuated total refraction Fourier transform infrared

bis-GMA Bisphenol glycidyl dimethacrylate

BP Benzophenone

BPO Benzoyl peroxide

CQ

Camphorquinone

DEGDMA Diethylene glycol dimethacrylate

DHEPTI Di-hydroxyethyl-p-toluidine

DMAEMA Dimethylaminoethyl methacrylate

EGDMA Ethylene glycol dimethacrylate

FTIR Fourier transform infrared spectroscopy

GIC Glass-ionomer cement

GMA

Glycidyl methacrylate

HEMA Hydroxyethyl methacrylate

IA Itaconic acid

ICP–ES Inductively coupled plasma emission spectroscopy

IR infrared spectroscopy

MA Maleic acid

NMR

Nuclear Magnetic Resonance

NVP

N-vinyl pyrrolidone

PMMA

Polymethyl methacrylate

x

RF Riboflavin

RMGICs Resin-modified glass-ionomer cements

SEM

Scanning electron microscopy

ST Safranin T

TAS

Transient absorption spectroscopy

TEOHA Triethanolamine

TLC Thin-layer chromatography

UDMA Urethane dimethacrylate

UV Ultraviolet

XPS X-Ray photoelectron spectroscopy

xi

CONTENTS Chapter Page

ABSTRACT iv

ACKNOWLEDGEMENTS

ABBREVIATIONS

vi

ix

I INTRODUCTION 1

1.1 Glass-Ionomer Cements 2

1.1.1 Composition of GICs 3

1.1.2 Water Settable Cement 5

1.1.3 Powder: Liquid Ratio 5

1.1.4 Working and Setting Times 7

1.1.5 Variations in Basic Glass Composition 8

1.1.6 Role of Glasses in GICs 8

1.1.7 Effect of Glass Particles Size on GICs 10

1.1.8 Role of Aluminum in Glasses 10

1.1.9 Phases in Glass Structure 12

1.1.10 Role of Liquid in GICs 12

1.1.11 Role of Polymer on Setting Time 14

1.1.12 Role of Tartaric Acid in Setting Time 14

1.1.13 Setting Reaction 15

1.1.14 Leaching Behaviour of Glasses During the Setting Reaction 17

1.1.15 Structure of Set Cement 18

1.1.16 Solubility and Disintegration 18

xii

1.1.17 Adhesive Characteristics 18

1.1.18 Surface Treatment of Glass Particles and Their Effect on the Setting Reaction 19

1.1.19 Participation of Glasses During Cement Formation 21

1.1.20 Types of Glass Ionomer Cements 21

1.1.21 Physical Properties of GICs 23

1.1.22 Literature on Glass Ionomer Cements 28

II CHEMISTRY OF POLYMERIZATION 29

2.1 Introduction 30

2.2 Polymerization Reactions 32

2.2.1 Condensation Polymerization 32

2.2.2 Addition Polymerization 33

2.3 Free-radical Polymerization 34

2.4 Polymerization Process 37

2.4.1 Activation 37

2.4.2 Initiation 37

2.4.3 Propagation 39

2.4.4 Termination 39

2.5 Activation of Resin Materials 40

2.6 Photopolymerization 42

2.6.1 Photoinitiated Free Radical Polymerization 42

2.6.2 Photoinitiated Cationic Polymerization 43

2.6.3 Photoinitiated “Living” Polymerization 43

2.7 Photophysical Process in Photoreactions 46

2.8 Photochemical Reactions of Riboflavin 48

xiii

2.8.1 Introduction 48

2.8.2 Spectral Properties 49

2.8.3 Redox and Protolytic Equilibria 52

2.8.4 Photochemical Reactions 52

2.8.4.1 Intramolecular photoreduction 53

2.8.4.2 Intermolecular photoreduction 54

2.8.4.3 Photosensitized reactions 56

III APPLICATIONS OF LASER FLASH PHOTOLYSIS, SPECTROSCOPY AND ELECTRON MICROSCOPY IN

PHOTOPOLYMERIZATION

58

3.1 Introduction 59

3.2 Spectral and Photophysical Properties of Riboflavin 60

3.3 Applications to Polymerization of 2-Hydroxyethyl methacrylate 61

3.4 Spectroscopic Techniques 63

3.4.1 UV and Visible Spectroscopy (UV/VIS) 63

3.4.2 Transient Absorption Spectroscopy (TAS) 64

3.4.3 Infrared Spectroscopy (IR) 64

3.4.4 Fourier Transform Infrared Spectroscopy (FTIR) 66

3.4.5 Raman Spectroscopy 68

3.4.6 Nuclear Magnetic Resonance Spectroscopy (NMR) 69

3.4.7 Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS-

NMR)

70

3.4.8 Time-of-Flight Secondary Ion Mass Spectroscopy (TOF–SIMS) 71

3.4.9 Fluorimetry 71

3.4.10 Atomic Absorption Spectroscopy (AAS) 71

3.4.11 Inductively Coupled Plasma Emission Spectroscopy (ICP–ES) 72

3.4.12 X-Ray Photoelectron Spectroscopy (XPS) 72

xiv

3.5 Electron Microscopy 73

3.5.1 Scanning Electron Microscopy (SEM) 73

3.5.2 Transmission Electron Microscopy (TEM) 75

OBJECT OF PRESENT INVESTIGATION 76

IV EXPERIMENTAL WORK 80

4.1 Materials 81

4.2 Methods 84

4.2.1 Thin-Layer Chromatography (TLC) 84

4.2.2 pH Measurements 84

4.2.3 Ultraviolet and Visible Spectrophotometry 85

4.2.4 Fluorescence Spectroscopy 85

4.2.5 Light Intensity Measurement 86

4.2.5.1 Procedure 87

4.2.5.2 Calculations 89

4.2.6 Polymerization of 2-Hydroxyethyl methacrylate (HEMA) 91

4.2.6.1 Radiation source 91

4.2.6.2 Method 91

RESULTS AND DISCUSSION 93

V POLYMERIZATION OF HEMA IN AQUEOUS SOLUTION 94

5.1 Riboflavin as Photoinitiator 98

5.1.1 Spectral Characteristics of RF Solutions 98

5.1.2 Photoproducts of Riboflavin 100

5.1.3 Assay of HEMA 101

5.1.4 Kinetics of Photopolymerization of HEMA 104

5.1.5 Effect of pH 104

xv

5.1.6 Effect of Viscosity 113

5.1.7 Effect of TEOHA Concentration 116

5.1.8 Mechanism of HEMA Polymerization 116

5.2 Camphorquinone as Photoinitiator 126

5.2.1 Spectral Characteristics of CQ Solutions 126


5.2.3 Kinetics of Polymerization 128





5.3 Safranin T as Photoinitiator 151

5.3.1 Spectral Characteristics of ST Solutions 151


5.3.3 Kinetics of Photopolymerization 153





VI POLYMERIZATION OF HEMA IN NONAQUEOUS SOLUTION 176

6.1 Types of Bimolecular Reactions 177

6.2 Solvent Effect on Polymerization Reactions 179

6.3 Photoinduced Electron Transfer 181

6.3.1 Long-distance Electron Transfer 181

6.4 Photoinitiated Polymerization 182

6.5 Effect of Dielectric Constant 184

xvi

6.5.1 Riboflavin as Photoinitiator 184

6.5.2 Camphorquinone as Photoinitiator 190

6.5.3 Safranin T as Photoinitiator 194

6.6 Effect of Viscosity 197

6.6.1 Riboflavin as Photoinitiator 197

6.6.2 Camphorquinone as Photoinitiator 198

6.6.3 Safranin T as Photoinitiator 202

6.7 Spectral and Structural Characteristics of Photoinitiators 202

6.8 Mechanism of Polymerization 203

CONCLUSIONS AND SUGGESTIONS 206

REFERENCES 211

AUTHOR’S PUBLICATIONS 254

CHAPTER I

INTRODUCTION

2

1.1. Glass-Ionomer Cements

The term ‘glass-ionomer cement’ (GIC) is used for an acid-decomposable glass

and a water-soluble acid, i.e., a polyelectrolyte such as polyacrylic acid, which is set by an

acid–base or neutralization reaction. It was first reported by Wilson and Kent (1971). GICs

were developed to incorporate the favourable properties of both silicate and

polycarboxylate cements (Wilson and McLean, 1988). The concept was to produce a

cement with characteristics of both the silicate cements and the polycarboxylate cements

(ability to bond chemically with tooth structure and friendly to pulp). The zinc

polycarboxylate cement came along during sixty’s (Smith, 1968). It was the first metal

polycarboxylate cement of practical purpose to be used in dentistry. The characteristic

properties of these cements are shown in Fig.1.

These materials have proven good adhesion with enamel and dentine and are

widely used in a variety of clinical applications. The main property of the GICs is releasing

fluoride over a long period of time, along with the adhesive characteristic of these

materials; it has an aesthetic quality as well.

The synonyms used for ionomer cements are as follows:

(1) Poly (alkenoate) cement, (2) glass ionomer cement (Millett and McCabe, 1996), (3)

acronym of alumino-silicate polyacrylic acid (ASPA) (Nagaraja and Kishore, 2005).

All these cements are available as: (i) Powder/liquid in bottles, (ii) Powder/distilled water

(water settable cement), (iii) Pre-proportioned powder/liquid in capsules (McCabe and

Walls, 2008).

3

1.1.1. Composition of GICs

In the composition of powder and liquid, the powder consists of sodium

alumino silicate glass with about twenty percent calcium fluoride. The composition is

similar to that used in a silicate material but has higher alumina-silica ratio (McCabe and

Walls, 2008; McLean, and Gasser, 1985a). This increases the reactivity of the glass with the

weak poly (acrylic acid) as compared to that of the phosphoric acid used in the silicate

glasses. These glasses contain more fluoride as compared to GICs (McCabe and Walls,

2008) (Fig. 2).

The earlier liquid used in a GIC was fifty percent aqueous solution of

polyacrylic acid. The problem with this solution was that it was very viscous and had a

tendency to gel. This problem can be overcome by using copolymers instead of

homopolymers by creating the steric hindrance to stop the hydrogen bonding, or vacuum

drying the polyacrylic acid (McCabe and Walls, 2008).

In the most current cements, the liquid contains polyacrylic acid in the form of

copolymer with itaconic acid, maleic acid and tricarballylic acid by using the aqueous

solution of polymers derived from the unsaturated alpha, beta-dicarboxylic acid. These

acids have four or five carbon atoms in a liquid to form dental cement which exhibits an

excellent setting performance, good strength, good physiological compatibility and

acceptable aesthetics (Schmitt et al., 1982). The copolymer has an average molecular

weight of less than 20,000, and a solution having a viscosity of less than 50 poise (Crisp et

al., 1977). Copolymerisation of all these acids tends to increase the reactivity of the liquid,

decreases the viscosity and reduce the tendency for gelation.

4

Fig. 1. Characteristic properties of glass ionomer cements.

Fig. 2. Glass-ionomer glass powder composition.

Silicate Cement

Fluoride release

Tooth colour

Translucency

Polycarboxylate

Adhesion to tooth structure

Glass Ionomer Cement

Glass Composition

Area

SiO2

CaF3 Al2O3

5

The various types of carboxylic acid units used in GIC liquid (Wilson and

McLean, 1988) are shown in Fig. 3. Tartaric acid added to the liquid component of GIC

improves the handling characteristics, which increases the working time and shortens the

setting time (Nicholson et al., 1988).

Water is the most important constituent of the GIC liquid and is responsible for

the acid- base reaction, it is the medium for reaction and it hydrates the reaction products.

The quantity of the water present in the liquid is very critical in the composition of GICs.

Mishandling during the operating procedures may affect the properties of the cement. Too

much water results in weak cement, whereas too little water is insufficient for the setting

reaction to go to completion (Anusavice, 2003). Therefore a proper proportion of water is

required for the setting of GICs.

1.1.2. Water Settable Cement

In this form of cement the poly (acrylic acid) copolymer is freeze dried, and

mixed with the glass ionomer powder. The liquid is water or water containing tartaric acid.

When the water is mixed with powder, the poly (acrylic acid) partly goes into the solution

to form the liquid acid. Then the chemical reaction takes place as mentioned for powder and

liquid. This cement is known as water settable cement (McCabe and Walls, 2008).

1.1.3. Powder: Liquid Ratio

The powder / liquid ratio should be high in order to optimize the strength and

lower solubility of the cement. The dentists should always follow the manufacturer’s

recommended ratio for optimum performance. An increase in powder / liquid ratio leads to

6

Fig. 3. Types of carboxylic acid units used in GIC liquid.

7

an increase in stiffness of mix, setting rate, compressive strength, superficial hardness and

resistance to acid attack (Crisp et al., 1976). For restorative purpose the recommended

powder:liquid ratio is 6.8:1. At this ratio the compressive strength is 195 MPa and diametral

strength is 16.0 MPa (Billington et al., 1990). The mechanical properties of GICs are

affected as the powder: liquid ratio decreases. At the ratio of 5:1 the compressive strength

and diametral strength are around 103 MPa and 7.5 MPa, respectively (Billington et al.,

1990).

The reduced porosity levels in the cement mass and extended working and

setting time occur when the powder / liquid is used below the manufacturers’ recommended

ratio. Since it causes reduction in the cement’s load bearing capacity, it fails at lower

compressive stress levels in the posterior region of the mouth (Fleming et al., 2003).

1.1.4. Working and Setting Times

The oscillating rheometer is used to observe continuous changes in viscosity

during the setting cycle. Pearson (1991) has determined the working and setting times of

different commercially available restorative GIC as 1.5 minutes and 5-6 minutes,

respectively.

According to ISO standard (9917) the setting time of restorative cement is a

minimum of 2 minutes and maximum of 6 minutes, whereas minimum setting time for

luting cement is 2.5 minutes and maximum is 8 minutes. For lining / base cement the

minimum and maximum setting time is 2 minute and 6 minutes, respectively (McCabe and

Walls, 2008).

8

1.1.5. Variations in Basic Glass Composition

Typical glasses for cement formation are calcium alumino-silicates with

variable composition. Calcium may be replaced wholly by strontium which has a similar

ion radius (Sr2+

=1.13A°, Ca2+

= 0.99A°). Strontium has a capability to replace calcium

without disrupting the glass structure and translucency and provide greater radiopacity

(Wilson and McLean, 1988).

Barium and lanthanum can partly replace colour, they are alkaline earth metal

and have a radio-opaque property but they do not perform the same role as strontium

because of greater ionic radii (Wilson and McLean, 1988).

1.1.6. Role of Glasses in GICs

Glasses are based on SiO2 - AlO3 - CaF2- AlPO4- Na3AlF6 composition. The

ratio of AlO3 / SiO2 is 1:2 or more and fluoride content is around 23% but commercially

available glasses have more sodium and less fluoride content (Wilson and McLean, 1988;

McLean and Gasser, 1985a).

The powder used in glass ionomer cements has similar composition as that of

the dental silicate cement. Glasses contain different metals like strontium, barium and zinc.

These powders are combined and fused at high temperatures, typically in the range of 1200

to 1550° C. After fusion of the constituent materials, the molten glasses can either be

poured onto a cool metal plate and then into water, or poured directly into the water. This

will form a coarse frit which is further ground, usually by dry milling in a rotatory ball mill,

until the glass powder will pass through a 45µm sieve (for a filling purpose) or a 15µm

sieve (fine grained particles for luting cement) (Hill and Wilson, 1988).

9

The modification of the basic glasses can increase the working time and

decrease the water sensitivity by partially removing the outer surface of the glass particles

around 10-100 µm with the help of acid (Schmitt et al, 1983). The glass component has

many functions in the cement. It acts as a source of ions for the initial cross-linking

reaction, and also provides an inorganic substance such as silicate for the post-hardening

maturation process. It also controls the setting rate and assists in the development of

translucency (Wilson, 1996a).

Fluoride also plays an important role in the composition of glasses. These

powder are combined and fused (at high temperature) with the fluoride flux which is

responsible for lowering the fusion temperatures (Wilson and McLean, 1988). Fluoride also

improves the handling characteristics of the cement paste and enhances the strength of the

set cement. It also has the therapeutic effect in the mouth. However, the optimum fluoride

release level of GIC against dental caries is not yet known due to the fact that the leaching

of fluoride from glass ionomer is a complex process that is not yet well understood.

In a previous study, it has been suggested that the elution of fluoride, which is

loosely bound in the GIC, originates from the initial acid-base reaction between the glass

and the polyacid (De Maeyer et al., 1998). Fluoride facilitates a refractive index match of

the glass to the formed inorganic-organic matrix, allowing translucent cements to be

produced. Fluoride ions also help to produce an amorphous phase separation of glass-

ionomer to give a very reactive glass droplet phase and less reactive matrix phase

(Culbertson, 2001). Acid treatment can modify the properties of glasses such as the

working time. It is responsible for the removal of ions from the surface of the glass particles

10

and reduced soluble fluoride. However, the presence of mildly acidic water increases the

soluble fluoride (Williams et al., 2002).

It is thought that the droplet phase provides cations for generating the salt-

bridges between the polymeric acid chains (Fig.4). The matrix phase also acts as filler,

forming a stable interface to which the polymeric acid chains can bond (Culbertson, 2001).

The other types of glasses have developed high strength hydrolytically stable cements based

on alumino-zinc-silicate glasses (Darling and Hill, 1994). Metal powder may also be

blended with the glass particles by sintering a mixture of the glass and the precious metal at

a temperature greater than 600° C to improve the wear resistance of the glasses (McLean et

al., 1985).

1.1.7. Effect of Glass Particles Size on GICs

Different sizes of the glass particles are used according to the clinical

application but they have some effect on the setting reaction as well. The particle size for

the restorative purpose is around 50 µm in diameter, while that for the luting and lining

materials is reduced to less than 20 µm (Noort, 2002). The setting reaction will be faster by

using fine particle size because they will increase the specific surface of the glass powder

(Wilson and McLean, 1988).

1.1.8. Role of Aluminium in Glasses

The structure of the silicate glass is very difficult to decompose by acid,

because it is a highly cross linked network of connected silicon and oxygen atoms and it

does not carry the electrical charges which make it free to acid attack. Aluminium is

incorporated into silica network by replacing silicon, thus producing the negative charge on

the network, which becomes more basic and more susceptible to acid attack (Wilson and

11

Fig. 4. Possible inter- and intramolecular Ca2+

or Al3+

carboxylates (salt-bridges or

molecular structures) on cured glass-ionomer cement, where X represent

OH or F anions.

12

McLean, 1988; Griffin and Hill, 1999). It may act as either a network former or network

modifier and depends on their co-ordination. If aluminium received four-fold co-ordination

in an oxygen polyhedron it act as a network former, or it may adopt six fold co-ordinations

so as to act as a network modifier. The glass component not only acts as a source of ions for

initial crosslinking, but also provides the inorganic substance for post hardening

(Lowenstein, 1954).

1.1.9. Phases in Glass Structure

The property and behaviour of ions may be modified by change in the structure.

In spinodal type of structure with continuously interlocking phases, the calcium is more

susceptible to acid attack as compared to the other phases (Wasson and Nicholson, 1990).

The flexural strength of the cement may be increased by inducing the disperse droplet

phase.

1.1.10. Role of Liquid in GICs

The main acids used in glass ionomer cements are polymers with carboxylic

acid functional groups. The liquid is an aqueous solution of polymers and copolymers of

acrylic acid with maleic acid and itaconic acid (Crisp and Wilson, 1977). Other monomers

have been used to make copolymers with acrylic acid for experimental glass ionomer

cements, such as 3-butene-1, 2, 3-tricarboxylic acid (Wilson and Nicholson, 1993). Itaconic

acid promotes reactivity between the glass and the liquid. It also prevents gelation of the

liquid which can result from hydrogen bonding between two polyacrylic acid chains

(Wilson and Nicholson, 1993). These polyelectrolytes used in glass-ionomer cement

(Culbertson, 2001) are shown in Fig 5. Polymaleic acid is a stronger acid than polyacrylic

acid and causes the cement to harden and lose its moisture sensitivity faster. This is due to

13

Fig. 5. Polyelectrolytes used in glass-ionomer formulations.

14

the number of carboxyl (COOH) groups, more carboxyl groups lead to more rapid

crosslinking (Wilson and McLean, 1988). The polyacid is typically concentrated to the

range 40-50 % for use, it may be either in the form of aqueous solution or blended vacuum

dried with the glass powder when the system is activated by mixing with either water or an

aqueous solution of tartaric acid.

1.1.11. Role of Polymer on Setting Time

Polymers affect the setting characteristic of the cement. The proportion of the

aqueous solution is very important in the setting time. Decreasing the proportion of aqueous

polymer solution it may reduce the setting time but at the same time it enhances the

compressive strength of the set cement (Crisp and Wilson, 1977). The molecular weights of

the poly acid affects setting rate, toughness, fracture toughness, wear and erosion resistance.

The polyacid chain is very important regarding the formation of the cement, the higher the

molecular weight, the better properties achievable. However, the higher molecular weight is

not practical due to its high viscosity (Wilson et al., 1989).

1.1.12. Role of Tartaric Acid in Setting Time

Tartaric acid is an extremely important ingredient of GICs. An isomer of

tartaric acid is unique in both prolonging the working time and increasing the setting rate It

acts to extend the working time and shortens the setting time by removing the ion from the

glass particles; it then complexes with the ions, preventing them from crosslinking the

polymer chain until the chains become more linear when crosslinking can occur more

readily (Cook, 1983b; Nicholson et al., 1988). Tartaric acid modifies the setting

characteristics of glass ionomer cements. It sharpens the setting time and also increases the

compressive strength, without effecting the working time.

15

1.1.13. Setting Reaction

The setting of glass ionomer cement is complex and may vary with the

composition. On mixing the component of the cement (either aqueous solution of polymer

with the glass powder or polymer / glass powder mixed with water), hydrogen ions are

released and attack the glass particles, converting there peripheries into silica-based

hydrogels and responsible the release of silicic acid and aluminium, calcium, sodium and

fluoride ions (Anusavice, 2003).

In the second phase, these ions migrate out of the hydrogel and into the aqueous

cement phase where they precipitate out as the pH increases. The resulting calcium and

aluminium polycarboxylates ionically crosslink the polymer chains to form the basic

cement matrix (Wilson and Nicholson, 1993). Initial setting (gelation) is due to the chain

entanglement as well as weak ionic cross linking which correspond with the viscoelastic

behaviour of the freshly set cement. The final maturation increases the mechanical

properties of the cement gradually with time (Wilson and Nicholson, 1993) (Fig. 6). A

number of different factors which affect the speed of the setting reaction and the final

strength of the glass-ionomer cement (Prosser et al., 1986; Hill et al., 1989) are as follows:

• The types of polymers or copolymers used in the formulation, which also consider

the concentration of water, ratio of powder/liquid and their molecular weight.

• Type of tartaric acid, either (+)-or meso tartaric acid, which effect on their setting

time and compressive strength of the cured cement.

• The choice of the type of CaFAlSi glass powder and additives

• The ratio of the polymeric acid / CaFAlSi glass powder (Prosser et al., 1986)

16

Fig. 6. Setting reactions of glass ionomer cement.

17

1.1.14. Leaching Behaviour of Glasses During Setting Reaction

During the setting reaction of GIC, the leaching of different cations and anions

from the cements take place. Cook (1983a) showed by atomic absorption spectroscopy that

aluminium was present in the matrix during the early stage of the setting reaction by

dissolving freshly prepared cement in a 3% percent potassium hydroxide solution. It was

concluded that both calcium and aluminium ions were present in the initial setting reaction.

Another study by Wasson and Nicholson (1990) showed the release of aluminium and

calcium by washing the cement with aqueous solution of acetic acid. Following acid

washing the solutions were analysed by Inductively Coupled Plasma Optical Emission

Spectroscopy (ICP-OES). Two glasses including the single phase oxide glass (MP4) and a

complex phase separated glass (G338) were studied. Very little changes were observed in

MP4 and indicated that acid attacks uniformly at the surface of the glass. In G338 glass,

aluminium was found to be released under all condition of time of washing and acid

concentration. However, the ratio was different in Al:Ca and Ca:Si. This result showed that

acid attack occurs preferentially at calcium rich site. The same results were observed by

Barry et al. (1979). They also indicated that the acid attack was at the calcium rich site.

Another concept for setting the glass ionomer cement was the micro structural

study by Hatton and Brook (1992). In this study the set glass-ionomer cements were

sectioned with a diamond knife and examined in the transmission electron microscope by

using a probe to see the behaviour of the ions release from the core to the depleted zone and

matrix. It was found that the ions released from core into the matrix, as well as in the glass

and in ion depleted zone around the individual glass particles. The silicon and phosphorus

was present throughout the matrix as well as in the depleted zone around the individual

18

glass particles. Another study by Wilson (1996b) showed that silicate and phosphate are the

main matrix formers. Two glasses including phosphate-containing and the other phosphate-

free were used and mixed with 40% aqueous solution of acetic acid. ICP analysis showed

after several days that the phosphate-free glass gave rise to aluminium and silicon, whereas

phosphate-containing glass gave a product having phosphorus and forms a mixed silicate /

phosphate structure.

1.1.15. Structure of Set Cement

The set cement consists of agglomerates of unreacted powder particles

surrounded by silica gel embedded in an amorphous matrix of hydrated calcium and

aluminium polysalt (Anusavice, 2003).

1.1.16. Solubility and Disintegration

The initial solubility and early moisture sensitivity of GICs is very high. The

best setting condition for GIC is a high humidity environment approximately (80%), but not

wet and should not be desiccated either, otherwise the cracking or crazing occurs due to

loosely bound water. The complete setting reaction take place in 24 hrs, therefore, the

cement should be protected from saliva in the mouth during this period.

1.1.17. Adhesive Characteristics

One of the basic properties of the glass ionomer is their adhesiveness. It

provides good adhesion to enamel and dentine. The exact mechanism has not been fully

understood, there may be a chelating of carboxyl groups of the polyacid with the calcium in

the apatite of enamel and dentine. The bond to enamel is always higher than that to dentine,

probably due to the greater inorganic content of enamel (McCabe and Walls, 2008) (Fig. 7).

Shear bond strength was about 4-10 MPa to both enamel and dentine without using any

19

conditioner. This shear bond strength was increased when using the conditioner or etchant

on the tooth surface (McCabe, 1998). The glass ionomers are relatively brittle, having a

flexural strength of only 15-20 MPa whereas the tensile strength of the cement itself is only

around 12 MPa (McCabe and Walls, 2008).

Another method used by manufacturers that apply the conditioner and primer on

the enamel and dentine before the placement of restorative materials, primer has capability

to partly demineralise the tooth surface and increase the wettability to enhance the shear

bond strength around 8-24 MPa (McCabe, 1998).

1.1.18. Surface Treatment of Glass Particles and Their Effect on the Setting Reaction

Different types of treatment applied to modify the surface of the glasses include

properties such as setting and working time. One of surface treatment is to apply complex

fluoride salt like Na3AlF6 or K3AlF6 which improved the physical properties of the glass

powder and the fluidity of the mixed cement. Another method to sharpen the setting time of

GIC is to induce a surface reaction of tartaric acid with the surface of the glass particles. To

adjust the working and the setting time of the GIC can be achieved by different method.

Heat treatment is one of these; however, the disadvantage of such thermal treatment is to

prolonged the setting time as well as the working time. Snap setting of these materials is

clinically important (Gasser, 1994).

Another approach to increase the working time is washing the reactive glass

particles with an acid to reduce the reactivity of the glasses and therefore, increasing the

working time of the cement (Schmitt et al., 1983).

20

Fig. 7. One possible mechanism of bonding to tooth substance for GIC.

CO-2 CO

-2 CO

-2 CO

-2

Polyacid chain

Tooth structure

Ca2+ Ca

2+

21

This has little or no effect on the setting time. The acid washing results in a

partial leaching of the outer surface of the glass particles; therefore, it produces the delay to

start of set and controls the initial acid reactivity in order to achieve the proper setting and

working time (De Maeyer et al., 1998).

1.1.19. Participation of Glasses During Cement Formation

The composition and structure of the glasses used in GICs are very important

regarding the performance of the final set cement. Working time, initial solubility, speed of

set, strength and cariostatic properties are largely determined by the glasses used, In

addition, they control radiopacity and optical characteristics such as translucency. In the

chemical reaction of GIC the hydrogen ions are formed from the ionization of the poly

(acrylic acid) in water and formation of the silica-based hydrogels around the involved

glass particles. Calcium and Aluminium ions migrate from silica hydrogel into the aqueous

cement phase where, as the pH increases, they precipitate out as polysalts. They ionically

crosslink the polyanion chain and cause the cement to harden. During setting reaction, ions

released from the glasses and an involved in the matrix formation but it is not known how

much the particle size and weights are affected (McCabe and Walls, 2008).

1.1.20. Types of Glass Ionomer Cements

A classification of different types of GICs according to their applications is: I -

luting cements, II - restorative esthetics, III - restorative reinforced cements, IV - lining

cements (Wilson and McLean, 1988). These types are described as follows.

(i) Type 1 Luting Cements

Dental luting agents provide the link between a fixed prosthesis and the

supporting prepared tooth structure; GICs have been in use for many years for luting

purpose. Traditionally, zinc phosphate cement is still used as a luting material, despite its

22

well documented disadvantages, like solubility and lack of adhesion. GICs have some

advantages over zinc phosphate cement such as better flow and lower solubility (Pluim et

al., 1984) along with the fluoride release which helps to prevent the recurrent caries even

though this is very minimal (Scoville et al., 1990). GICs were shown to have the highest

retentive value among the luting cements (McComb, 1982; Young et al., 1985

(ii) Type II Restorative Esthetics

This is slow setting cement and needs to be more in isolation from the oral

environment for up to 24 hours in order to produce better physical properties and

translucency (Mount, 1990). Polishing and contouring of the restoration should always be

performed under air-water spray using a very fine diamond to begin with and finishing

with aluminum oxide discs (McKinney et al., 1987; Mount, 1991).

(iii) Type III Restorative Reinforced Cements

Due to the short setting time of this cement, e.g. Ketac silver, it can be

finished five minutes after the start of mix. This can be used only in areas where esthetics

is of no concern due to lack of translucency and esthetic appeal (McLean and Gasser,

1985b). This type has improved physical properties as compared to the conventional glass

ionomer cement, such as higher compressive strength, (Murray et al., 1986), higher

compressive fatigue limit (Walls et al., 1987) and better wear resistance (Moore et al.,

1985). Fracture resistance is similar to the unreinforced type (Osman et al., 1986). This

reinforced GICs have a number of clinical applications such as for cavity base, (Kanca,

1988; Scherer et al., 1990) restorations (class V, minimal class II., e.g. tunnel preparation

(McLean, 1990) and primary teeth (Croll et al.,1988; Croll, 1989, 1990), sealants, repairs

of castings (McLean, 1990) and temporary or emergency procedures (Burke, 1990). These

23

cements are commonly used for core buildups and the area where a moderate strength and

cariostatic effects are required. However, the tensile strength is not high, therefore, this

requires a considerable support from remaining tooth structure (Mount, 1991).

(iv) Type IV Lining Cements

These types of cements are used for conventional lining or as a base under

restorations and a dentin substitute (Kanca, 1988; Scherer et al., 1990; McLean et al.,

1985; Godoy et al., 1998; Holton et al., 1990). We need a low powder/liquid ratio if used

as conventional lining agent in order to make it flow easily over the pulpal floor. This type

of GIC has both the chemically and light activated forms (Tyas, 1989). The light activated

glass ionomer cements that contain polyalkenoic acid may be superior in strength to the

hand mixed cements (Tay and Lynch, 1990). Due to lack of fracture resistance, this

cement is not recommended for use in high stress bearing areas, where other restorative

materials like composite resin or dental amalgam are recommended. These are placed over

the glass ionomer cement and are responsible to bear the major stresses of the occlusal

forces (Kanca, 1988; Scherer et al., 1990; McLean et al., 1985).

1.1.21. Physical Properties of GICs

It is well known that GICs possess certain unique properties, such as direct

adhesion to the moist tooth structure and base metals due to capability of crosslinking with

calcium ions in tooth or metal ions in base metals (Wilson and Kent, 1971; Wilson and

McLean, 1988; Crisp et al., 1975; Katsuyama et al., 1993), anticariogenic properties due

to fluoride release (Wilson and Nicholson, 1993), minimized microleakage at the tooth–

enamel interface due to low shrinkage, thermal compatibility with tooth enamel and dentin

because of low coefficient of thermal expansion similar to that of the tooth structure

24

(Davidson, 1999; Mount, 1994), biologically compatible and low cytotoxic effect which

make them useful as a restorative material (McLean et al., 1994; Wilson and Kent, 1972;

Wilson, 1989). However, the problem associated with the existing conventional

formulation of GIC in which the materials set sluggishly, showed relatively prolonged

sensitivity to moisture, opacity, brittleness and low mechanical properties which make the

cement unsuitable for use in high stress bearing areas. Therefore, efforts for improvement

have been made in several ways to enhance their physical properties to overcome these

problems (Wilson and Kent, 1971; Wilson and McLean, 1988; Crisp et al., 1975;

Katsuyama et al., 1993; Wilson and Nicholson, 1993; Davidson, 1999; Mount, 1994;

McLean et al., 1994; Wilson and Kent, 1972; Wilson, 1989; Smith, 1999; Nicholson,

1998). These properties have been improved by several changes to the cement system,

either by bringing the changes in basic glass powder preparations or in the chemical

composition of polyacid which are responsible to form the organic matrix (Culbertson,

2001). Since these early materials, numerous modifications have been made to improve

their properties. They are:

(i) Different kinds of polymers, such as acrylic acid–maleic acid

(AA-MA), and acrylic acid-itaconic acid (AA-IA) as the polyacid component.

(ii) The use of dried polymer powders blended with the glass, and activated by the

addition of water (Prosser et al., 1984).

(iii) The development of cermet-containing cements, in which the filler consists of a

cermet (i.e. a ceramic-metal hybrid, in this case being calcium fluoroaluminosilicate glass

fused to silver) rather than a pure glass (McLean and Gasser, 1985b). These cermets are

used in areas where radiopacity is required, and for core build-up under crowns.

25

(iv) Metal-reinforced cements, where a metal, such as silver-tin alloy (Simmons,

1983) or stainless steel (Williams et al., 1992) is added to an otherwise conventional

glass-ionomer in an attempt to reinforce the set cement.

(v) Resin-modified cements, in which the conventional acid-base reactions are

involved with monomers and the initiator is responsible for photochemical polymerization

(Nicholson, 1998).

In order to improve toughness, speed of setting and resistance to dehydration,

hybrid types of materials were introduced in the late 1980s, in which some of the water

content was replaced by water-soluble polymers or monomer in order to achieve greater

early strength and speed up the initial set. These materials are known as resin-modified

glass-ionomer cements (RMGICs), these cement are responsible for combining the acid-

base cross-linking reaction of the metal ion-polyacid with the cross-linking polymerization

of a monomer system (Antonucci et al., 1988; Mitra, 1988). The RMGICs contain not

only the components of the glass-ionomer cement; polyacid, acid-degradable glass and

water, but also contain a water-loving monomer usually 2-hydroxyethyl methacrylate

(HEMA) along with the compatible polymerization initiators. RMGICs have two possible

setting mechanisms, the acid–base reaction of the conventional glass-ionomer cement and

a free-radical addition polymerization of the monomer. The resin-modification was

intended to provide a material with improved mechanical properties and a command cure

facility while still maintaining the clinical advantages of the conventional GICs, such as

adhesion and fluoride release offering some protection against caries. In essence, the

RMGICs are glass-ionomer cements with the incorporation of a small quantity of

monomers as well as initiators involved in the polymerization reaction (Sidhu, 2010). The

26

influence of HEMA plays a role on the optical properties of HEMA-added dental glass

ionomer (HAGI), incorporation of 30–40% HEMA is recommended when formulating

HAGI (Lim et al., 2009).

It has been shown that due to steric hindrance, a very close attachment of

carboxylic acid groups to the polymeric backbone of PAA resulted in a very rigid matrix

(Culbertson, 2001, 2006; Xie et al., 1998). Subsequently, not all the carboxylic acid

groups will convert to carboxylate groups and very little polysalt bridge (Ca+2 di- and

Al+3 tricarboxylate complexes) are formed. Recent studies have shown that this problem

may be overcome by the incorporation of different monomers, such as N-vinylpyrrolidone

(NVP), and amino acid derivatives as spacers between the carboxylic acid groups, which

is responsible to make the polymeric backbone more flexible (Culbertson, 2001; Xie et al.,

2006a, 2006b) and provide the greater chance for the acid–base reaction (allowing greater

freedom of COOH groups to react with Al and Ca ions), resulting in more complete

polysalt bridge formation by improved filler–polymer interaction. This approach was

found to improve the mechanical strength of GICs (Wilson and Kent, 1971; Wilson and

McLean, 1988; Crisp et al., 1975; Katsuyama et al., 1993; Wilson and Nicholson, 1993;

Davidson, 1999; Mount, 1994; McLean et al., 1994; Wilson and Kent, 1972; Wilson,

1989; Smith, 1999; Nicholson, 1998; Culbertson, 2001, 2006).

It has been found that amino acid polyelectrolytes are promising additives to

GIC polyacids (Culbertson, 2001, 2006; Xie et al., 1998; 2006a, 2006b). Amino acid

additives play an important role in both acid–base reactions in the cement matrix and

adhesion to the tooth structure. It has the capability to enhance the mechanical properties

and bond strength to the dentin due to the presence of amine groups and more carboxylic

27

acid groups in the structure of the final set cement. Kao et al. (1996) were the first

research group to report the use of N-acryloyl-substituted amino acid monomers and

evaluated their physical properties in the preparation of GIC. The incorporation of amino

acid in GICs is responsible to make better mechanical and working properties (Kao et al.,

1996). Wu et al. (2003) synthesized the amino acid derivatives (methacryloyl and

acryloyl) and incorporated in the formulations of conventional glass ionomers. These

amino acid modification (amino acid-modified GICs) showed higher compressive strength

(193–236 MPa) and significantly higher flexural strength (55–71 MPa) in comparison to

the commercial Fuji II glass ionomer (compressive strength 191 MPa and flexural strength

16 MPa). Xie et al. (2006a) reported that the resin-modified glass ionomer has increased

its mechanical strength by the incorporation of amino acid derivatives, in which the

compressive strength (20%), diametral tensile strength (70%) and flexural strength (93%)

(Xie et al., 2004a, 2004b). With the different molar ratios of acrylic acid (AA)–itaconic

acid (IA)–NVP polymers may be responsible to enhance their mechanical properties of the

GICs (Culbertson, 2001, 2006; Xie et al., 1998). Some other study reported that AA–MA–

NVP (MA: Maleic Acid) polymer resulted in modified GICs with higher compressive

strength and diametral tensile strength in comparison to Fuji IX commercial GIC

(Yamazaki et al., 2005).

Since the developments of GICs more than thirty years ago, these materials

have involved many different applications in clinical dentistry. The merits and demerits of

GICs system have proved by different clinical experiences. This has resulted in improved

formulations and more controlled techniques. It is difficult to produce an ideal material, but

with continuous development it may be possible.

28

1.1.22. Literature on Glass Ionomer Cements

A. Chapters

Andreasen et al. (2006), Anusavice (2003), Bhat and Nandish (2006), Cahn et

al. (1992), Eliades et al. (2003), Ferracane (2001), Ingle et al. (2008), Ireland (2006),

McCabe and Walls (2008), Nicholson (2002), O′Brien (2002), Phinney and Halstead

(2008), Power and Sakaguchi (2006), Roberson et al. (2006), Schmalz and Arenholt

(2009), Smith et al. (1996), Soratur (2002), Subbarao (2007), Van Noort (2002).

B. Books

Chuin (1983), Combe (2006), Darling (1993), Davidson (1999), Gillingham

(2004), Katsuyama et al. (1993), Kilpatrick (1993), Mount (1994), Mount (2002),

Nicholson (1991), Oen et al. (1990), Wilson and McLean (1988).

C. Reviews:

AI-Badry and Kamel (1994), Buck (2002), Cho and Cheng (1999), Croll and

Nicholson (2002), Culbertson (2001), D’Alpino et al. (2006), Kovarik et al. (2005),

Kramer and Frankenberger (2007), Millett and McCabe (1996), Nicholson (1998),

Nicholson (2007), Nicholson and Beata (2009), Ngo (2010), Rosenstiel et al. (1998),

Simonsen (1996), Walls (1986).

D. Specifications

ANSI/ADA (1989), American National Standards Institute / American Dental

Association; BSI (1981), British Standards Institution Specification; ISO (1986),

International Standards Organization; ISTC (1991), International Standard Technical

Committee.

CHAPTER II

CHEMISTRY OF

POLYMERIZATION

30

2.1. Introduction

A polymer is a long chain organic molecule that is made up of many units

(mer = unit) and the oligomer is a short polymer composed of two, three or four mer

units. When many identical, small molecules chemically react, adding together like

beads on a string, they produce a long chain of many mers, or a polymer (Ferracane,

2001; O′Brien, 2002). If the reactions occur between two different but compatible

monomers, the polymeric product is called a copolymer. This copolymer may be

either random (mers do not appear in specific order) or block (large numbers of one

type of mer appear arranged in sequence). Atoms along the length of any polymer are

joined together through strong, primary covalent bond (O′Brien, 2002). The ability to

add many different types of monomers together to make copolymers gives the

polymer chemist a tremendous ability to “tailor- make” molecules for specific

applications. For example a patient who cannot tolerate a hard denture base made

from poly(methyl methacrylate) (PMMA) requires a soft liner to be placed on the

tissue surface of the dentures. There are many rubber-like polymers compatible with

the PMMA, but they would be too flexible and have limited durability for dentures. A

copolymer can be produced, however, by having the molecules that produce the hard

denture base react with the rubbery material, resulting in the liner with properties

intermediate between the two, i.e., not too soft or too hard. Polymers of different

types are used in dentistry as denture bases and artificial teeth, denture liners or

tissues conditioners, composite restorative and pit and fissure sealants, impression

materials, custom trays for impression, temporary restoratives, mouth guards,

maxillofacial prostheses, space maintainers, veneers cements, and adhesives. They

31

can be processed in the laboratory, in the office, or in the cavity, and they can be

produced with nearly ideal esthetics (Ferracane, 2001; Van Noort, 2002; Power and

Sakaguchi, 2006).

The molecular weight of polymers plays an important role in determining its

physical properties. Chemically, polymers are long-chain molecules of very high

molecular weight. For example the hard denture base resins have been reported to

have viscosity average molecular weight ranging from 150 000 to 600 000 (Cahn et

al., 1992). However, improving the physical properties by increasing the molecular

weight which is responsible to increase the degree of polymerization until a certain

molecular weight is reached (Subbarao, 2007; Soratur, 2002). Polymers may exist in

the crystalline form or in an amorphous state. Many polymers crystallize, and the

size, shape, and organization of the crystals depend on how the polymer was

crystallized. These polymers are not wholly crystalline solids, but are composed of a

large number of small crystalline regions in close proximity to one another, in an

amorphous matrix (Van Noort, 2002). Other polymers are amorphous, often because

their chains are too irregular to permit regular packing. The onset of chain molecular

motion heralds the glass transition and softening of the polymer from the glassy

(plastic) state to the rubbery state. Mechanical behavior includes such basic aspects as

modulus, stress relaxation, and elongation to break (Sperling, 2006).

Degradation of the resin matrix material is influenced by the degree of cure.

An increased degree of cure influences degradation at the filler surface by increasing

the resin density and decreasing the diffusion rate through the matrix material. When

32

a resin composite is placed in ethanol, monomer is leached during a shorter time

interval than if it were placed in water. When monomer molecules leach into alcohol,

alcohol molecules diffuse into the resin. During this process, matrix will swell and

retain alcohol molecules within the resin network. Because of the space alcohol

molecules occupy, they will increase the distance between the polymer chains. Due to

increase in chain spacing results in weaker polar interaction between the separated

chains, which results in a softer and more wear-susceptible matrix (Eliades et al.,

2003).

2.2. Polymerization Reactions

Monomers react to forms polymers by a chemical reaction called

polymerization. Most polymerization reactions are of two types: condensation and

addition polymerization and are described as follows.

2.2.1. Condensation Polymerization

Condensation polymers are any kind of polymers formed through

a condensation reaction, releasing small molecules or low molecular weight as by-

products such as water or methanol, as opposed to addition polymers which involve

the reaction of unsaturated monomers. The type of end product resulting from a

condensation polymerization is dependent on the number of functional end groups of

the monomer which can react. Generally condensation polymers

include polyamides, polyacetals and polyesters and the most commonly known

condensation polymers are proteins, fabrics such as nylon, silk, or polyester

(Ferracane, 2001). Materials that set by the condensation mechanism use for taking

the impression of some dental prosthesis, for example, the dental crown and dental

33

bridges including polysulphide rubber and silicone rubber impression materials

(O′Brien, 2002; Anusavice, 2003; Roberson et al., 2006).

2.2.2. Addition Polymerization

Most dental resins are polymerized by a mechanism in which monomers add

sequentially to the end of a growing chain. Addition polymerization starts from an

active center, adding one monomer at time to rapidly from a chain. In theory the

chain can grow indefinitely until the entire monomer is exhausted. The process is

simple, but it is not easy to control (Anusavice, 2003). Addition reactions usually

involve monomers containing carbon–carbon double bonds (C=C). The C=C bond is

of high energy and is relatively unstable, so it reacts rather easily with other

molecules (Ferracane, 2001). The reactive species which is involved in the addition

reaction may be ionic in nature or it may be a free radical. Although most addition

polymerizations processes in dental materials may be characterized as free radicals

processes, other mechanisms involving the growth of chains through ionic species

such as anions and cations are also used. Cationic ring opening reactions involving

imine and oxirane groups may be used to produce addition polymers. This ring

opening polymerization of imines is employed in the setting of certain impression

materials whilst the ring opening polymerization of oxiranes are closely related

sioloranes and are being used in newly developed resin matrix composite materials. A

key feature of the ring opening process is that it produces a slight expansion, which

reduces the overall contraction resulting from the conversion of monomer molecules

to polymers (McCabe and Walls, 2008). Materials that sets by this reaction include

34

poly(methyl methacrylate), used in the construction of dentures, and bisphenol

glycidyl dimethacrylate (bis-GMA), a common component of the matrix of resin

composites (Cahn et al., 1992; O′Brien, 2002).

2.3. Free-radical Polymerization

Free-radical polymerization reaction usually occurs with unsaturated

molecules containing double bonds (Power and Sakaguchi, 2006). The activator can

be light, heat, or a chemical (peroxide), and all are used in dentistry (O′Brien, 2002).

The first step in the polymerization is to break down or activate the initiator and cause

it to initiate the reaction by joining with a monomer and making it reactive. When the

monomer is attacked by the initiator, the C=C double bond splits, leaving the

molecule with C-C single bond and a free, unpaired electron (Anusavice, 2003;

Power and Sakaguchi, 2006; Sperling, 2006; Ferracane, 2001). This type of reaction

begins at many sites throughout the mixture of monomers, one molecule at a time is

added and a chain rapidly builds up, which can grow almost indefinitely as long as

the supply of the building block is available, finally as fewer monomers remain and

the viscosity of the mixture increases, the reaction enters the final stage where it

stops, or terminates, by the combination of the remaining free radicals (McCabe and

Walls, 2008; Sperling, 2006).

Two more important points are always to be considered during the reaction of

addition type polymerization. First every time C=C bond is broken, heat is evolved.

Second, as each monomer reacts, the entire mass of polymer shrinks. This follows

from the facts that two molecules held together by a primary chemical bond, such as a

covalent C=C linkage, occupy less total volume than they did when they were next

35

to another is an overall shrinkage of polymers used as dental restoratives or

prosthetics devices (Ferracane, 2001; Anusavice, 2003).

Free radical polymerization reactions can be inhibited by the presence of any

material that will react with a free radical, thus decreasing the rate of initiation and

increasing the rate of termination. Decreasing the rate of initiation retards the

polymerization reaction, and increasing the rate of termination decreases the degree

of polymerization or the molecular weight of the final polymer. Such materials as

hydroquinone, eugenol, or large amounts of oxygen will inhibit or retard the

polymerization. Small amounts of hydroquinone are used to protect the

methylmethacrylate monomer from premature polymerization, which prolongs the

shelf life of the monomer (Power and Sakaguchi, 2006).

The majority of monomers which can be polymerized by a free radical

addition mechanism are of the alkenes type. That is, they contain a C=C double bond.

These monomers can be represented by the general formula and some of the familiar

monomers which can be obtained by substituting for X and Y are given in Table 1.

36

Table 1

General formula for alkene molecules which are capable of polymerizing to form

polymers. Example of some specific monomers are given as under:

X Y Monomer Polymer

H H Ethylene Polyethylene

H Cl Vinyl chloride Polyvinyl chloride

H Phenyl Styrene Polystyrene

H –CH=CH2 Butadiene Polybutadiene

H –CO2CH3 Methylacrylates Polymethylacrylate

CH3 –CO2CH3 Methylmethacrylate Polymethylmethacrylate

37

2.4. Polymerization Process

The polymerization process follows a well documented pattern which consists

of four main stages-activation, initiation, propagation and termination (Soratur, 2002;

Van Noort, 2002; Anusavice, 2003; Power and Sakaguchi, 2006; Sperling, 2006;

Subbarao, 2007; McCabe and Walls, 2008).

2.4.1. Activation

This involves the production of free radicals by decomposition of the peroxide

initiator using either thermal activation (heat), chemical activators or radiation of a

suitable wavelength if a radiation activated initiator is present. In simplified, general

terms it may be expressed as follows

2.4.2. Initiation

Initiation is a step in which free radicals, formed on activation, react with

monomer units to create the initial end of a polymer chain. This is illustrated for the

specific case of the benzoyl peroxide radical (RO .) and the methacrylates monomer

(M) (it should be pointed out that hydrogen peroxide undergoes the same reaction on

a wound, giving a burning sensation as the free radicals “kill the germs.”) (Sperling,

2006) in Fig. 8. The reaction may be given in simplified general terms as follows

38

Fig. 8. The reaction of a benzoyl peroxide radical with methylmethacrylate to

form a new species. This is the initiation reaction in free radical

polymerization of methylmethacrylate (McCabe and Walls, 2008).

39

Where the symbol M represents one molecule of the monomer. It can be seen from

the above equation and Fig 1 that the initiation reaction is an addition reaction

producing another active free radical reaction species which is capable of further

reaction.

2.4.3. Propagation

Following initiation, the new free radicals are capable of reacting with further

monomer molecules to form the growing chain. Each stage of the reaction produces a

new reactive species capable of further reaction as illustrated in the following

equation

A general equation for the propagation reaction may be written as follows

Where the value of n defines the number of monomer added and hence the length of

the chain and the molecular weight (McCabe and Walls, 2008).

2.4.4. Termination

Termination is the conclusion of the process as a result of steric hindrance,

lack of monomer molecules or other problems. Other examples of termination involve

the reaction of growing chains with molecules of initiators, dead polymer, impurity or

40

solvent, if present. One example of termination is the combination of two growing

chains to form one dead chain as follows

Light intensity plays an important role for the activation of the materials. Increased

light intensity increases the probability of effective activation whereas low light

intensity does not cause activation (Anusavice, 2003; Power and Sakaguchi, 2006;

McCabe and Walls, 2008). Activation and initiation do occur happen quickly, early

propagation rates involve 100,000 to 1 million monomer reactions per second

(Roberson et al., 2006).

2.5. Activation of Resin Materials

For the early resin composites, polymerization was initiated by the mixing of

two pastes. One paste contained an activator, such as a tertiary amine, most

commonly di-hydroxyethyl-p-toluidine (DHEPTI), which was used to split the

initiator, usually benzoyl peroxide, found in the second paste (Cahn et al., 1992). The

UV light, with a wavelength of approximately 365 nm, splits benzoin methyl ether

into free radicals, without the presence of tertiary amines, thus starting the

polymerization. In this way, only one paste of composite was necessary and

polymerization would not start until it was activated by the UV light (McCabe and

Walls, 2008). Unfortunately, there were some serious drawbacks attached to the UV

light-curing systems. Because of the spectral distribution of UV light it may cause

damage to the eye and soft tissue burns (Xu et al., 2008). The depth of cure achieved

with UV light-curing units was limited due to high light absorption in the resin

composite. However, the benefits of having one paste resin composite, which sets on

41

demand by the curing light, gave rise to the development of visible light activated

resin composites. Dart et al. (1978) at ICI in London, received a patent for the visible

light curing of resins. In modern light-cured resin composites, the most common

photoinitiator is camphorquinone (CQ). This CQ can itself photoinitiate

polymerization, however, only at a low rate. In order to accelerate the polymerization,

amines are used as coinitiators (Jakubiak et al., 2003). CQ is an α diketone that

feature is a conjugated dicarbonilic group and also sensitive to visible range of

spectrum with a wavelength in the blue range, approximately 420 to 490 nm with a

peak at 470 nm (Alvim et al., 2007). After irradiation, it works as a source of free

radicals for the curing process. The absorption of visible light by CQ raises the

molecule to an excited state, known as the “triplet-state”, with very short half-life

(Tsai and Charney, 1969). If the excited CQ interacts with an amine molecule, it

results in an excited state complex, called “exciplex” (Stansbury, 2000). In this state,

CQ can abstract a hydrogen atom from the tertiary amine, resulting in the formation

of a free Radical (Jakubiak et al., 2007; Schneider et al., 2009). CQ is a solid yellow

compound with an unbleachable chromophore group, so that large amounts of CQ in

resin formulations lead to an undesirable yellow color, affecting the final aesthetic

appearance of the cured material (Rueggeberg et al., 1997; Shin and Rawls, 2009).

The concentration of CQ photosensitizer is in the range 0.17–1.03 mass% of the resin

phase and that of dimethylaminoethyl methacrylate (DMAEMA) reducing agent is

0.86–1.39 mass% (Taira et al., 1988).

42

2.6. Photopolymerization

The formation of polymers by photoinitiated polymerization is the basis for

the majority of the commercial applications of photopolymer technology (Peiffer,

1997). Radiation curing, ultraviolet (UV) curing, light (visible) curing, laser curing

and photocuring became very attractive phenomena with the development of new

technologies in 1990s. The word “curing” refers, in this case, to the use of a luminous

“radiation as an energy source to induce the rapid conversion of specially formulated

100% liquid to solids”. Curing can also be achieved with electron beams, x-rays,

plasmas, and microwaves (Fouassier, 1995). Photoinitiated polymerization can result

in a number of different reaction schemes including free radical, cationic, and living

polymerizations (Reiser, 1989; Fouassier, 1995).

2.6.1. Photoinitiated Free Radical Polymerizations

The most common type of photoinitiated polymerization is free radical

polymerization. The basic components of this reaction usually include acrylate or

methacrylic monomers or oligomers, along with a free-radical generating

photoinitiator (Reiser, 1989; Fouassier, 1995).

The polymerization begins with the formation of a free radical species through

the absorption of light by the photoinitiators, then proceeds by a typical chain-growth

mechanism or crosslinking (Fouassier, 1995; Kindernay et al., 2002). In order to gain

a better understanding of the effect of chain-length-dependent termination and chain

transfer agents in photoinitiated polymerizations, Bowman and coworkers have

studied the photoinitiated polymerizations of 2-hydroxyethyl methacrylate (HEMA)

lightly crosslinked with diethylene glycol dimethacrylate (DEGDMA) (Hacioglua et

43

al., 2002). The photoinitiator used in the polymerization of HEMA by various

workers are listed in Table 2.

2.6.2. Photoinitiated Cationic Polymerizations

As previously mentioned, airborne free radical polymerizations can be

inhibited by the presence of oxygen. The use of cationic mechanisms circumvent this

problem since cationic photopolymerizations are insensitive to oxygen, minimal

sensitivity to water, low shrinkage during curing and dark curing behaviour (Decker

and Bendaikha, 1998; Li et al., 2010). Onium salts were generally employed as

photoinitiators for cationic polymerization (Gomez et al., 2007).

2.6.3. Photoinitiated “Living” Polymerizations

Bowman and coworkers have experimented with the living

photopolymerization of methacrylates (Kannurpatti et al., 1997; Lovell et al., 2001;

Hutchison et al., 2002). By using an iniferter such as 2,2-dimethoxy-2-

phenylacetophonone in combination with tetraethylthiuram disulfide, this group

synthesized polymers with narrow molecular weight distributions and no trapped

radicals. An iniferter is a compound that can initiate, chain transfer and terminate

radical polymerizations. The polymerization proceeds in a stepwise fashion with

reversible radical termination by tetraethylthiuram disulfide. Bowman (2008) has

photopolymerized 2-hydroxyethyl methacrylate (HEMA), diethylene glycol

dimethacrylate, and triethyleneglycol dimethacrylate using this or a similar method.

Bowman’s group has also performed living polymerizations to graft acrylic acid onto

polypropylene (PP) membranes (Ma et al., 2000).

44

Table 2

Photoinitiators used in the polymerization of 2-hydroxyethyl methacrylate (HEMA)

46

In biomaterial application, photoinitiated free-radical polymerization was used to

create an adhesive and an insulating material for autonomic nerve activity recording

(Toru et al., 2002).

2.7. Photophysical Processes in Photoreactions

The photophysical processes involved in photoreactions have been explained

by Moore (1996) and are expressed by an energy level diagram (Fig. 9) and through

the following equations.

A UV or visible light-induced chemical process starts with the excitation of a

molecule from their ground state (Do) to the reactive excited states, by the absorption

of photons of particular wavelengths. The molecule, Do, in the ground, upon the

absorption of radiation state is excited to a higher energy level, 1D (excited-singlet

state) (2.8). The excited-singlet state is deactivated on dissipating energy by a variety

of competing physical processes. The energy dissipation may be either by internal

conversion (1C) (2.9) (a non-radiative transition between states of like multiplicity) or

by photon emission (fluorescence) resulting in return to Do (2.10). Even if the

excitation of a molecule occurs to an excited state higher than the first, 1C will

always convert molecule to the 1D level (within picoseconds) before the occurrence

47

Fig. 9. Energy level of molecules, showing electronic transitions involving

fluorescence, phosphorescence, internal conversion (IC) and intersystem

crossing (ISC) (Moore, 1996).

48

of fluorescence. Thus the emission wavelength of fluorescence is the same,

irrespective of the wavelength of excitation. Any excess energy within a particular

electronic state is dissipated as heat on collision with the neighboring molecules. This

has been referred to as vibrational relaxation (VR). In the excited-singlet state (life-

time in nanosecond), the ionization potential of the molecule is reduced, and the

excited electron is more easily removed than it is from the ground state molecule

(2.11). This process is more likely to occur if high energy UV is used (i.e. < 300 nm)

and if the molecule is in the anionic state. Alternatively, intersystem crossing (ISC)

may occur from the excited-singlet state to a metastable excited-triplet state, 3D, in

which the electron spin is parallel (2.12). The excited-triplet state, because of its

longer life-time (microsecond to second or even longer), could diffuse to a significant

distance in fluid media and therefore has a much higher probability of interaction

with neighboring molecules. If no interaction occurs, it decays back to the ground

state by a further ISC process (2.13), or by the emission of fluorescence (2.14).

2.8. Photochemical Reactions of Riboflavin

2.8.1. Introduction

In view of the importance of riboflavin (vitamin B2) (1) as a photoinitiator in

the polymerization of acrylamide (Oster et al., 1957) and 2-hydroxyethylmeth-

acrylate (Bertolotti et al., 1999; Orellana et al., 1999; Encinas et al., 2001; Encinas

and Previtali, 2006), and its high photosensitivity to visible light, it is necessary to

consider the photochemical reactions of riboflavin. The photochemistry of riboflavin

has been reviewed by a number of workers (Oster et al., 1962; Holmstrom, 1964a;

Penzer, 1970; Penzer and Radda, 1967, 1971; Song, 1971; Hemmerich, 1976; Muller,

49

1981; Heelis, 1982, 1991; Fasihullah, 1988; Tollin, 1995; Ahmad and Vaid, 2006;

Ahmed, 2009; Ahmad et al., 2010a; Encinas and Previtali, 2006; Silva and Edwards,

2006; Silva and Quina, 2006; Garcia et al., 2006).

2.8.2. Spectral Properties

Riboflavin exhibits absorption in the ultraviolet and visible region with

maxima at 223, 267 (Є 32500 M−1

cm−1

), 374 (Є 10600 M−1

cm−1

) and 444 nm (Є

12500 M−1

cm−1

) in aqueous solution (British Pharmacopoeia, 2009; Du et al., 1998;

Dawson et al., 1986). The molar absorptivity of riboflavin at the maxima are greater

than 104 M

−1 cm

−1 indicating the π-π* electronic transition. The positions of the

absorption maxima and the molar absorptivities of flavin chromophore are influenced

by the solvent and pH (Ahmad et al., 2004b, 2006b; Ahmad and Vaid, 2006).

Riboflavin gives an intense yellow green fluorescence at 520 nm in aqueous solution

(Weber, 1950). The fluorescence of riboflavin is due to the neutral form of the

molecule where as the ionized forms (cation and anion) are non-fluorescent (Drossler

et al., 2002). The quantum yield of riboflavin is affected by the polarity of the

medium, e.g., 0.26 (water), 0.30 (ethanol), 0.36 (pyridine) (Koziol, 1966a).

Riboflavin is highly photolabile is less polar solvents (Koziol, 1966b). The

spectroscopic and photophysical properties of riboflavin and analogues in aqueous

and organic solvents have been reviewed by Muller (1981), Heelis (1991) and Ahmad

and Vaid (2006). The spectroscopic properties of the excited singlet-state and excited-

triplet state are given in Table 3 and 4, respectively.

50

Table 3

Spectroscopic and photophysical properties of the excited-singlet state of selected flavins*.

51

Table 4

Spectroscopic and photophysical properties of the excited-triplet state of lumiflavin

and lumichromes*.

52

2.8.3. Redox and Protolytic Equilibria

The redox and protolytic equilibria of the ground and excited states of flavins

(Muller, 1983) are as follows.

Thus the flavin can exist in the cationic, neutral and anionic forms in the ground and

excited states and can undergo one and two electron reductions in the ionized forms.

2.8.4. Photochemical Reactions

Riboflavin is highly photosensitive and is degraded by several photochemical

reactions including intramolecular photoreduction and Intramolecular photoaddition

(Heelis, 1981, 1991; Ahmad and Vaid, 2006). The nature and rates of these reactions

is affected by factors such as pH of the medium (Cairns and Metzler, 1971; Schuman

53

Jorns et al., 1975; Ahmad and Rapson, 1990; Ahmad et al., 2004a; Hozler et

al.,2005), polarity of the medium (Song, 1971; Moore and Ireton, 1977; Schmidt,

1982; Ahmad and Tollin, 1981a; Ahmad et al., 2006b), buffer type and concentration

(Holmstrom and Oster, 1961; Holmstrom 1964b; Schuman Jorns et al., 1975; Ahmad

et al 2004a, 2005, 2008, 2010b), ionic strength (Sato et al., 1984), oxygen content

(Treadwell et al., 1968; Schuman Jorns et al., 1975; Ahmad et al., 2004b),

complexation (Ahmad et al., 2008; 2009) and light intensity and wavelengths (Sato et

al., 1982; Ahmad and Rapson, 1990; Ahmad et al., 2006a). The photolysis of

riboflavin is mediated by both excited-singlet and excited-triplet states and occurs

through several pathways (Song, 1971; Cairns and Metzler, 1971; Schuman Jorns et

al., 1975; Ahmad and Tollin, 1981b; Ahmad and Vaid, 2006). The life-time of the

flavin excited-singlet (1Fl) is approximately 5 ns (Wahl et al., 1974) and that of the

flavin excited-triplet state (3Fl) is 1 ns (Namen and Tegner, 1986). The

Intramolecular and intermolecular photoreduction of riboflavin or other flavins is of

interest in the present studies.

2.8.4.1. Intramolecular photoreduction

This is the normal pathway of riboflavin photolysis in aqueous and organic

solvents. It has been suggested that the intramolecular photoreduction occurs through

dehydrogenation of the N(10)-ribityl side chain in the excited state to give ketonic or

aldehydic functions and simultaneous reduction of the flavin (isoalloxazine) ring.

This is followed by the oxidation of the side-chain to give formylmethylflavin (2).

This compound is an intermediate in the photolysis sequence of riboflavin (Moore et

al., 1963; Smith and Metzler, 1963; Cairns and Metzler, 1971; Ahmad and Vaid,

54

2006; Ahmad et al., 2006b) and is reported to hydrolyse to lumichrome (3) and

lumiflavin (4) (Song et al., 1965; Ahmad et al., 1980; 2006a;) and is oxidized to

carboxymethylflavin (5), (Fukumachi and Sakurai 1954; Treadwell et al., 1968;

Ahmad et al., 2006b). The chemical structures of riboflavin and photoproducts are

shown in Fig.10. The primary process involved in the photolysis of riboflavin

requires the abstraction of a hydrogen atom from the α-CH group of the ribityl side-

chain to give a biradical intermediate (Heelis, 1991) that disproportinates leading to

the formation of the reduced 2′-keto flavin. This is oxidized to give

formylmethylflavin or similar compounds (Fig.10).

2.8.4.2. Intermolecular photoreduction

The biological oxidation-reduction reactions of riboflavin occur through

participation of certain enzymes including FMN and FAD, the prosthetic group of

which is riboflavin. The intermolecular photoreduction of flavins involves the

addition of two electrons from external donors to give a fully reduced flavin (6)

A number of compounds such as amino acids, thiols and aldehydes are known as

electron donors in these reactions. Molecular oxygen can oxidize the reduced flavins

back to the oxidized form under favourable conditions (Heelis, 1982, 1991).

• The intermolecular photoreduction of flavins occurs through a one-electron

transfer from a donor to the flavin triplet state (3Fl) to produce a semiquinone

radical (FlH) followed by dismutation of the radical to form the fully reduced

flavin.

55

Riboflavin (1) Formylmethylflavin (2)

Lumichrome (3) Lumiflavin (4)

Carboxymethylflavin (5) 1,5- Dihydroflavin (6)

Fig. 10. Chemical structure of riboflavin and photoproducts.

56

The technique of laser flash photolysis has been used to understand the mechanism of

intermolecular electron transfer reactions involving flavins (Tollin, 1995).

2.8.4.3. Photosensitized reactions

Photosensitization is a process by which a photophysical alteration occurs in

one molecular entity as a result of initial absorption of radiation by another molecular

entity called a photosensitizer. In the presence of air and light riboflavin can initiate

photosensitized reactions involving a number of electron donors (substrates) (Heelis,

1991; Silva and Edwards, 1996; Ahmad and Vaid, 2006; Encinas and Pervitali, 2006;

Silva and Quina, 2006; Garcia et al., 2006; Rivlin, 2007). The photosensitized

oxidation reactions involve Type I or Type II, mechanisms (Taylor and Radda, 1971;

Silva et al., 1994; Karlsen, 1996).

In Type I mechanism, the substrate reacts with the sensitizer excited state

(either singlet or triplet) to give radicals or radical ions, respectively, by hydrogen

atom abstraction or electron transfer to oxygenated products. In Type II mechanism,

the excited sensitizer reacts with triplet oxygen (3O ) to form singlet molecular oxygen

(1O ) which then reacts with substrate to form the products (Tonnesen, 1996). The

type I and type II mechanisms of the photosensitized oxidation reactions (Silva and

Quina, 2006) may be expressed as follows.

2

2

57

Type I mechanism (in presence of low oxygen concentration)

Type II mechanism (in presence of high oxygen concentration)

RF, 1RF and

3RF represent riboflavin in the ground state, singlet state and triplet state,

respectively, RF•−

, RFH• and RFH2 are the radical anion, the semiquinone radical and

the reduced form of riboflavin; SH is the reduced substrate and SH•+

, S• and

1O2 are

the radical cation, the radical and singlet oxygen, respectively.

The riboflavin-senstized photoreactions include photooxidation,

photodecarboxylation, photoisomerization, photomonomerization,

photopolymerization, photodegradation, photoinitiation, photoinactivation,

photoinduction and photomodification (Ahmad and Vaid, 2006; Silva and Quina,

2006; Garcia et al., 2006; Encinas and Pervitali, 2006; Ahmed, 2009).

CHAPTER III

APPLICATIONS OF LASER

FLASH PHOTOLYSIS,

SPECTROSCOPY AND

ELECTRON MICROSCOPY IN

PHOTOPOLYMERIZATION

59

3.1. Introduction

The glass ionomer cement (GIC) or glass polyalkenoate cement (GPC) was

developed by Wilson and Kent (1971, 1972) and is used in dentistry as restorative

material. It is also used as an adhesive, fissure sealant and tooth filling material. GIC

is formed by the acid-base reaction of an ion leachable fluoro-alumino-silicate glass

with an acidic polyelectrolyte usually poly (acrylic acid) (Wilson, 1978; Zainuddin et

al., 2008). The setting reaction of the GICs involves gelation of the polyacid by cross-

linking of the carboxyl groups with cations liberated from the glass to form a polysalt

matrix (Crisp and Wilson, 1974a, 1974b; Crisp et al., 1974). In addition to this, the

setting mechanism in the resin-modified glass ionomer cements (RMGICs) involves a

free-radical polymerization reaction leading to the formation of a polymer network

(Wilson, 1990). The photoinitiated polymerization of vinyl monomers such as

hydroxyethyl methacrylate (HEMA) by riboflavin (RF) has been studied by laser

flash photolysis and transient absorption spectroscopy. The object of this study is to

highlight the applications of: 1) laser flash photolysis in the photoinitiated

polymerization processes of GICs; 2) spectroscopic techniques in the structural

characterization of GICs components; and 3) electron microscopy in the study of

surface properties of GICs.

The photoinitiated polymerization of vinyl monomers has been the subject of

extensive study over the past fifty years (Oster and Yang, 1968; Slifkin, 1971; Eaton,

1986; Paczrkowski et al., 1999) and the application of laser flash photolysis has

facilitated the understanding of the excited state interactions of photoinitiators and the

monomers (Previtali et al., 1994; Bertolotti et al., 1999; Encinas and Previtali, 2006)

60

leading to the polymerization process. RF has widely been used as a photoinitiator in

vinyl polymerization and its photochemistry has been reviewed by many workers

(Penzer and Radda, 1971; Song, 1971; Hemmerich, 1976; Muller, 1981; Heelis, 1982,

1991; Tollin, 1995; Holzer et al., 2005; Ahmad and Vaid, 2006).

The earlier applications of laser flash photolysis to flavin photochemistry has

played a significant role in the understanding of the flavin-mediated electron transfer

reactions (Schreider et al., 1975; Heelis, et al., 1980, 1981, 1985; Ahmad and Tollin,

1981a, 1981b; Ahmad and Vaid, 2006; Ahmad et al., 1981, 1982; Traber et al., 1981;

Karen et al., 1983; Hazzard et al., 1987; Tollin, 1995; Tollin, et al., 1993). Further

studies have been carried out on the photoinitiated free radical polymerization

electron-transfer reactions (Seigalski and Paczkowski, 2005), excited state interaction

of flavins with amines (Encinas and Previtali, 2006), and fluorescence quenching

mechanisms of flavins (Karen et al., 1983). These electron transfer reactions

constitute the basis of photoinitiated polymerization processes involved in GICs.

3.2. Spectral and Photophysical Properties of Riboflavin

In view of the importance of RF as a major photoinitiator in polymerization

processes of GICs, a knowledge of the spectral and photophysical properties of RF is

necessary for the understanding of laser flash photolysis studies. The UV and visible

absorption spectra of RF in aqueous and organic solvents have been reported by

Koziol (1966a), Heelis (1982, 1991), Ahmad and Rapson (1990), Du et al. (1998),

Sikorska et al. (2005) and Ahmad et al. (2006a). It exhibits peaks around 220, 265,

375 and 445 nm in aqueous solution and all the absorption maxima possess high

molar absorptivities (>104 M

–1 cm

–1), corresponding to π–π* transitions. RF emits an

61

intense yellow-green fluorescence at 520–530 nm in aqueous and organic solvents

with a quantum yield of 0.26–0.30. The cationic and anionic forms of the molecule

are non-fluorescent (Sun et al., 1972; Heelis, 1982; Visser and Muller, 1979; Drossler

et al., 2002).

3.3. Applications to Polymerization of 2-Hydroxyethyl methacrylate

Laser flash photolysis has been applied to study the polymerization of 2-

hydroxyethyl methacrylate (HEMA) in a methanolic solution using safranin T as the

photoinitiator in the presence of tertiary aliphatic amines. The polymerization rate is

increased with the amine concentration to a maximum and then decreased slowly.

Safranin T in the presence of amines yields the triplet species 3SH

+, 3

S+ and the

unprotonated semireduced radical, SH•. The polymerization involves the interaction

of the excited triplet state of safranin with the added amine (Previtali et al., 1994).

A detailed investigation of the polymerization of HEMA in methanol,

photoinitiated by riboflavin in the presence of triethanolamine (TEOHA), has been

conducted by laser flash photolysis (Bertolotti et al., 1999). The transient absorption

spectrum of photolysed solutions is ascribed to the triplet–triplet absorption of RF.

The triplet decay is accompanied by the appearance of a long-lived absorption band

in the 500–600 nm region due to the formation of the semireduced RF radical and is

shortened at 670 nm in the presence of TEOHA. The results indicated that the triplet

quenching by TEOHA (bimolecular quenching rate constant 8 × 108

M–1

s–1

) is due to

the electron transfer from the amine to the triplet of RF with the formation of

semireduced flavin (RF•−

) and semioxidised amine (Am•+

). The protonation of the RF

62

radical anion by proton transfer from the amine radical cation produces the RF neutral

radical.

3RF + Am → (RF

• − + Am

• +) → RF H• + Am (−H)• (3.1)

The interaction of triplet excited state of RF with the amine results in the

polymerization of HEMA. In the light of the previous work and further studies a

photoinitiation mechanism of HEMA polymerization by RF/ TEOHA in aqueous

solution has been proposed (Orellana et al., 1999). It has been suggested that the free

radicals produced in the photoinduced electron transfer from TEOHA to excited RF

lead to the polymerization of HEMA. The polymerization rates increase with the

amine concentration and are maximum at an amine concentration of 0.01 M. Based

on the time-resolved photolysis studies a scheme of the photochemical behavior RF in

the presence of amine (Am) and monomer (HEMA) in aqueous solution at pH 9 has

been presented (Orellana et al., 1999). It has been proposed that the interaction

between the RF triplet state and HEMA does not lead to polymerization. The process

involves RF triplet quenching by the amine to produce radicals initiating the

polymerization reaction.

The RF initiated photopolymerization rates of HEMA have been determined

using amines of different chemical structure as co-initiators (Encinas et al., 2001).

The results have been explained in terms of the dependence of the production of

amine radicals and their reactivity towards the monomer double bonds with the

structural features of the amines. Photochemical studies of 4-2 (2-hydroxy-3-

trimethylammoniumpropoxy) thioxanthone (TXA) in water (pH 9.5) using laser flash

63

photolysis indicated the dependence of the polymerization rate of HEMA on the

amine concentration. This has been explained in term of the interaction of the excited

state of TXA with the amine in different media. The excited singlet and triplet states

are quenched by electron transfer from the amine to TXA forming a charge transfer

intermediate. This is followed by rapid proton transfer from the amine radical cation

to the TXA radical anion to give the ketyl and amine neutral radicals:

TXA* + Am → (TXA

• − + Am

• +) → TXAH• + Am (−H)• (3.2)

↓

TXA + Am

The amine radical [Am (−H)•] originating from the interaction between TXA

triplet state and the co-initiator is involved in the initiation of the polymerization of

vinyls monomers (Valdebenito and Encinas, 2003).

3.4. Spectroscopic Techniques

3.4.1. UV and Visible Spectroscopy (UV/VIS)

The UV and visible absorption characteristics of RF have been reviewed in

Section 3.2. Spectral changes occurring in the UV and visible region on the

photolysis of aqueous solutions of RF (Ahmad and Vaid, 2006; Ahmad et al., 2004a,

2008; Holzer et al., 2005) and formylmethylflavin (Ahmad et al., 1980, 2006b) have

been studied. Variations in the UV and visible spectra of lumichrome in the presence

of various concentration of butylamine (Encinas et al., 2002), of 2-hydroxyethyl

methacrylate (HEMA) / hydroxytelechelic polybutadiene (HTBP) / 2,2-dimethyl-2-

hydroxyacetophenone (Darocur 1173) mixture (Desilles et al., 2005) and of the

absorption coefficient of a dental resin as a function of wavelength at different

64

illumination times (Chen et al., 2007) have been reported. The visible absorption

spectra of methylene blue and thionine in aqueous solutions and in poly-2-

hydroxyethyl methacrylate hydrogel (PHEMA) matrix in the presence of surfactant

(SDS and TAB) have been studied (Ghanadzadeh Gilani et al., 2008).

3.4.2. Transient Absorption Spectroscopy (TAS)

Several studies have been reported on transient absorption measurement in the

UV and visible region on the excited state interactions of flavins in the initiation of

vinyl polymerization (Encinas and Previtali, 2006). The technique has been useful in

the identification of the excited state species and in the understanding of their role in

the mechanism of the reaction.

The measurements of transient absorption spectra of excited state species

include those of the monoprotonated safranin triplet state, 3SH

+ (λmax 830 nm),

unprotonated safranin triplet state, 3S (λmax 420 nm) (Previtali et al., 1994), riboflavin

triplet state, 3RF (λmax 670 nm), semireduced riboflavin radical, RFH•

(λ max 500–

600 nm) (Bertolotti et al., 1999), lumichrome triplet state, 3LC (λmax 370 nm),

semireduced lumichrome radical, LCH• (λmax 450 nm) (Encinas et al., 2002) and 4-(2-

hydroxy-3-trimethylammoniumpropoxy)thioxanthane triplet state, 3TXA (λmax 580

nm) (Valdebenito and Encinas, 2003).

3.4.3. Infrared Spectroscopy (IR)

The application of IR spectroscopy in the study of the acid-base reaction in

GICs is of qualitative nature. IR has been used to study the interaction between

cations and carboxylate groups in various media. The IR spectra exhibit C=O peak at

1700 cm–1

as a result of the acid neutralization (Crisp et al., 1974). An IR

65

spectroscopic study of the formation of cements between metal oxides and aqueous

solutions of poly (acrylic acid) indicates that oxides of divalent metals form cement

gels more readily than the oxides of trivalent metals. Structural information can be

obtained from the observed frequency shift of the absorption band of the carboxylate

groups (~1700 cm–1

) in the metal poly (acrylate) gel relative to those of the purely

ionic form (Crisp et al., 1976). The IR spectra during hardening of GICs show a shift

of the characteristic band of the silicate network (1000 cm–1

) towards high frequency,

with time (Matsuya et al., 1996).

The setting of a hybrid cement composed of ethyl cyanoacrylate and a glass

ionomer cement (ECGIC) has been studied by mid-IR spectroscopy. In the spectrum

of ECGIC, both the acid-base reaction (of the GIC components) and the

cyanoacrylate polymerization have been identified by their characteristic peaks. The

spectrum indicates that the cyanoacrylate polymerization is complete within 6–7 min,

while the acid-base reaction continues 45 min after mixing. IR spectroscopy is

particularly valuable in the development of dental materials, where correct setting is

of fundamental importance to achieve the desired mechanical properties (Tomlinson

et al., 2007). The photoinduced polymerization of poly (ethylene glycol)

dimethacrylate and poly (ethylene glycol) monomethacrylate in imidazolium-based

ionic liquids (ILS) has been studied by IR. An interaction between the monomer

carbonyl group and C2–H of the imidazolium ring is indicated by a shift of the IR

absorption peaks of the two groups. The relative strength of polar interactions

depends on the position of the C=O absorption. The polymerization rates are directly

66

related to the magnitude of the monomer/IL interaction (Andrezejewska et al., 2009;

Woecht et al., 2008).

3.4.4. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR spectroscopy has been used to study the role of certain ingredients in the

setting of GICs. It has been confirmed that (+)-tartaric acid incorporated into GICs to

control the setting characteristics, reacts more readily with the glass than poly(acrylic

acid), resulting in the delay of the setting of the cement. (+)-Tartaric acid not only

reacts rapidly to yield calcium tartrate (1385 cm–1

), but also enhances the rate of

formation of aluminum polyacrylate (1600 and 1460 cm–1

) within the cement. FTIR

is capable of showing distinct differences between the rates at which the various

bands appear in different formulations, of demonstrating the relative ease of tartrate

formation, and of showing the corresponding suppression of polyacrylate formation

(Nicholson et al., 1988).

A FTIR study on the free radical setting mechanism of resin modified GICs

(RMGICs) has shown that the curing efficiency is reduced in commercial products

immediately after irradiation. Non-irradiated specimens after the dark storage

demonstrated higher carboxylate yields compared to their irradiated analogs.

Photopolymerization seems to greatly reduce the acid-base reaction during the early

setting stages of RMGICs (Kakaboura et al., 1996). FTIR and Raman studies indicate

different peak shifts for the polyacid neutralization and polymerization reactions

(Wan et al., 1999). The physical and chemical changes of aesthetic restorative

materials in artificial saliva have been investigated using FTIR spectroscopy. The

spectra show major chemical changes in a GIC in P–NH2 group bonding (900–1000

67

cm–1

) which may be responsible for increase in microhardness after immersion in the

artificial saliva solution. A direct correlation between the surface roughness and

spectral changes of the tested material has not been observed (Yip and To, 2005).

These authors have summarized the FTIR studies of the reaction mechanism and

chemical changes of GICs conducted by Lee et al. (1995a, 1995b), Papagiannoulis et

al. (1997), Eliades et al. (1998) and DeMaeyer et al. (2002). FTIR spectroscopy has

been applied to the structural study of ethylhydroxymethylacrylate (EHMA), an

isomeric analogue of HEMA (Antonucci et al., 2000) and to determine the effect of

water on the polymerization of HEMA (Jafarzada Kashi et al., 2007). The degree of

conversion of HEMA is decreased by increasing the water content in HEMA solution.

A new diamond attenuated total reflectance Fourier transform infrared (ATR FTIR)

method has been developed to quantify the process taking place in a RMGIC at 1 mm

depth from the cement/water interface. Various spectral changes have been observed

due to 90% polymerization within 1 min after 20 s exposure to a dental light. On the

completion of polymerization further IR peak shifts (1300 and 1322 cm–1

), with time,

are observed. A comparison of the spectral changes in RMGIC during setting with

those of the conventional GIC indicates that these could be assigned to water sorption

and / or polyacid neutralization. Such behavior suggests two separate diffusion

mechanism for acid neutralization (Young, 2002). The technique has been used to

quantify polyacid neutralization and light-catalyzed polymerization rates in various

GICs, RMGICs and compomers. At 150 s after light exposure, levels of methacrylate

polymerization (1730 cm–1

) on the lower surface of 1 mm thick specimens are 97–

98% for the GICs and RMGICs and 47% for the compomers. After light exposure,

68

polymerization rates for compomers decrease linearly with inverse time. Reaction

mechanisms have been discussed and used to interpret the mechanical properties,

fluoride release and adhesion to tooth structure (Young et al., 2004). FTIR data

confirm that RMGIC Vitrebond (VB) exhibits the carboxylate cross linking reactions

of a true glass ionomer (Mitra et al., 2009).

3.4.5. Raman Spectroscopy

The Raman spectrum of poly (acrylic acid) and the various peak assignments

have been reported by Dong et al. (1997). Raman spectra of various combinations of

GIC components have been compared with those of the reactants and the salts of poly

(acrylic) and tartaric acids. The components consisted of a fast-setting acid-

degradable dental glass, poly (acrylic acid) and /or tartaric acid. The spectra obtained

on the addition of water to the glass and tartaric acid indicates the loss of acid and

formation of tartarates within a few seconds of mixing. After about 1 h, Raman peaks

associated with the ionized species disappear. Raman spectra show changes during

the reaction between a very fast setting fluoro-alumino-silicate glass, poly (acrylic

acid), tartaric acid and water at various acid and water ratios (Young et al., 2000).

The degree of polymerization of HEMA in the presence of camphorquinone as

photoinitiator has been measured using micro-Raman spectroscopy (Wang et al.,

2006).

Raman spectroscopic methods have been developed for characterizing the

rates of both the polyacid glass (GIC) reaction and the light-catalyzed polymerization

process in hybrid materials. These methods have been applied to commercial

RMGICs and the acid modified composite (compomer). Several Raman and FTIR

69

peaks have been identified that can be used to quantify any polymerization process.

The C=C stretching peak at1634 cm–1

of the methylacrylate group is one of the most

intense in the Raman spectra. It is well removed from any other intense peak in

RMGI and its height can be used for quantitative analysis. It has been observed that

after 20 s of cure this peak height is 10% of that observed before cure. Raman

scattering intensity is generally proportioned to the concentration (Young et al.,

2009).

3.4.6. Nuclear Magnetic Resonance Spectroscopy (NMR)

The structural changes during hardening of GICs and the role of the silica

phase in the hardening of the cement have been investigated by the application of

solid state NMR spectroscopy. The 27

Al NMR analysis of the cement samples after

the comprehensive strength measurements shows that Al3+

ion is tetrahedrally

coordinated by oxygen in the original glass but a part of the Al3+

ion is octahedrally

coordinated after hardening to form Al polyacrylate gel. The chemical shift of Si in

the 29

Si NMR spectra also changes during hardening. The variation in the chemical

shift indicates the structural changes in the silicate network (Matsuya et al., 1996).

The quantitation of five-and six-coordinated Al3+

ions in aluminosilicate and fluoride-

containing glass has been achieved by high field, high resolution 27

Al NMR (Stebbins

et al., 2000). NMR has also been used to study the setting mechanisms of GICs

(Maeda et al., 1993; Pires et al., 2004). The polymerization efficacy of self-etching

dental adhesives in different solvent evaporation conditions have been studied by

NMR spectroscopy. Temperature increase after the photopolymerization reaction is

dominant towards the effect of the drying step for solvent evaporation. Attempts to

70

remove the solvents did not increase the extent of polymerization (Numes et al.,

2006).

3.4.7. Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy (MAS–

NMR)

MAS–NMR has been used for the structural characterization of various GICs.

The 29

Si NMR spectra are consistent with Q4 (3Al) and Q

4 (4Al) units being present

and agree with the low non-bridging oxygen contents. The 27

Al NMR spectra

typically exhibit three distinct sites at 45-60, 20 and 0 ppm, attributed to Al (IV), Al

(V) and Al (VI) coordinate aluminium.

The 31

P NMR spectra show a chemical shift between –8 and –23 ppm

indicating a PO4 tetrahydra surrounded by 1–4 Al moieties. The 19

F NMR spectra

indicate the presence of Al-F-Ca (n) and Al-F-Sr (n) in different glasses. A peak

corresponding to Al-F-Na (n) is present in all glasses. The intensity of the peak is

approximately proportional to the sodium content (Stamboulis et al., 2004). Similar

studies on the structural characterization of fluorine containing glass by 19

F, 27

Al,

27Al,

29Si and

31P MAS–NMR have been reported (Kiczenski et al., 2004; Stamboulis

et al., 2005; Matsuya et al., 2007).

A 27

Al MAS–NMR study on the setting reaction of GIC has clearly shown the

formation of six coordinate, aluminium Al (VI) cross-linking to carboxyl groups in

the poly(acrylic acid). It has been found that the composition of original glass exerts a

substantial effect on the setting behaviour of the cements and the presence of

phosphorus has a strong influence in the setting reaction (Zainuddin et al., 2008).

71

3.4.8. Time-of-Flight Secondary Ion Mass Spectroscopy (TOF–SIMS)

An application of TOF–SIMS to the study of the adsorption of monomers to

hydroxyapatite (HA) has been reported (Smith, 1998). The technique indicates that

increased hydrophobicity and reduced chain flexibility and mobility in functional

monomers and polymers reduce intrinsic bonding to HA on adsorption.

3.4.9. Fluorimetry

The fluorescence decay profiles of RF in HEMA / water (1:2), water, pH 9

(λex 440 nm, λem 525 nm) (Orellana et al., 1999) and in HEMA / methanol in the

absence and presence of triethylamine (TEA) (Encinas et al., 2001) and of TXA in

water pH 9.5, HEMA / water (1:4), HEMA / acetonitride (1:1) (λex 382–392 nm, λem

382–472 nm) (Valdebenito and Encinas, 2003) have been determined. Stern-Volmer

plot for the fluorescence quenching of lumichrome singlet state (1LC) (λex 450 nm,

λem 550 nm) (Encinas et al., 2002), and Rehm-Weller plot for the quenching of the

fluorescence of neutral red and thionine dyes (Buchwieser et al., 1991) have been

reported. The fluorescence quantum yield of riboflavin in neutral water (pH 7) is ≈

0.26. The cationic form (pH –1.09) is non-fluorescent and the quantum yield of the

anionic form (pH 13.35) is ≈ 0.001155 (Drossler et al., 2002).

3.4.10. Atomic Absorption Spectroscopy (AAS)

AAS has been used to determine the elements involved in the setting process

of GICs. In freshly prepared cements (dissolved in 3% potassium hydroxide solution),

the sequence of release of elements shows aluminium to be the first in the setting

matrix (Cook, 1983a). The analysis of different cations such as potassium and

72

rubidium on their interaction with GICs and uptake from aqueous solution has been

carried out by this technique (Hadley et al., 2001).

3.4.11. Inductively Coupled Plasma Emission Spectroscopy (ICP–ES)

In a comparative study of the composition of mineral trioxide aggregate

(MTA) and Portland cement, ICP–ES has been used to evaluate the chemical

elements present. The results show similarities between the materials, except that

there is no detectable quantity of bismuth in Portland cement. It has been concluded

that there is no significant difference between the other elements in both Portland and

MTA and Portland cement may be used as an option to MTA in dental practice

(Funteas et al., 2003). ES has been used to study the conventional RMGICs by

monitoring the peak intensities of chromium ions in the aluminium oxide matrix as a

function of the reaction time (due to poly-salt formation). Emission spectra have been

used to compare the different setting reactions with and without light irradiation. The

differences of the setting reactions are based on the formation of the polymer network

due to light–induced polymerization (Jin, 2000).

3.4.12. X-Ray Photoelectron Spectroscopy (XPS)

In spectroscopic studies of the stability of the chemisorption of monomers on

clean sintered hydroxyapatite (HA) surfaces, the C peaks in the energy spectrum of

HA shows evidence of C–H bonding at 285 eV together with several minor peaks for

oxygenated components indicative of the ubiquitous presence of C contaminants. A

characteristic peak for –COOH group (209 eV) was present in another specimen

having a layer of poly (acrylic acid) adsorbed from solution and dried. XPS shows

that considerable amounts of poly (acrylic acid) remain adsorbed to HA even after

73

boiling the HA specimens in water for 2 h (Smith, 1998). On the basis of XPS of a

self-cured GIC, it has been proposed that an ion exchange layer is formed by the

reciprocal diffusion of the different elements forming the GIC and dentine, with the

exception of collagen (Tanumiharaja et al., 2001). XPS studies have shown that the

methacrylated copolyalkenoic acid component present in RMGIC VB and VBP are

chemically bonded to the calcium in HA (Mitra et al., 2009).

3.5. Electron Microscopy

3.5.1. Scanning Electron Microscopy (SEM)

A detailed study of the working and setting times of GICs after mixing by the

measurement of scanning electron micrographs at different time intervals has been

conducted (Pearson, 1991). It has been found that the surface of the GICs at 10 min

finish causes some disturbance to the structure of the restoration and produces fault

lines. These lines are not apparent on the specimen finish after 24 h. The effect of

etching the GICs at 10–60 s immediately after mixing shows that longer the etch

time, the more damage caused to the subsurface. The enamel surfaces exposed to

different orthodontic bonding systems have been evaluated by SEM. The bonding

system with self-etching primers or conditioners with poly (acrylic acid) might offer

potential benefits compared with the conventional acid etching and priming because

of fewer irreversible changes to the enamel surface (Fjeld and Ogaard, 2006).

The strengthening of GIC by compounding short fibers with CaO–P2O5–

SiO2–Al2O3 (CPSA) glass has been investigated to observe the microstructure of the

fractured surface (after diametral tensile strength measurement) of the set cement.

CPSA glass short fibers are more effective in the strengthening of GIC than the

74

electric glass fibers (Kobayashi et al., 2000). The fractured surface of commercial

GICs has been examined using SEM to ascertain the relationship between the

mechanical properties and the microstructures of the cements (Xie et al., 2000). The

mechanical properties and morphology of polycarbonate/ethylene-1-octylene

copolymer (PC/POE) binary blends and PC/POE/ ionomer ternary blends have been

studied by SEM of fractured surfaces. The tensile strength and elongation at break of

the PC/POE blends decrease with increasing the POE content. The PC/POE weight

ratio has a greater effect on the morphology of PC/POE blends (Li et al., 2003).

Twelve specimens of mineral trioxide aggregate (MTA) and Portland cement material

have been examined for the percentage composition of chemical elements. In spite of

chemical similarity, a difference in the texture and particles of each material is

observed by SEM (Oliveira et al., 2007).

Field-emission electron microscopy (FE–SEM) has been applied to the

evaluation of GIC-dentine interface morphology after using different surface

conditioners. All the GICs tested demonstrate intimate adaptation to the dentine

surface whether it is conditioned or not (Tanumiharja et al., 2001). A high resolution

SEM study in conjunction with tensile bond strength analysis has been used to

correlate the morphologic findings to the bonding potential of a restorative system.

The binding procedure shows favorable results at ultra-structural level when the

compomer is applied to both the enamel and dentine surface for 30 s (Breschi et al.,

2001). SEM images show evidence of micromechanical bonding at the interface

between VBP and the tooth (Mitra et al., 2009).

75

Electron probe microanalysis has been employed to study the ion exchange of

selected elements (e.g., Ca, P) between dentine and GICs. The element penetration

into the tooth structure and GIC exceeds beyond the “ion exchange layer” as observed

in SEM studies (Knight et al., 2007).

3.5.2. Transmission Electron Microscopy (TEM)

The ultrastructure of the hardened GIC has been observed by TEM in

conjunction with an energy-dispersive X–ray microanalyser. The existence of a

metal-ion-depleted and Si-rich layer (siliceous hydrogel) on the glass surface in the

hardened cement has been confirmed (Hatton and Brook, 1992).

The existence of the glass ionomer phase in a conventional GIC, a RMGIC, a

giomer, a compomer and a composite, before and after water uptake, has been studied

by TEM. The variable extent of glass ionomer phase is determined by differences in

resin composition of the restoratives. Based on the ultrastructural manifestation of the

glass ionomer phase, the compomer is very similar to a resin composite. Unlike a

compomer, a giomer containing a pre-reacted glass-ionomer phase behaves more like

a RMGIC. It may occupy a position between the two materials within the spectrum of

hybrid tooth-colored restoratives (Tay et al., 2001).

A review on the applications of laser flash photolysis, spectroscopy and

electron microscopy in photopolymerization reactions and development of glass

ionomer dental cements has been presented by Ahmad et al. (2011).

76

OBJECT OF PRESENT INVESTIGATION

The polymerization of acrylic acid derivatives is being investigated since

1950s. The use of photoinitiators has facilitated the polymerization reactions

involving a radical mechanism. Several workers have studied the polymerization of 2-

hydroxyethyl methacrylate (HEMA), a widely used material in glass ionomer

cements, using different photoinitiators. So far a comparative study of the reactivity

of photoinitiators in the polymerization of HEMA has not been reported. In view of

this fact, the present study has been designed to evaluate the comparative reactivity of

the different photoinitiators such as riboflavin, camphorquinone and safranin T,

alongwith triethanolamine as a co-initiator, in the polymerization of HEMA in both

aqueous and organic solvents is to be carried out. It is intended to develop a simple

analytical method for the determination of HEMA in polymerized solutions during

the initial stages of the reactions (~5% concentration change) to avoid any volume

changes in the solution due to further reactions. It has been assumed that the rate of

initiation is equal to the rate of termination at low conversion of HEMA using a low

intensity radiation source. Under these conditions the reaction may follow an apparent

first-order kinetics. The rates of polymerization reactions in the presence of these

photoinitiators would be determined at various pH values to observe any effect with a

change in pH. The effect of a change in HEMA concentration on the reaction rate

would also be studied. An important aspect of the work would be to make a

comparison of the rates of polymerization in aqueous and organic solvents and to

evaluate the effect of solvent parameters such as dielectric constant (a measure of

77

polarity) and viscosity (a measure of rheological behavior) of the medium. It is also

intended to ascertain the effect of triethanolamine concentration on the rate of

polymerization and to determine the bimolecular rate constant for the interaction of

HEMA and triethanolamine during the reactions. The kinetic data may provide useful

information on the reactivity of the photoinitiators and their efficacy in the

polymerization of HEMA in aqueous and organic solvents. The variations in rates for

different photoinitiators may throw light on the mode of these reactions and hence the

choice of the best medium to achieve optimum polymerization.

78

PROPOSED PLAN OF WORK

An outline of proposed plan of work on the polymerization of 2-hydroxyethyl

methacrylate (HEMA) is presented as follows.

1. Development of an analytical method for the assay of HEMA during the

initial stages of the polymerization reactions under specified conditions.

2. Performance of polymerization reactions on HEMA in aqueous solutions

in neutral and alkaline region using different photoinitiators, e.g.,

riboflavin, camphorquinone and safranin T, in the presence of

triethanolamine (TEOHA) as a co-initiator.

3. Study of the kinetics of polymerization reactions at various pH values

within about 5% change of HEMA concentration using a low intensity

radiation source.

4. Study of the effect of variation in HEMA concentration on the rates of

polymerization reactions at various pH values.

5. Performance of polymerization reactions on HEMA in organic solvents in

the presence of different photoinitiators as described under 2.

6. Study of the effect of solvent characteristics such as dielectric constant

and viscosity and development of correlations between the polymerization

rate constants and solvent parameters.

79

7. Study of the effect of TEOHA concentration on the rate of polymerization

in aqueous and organic solvents and determination of the bimolecular rate

constants for the interaction of HEMA and TEOHA during the reactions.

8. Comparison of the reactivity and efficacy of the photoinitiators used on

the basis of the kinetic data and to throw light on the mode of

polymerization reactions of HEMA in various media.

CHAPTER IV

EXPERIMENTAL

WORK

81

4.1. Materials

2-Hydroxyethyl methacrylate (HEMA) ≥99+% Aldrich

Synonym: 1, 2-Ethanediol mono(2-methylpropenoate)

Linear formula: CH2=C (CH3) COOCH2CH2OH Mr 130.14

Density: 1.073g/ml at 25˚C

It was distilled under reduced pressure and was stored in a refrigerator at 2-8˚C.

Triethanolamine ≥99% Merck

Density: 1.124 g/ml at 20˚C

Empirical formula: C6 H15 NO3 Mr 149.19

It was distilled under reduced pressure before use.

Camphorquinone 97% Aldrich

Synonym: 2,3 Bornanedione,

Empirical formula: C10 H14 O2 Mr 166.22

It was stored in the dark in a desiccator.

Safranin T Fluka

Synonym: Basic Red 2, Safranin O, 3,7-Diamino-2,8-dimethyl-5

phenylphenazinium chloride

Empirical formula: C20H19ClN4 Mr 350.84

Storage: It was stored in the dark in a desiccator.

82

Riboflavin: (3,10-dihydro-7,8-dimethyl-10-[(2S,3S,4R)-2,3,4,5-tetrahydroxypentyl]

benzopetridine-2,4-dione) Merck

Empirical formula: C17 H2ON4O6 Mr 376.4

It was found to be chromatographically pure and was stored in the dark in a

desiccator.

Lumiflavin (7,8,10-trimethylisoalloxazine) Sigma

Empirical formula: C13H12N4O2 Mr 256.3

It was stored in a light resistant container in the desiccator below O˚C.

Lumichrome (7,8-dimethylalloxazine) Sigma


It was stored in the dark in a desiccator.

Formylmethylflavin (7,8-dimethyl-10-formylmethylisalloxazine)


It was synthesized according to the method of Fall and Petering (1956) by the

periodic acid oxidation of riboflavin. The material was recrystallized from absolute

methanol, dried in vacuo and stored in the dark in a refrigerator.

Potassium Ferrioxalate

Empirical formula: K3Fe(C2O4)3.3H2O Mr 491.2

Potassium ferrioxalate was prepared by the method of Hatchard and Parker (1956) by

mixing three volume of 1.5 M potassium oxalate with one volume of 1.5 M ferric

83

chloride with vigorous stirring. The yellow green precipitated compound was

recrystallized twice from water, dried at 45˚C and stored in the dark in a desiccator.

Buffer Solutions

The following buffer solutions were used:

Phosphate buffer, pH 6.0-8.0; Borate buffer, Na2B4O7-HCl, pH 9.0.

The ionic strength was kept constant in each case (0.02 M).

Reagent/Solvents

All reagents and solvents were of analytical grade or of the purest form available

from BDH//Merck.

Water

Water was purified using a Milli-Q-system.

84

4.2. Methods

4.2.1. Thin-Layer Chromatography (TLC)

The following TLC conditions were used for the separation and identification

of riboflavin and photoproducts.

Adsorbent: a) Silica gel GF 254 (Merck precoated plates)

b) Microgranular CC 41 cellular powder, 10-20 micron

(Whatman) 30g /150 ml distilled water

Layer thickness: 250-µm

Solvent systems: I1; 1-butanol-acetic acid-water (4:1:5, v/v, organic phase) / silica

gel (Treadwell et al., 1968).

I2; 1-butanol-1-propanol-acetic acid-water (50:30:2:18, v/v) /

cellulose powder (Ahmad et al., 1980).

Temperature: 25-27 ˚C

Location of spots: UV light, 254 and 365nm (Unitec lamp, UK)

4.2.2. pH Measurements

An Elmerton LCD display pH meter (model-CP 501, sensitivity ± 0.01 pH

unit, Poland) with appropriate electrodes was used for pH measurements.

The electrodes were calibrated at 25˚C using the following buffers:

Phosphate (0.2 M) = pH 6.86

Disodium tetraborate (0.2 M) = pH 9.18

85

4.2.3. Ultraviolet and Visible Spectrometry

All absorbance and spectral measurements were performed on a Shimadzu

UV visible recording spectrophotometer (model UV-1601) using matched silica cells

of 1cm path length and appropriate control solutions in the reference beam. The base

line correction was made automatically by the built-in baseline memory at the

initializing period. The auto-zero adjustment was performed by one-touch operation.

The instrument calibration was periodically carried out using the following standards.

Wavelength scale: Automatic at the initializing period.

Absorbance scale: 0.050 g/l of K2Cr2O7 in 0.005 M H2SO4

Absorbance at 257 nm = 0.725

350 nm = 0.539 ± 0.005 (Rand, 1969)

Riboflavin solution, pH 4.0 (acetate buffer)

A (1%, 1cm) at 444 nm = 328

(British Pharmacopoeia, 2009).

4.2.4. Fluorescence Spectroscopy

Fluorescence measurements of RF in various HEMA solutions (aqueous and

organic solvent) were carried out at room temperature (~25 ºC) using a Spectramax 5

fluorimeter ( Molecular Devices, USA) in the end point mode using λex = 374 nm and

λem = 520 nm (Song and Metzler, 1967). The fluorescence was recorded at relative

fluorescence unit using a pure 0.05 mM RF solution as standard. The solutions were

86

protected from light with aluminum foil to prevent photolysis of riboflavin during

handling.

4.2.5. Light Intensity Measurement

The potassium ferrioxalate actinometer (Parker, 1953; Hatchard and Parker,

1956) has been found as the most useful solution phase actinometer and may be used

over a wide range of wavelengths in the UV and visible region (254–577 nm). It has

been used by Holmstrom and Oster (1961), Byrom and Turnbull (1967), McBride and

Moore (1967), Ahmad (1968) and Ahmad et al., (2004a, 2004b, 2005, 2006a, 2006b,

2008, 2009, 2010a) for the measurement of light intensity in the photolysis of

riboflavin and analogues. This actinometer has been employed for light intensity

measurements of the radiation source employed in this study.

In the actinometer the solutions of potassium ferrioxalate in sulphuric acid are

irradiated. This results in the reduction of ferric ion to ferrous:

On the completion of irradiation the amount of Fe2+ ions produced may be measured

spectrometrically by the formation of the red ferrous-phenanthroline complex and the

determination of its absorbance at 510 nm.

It is important to ensure that the actinometer solution completely absorbs the

radiation during the period of the measurement. The actinometry is performed before

87

and after irradiation and the lamp assumed to maintain the mean during the

experiment.

4.2.5.1. Procedure:

An oxygen free 0.006 M solution of potassium ferrioxalate (2.947 g/1) in 0.05

M H2SO4 is placed in the reaction vessel and irradiated with a General Electrics 15 W

fluorescent lamp (emission in the visible region) used for the photolysis. The

irradiation of this solution is carried out under nitrogen (90–120 bubbles/minute). The

temperature of the reaction vessel is maintained at 25 ± 1˚C by circulating water from

a thermostat cooling unit.

An aliquot of the photolysed solution of potassium ferrioxalate (1–2 ml) is

pipetted out at appropriate intervals (upto 15 minutes) into a 20 ml volumetric flask,

to which 1.8 ml of 0.5 M H2SO4 + 2 ml of 0.1% 1:10-phenanthroline + 1 ml of buffer

(60 ml M CH3COONa + 36 ml 0.5 M H2SO4) are added. The flask is made upto the

mark with distilled water (final pH 3.5), well shaken to mix and allowed to stand for

one hour in the dark to develop the coloured complex. The absorbance of the

phenanthroline-ferrous complex is measured in a 1 cm cell at 510 nm using the

appropriate solution in the blank. The amount of Fe2+ ions formed is determined from

the calibration graph. The calibration graph is constructed in a similar manner using

several dilutions of 1× 10−6 M/ml of Fe2+ in 0.05 M H2SO4 using a standardized 0.1

M FeSO4 in 0.05 M H2SO4 (Fig.11). The value of the molar absorptivity of Fe2+

complex, as determined from the slope of the calibration graph, is equal to

88

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12

Concentration of Fe++

× 105 M

Ab

sorb

an

ce a

t 51

0 n

m

Fig. 11. Calibration graph for the determination of K3Fe(C2O4)3.

89

1.11×104 M−1 cm1 and is in agreement with the value reported by Parker (1953).

Employing the known values of the quantum yield for ferrioxalate actinometer at

various wavelengths (Hatchard and Parker, 1956), the number of Fe2+ ions formed

during the photolysis, the time of exposure and the fraction of the light absorbed by

the length of the actinometer solution, the light intensity incident just inside the front

window of the photolysis cell can be calculated. In the present case it has been

assumed that total absorption of the light takes place.

4.2.5.2. Calculations:

The number of Fe2+ ions formed during photolysis (nFe2+) is given by the

equation:

where V1 = volume of the actinometer solution irradiated (ml),

V2 = volume of the aliquot taken for analysis (ml),

V3 = final volume to which the aliquot V2 is diluted (ml),

1 = path length of the spectrophotometer cell used (1cm),

A = measured absorbance of the final solution at 510 nm,

ε = molar absorptivity of the Fe2+ complex (1.11 × 104 M−1 cm−1)

90

The number of quanta of the radiation source absorbed by the actinometer solution,

nabs, can be determined as follows:

where Φ = quantum yield for the Fe2+ formation at the desired wavelength (Hatchard

and Parker, 1956).

Considering t as the irradiation time of the actinometer in seconds, the number of

quanta per second per cell, nabs, is determined by the equation:

The average intensity of the lamp used has been determined as 2.1±0.2×1016quanta

s−1.

91

4.2.6. Polymerization of 2-Hydroxyethyl methacrylate (HEMA)

4.2.6.1. Radiation source

The choice of the radiation source for a photochemical reaction is an

important factor for the desired reaction to occur and depends on the chemical nature

and absorption characteristics of the reactants and the duration of the required change

in the reactants. It also depends on the total output of energy and the spectral and

relative energy distribution of the source. Since riboflavin (RF), camphorquinone

(CQ) and safranin T (ST) have been used as photoinitiators in this reaction, a General

Electrics 15 W fluorescent lamp (emission in the visible region) was used for the

photochemical work. The visible radiations are absorbed by all the photoinitiators

used in this work (absorption maxima: RF 445 nm (Heelis, 1982); CQ 468 nm

(Jakubiak et al., 2003); ST 520 nm (Previtali et al., 1994). The low intensity radiation

source has been used to determine the chemical change during the initial stages of the

reactions for kinetic purposes.

4.2.6.2. Method

A 1-3 M solution of 2-hydroxyethyl methacrylate (HEMA) containing

appropriate concentrations of the photoinitiators and triethanolamine (absorbance of

all the solutions at the absorption maxima of the photoinitiators is ~ 0.125 to avoid

inhomogenous free radical distribution (Alverez et al., 1998)) and 0.0025-0.01 M

triethanolamine (TEOHA) in water (pH 6.0-9.0 maintained with 0.02 M buffer) or in

organic solvents, was placed in a 10 ml volumetric flask and immersed in a water

bath maintained at 25±1˚C. The solution was degassed with nitrogen for 20 minutes

92

and then irradiated with the 15 W lamp (emission in the visible region) fixed

horizontally at a distance of 30 cm from the centre of the flask in a radiation chamber.

A gentle stream of nitrogen gas was continuously bubbled into the solution during

irradiation. An appropriate volume of the aliquot of the irradiated solution was

pipetted out at various intervals for spectrometric assay of HEMA.

Aqueous solutions of the photoinitiators (RF, CQ, ST) containing the same

concentration as used during the polymerization of HEMA were also irradiated under

identical conditions to determine any loss of the photoinitiator and formation of

photoproducts during irradiation. Spectral measurements and thin-layer

chromatography of the irradiated solutions were carried out to detect any

photodegradation of the photoinitiators.

RESULTS

AND

DISCUSSION

CHAPTER V

POLYMERIZATION OF

HEMA IN AQUEOUS

SOLUTIONS

95

Acrylic acid derivatives including 2-hydroxyethyl methacrylate monomer

(HEMA) (Mitra, 1991), urethane dimethylacrylate monomer (Atai et al., 2007),

proline modified acrylic acid copolymer

(Moshaverinia et al., 2009a),

N-

vinylcaprolactam-containing acrylic acid terpolymer (Moshaverinia et al., 2009b), N-

vinylpyrrolidone modified acrylic acid copolymer (Moshaverinia et al., 2008), and

polyurethane acrylate monomer (Yaobin et al., 2006) have been synthesized for

dental cement applications. These derivatives are intended to undergo polymerization

on exposure to light and form a hardened mass (cement). Among these derivatives,

HEMA is widely used in glass ionomer cements (GICs) that are employed as

restorative materials (Smith, 1998). Various types of GICs containing HEMA have

been developed as light cure restorative materials (Lee et al., 2010).

The photoinitiated polymerization of vinyl polymers has been studied since

1950s (Oster et al., 1957, 1958; Eaton, 1986; Lissi and Encinas, 1991; Paczkowski et

al., 1999; Encinas and Previtali, 2006). The process involves the participation of

photoinitiators that absorb in the visible region. The kinetics of photopolymerization

reactions has been discussed by several workers (Encinas and Previtali, 2006; Encinas

et al., 1996; Watts and Cash, 1991; Watts et al., 2003; Watts, 2005; Andrezejewska,

2001). The medium characteristics, ionization behavior of reacting species and

efficiency of the photoinitiators are among the major factors that influence the rate of

polymerization reactions (Alvarez et al., 1996). In most cases the

photopolymerization rates of HEMA have been determined dilatometrically

(Bertolotti et al., 1999; Orellana et al., 1999; Encinas et al., 2001). This method has

been used to study the effect of solvent on the rates of polymerization of HEMA

96

photoinitiated by azo compounds, (Alvarez et al., 1996; Valdebenito and Encinas,

2003) camphorquinone, (Wang et al., 2006) and pyrene derivatives (Encinas et al.,

1993). The light activated composite for restorative purpose are one-paste system

which contain a photoinitiator that is sensitive to blue light (470 nm). The curing of

composite resins in this case is initiated by the use of special dental curing lamps that

emit light at 470 nm with a light intensity of 500 Wm–3

(Watts et al., 1984). The

polyacid-modified composite resins are light activated and cured by free-radical

reactions using a photoinitiator and subsequently made to set by a dental curing lamp

that emits at 470 nm in the same way as the true composite resins (Nicholson, 2002).

The present studies are concerned with the polymerization of HEMA using

riboflavin, camphorquinone and safranin T as photoinitiator and triethanolamine

(TEOHA) as a co-initiator in aqueous and organic solvents. The chemical structure of

HEMA and photoinitiators are given in Fig.12. The chemical structure of riboflavin is

shown in Fig. 10. It is intended to conduct a comparative study of the reactivity of

these photoinitiators on the basis of the kinetic data obtained on the polymerization of

HEMA in the pH range of 6.0–9.0 and to throw light on the interaction of HEMA and

TEOHA resulting in the polymerization process. An important aspect of this work is

to study the effect of solvent characteristics such as dielectric constant and viscosity

on the rate of polymerization reactions. These parameters may influence the reactivity

of the photoinitiators in a particular medium (i.e. degree of polarity), depending upon

the nature and life-times of the radicals formed and the extent of their interactions

with HMEA. In addition to this the viscosity of the medium would determine the

magnitude of diffusional processes of the species involved and hence the rate of the

97

Fig. 12. The chemical structures of HEMA, TEOHA and photoinitiators.

98

reactions. The study would provide information on the photoinitiated reactions and

interactions of HEMA with excited species in the pH range 6.0–9.0 and in organic

solvents. It would facilitate the understanding of polymerization processes in light-

cure GIC systems. The nature and mechanisms of polymerization reactions using a

photoinitiator have been discussed in Section 6.2–6.4. The polymerization reactions

of HEMA involving different photoinitiators in aqueous medium and the effect of pH

and viscosity on these reactions are presented in the following sections.

5.1. Riboflavin as Photoinitiator

Riboflavin (RF) has been used as a photoinitiator in the polymerization of

HEMA along with triethanolamine (TEOHA) as a co-initiator to form a redox pair

involved in the process (Bertolotti et al., 1999; Orellana et al., 1999; Encinas et al.,

2001; Porcal et al., 2003). RF is an efficient electron acceptor and meditates in

numerous photochemical and biological electron transfer reactions (Ahmad and

Tollin, 1981a, 1981b; Ahmad et al., 1981, 1982; Ahmad and Vaid, 2006; Heelis,

1982, 1991; Tollin, 1995; Tollin et al., 1993; Scigalski and Paczkowski, 2005).

5.1.1. Spectral Characteristics of RF Solutions

RF exhibits an absorption maximum at 444 nm (ε = 12500 M-1

cm-1

) (Ahmad

et al., 2004a) in the visible region. The absorption spectrum of RF (pH 7.0) is shown

in Fig. 13. A concentration of 1 × 10-5

M (absorbance at 444 nm = 0.125) was used

as a photoinitiator along with 0.0025–0.01 M TEOHA in the polymerization of

HEMA solutions to avoid inhomogenous free radical distribution (Alvarez et al.,

99

Fig. 13. Absorption spectrum of riboflavin at pH 7.0.

100

1998). The spectral characteristics of the photolysed solutions of RF (pH 9.0) during

irradiation (5 min) showed a minor loss of absorbance at 444 nm in the visible region

indicating the formation of LC as the major final photoproduct through FMF as an

intermediate in this reaction (Ahmad and Vaid, 2006; Ahmad et al., 1980; Ahmad et

al., 2006b). Since RF solutions are used at a low concentration (1×10–5

M) in the

polymerization reactions (Bertolotti et al., 1999), any minor spectral changes during

the reaction would not affect the efficacy of RF as a photoinitiator.

5.1.2. Photoproducts of Riboflavin

RF is highly sensitive to light (DeRitter, 1982) and may undergo

photodegradation during irradiation with the visible light. It was felt necessary to

irradiate RF solution and to examine the photoproducts formed in this reaction which,

to some extent, would cause a loss in RF concentration and hence its efficacy as a

photoinitiator. The TLC of the photolysed solutions of RF (1×10–5

M), in the presence

of HEMA, using solvents systems (a) and (b) (Section 4.2.1), indicated the presence

of trace amounts of FMF, LC and LF at pH 9.0 only. In the alkaline solutions the

photodegradation of RF is much greater than that in the acid region (Ahmad and

Vaid, 2006), however, under the present irradiation conditions (~1 min), there would

be negligible loss in RF concentration. The photoproducts of RF observed are known

and have previously been reported (Ahmad and Vaid, 2006; DeRitter, 1982; Ahmad

and Rapson, 1990).

101

5.1.3. Assay of HEMA

A UV spectophotometric method has been used for the assay of HEMA at 208

nm during the photopolymerization reactions. This wavelength corresponds to the

absorption maximum of HEMA (Table 5) and on appropriate dilution would give the

concentration of HEMA monomer during the reaction. The validity of Beer’s Law

was confirmed in the desired concentration range prior to the assay and the content of

HEMA in photopolymerized solutions was determined using 7980 M–1

cm–1

as the

value of molar absorptivity at the analytical wavelength (Table 5). The reproducibility

of the method was confirmed by the assay of known amounts of HEMA in the

concentration range likely to be found in photopolymerized solutions. The values of

RSD for the assay indicate the precision of the method within ± 3%. The results of a

typical assay of HEMA in photopolymerized solutions at pH 9.0 are reported in Table

6. This is a new method for the determination of HEMA in photopolymerized

solutions since most of the previous workers have used dilatometric method for this

purpose (Bertolotti et al., 1999; Orellana et al., 1999; Encinas et al., 2001). In some

cases the rates of polymerization of HEMA have been measured using gas

chromatography (Beers et al., 1999), Raman spectroscopy (Wang et al., 2006), ATR-

FTIR spectroscopy (Guo et al., 2008) and differential scanning calorimetry (Jakubiak

et al., 2003; Anderzejewska et al., 2009). The assay of HEMA has been carried out

during the initial stages of the reaction assuming that a negligible change in volume

occurs in the medium during which does not affect the accuracy of the assay method.

102

Under the Assay conditions of HEMA, on the dilution of photolysed

solutions, the maximum TEOHA concentration used (0.01 M) would be of the order

of 10×-6 M. It would probably be too low to undergo complexation with HEMA in a

molar ratio. The UV spectra of HEMA at such a dilution did not show any change in

UV absorption in the presence of TEOHA. This suggest that no possibility of

interaction between these compounds to effect the rate constant.

The error on the Assay of HEMA was determined to be within ± 3%. In the

Assay of HEMA in photolysed solutions, the analytical error would remain the same

but the magnitude of Assay values would be affected by other factors such as the

variations in light intensity, changes in volume of HEMA, overall analytical errors in

measurement etc. Therefore, the error in the measurement of rate constants may

exceed the 3 % limit as indicated with the rate constants (Table 7-9). Under these

conditions the values of rate constant may be considered as statistically meaningful.

Table 5. Calibration data for HEMA showing linear regression analysis.a

λ max 208 nm

Concentration range 0.1–1.0 × 10–4

M

Slope 7980

SE (±) of slope 0.0112

Intercept 0.0010

Correlation coefficient 0.9995

Molar absorptivity (ε) 7980 M–1

cm–1

a values represent a mean of five determinations.

103

Table 6. Polymerization of HEMA (1 M) at pH 9.0 in presence of RF as

photoinitiator at various TEOHA concentrations.

TEOHA Concentration (M) Time

(sec) 0.0025 0.005 0.0075 0.01

0 1.000 1.000 1.000 1.000

10 0.997 0.996 0.993 0.992

20 0.994 0.991 0.988 0.985

30 0.992 0.988 0.983 0.975

40 0.990 0.983 0.975 0.968

50 0.987 0.976 0.967 0.959

60 0.985 0.971 0.963 0.947

104

5.1.4. Kinetics of Photopolymerization of HEMA

The kinetics of photopolymerization at low conversion of HEMA has been

studied by UV spectrometry as described above. The assay data on HEMA during the

reactions were subjected to kinetic treatment and the photolysis of HEMA (a measure

of photopolymerization) was found to follow pseudo first-order kinetics in the initial

stage of the reaction at 0.01 M TEOHA concentrations (Orellana et al.,1999) at pH

6.0–9.0 (Fig. 14–18). The apparent first-order rate constant (kobs) for the

polymerization of HEMA at 1–3 M in the presence of various concentrations of

TEOHA are reported in Table 7–9, respectively. The polymerization of HEMA at low

conversion remains homogenous since the polymer remains soluble in monomer-rich

aqueous solutions (Encinas et al., 1996). The steady-state assumption of the rate of

initiation being equal to the rate of termination is considered valid only at a low

conversion of monomer (Jakuubiak et al., 2003) and has been observed in the present

study. The value of kobs reported represents the rate of initiation in this case. During

the initial stages of the reaction (within ~5% change), the average degree of

polymerization as well as the viscoelastic properties of HEMA would almost be

constant on changing the co-initiator concentration (0.0025–0.01 M). Under these

conditions, the shrinkage properties of the polymerized solution would largely remain

unaffected.

5.1.5. Effect of pH

The apparent first-order rate constants (kobs) for the photopolymerization of

HEMA indicate that the rate of reaction is enhanced with an increase in pH from 6.0–

105

R2 = 0.993

R2 = 0.995

R2 = 0.993

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60 70 80 90 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 14. First-order plots for the polymerization of HEMA in the presence of RF at

pH 6.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

106

R2 = 0.991

R2 = 0.991

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60 70 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

107

R2 = 0.989

R2 = 0.996

R2 = 0.989

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60 70 80Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.5. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

108

R2 = 0.995

R2 = 0.991

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60 70

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 8.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

109

R2 = 0.992

R2 = 0.993

R2 = 0.991

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 9.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

110

Table 7. The apparent first-order rate constant (kobs) for polymerization of HEMA (1

M) at pH 6.0–9.0 in presence of RF as photoinitiator at various TEOHA

concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 1.05±0.084 1.66±0.099 2.97±0.14 3.70±0.15

7.0 1.61±0.080 2.42±0.12 4.03±0.16 5.05±0.19

7.5 1.97±0.098 3.13±0.15 5.07±0.20 6.32±0.16

8.0 2.49±0.12 3.83±0.14 5.72±0.25 7.89±0.25

9.0 2.48±0.10 4.89±0.16 6.41±0.22 8.87±0.28

111



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.93±0.061 1.45±0.075 2.32±0.092 2.84±0.12

7.0 1.18±0.058 2.24±0.091 3.31±0.12 3.98±0.15

7.5 1.38±0.069 2.86±0.11 4.28±0.15 5.18±0.21

8.0 1.92±0.086 3.23±0.099 5.32±0.20 6.44±0.26

9.0 2.17±0.11 3.57±0.13 5.98±0.18 7.63±0.22

112



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.86±0.052 1.25±0.085 1.94±0.12 2.45±0.14

7.0 1.03±0.050 1.98±0.011 2.82±0.15 3.43±0.12

7.5 1.13±0.068 2.45±0.13 3.60±0.12 4.38±0.21

8.0 1.62±0.11 2.77±0.16 4.56±0.23 5.61±0.25

9.0 2.01±0.099 3.28±0.14 5.39±0.21 6.68±0.22

113

9.0. Since the photopolymerization of HEMA has been carried out in the presence of

TEOHA, a decrease in the ionization of TEOHA (pKa 7.82) (Albert and Serjeant,

1962) from 98.5–6.1% at pH 6.0–9.0 would affect the rate of photopolymerization

since it would facilitate electron transfer from TEOHA to RF triplet state (Orellana et

al., 1999), ultimately resulting in the polymerization of HEMA. The rate-pH profiles

for the photopolymerization of HEMA at 1–3 M concentrations are shown in Fig. 19.

These profiles represent sigmoid type curves as a consequence of an acid / base

dissociation of HEMA molecule. The profiles indicate a great enhancement in the

rate, with pH, in the range of 6.0–9.0, as observed earlier by Valdebenito and Encinas

(2003). An increase in the rate of polymerization, with pH, is attributed to the

presence of a labile proton on the hydroxyl group of HEMA (Jakubiak et al., 2003).

The relative decrease in the rate of polymerization of HEMA (6.0–9.0) from 1–3 M

appears largely to be due to the viscosity effect as discussed below.

5.1.6. Effect of Viscosity

It has been observed that the rate of photopolymerization of HEMA in

aqueous solution (pH 6.0–9.0) is affected by a change in the viscosity of the medium

on increasing the HEMA concentration (1–3 M). The plots of kobs at different HEMA

concentrations versus the inverse of viscosity of the solutions in the pH range 6.0–9.0

are linear (Fig. 20), indicating that an increase in viscosity leads to a decrease in the

rate of polymerization of HEMA probably as a result of flavin triplet quenching

(Ahmad and Tollin, 1981b). In addition to this the rate is controlled by solute

diffusion and would be affected by an increase in the viscosity of the medium. A

114

1 M

2 M

3 M

0.0

2.0

4.0

6.0

8.0

10.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

ko

bs

× 1

04

(s–

1)

Fig. 19. Rate-pH profile for polymerization of HEMA (1–3 M) in presence of RF and

0.01 M TEOHA.

115

Fig. 20. Plots of kobs for polymerization of HEMA in presence of RF and 0.01 M

TEOHA versus inverse of solution viscosity. HEMA: (●) 1.0 M, (■) 2.0 M,

(▲) 3.0 M.

pH 6.0

pH 7.0

pH 7.5

pH 8.0

pH 9.0

0.0

2.0

4.0

6.0

8.0

10.0

0.4 0.5 0.6 0.7 0.8 0.9 1.0

Viscosity (mPa.s)–1

ko

bs

× 1

04

(s–

1)

116

small change in the viscosity of HEMA solutions with an increase in pH has been

reported (Valdebenito and Encinas, 2003). Thus the HEMA concentration appears to

be one of the controlling factors in the rate of polymerization.

5.1.7. Effect of TEOHA Concentration

TEOHA plays an important role in the polymerization of HEMA and the

reaction does not occur in the absence of the co-initiator. RF radicals interact with

TEOHA to cause the polymerization reaction. It has been reported that the rate of

photopolymerization of HEMA is maximum in the presence of 0.01 M TEOHA

(Orellana et al., 1999). In order to observe the effect of TEOHA concentration

(0.0025–0.01 M) and its interaction with HEMA (1–3 M) in the initial stages of

polymerization, the kobs values were determined for the reactions carried out at

various TEOHA concentrations (Fig. 21–25). The plots of kobs versus TEOHA

concentrations were found to be linear (Fig. 26) and the slopes yielded the second-

order rate constant (k2) for the interaction of TEOHA with HEMA. The k2 values

(Table 10) have been found to increase with an increase in pH as explained above for

the polymerization reactions at pH 6.0–9.0. Thus the rate of polymerization is

dependent on the fate of amine radical produced during the reaction and their

interaction with HEMA.

5.1.8. Mechanism of HEMA Polymerization

A photoinitiation mechanism of HEMA polymerization by RF / TEOHA

system in aqueous solution has been proposed by Orellana et al. (1999). Based on the

117

R2 = 0.987

R2 = 0.990

R2 = 0.992

R2 = 0.993

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.992

R2 = 0.993

R2 = 0.992

R2 = 0.995

0.280

0.290

0.300

0.310

log

co

nce

ntr

ati

on

(M

)

R2 = 0.989

R2 = 0.992

R2 = 0.991

R2 = 0.993

0.460

0.464

0.468

0.472

0.476

0.480

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 21. First-order plots for the polymerization of HEMA (1–3 M) in presence of RF

as photoinitiator at different TEOHA concentrations (pH 6.0). (×) 0.0025 M,

(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

2M

3M

1M

118

R2 = 0.994

R2 = 0.991

R2 = 0.992

R2 = 0.991

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.993

R2 = 0.992

R2 = 0.990

R2 = 0.991

0.280

0.290

0.300

0.310

log

co

nce

ntr

ati

on

(M

)

R2 = 0.990

R2 = 0.992

R2 = 0.992

R2 = 0.990

0.460

0.464

0.468

0.472

0.476

0.480

0 10 20 30 40 50 60 70 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

119

R2 = 0.990

R2 = 0.991

R2 = 0.990

R2 = 0.993

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.990

R2 = 0.992

R2 = 0.993

R2 = 0.996

0.275

0.285

0.295

0.305

log

co

nce

ntr

ati

on

(M

)

R2 = 0.992

R2 = 0.991

R2 = 0.992

R2 = 0.989

0.450

0.460

0.470

0.480

0 10 20 30 40 50 60 70 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

120

R2 = 0.993

R2 = 0.990

R2 = 0.992

R2 = 0.996

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.993

R2 = 0.995

R2 = 0.991

R2 = 0.990

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.989

R2 = 0.991

R2 = 0.994

R2 = 0.992

0.450

0.460

0.470

0.480

0 10 20 30 40 50 60 70

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

121

R2 = 0.993

R2 = 0.989

R2 = 0.993

R2 = 0.994

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.992

R2 = 0.992

R2 = 0.988

R2 = 0.993

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.991

R2 = 0.991

R2 = 0.992

R2 = 0.990

0.450

0.460

0.470

0.480

0 10 20 30 40 50 60

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

122

R2 = 0.990

R2 = 0.991

R2 = 0.993

R2 = 0.993

R2 = 0.994

0.0

2.0

4.0

6.0

8.0

10.0

ko

bs

× 1

04

(s–

1)

R2 = 0.991

R2 = 0.992

R2 = 0.994

R2 = 0.992

R2 = 0.995

0.0

2.0

4.0

6.0

8.0

10.0

ko

bs

× 1

04

(s–

1)

R2 = 0.989

R2 = 0.991

R2 = 0.993

R2 = 0.994

R2 = 0.994

0.0

2.0

4.0

6.0

8.0

10.0

0.000 0.002 0.004 0.006 0.008 0.010

TEOHA concentration (M)

ko

bs

× 1

04

(s–

1)

Fig. 26. Second-order plots for polymerization of HEMA (1–3 M) in presence of RF

at various TEOHA concentrations (0.0025–0.01 M). pH: (●) 6.0, (■) 7.0,

(▲) 7.5, (♦) 8.0, (*) 9.0.

1M

2M

3M

123

Table 10. Second-order rate constants (k′) for the interaction of TEOHA (0.0100 M)

with HEMA (1–3 M) at pH 6.0–9.0 in the presence of RF.

k'×102 M

–1 s

–1

pH Monomer:

water

ratio

(1.21:10)

Correlation

coefficient

Monomer:

water

ratio

(2.42:10)

Correlation

coefficient

Monomer:

water

ratio

(3.63:10)

Correlation

coefficient

6.0 3.70 0.993 2.84 0.995 2.45 0.993

7.0 5.05 0.991 3.98 0.991 3.43 0.990

7.5 6.32 0.993 5.18 0.996 4.38 0.989

8.0 7.89 0.996 6.44 0.990 5.61 0.992

9.0 8.87 0.994 7.63 0.993 6.68 0.990

124

time-resolved photolysis studies a scheme of the photochemical behavior of RF in the

presence of amine and monomer in aqueous solution has been presented (Scheme 1).

The process involves RF triplet quenching by the amine to produce radicals initiating

the polymerization reaction. It has been suggested that the free radicals produced in

the photoinduced electron transfer from TEOHA to excited RF form a radical pair

which ultimately leads to the polymerization of HEMA. The various steps involved in

the photopolymerization process are shown in the scheme. The free radicals may also

be produced from other photoinitiators such as ketones and add to HEMA to initiate

polymerization of the monomer (Dutta and Bhat, 1996). The use of camphorquinone

and safranin T as photoinitiators in the polymerization of HEMA is described in the

following sections.

125

Scheme 1. Photopolymerization of HEMA in presence of RF and TEOHA.

126

5.2. Camphorquinone as Photoinitiator

The results of the reactivity of RF as a photoinitiator in RF / TEOHA system

in the polymerization of HEMA have been presented in Section 5.1. Camphorquinone

(CQ) has previously been used as a photoinitiator in this reaction by several workers

(Corrales et al., 2003; Jakubiak et al., 2003; Wang et al., 2006; Chen et al., 2007;

Guo et al., 2008). It has been used as a photoinitiator alongwith TEOHA in the

polymerization of HEMA in these studies. CQ has been employed as a photoinitiator

in the polymerization of HEMA in both aqueous and organic solvents. In the present

work CQ / TEOHA system has been used for the polymerization of 1–3 M HEMA at

pH 6.0–9.0 to compare the kinetic data with those of RF. The CQ / amine

photoinitiator system is widely used for the polymerization of dental restorative

materials (Watts, 2005).

5.2.1. Spectral Characteristics of CQ Solutions

CQ possesses an absorption maximum at 468 nm (ε = 46 ± 2 M–1

cm–1

) (Chen

et al., 2007) in the visible region (Fig. 27). This is similar to that of RF (445 nm),

however, the value of molar absorptivity of CQ at the absorption maximum is much

lower than that of RF and this may affect its reactivity as a photoinitiator compared to

that of RF. In order to maintain the same absorbance (0.125 as that of RF in the

HEMA solution, a 2.7×10–3

M concentration of CQ was used during the reaction. The

low absorbance of the photoinitiators is necessary to avoid inhomogenous free radical

distribution (Alvarez et al., 1998). The spectra of the photolysed solutions of CQ

alone at various pH values during irradiation (5 min) did not show any change in

127

Fig. 27. Absorption spectra of camphorquinone at pH 7.0.

128

absorbance at 468 nm in the visible region. This may be due to the use of a low

intensity irradiation source (15 W lamp) which does not produce any change in the

molecule during the short period of exposure to light. It may be concluded that

aqueous solutions of CQ are less sensitive to light compared to RF and would remain

stable during the polymerization reactions.


The concentrations of HEMA (1–3 M) in aqueous solutions polymerized in

the presence of CQ as a photoinitiator were determined by measurement of

absorbance at 208 nm as in the case of RF (Section 5.1.3). The result of a typical

assay of HEMA during the reactions at pH 9.0 is presented in Table 11. The values

show a gradual loss in the concentration of HEMA, with time, indicating the

conversion of the monomer. The loss of HEMA has been found to increase with pH

suggesting that the polymerization reaction is facilitated in the alkaline medium. The

analytical data have only been obtained at low conversion of HEMA.

5.2.3. Kinetics of Polymerization

The kinetics of polymerization of HEMA in the presence of CQ has been

studied in the initial stages of the reaction (~5% change) for reasons stated in the case

of RF as a photoinitiator. The assay data at various pH values were found to comply

with pseudo first-order kinetics (log concentration versus time plots are linear) and

the apparent first-order rate constants (kobs) (Table 12–14) were determined from the

slopes of the straight lines (Fig. 28–32). The plots were found to be linear only within

129

Table 11. Polymerization of HEMA (1 M) at pH 9.0 in presence of CQ as



(sec) 0.0025 0.005 0.0075 0.01

0 1.000 1.000 1.000 1.000

10 0.997 0.995 0.992 0.990

20 0.995 0.989 0.987 0.983

30 0.992 0.985 0.981 0.974

50 0.988 0.978 0.970 0.959

70 0.984 0.971 0.962 0.947

130


M) at pH 6.0–9.0 in presence of CQ as photoinitiator at various TEOHA

concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 1.01±0.072 1.84±0.095 2.80±0.17 3.35±0.15

7.0 1.32±0.081 2.34±0.097 3.31±0.15 4.04±0.16

7.5 1.79±0.099 3.12±0.11 4.03±0.16 5.54±0.21

8.0 1.81±0.092 4.07±0.13 5.14±0.18 6.97±0.19

9.0 2.28±0.11 4.15±0.15 5.49±0.21 7.78±0.27

131



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.87±0.051 1.38±0.062 2.18±0.11 2.72±0.17

7.0 0.98±0.062 1.56±0.069 2.73±0.14 3.51±0.16

7.5 0.72±0.044 2.03±0.082 3.47±0.12 4.65±0.21

8.0 1.38±0.062 3.10±0.15 3.97±0.21 5.89±0.25

9.0 1.55±0.075 2.82±0.13 5.29±0.24 6.78±0.27

132



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.45±0.020 1.06±0.039 1.92±0.15 2.33±0.12

7.0 0.90±0.042 1.38±0.068 2.02±0.11 2.86±0.15

7.5 0.76±0.036 1.70±0.081 2.68±0.14 4.01±0.18

8.0 1.09±0.042 2.26±0.098 4.14±0.22 5.12±0.24

9.0 1.62±0.084 3.17±0.14 4.13±0.20 5.89±0.26

133

R2 = 0.987

R2 = 0.989

R2 = 0.989

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 25 50 75 100 125

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 28. First-order plots for the polymerization of HEMA in the presence of CQ at

pH 6.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

134

R2 = 0.989

R2 = 0.989

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

135

R2 = 0.993

R2 = 0.988

R2 = 0.987

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.5. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

136

R2 = 0.994

R2 = 0.992

R2 = 0.989

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 20 40 60 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 8.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

137

R2 = 0.995

R2 = 0.990

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 10 20 30 40 50 60 70

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 9.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

138

5% concentration loss of HEMA, i.e. low conversion of the monomer (Jakubiak et al.,

2003). Above this level, the reaction is complicated resulting in volume changes and

non-linear behavior. In the majority of cases dilatometric method has been used to

study the polymerization of HEMA to account for any change in the volume of the

solutions (Bertolotti et al., 1999; Orellana et al., 1999; Encinas et al., 2001). In the

present case the UV spectrometric method has been found to be satisfactory for

kinetic purposes in the concentration range involving low conversion of HEMA.

5.2.4. Effect of pH

The present study of the polymerization of HEMA using CQ as a

photoinitiator has been carried out in the pH range of 6.0–9.0 as in the case of RF.

The values of apparent first-order rate constants (kobs) for the reactions (Table 12–14)

have been found to gradually increase with pH showing a relatively fast change

between pH 7.0–8.0. This is probably due to the deprotonation of TEOHA (pKa 7.82)

(Albert and Serjeant, 1962) and consequently the ease in electron transfer from

TEOHA to RF triplet state. It is followed by the interaction of amine radicals with

HEMA to undergo polymerization. The rate is highest at pH 9.0 probably due to the

basic nature of the neutral amine radical. This behavior has already been explained in

the case of the reactivity of RF as a photoinitiator in this reaction (Section 5.1.5). The

rate-pH profiles for the polymerization reaction of HEMA carried out at 1–3 M

concentrations are shown in Fig. 33. An explanation for the increase in the rate of

reactions on increasing the pH from 6.0–9.0 is based on the availability of a labile

proton on the molecule (Beers et al., 1999) and has been discussed in the case of RF

139

1 M

2 M

3 M

0.0

2.0

4.0

6.0

8.0

10.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

ko

bs

× 1

04

(s–

1)

Fig. 33. Rate-pH profile for polymerization of HEMA (1–3 M) in presence of CQ and

0.01 M TEOHA.

140

earlier (Section 5.1.5).


The effect of viscosity on the rate of polymerization of HEMA on changing

the concentration from 1–3 M in the presence of CQ is prominent (Fig. 34) and is

similar to that observed in the case of RF. However, the apparent first-order rate

constants obtained in the presence of CQ are considerably lower than those of RF

under the same conditions (Table 12–14). This appears to be due to a difference in the

reactivity of the triplet states of the two photoinitiators. The increase in the viscosity

of HEMA solutions would cause quenching of the triplet state and hence a decrease in

the rate of interaction between the CQ triplet state and the amine leading to the

formation of a radical ion pair and further reactions. Furthermore, the solute diffusion

processes would also be affected by the viscosity of the medium and thus would

influence the rate of polymerization of HEMA. It is, therefore, necessary to take into

consideration the effect of viscosity on the rate of polymerization of HEMA and

consequently on the yield of the polymer.


In order to compare the effect of TEOHA concentration on the rate of

polymerization of HEMA in the presence of CQ, with that of RF, the reactions were

carried out at 0.0025–0.01 M TEOHA concentrations (Fig. 35–39). The second-order

rate constants for the interaction of HEMA and TEOHA under these conditions (Fig.

40) are reported in Table 15. These rate constants are lower than those obtained

141

pH 6.0

pH 7.0

pH 7.5

pH 8.0

pH 9.0

0.0

2.0

4.0

6.0

8.0

10.0

0.4 0.5 0.6 0.7 0.8 0.9 1.0


ko

bs

× 1

04

(s–

1)

Fig. 34. Plots of kobs for polymerization of HEMA in presence of CQ and 0.01 M


(▲) 3.0 M.

142

R2 = 0.986

R2 = 0.988

R2 = 0.989

R2 = 0.987

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.990

R2 = 0.987

R2 = 0.989

R2 = 0.989

0.280

0.290

0.300

0.310

log

co

nce

ntr

ati

on

(M

)

R2 = 0.985

R2 = 0.991

R2 = 0.989

R2 = 0.989

0.460

0.464

0.468

0.472

0.476

0.480

0 25 50 75 100 125

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 35. First-order plots for the polymerization of HEMA (1–3 M) in presence of CQ


(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

143

R2 = 0.989

R2 = 0.992

R2 = 0.992

R2 = 0.989

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.986

R2 = 0.990

R2 = 0.992

R2 = 0.989

0.280

0.290

0.300

0.310

log

co

nce

ntr

ati

on

(M

)

R2 = 0.987

R2 = 0.986

R2 = 0.989

R2 = 0.990

0.460

0.464

0.468

0.472

0.476

0.480

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

144

R2 = 0.991

R2 = 0.993

R2 = 0.991

R2 = 0.993

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.992

R2 = 0.990

R2 = 0.987

R2 = 0.988

0.275

0.285

0.295

0.305

log

co

nce

ntr

ati

on

(M

)

R2 = 0.988

R2 = 0.988

R2 = 0.990

R2 = 0.987

0.450

0.460

0.470

0.480

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

145

R2 = 0.990

R2 = 0.991

R2 = 0.990

R2 = 0.994

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.988

R2 = 0.988

R2 = 0.992

R2 = 0.992

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.984

R2 = 0.991

R2 = 0.993

R2 = 0.989

0.450

0.460

0.470

0.480

0 20 40 60 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

146

R2 = 0.991

R2 = 0.989

R2 = 0.992

R2 = 0.995

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.986

R2 = 0.991

R2 = 0.991

R2 = 0.990

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.991

R2 = 0.986

R2 = 0.987

R2 = 0.990

0.450

0.460

0.470

0.480

0 10 20 30 40 50 60 70

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

147

R2 = 0.992

R2 = 0.989

R2 = 0.989

R2 = 0.990

R2 = 0.993

0.0

2.0

4.0

6.0

8.0

10.0

ko

bs

× 1

04

(s–

1)

R2 = 0.993

R2 = 0.992

R2 = 0.990

R2 = 0.991

R2 = 0.989

0.0

2.0

4.0

6.0

8.0

10.0

ko

bs

× 1

04

(s–

1)

R2 = 0.988

R2 = 0.991

R2 = 0.990

R2 = 0.990

R2 = 0.994

0.0

2.0

4.0

6.0

8.0

10.0

0.000 0.002 0.004 0.006 0.008 0.010


ko

bs

× 1

04

(s–

1)

Fig. 40. Second-order plots for polymerization of HEMA (1–3 M) in presence of CQ


(▲) 7.5, (♦) 8.0, (*) 9.0.

1M

2M

3M

148


with HEMA (1–3 M) at pH 6.0–9.0 in the presence of CQ.

k'×102 M

–1 s

–1

pH Monomer:

water

ratio

(1.21:10)

Correlation

coefficient

Monomer:

water

ratio

(2.42:10)

Correlation

coefficient

Monomer:

water

ratio

(3.63:10)

Correlation

coefficient

6.0 3.35 0.987 2.72 0.989 2.33 0.989

7.0 4.04 0.989 3.51 0.989 2.86 0.990

7.5 5.54 0.993 4.65 0.988 4.01 0.987

8.0 6.97 0.994 5.89 0.992 5.12 0.989

9.0 7.78 0.995 6.78 0.990 5.89 0.990

149

for the interaction of HEMA and TEOHA in the presence of RF and also decrease

with an increase in HEMA concentration at all pH values. However, these rate

constants also increase with an increase in pH suggesting a higher interaction of the

amine radicals with HEMA in the alkaline region. The degree of interaction of the

amine radicals with HEMA would depend upon the reactivity and radical formation

of the photoinitiator in this reaction.


The mechanism of HEMA polymerization using CQ as a photoinitiator is

similar to that suggested for the polymerization reaction in the presence of RF

(Orellana et al., 1999) and has been discussed by Jakubaik et al. (2003).

Photoinitiated CQ-amine polymerization has been suggested to occur through

electron / proton transfer. It involves the formation of reactive radicals that initiate the

polymerization, by hydrogen atom abstraction from the amine (AH) by the triplet

state of CQ, mediated by photoinduced electron transfer through a radical ion pairs.

The radicals thus formed attach the C=C bond of the monomer to initiate

polymerization (Stansbury, 2000). The efficiency of the polymerization depends not

only on the efficiency of the quenching of the triplet state of CQ by AH, but also on

the relative importance of processes leading to the formation of radicals that are able

to initiate the polymerization. The mechanism of the polymerization of HEMA

involving CQ / TEOHA system is similar to that of Orellana et al. (1999) as presented

in Scheme 2.

150

Scheme 2. Photopolymerization of HEMA in the presence of CQ and TEOHA.

151

5.3. Safranin T as Photoinitiator

The results of the reactivity of RF and CQ as photoinitiators in the

polymerization of HEMA and the effects of pH, viscosity and TEOHA concentration

on the rates of these reactions have been discussed in Sections 5.1 and 5.2,

respectively. In order to evaluate the reactivity of safranin T (ST) as a photoinitiator

in this reaction under the present condition and its comparison with that of RF and

CQ as photoinitiators, the polymerization of HEMA in the presence of ST / TEOHA

system at pH 6.0–9.0 has been studied. The use of ST as a photoinitiator in the

polymerization of HEMA in aqueous and organic solvents has previously been

reported by some workers (Previtale et al., 1994; Encinas et al., 1996; Gomez et al.,

2003). The present work focuses on the kinetic aspects of these reactions.

5.3.1. Spectral Characteristics of ST Solutions

ST exhibits an absorption maximum at 520 nm (ε = 2900 M–1

cm–1

) in the

visible region (Dutta and Bhat, 1996) (Fig. 41). It would have a lower energy of the

excited triplet state compared to that of the RF and CQ which have relatively lower

absorption maxima (445 and 468 nm, respectively). A 4.31×10–5

M ST concentration

in HEMA solutions (absorbance = 0.125) has been used for the polymerization

reactions. Similar ST solutions (pH 6.0–9.0) on irradiation for a period of 1–5 min did

not show any absorbance change at the maximum indicating that ST concentration

remains constant during the reactions and the photoinitiator is stable to light during

irradiation in the polymerization reactions.

152

Fig. 41. Absorption spectrum of safranin T at pH 7.0.

153


The same UV spectrophotometric method has been used for the assay of

HEMA during the polymerization reactions carried out in the presence of ST as in the

case of RF and CQ. The concentration values of HEMA for the reactions at 1–3 M

concentration in the pH range of 6.0–9.0 are given in Table 16. The values decrease,

with time, as a result of the loss of HEMA during the initial stages of the reactions as

observed in the case of RF and CQ. However, the magnitude of change is lower,

probably due to a difference in the reactivity of ST. The increase in the loss of

HEMA, with pH, is in accordance with the polymerization behavior observed in the

presence of RF and CQ and may involve a similar mechanism.

5.3.3. Kinetics of Photopolymerization

The loss of HEMA during the polymerization reactions in the presence of ST

also follows pseudo first-order kinetics within 5% concentration change at all pH

values (Fig. 42–46) as observed in the case of RF (Section 5.1) and CQ (Section 5.2).

Beyond this level the data do not fit the first-order linear plot suggesting a change in

the nature and mode of the reaction. The values of apparent first-order rate constants

determined at 6.0–9.0 for the polymerization of 1–3 M HEMA are reported in Table

17–19. The values of the rate constants are affected by the pH and viscosity of the

solutions in a manner similar to that observed in the case of data obtained in the

presence of RF and CQ as photoinitiators. This kinetic behavior of HEMA

polymerization has been explained earlier for the reactions carried out in the presence

of RF and CQ. The apparent first-order rate constants (kobs) determined for the

154

Table 16. Polymerization of HEMA (1 M) at pH 9.0 in presence of ST as



(sec) 0.0025 0.005 0.0075 0.01

0 1.000 1.000 1.000 1.000

10 0.997 0.994 0.991 0.991

20 0.995 0.991 0.987 0.986

40 0.991 0.985 0.979 0.971

60 0.987 0.979 0.971 0.964

80 0.983 0.974 0.959 0.951

155

R2 = 0.991

R2 = 0.991

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 30 60 90 120 150 180

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 42. First-order plots for the polymerization of HEMA in the presence of ST at

pH 6.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

156

R2 = 0.993

R2 = 0.988

R2 = 0.989

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 30 60 90 120 150

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

157

R2 = 0.989

R2 = 0.990

R2 = 0.990

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 30 60 90 120

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 7.5. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

158

R2 = 0.988

R2 = 0.991

R2 = 0.991

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 8.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

159

R2 = 0.990

R2 = 0.994

R2 = 0.991

-0.100

0.000

0.100

0.200

0.300

0.400

0.500

0 20 40 60 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)


pH 9.0. HEMA: (●) 1.0 M, (■) 2.0 M, (▲) 3.0 M.

160


M) at pH 6.0–9.0 in presence of ST as photoinitiator at various TEOHA

concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.78±0.045 1.36±0.060 2.03±0.095 2.57±0.13

7.0 1.00±0.052 1.53±0.072 2.32±0.12 2.98±0.14

7.5 1.47±0.061 2.49±0.11 3.64±0.16 4.45±0.22

8.0 1.57±0.068 3.01±0.15 3.98±0.20 5.45±0.21

9.0 2.09±0.099 3.17±0.15 4.85±0.24 6.08±0.27

161



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.51±0.026 0.90±0.042 1.47±0.068 2.14±0.096

7.0 0.59±0.024 1.05±0.048 1.88±0.085 2.50±0.12

7.5 0.64±0.031 1.63±0.072 2.60±0.12 3.79±0.17

8.0 1.34±0.060 2.58±0.12 3.38±0.17 4.73±0.22

9.0 1.56±0.081 2.87±0.14 3.87±0.18 5.44±0.25

162



concentrations.

kobs × 104 s

–1

pH 0.0025 M 0.0050 M 0.0075 M 0.010 M

6.0 0.29±0.012 0.77±0.042 1.19±0.058 1.79±0.085

7.0 0.49±0.028 0.94±0.048 1.40±0.065 2.10±0.10

7.5 0.63±0.029 1.41±0.068 2.17±0.11 3.28±0.14

8.0 1.00±0.055 1.85±0.072 2.88±0.15 4.14±0.21

9.0 1.22±0.058 2.32±0.12 3.22±0.157 4.81±0.25

163

polymerization reactions in the presence of ST are slightly lower than those obtained

in the presence of CQ indicating similar reactivities of the two compounds.

5.3.4. Effect of pH

Similar to the study of the polymerization reactions of HEMA carried out in

the presence of RF and CQ, the present work is also based on the reactions conducted

in the pH range of 6.0–9.0 to compare the reactivities of these photoinitiators. The

values of the apparent first-order rate constants (kobs) are close to those obtained in the

presence of CQ and increase with pH, for similar reason as stated in the case of RF

and CQ. On the basis of the values of these rate constants, the reactivity of ST

appears to be lower than that of RF and CQ. The rate-pH profiles for the

polymerization reactions in the presence of ST (Fig. 47) are similar to those of RF

and CQ, indicating a similar mechanism involved in these reactions. However, the

difference in the magnitude of rate constants is due to the differences in the reactivity

and radical formation of the photoinitiators. Thus the highest rate of polymerization

of HEMA also appears to be at pH 9.0 in this case or above as observed by

Valdebenito and Encinas (2003).


The photochemical reactions involving free radical or radical ion pairs are

generally affected by the viscosity of the medium (Turro et al., 2010). This has also

been observed in the polymerization reactions of HEMA carried out at 1–3 M

concentrations using RF and CQ as photoinitiators. In a similar manner the rate

constants for the polymerization reactions in the presence of ST have been found to

164

1 M

2 M

3 M

0.0

2.0

4.0

6.0

8.0

10.0

4.0 5.0 6.0 7.0 8.0 9.0 10.0

pH

ko

bs

× 1

04

(s–

1)

Fig. 47. Rate-pH profile for polymerization of HEMA (1–3 M) in presence of ST and

0.01 M TEOHA.

165

decrease with an increase in the viscosity of the medium (Table 17–19). A gradual

decrease in rates for the reactions carried out at 1–3 M HEMA concentrations have

been observed (Fig. 48). As explained earlier in the case of RF (Section 5.1.6) and

CQ (Section 5.2.5), the decrease in the rate constant in viscous media is probably a

result of triplet state quenching and changes in diffusional processes of the species

involved. This would result in a relatively low rate of interaction of the excited state

species and consequently a low rate of polymerization compared to that of RF and

CQ. These results indicate that it may be necessary to keep into consideration the

effect of viscosity on the rate of polymerization reactions and ultimately the yield of

the polymer.


The rates of polymerization reactions of HEMA in the presence of a fixed

concentration of ST and variable concentrations of TEOHA at pH 6.0–9.0 (Fig. 49–

53) are lower than those of RF and CQ indicating a lower reactivity of ST compared

with that of the other two photoinitiators. This would suggest a lower rate of

formation and interaction of the amine radicals and HEMA in the presence of ST

compared to that of RF and CQ. The second-order rate constants for the interaction of

HEMA (1–3 M) and TEOHA (0.0025–0.01 M) (Fig. 54) are reported in Table 20 and

are similar to those obtained in the presence of CQ under the same conditions.

However, the values are considerably lower than those obtained in the presence of

RF. Thus the photochemical behavior of ST in the polymerization reaction of HEMA

is similar to RF and CQ but its reactivity is lower than the two photoinitiators. This is

166

pH 6.0

pH 7.0

pH 7.5

pH 8.0

pH 9.0

0.0

2.0

4.0

6.0

8.0

10.0

0.4 0.5 0.6 0.7 0.8 0.9 1.0


ko

bs

× 1

04

(s–

1)

Fig. 48. Plots of kobs for polymerization of HEMA in presence of CQ and 0.01 M


(▲) 3.0 M.

167

R2 = 0.992

R2 = 0.993

R2 = 0.988

R2 = 0.991

-0.030

-0.020

-0.010

0.000

0 30 60 90 120 150 180

log

co

nce

ntr

ati

on

(M

)

R2 = 0.986

R2 = 0.991

R2 = 0.987

R2 = 0.991

0.280

0.290

0.300

0.310

0 30 60 90 120 150 180

log

co

nce

ntr

ati

on

(M

)

R2 = 0.987

R2 = 0.993

R2 = 0.992

R2 = 0.990

0.460

0.464

0.468

0.472

0.476

0.480

0 30 60 90 120 150 180

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 49. First-order plots for the polymerization of HEMA (1–3 M) in presence of ST


(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

168

R2 = 0.987

R2 = 0.992

R2 = 0.994

R2 = 0.993

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.990

R2 = 0.992

R2 = 0.990

R2 = 0.988

0.280

0.290

0.300

0.310

log

co

nce

ntr

ati

on

(M

)

R2 = 0.989

R2 = 0.988

R2 = 0.988

R2 = 0.989

0.460

0.464

0.468

0.472

0.476

0.480

0 40 80 120 160

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

169

R2 = 0.990

R2 = 0.991

R2 = 0.994

R2 = 0.989

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.987

R2 = 0.994

R2 = 0.992

R2 = 0.990

0.275

0.285

0.295

0.305

log

co

nce

ntr

ati

on

(M

)

R2 = 0.989

R2 = 0.989

R2 = 0.991

R2 = 0.990

0.450

0.460

0.470

0.480

0 30 60 90 120

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

170

R2 = 0.985

R2 = 0.989

R2 = 0.987

R2 = 0.988

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.987

R2 = 0.990

R2 = 0.988

R2 = 0.991

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.985

R2 = 0.990

R2 = 0.995

R2 = 0.991

0.450

0.460

0.470

0.480

0 20 40 60 80 100

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

171

R2 = 0.994

R2 = 0.987

R2 = 0.987

R2 = 0.990

-0.030

-0.020

-0.010

0.000

log

co

nce

ntr

ati

on

(M

)

R2 = 0.994

R2 = 0.987

R2 = 0.989

R2 = 0.994

0.260

0.280

0.300

0.320

log

co

nce

ntr

ati

on

(M

)

R2 = 0.988

R2 = 0.990

R2 = 0.990

R2 = 0.991

0.450

0.460

0.470

0.480

0 20 40 60 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)



(■) 0.005 M, (▲) 0.0075 M, (●) 0.01 M TEOHA.

1M

2M

3M

172

R2 = 0.996

R2 = 0.992

R2 = 0.991

R2 = 0.995

R2 = 0.990

0.0

2.0

4.0

6.0

8.0

10.0

0.000 0.002 0.004 0.006 0.008 0.010

ko

bs

× 1

04

(s–

1)

R2 = 0.991

R2 = 0.992

R2 = 0.990

R2 = 0.994

R2 = 0.995

0.0

2.0

4.0

6.0

8.0

10.0

0.000 0.002 0.004 0.006 0.008 0.010

ko

bs

× 1

04

(s–

1)

R2 = 0.987

R2 = 0.992

R2 = 0.989

R2 = 0.995

R2 = 0.993

0.0

2.0

4.0

6.0

8.0

10.0

0.000 0.002 0.004 0.006 0.008 0.010


ko

bs

× 1

04

(s–

1)

Fig. 54. Second-order plots for polymerization of HEMA (1–3 M) in presence of ST


(▲) 7.5, (♦) 8.0, (*) 9.0.

1M

2M

3M

173


with HEMA (1–3 M) at pH 6.0–9.0 in the presence of ST.

k'×102 M

–1 s

–1

pH Monomer:

water

ratio

(1.21:10)

Correlation

coefficient

Monomer:

water

ratio

(2.42:10)

Correlation

coefficient

Monomer:

water

ratio

(3.63:10)

Correlation

coefficient

6.0 2.57 0.991 2.14 0.991 1.79 0.990

7.0 2.98 0.993 2.50 0.988 2.10 0.989

7.5 4.45 0.989 3.79 0.990 3.28 0.990

8.0 5.45 0.988 4.73 0.991 4.14 0.991

9.0 6.08 0.990 5.44 0.994 4.81 0.991

174

due to a lower energy of the wavelength of irradiation (520 nm) compared with those

of RF (445 nm) and CQ (468 nm). These rate constants are affected by the pH and

viscosity of the medium and are a function of the concentration of TEOHA used in

the reactions. The rates of polymerization are increased with an increase in TEOHA

concentration. It has been reported that the rate of polymerization of HEMA is

maximum in the presence of 0.01 M TEOHA (Orellana et al., 1999). In the present

case also the maximum concentration of TEOHA used in the polymerization reaction

is 0.01 M and its interaction with HEMA has been studied.


The kinetic behavior of the polymerization reactions of HEMA using RF, CQ

and ST as photoinitiators is similar with respect to pH, viscosity and TEOHA

concentration suggesting a similarity in the mechanism of these reactions. However,

the difference in the rates of polymerization in the presence of these photoinitiators is

probably due to variation in the reactivity of the individual photoinitiators and its role

in the process. The primary reaction in the polymerization process involves the

formation of the photoinitiator (e.g. RF) and amine radicals and subsequently the

interaction of amine radicals with HEMA to undergo polymerization as presented in

Scheme 1 (Orellana et al., 1999). A similar mechanism for the polymerization of

HEMA in the presence of ST has earlier been suggested by Encians et al. (1996) and

is presented in Scheme 3. It describes only the formation of amine radicals and no

further reactions to proceed to the polymerization of HEMA. This step was later

included in the scheme of Orellana et al. (1999) as given in Scheme 1.

175

Scheme 3. Photopolymerization of HEMA in the presence of ST and TEOHA.

CHAPTER VI

POLYMERIZATION OF

HEMA IN NONAQUEOUS

SOLUTIONS

177

The influence of solvent on the rates and mechanism of chemical reactions is

of great importance and has been discussed by many workers (Amis and Hinton,

1973; Reichardt, 1988; Buncel et al., 2003; Sinko, 2006; Turro et al., 2010). The

majority of the chemical reactions occurring in solution media are affected by solvent

characteristics such as dielectric constant, viscosity and solvating power. These

properties play an important role in determining the rate as well as the equilibrium of

the reaction. The transition state theory may predict the effects of changes in solvent

polarity on the rates of chemical reactions. It can further predict the ability of the

solvent to interact with the solute and thus solvate charged species undergoing the

reaction.

6.1. Types of Bimolecular Reactions

The various types of bimolecular reactions in organic solvents occur as

follows.

a. Neutral nonpolar reactants giving neutral nonpolar products

In this case the transition state would also be nonpolar, and in such a case a

change in solvent polarity would have little effect on the rate of reaction.

b. Neutral reactants giving charged products

In these reactions the transition state would be more polar than the initial state.

This results from the partial charge separation in the transition state. In this

case an increase in solvent dielectric constant (polarity) would result in an

increase in the rate of the reaction. The following quaternization reaction is an

example of this type of behavior in bimolecular reaction (the transition-state

structure in this reaction is enclosed in brackets and the dashed lines signify

178

the bonds that are partially formed or broken). In the experimental approach

solvent polarity can be the changed by replacing one solvent by another (Eq.

6.1).

c. Neutral molecule-ion reactions

In these reactions the transition state would be less polar than the initial state.

This would occur because the same charge would be spread over a greater

volume in the transition state. Thus an increase in the polarity of the solvent

would result in a decrease in the rate of reaction. In some cases the results of

this type of reaction may be difficult to interpret because as a result of

alteration of the reaction medium, factors other than the polarity are also

changed. This behavior would complicate the interpretation of these reactions.

d. Ion-ion reactions

If the reactants are two ions of opposite charge, then the transition state would

have less or no charge and, therefore, less polar than the initial state. In this

reaction an increase in the polarity of solvent would tend to decrease the rate.

If the two ionic reactants have the same charge, the opposite would occur

(Connors et al., 1986).

The activation energy of a reaction is dependent on the type of the reaction. It

does not change rapidly from one solvent to the other. The entropy of activation is

always negative and changes with the nature of the solvent. It becomes more negative

179

with a decrease in solvent polarity. Thus the reactions that produce products more

polar than the reacting species proceed well in polar solvents (Amis and Hinton et al.,

1973).

6.2. Solvent Effect on Polymerization Reactions

In relevance to polymerization reactions, it is useful to examine a plausible

sequence of steps that might occur along a reaction coordinate as a donor approaches

the acceptor (for which either the donor or the acceptor is electronically excited). In

such a case the reaction concludes with the electron transfer to form a radical ion pair.

The separation of the radical ion pair into free radicals and their rate of interaction

with the monomer would depend upon the solvent characteristics including polarity

and viscosity. The reaction would include a number of possible intermediate that

might exist along the reaction coordinate (Eq. 6.2).

Assuming the donor as an electronically excited species (*D) and that *D

encounters A and this leads to the formation of an encounter complex (*DA)ex. This

then forms an exciplex with delocalized excitation, symbolized as *(D, A)ex. In the

next stage, electron transfer takes place in the exciplex which is converted to a

contact radical ion pair (CRIP, symbolized as D●+

, A●¯ ). This is followed by insertion

of one or more solvent molecules between the partners of the radical ion pair to form

180

a solvent separated radical ion pair (SSRIP, symbolized as D●+

(S) A●¯ ). Finally the

ions separate to become free radical ions (FRIs) in the bulk solvent (FRI, D●+

, A●¯ ).

At various stages, each step along this hypothetical pathway will compete with other

possible pathways, such as chemical reactions or back electron transfer. Thus, the

radical ion pairs, CRIP, SSRIP, and FRI appear to be similar chemical species, since

they both formally consist of a D●+

and A●¯ units. However, they may possess vastly

different rates for back electron transfer to form D + A because of the distance

dependence of electron transfer and the reorganization energies involved in a

particular solvent.

The energetic term associated with electron transfer have a strong dependence

on the distance between charges

where Sp refers to a rigid molecular framework that serves as a spacer and allows

electron transfer in liquids but prevent diffusion D and A.

From (Eq. 6.5), it could be conceived that nascent radical ion pairs may have different

initial structures (Eq. 6.6) depending on the nature of the reaction that generated them

and the solvent properties for stabilizing the development of charges. In certain cases

D and A may be able to form a ground-state electron-transfer stabilized complex and

electron transfer may occur directly following excitation of this complex (Eq. 6.6).

181

In this case, the partners of the initial radical ion pair (D●+

, A●¯ ) must be a

geminate CRIP, since they were in contact in order to form a ground-state CT

complex (GSCTC). In this way, the initial geminate radical ion pair (D●+

, A●¯ ) is

formed with no solvent molecules separating the partners of the pair (Turro et al.,

2010). This treatment may apply to polymerization reaction in solution media

depending upon the nature of the solvent in which the reaction is being carried out.

6.3. Photoinduced Electron Transfer

Miller and Closs (1984, 1986, 1988) have demonstrated the validity of the

fundamental principles of Marcus’ theory (Marcus, 1956, 1959, 1960) of electron

transfer for a nonphotochemical system. The principles of this theory have been

applied to explain the nature of photochemical reactions. In the majority of examples

for application of Marcus theory to photochemical reactions, the key electron-transfer

steps are frequently back electron transfers from “ground-state” D●+

, A●¯ radical ion

pairs produced by photoinduced electron transfer. The concepts of Marcus theory

influence the efficiency of charge separation and the yield of final products in a

photoreaction controlled by electron transfer energetics.

6.3.1. Long-distance Electron Transfer

This involves electron transfer reactions that occur over distances significantly

larger than the sum of the van der Walls radii and of the electron donor and the

182

acceptor. An understanding of these electron transfer reactions is fundamental for an

understanding of important electron-transfer processes such as photopolymerization

and photosynthesis. The understanding of the mechanism of these processes requires

a knowledge of factors such as how the electron-transfer rate depends on the distance

of separation from the electron donor and acceptor (RDA), on the relative orientation

of D and A (which determines the effective orbital overlap), and on the nature of

intervening medium (solvent, rigid spacer, flexible spacer, supramolecular medium)

that separate the electron donor and acceptor. The rate constants for electron transfer

depend on the electronic coupling between the donor (D) and acceptor (A) involved

in an electron-transfer reaction. The magnitude of electronic coupling depends on the

wave functions of D and A. The mechanism of long distance electron-transfer

reactions have been discussed by Turro et al. (2010) in detail.

6.4. Photoinitiated Polymerization

Great interest has been shown in the polymerization of vinyl monomers

induced by light and the subject has been reviewed by several workers (Oster and

Yang, 1968; Eaton, 1986; Paczkowski et al., 1999; Encinas and Previtali, 2006). The

kinetic of free-radical polymerization can be represented by classical scheme that

comprises the photochemical radical production, the initiation chain, the propagation

and the termination steps. A key in these processes is the involvement of the

photoinitiator, which produces radicals through the photochemical process and

ultimately leads to polymerization. Among the principle characteristics of a

photoinitiators is a high absorption in a visible region (e.g., riboflavin). The

photoinitiators that absorb in the visible light allow (i) to find spectral windows when

183

pigmented formulations have to be polymerized, (ii) the use of the sun light for a

curing of outdoor coationgs. (iii) to imply laser beams as light source. Also

photoinitiators that can be used in polymerization in aqueous media are of interest

because of their ecological advantages. Riboflavin (vitamin B2) absorbs in the 400-

500 nm region, and fulfils these requirements. It has been used as a photoinitiator in

the polymerization of hydroxyethyl methacrylate (HEMA) by Oster et al., 1957;

Bertolotti et al., 1999; Orellana et al., 1999 and Encinas, 2001, and in the present

study along with other photoinitiators (i.e., camphorquinone and safranin T).

The efficacy of photoinitiators in the polymerization of HEMA may be

affected by medium characteristics including the polarity and viscosity and the extent

of radical formation. Several studies have been carried out on the effect of solvent on

photopolymerization of HEMA using dilatometry (Encinas et al., 1996; Valdebenito

and Encinas, 2003), gas chromatography (Beers et al., 1999), Raman spectroscopy

(Wang et al., 2006), ATR-FTIR spectroscopy (Guo et al., 2008), and differential

scanning calorimetry (DSC) (Anderzejewska et al., 2009). The present work is based

on the study of the effect of solvent dielectric constant and viscosity on the rate of

photopolymerization of HEMA in aqueous and organic solvents using a UV

spectrophotometric method. Riboflavin (λmax = 445 nm) (Heelis, 1982),

camphorquinone (λmax = 468 nm) (Jakubiak et al., 2003), and safranin T ((λmax = 520

nm) (Dutta and Bhat, 1996) and triethanolamine as co-initiator (Encinas et al., 1996;

Valdebenito and Encinas, 2003; Encinas et al., 2009) in the reaction. The study

throws light on the effect of solvent characteristics, interactions and kinetics of

HEMA polymerization. A comparative study of the reactivity of different

184

photoinitiators in aqueous and organic solvents has been made to highlight the effect

of solvent parameters on the kinetics of these reactions.

6.5. Effect of Dielectric Constant

The rate of the reactions between dipolar molecules is dependent on the

dielectric constant, D, of the medium (Sinko, 2006).

ln k = ln k D =∞ − K (1/D) (6.7)

where kD=∞ is the rate constant in a medium of infinite dielectric constant. The

dielectric constant of the medium is approximately equal to the dielectric constant of

the solvent in dilute solutions. A decrease in dielectric constant of the medium tends

to decrease the rate of reaction, and conversely.

6.5.1. Riboflavin as Photoinitiator

The polymerization reactions of HEMA were carried out in aqueous and

organic solvents containing RF and 0.01 M TEOHA. The values of HEMA

concentrations in water, acetonitrile, methanol and ethanol determined during

photopolymerization are reported in Table 21. The values for the reactions in 1-

propanol and 1-butanol could not be obtained due to the insolubility of RF in these

solvents. The apparent first-order rate constants (kobs) for the polymerization of

HEMA in the presence of RF as photoinitiator and TEOHA as a co-initiator were

determined from the slopes of the straight lines of log concentration versus time plots

in water and organic solvents (Fig. 55) and are reported in Table 22. The

determination of kobs in this study is based on the steady-state assumption that the rate

of initiation being equal to the rate of termination is considered valid only at a low

185

Table 21. Polymerization of HEMA in the presence of RF and TEOHA in aqueous

and organic solvents.

HEMA concentrations (M) Time

(sec) water acetonitrile methanol Ethanol

0 1.000 1.000 1.000 1.000

10 0.993 0.995 0.996 0.997

20 0.990 0.991 0.993 0.993

30 0.986 0.988 0.990 0.990

40 0.980 0.984 0.986 0.987

50 0.976 0.980 0.983 0.985

60 0.969 0.977 0.980 0.982

70 0.966 0.974 0.976 0.979

80 0.959 0.970 0.971 0.974

186

R2 = 0.994

R2 = 0.994

R2 = 0.996

R2 = 0.993

-0.030

-0.020

-0.010

0.000

0.010

0 10 20 30 40 50 60 70 80

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 55. First-order plots for the polymerization of HEMA in the presence of RF in

aqueous and organic solvents. (×) Water; (●) acetonitrile; (▲) methanol; (♦)

ethanol.

187

Table 22. Apparent first-order rate constants (kobs) for the polymerization of HEMA

in water and organic solvents. Effect of dielectric contants.

kobs × 104 (s

–1)a

Solvent

Dielectric

Constant

(25ºC) RF CQ ST

Water 78.5 5.05 4.04 2.98

Acetonitrile 37.5 3.71 3.02 2.10

Methanol 32.6 3.53 2.90 1.93

Ethanol 24.3 3.12 2.62 1.71

1-Propanol 20.1 – 2.45 1.56

1-Butanol 17.1 – 2.39 1.45

a The values of rate constants are relative and depend on specific experimental

conditions including light intensity.

188

conversion of monomer (Jakubaik et al., 2003). In the present case the values of kobs

have been determined from the analytical data obtained on HEMA assay during the

initial stages of the reaction (within ~5% change), on exposure to a low intensity

radiation source. Under these conditions the first-order plots have been found to be

linear in the desired range. In order to develop a correlation between kobs and

dielectric constant (D) of the medium, a plot of kobs versus dielectric constant of the

solvents was constructed (Fig. 56). It was observed that the rate of reaction is

dependent upon the solvent and is a linear function of the dielectric constant of the

medium, i.e. increasing with an increase in the solvent dielectric. Since RF is used as

a photoinitiator in this reaction, the behavior of RF could be explained on the basis of

the existence of a polar flavin (Fl) intermediate, which would facilitate the

polymerization reaction with an increase in the polarity of the medium. A strong

evidence for the presence of such an intermediate has been presented by Ahmad and

Tollin (1981a) who studied the solvent effect on flavin electron transfer reactions

using laser flash photolysis. According to these workers the reduction of flavin triplet

(3Fl) by the substrate (amine (AH) in this case) proceeds via a dipolar intermediate in

water and organic solvents and, therefore, the rate is increased with an increase in

solvent dielectric.

The extent of solvent interaction of the dipolar intermediate would determine the

degree to which it leads to the formation of separate radicals. In these reactions the

189

RF

CQ

ST

0.0

1.0

2.0

3.0

4.0

5.0

0 10 20 30 40 50 60 70 80

Dielectric constant

ko

bs ×

10

4 (

s–1)

Fig. 56. Plots of kobs for polymerization of HEMA against dielectric constant. (×)

Water; (●) acetonitrile; (▲) methanol; (♦) ethanol; (■) 1-propanol; (∗) 1-

butanol.

190

primary photochemical process is considered as being the electron transfer between

reactants. In such a case the transition state would be more polar than the reactant and

the rate of reaction would increase with the dielectric constant of the medium as

observed in the case of the photolysis of formylmethylflavin (Ahmad et al., 2006). It

needs to be emphasized that the flavin excited triplet state lifetimes vary depending

on the solvent characteristics (Ahmad and Tollin, 1981a) and this would affect the

rate of reaction. Thus the polarity of the medium in which the polymerization of

HEMA is being carried out would exert an effect on the rate of reaction depending

upon the polar character of the medium. Water, with the highest dielectric constant,

appears to be the best medium for carrying out the polymerization of HEMA to obtain

a greater yield than those of the organic solvents. The dependence of the

polymerization rate on the solvent may be due to changes in the initiation,

propagation and / or termination steps and has been discussed with reference to the

effect of azo compounds by Encinas et al. (1996).

It has been reported that the rates of polymerization of HEMA are decreased

with a decrease in medium polarity, i.e. from water to acetonitrile, as a result of

singlet quenching in organic solvents (Encinas et al., 1996; Valdebenito and Encinas,

2003). The present study is in agreement with the observations of these workers and a

decrease in the fluorescence of RF has been found in organic solvents (Section 6.6.1).

6.5.2. Camphorquinone as Photoinitiator

The results of the effect of solvent dielectric constant on the rate of

polymerization of HEMA in the presence of CQ as photoinitiator may be considered

191

on the basis of the data discussed above in the case of RF as a photoinitiator. The

CQ/amine photoinitiator system for generating radicals has been widely used for the

polymerization of dental restorative materials. In this system the absorption of light in

the visible region leads to the n–π* transition of the dicarbonyl group (Tsai and

Charney, 1969). The excited state interacts with TEOHA to form an exciplex. The

exciplex accepts an electron from TEOHA to produce a radical ion pair. This

abstracts a hydrogen atom from the TEOHA resulting in the formation of the primary

radical. This radical would attack the C=C bonds of the monomer (HEMA) to

undergo polymerization. The CQ radical may retard polymerization through

termination reaction (Stansbury, 2000). The theory of processes involved in

photopolymerization has been discussed in Section 6.2 and 6.3.

The assay of HEMA in aqueous and organic solvents in the presence of CQ is

reported in Table 23. The apparent first-order rate constants determined for the

polymerization reactions in the presence of CQ as a photoinitiator (Fig. 57) are given

in Table 22. The polymerization behavior of HEMA in aqueous and organic solvents

is similar to that observed in the presence of RF with respect to the effect of dielectric

constant (Fig. 56). However, the values of kobs in this case are lower than those

observed for RF and may be due to a lower reactivity of the excited state and

subsequent radical formation in this reaction. In view of the structural consideration

(C=O groups), the polar character of the radicals formed from CQ would be lower

than that of RF (a highly conjugated system), resulting in lower rate constants for the

reactants. The effectiveness of CQ / TEOHA system depends on the H-atom donor

ability of the amine in a particular environment, and the subsequent interaction of the

192

Table 23. Polymerization of HEMA in the presence of CQ and TEOHA in aqueous



(sec) Water acetonitrile methanol ethanol 1-propanol 1-butanol

0 1.000 1.000 1.000 1.000 1.000 1.000

20 0.992 0.993 0.995 0.995 0.996 0.997

40 0.985 0.985 0.990 0.991 0.991 0.993

60 0.978 0.982 0.985 0.986 0.985 0.988

80 0.967 0.976 0.980 0.981 0.982 0.984

100 0.962 0.970 0.974 0.976 0.978 0.979

120 0.953 0.964 0.966 0.969 0.971 0.972

140 0.945 0.957 0.960 0.964 0.966 0.968

193

R2 = 0.997

R2 = 0.995

R2 = 0.992

R2 = 0.995

R2 = 0.994

R2 = 0.991

-0.030

-0.020

-0.010

0.000

0 20 40 60 80 100 120 140

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 57. First-order plots for the polymerization of HEMA in the presence of CQ in


ethanol; (■) 1-propanol; (∗) 1-butanol.

194

photoinitiator excited species with the monomer (HEMA) to undergo polymerization

(Guo et al., 2008). The rates of the reaction would be affected by the nature and

polarity of the medium involved and the extent of radical formation.

6.5.3. Safranin T as Photoinitiator

The assay of HEMA in aqueous and organic solvents in the presence of CQ is

reported in Table 24. The apparent first-order rate constants for the polymerization of

HEMA in ST / TEOHA system in aqueous and organic solvent (Fig. 58) are reported

in Table 22. A plot of these rate constants (kobs) as a function of the solvent dielectric

constant is shown in Fig. 56. These results indicate that the reactivity of ST is lower

than that of RF and CQ as photoinitiators. Apart from a consideration of the excited

state polarization behavior of this molecule and subsequent polarity of the

intermediate involved in this reaction, the visible absorption maximum (520 nm) of

ST is higher than that of RF (445 nm) and CQ (468nm). This would provide a

relatively less energy for the activation of the molecule and would probably have a

lower efficiency compared with that of the other two photoinitiators. Thus the rates of

polymerization of HEMA in this case are lower than those obtained in the presence of

RF and CQ. The polymerization of HEMA in methanolic solutions using the

photoinitiator ST in the presence of tertiary aliphatic amines has been studied by

Previtali et al. (1994). The polymerization rates increase with the amine concentration

reaching a maximum, and further amine addition slowly reduces the polymerization

rate. The results indicate that the polymerization of HEMA involves the interaction of

the excited triplet state of ST with the amine and this would be affected by the solvent

characteristics.

195

Table 24. Polymerization of HEMA in the presence of ST and TEOHA in aqueous



(sec) Water acetonitrile methanol ethanol 1-propanol 1-butanol

0 1.000 1.000 1.000 1.000 1.000 1.000

30 0.992 0.993 0.994 0.994 0.995 0.996

60 0.984 0.985 0.987 0.988 0.990 0.993

90 0.972 0.978 0.980 0.982 0.987 0.989

120 0.964 0.973 0.975 0.977 0.980 0.985

150 0.958 0.968 0.970 0.974 0.976 0.979

180 0.948 0.963 0.967 0.970 0.973 0.974

196

R2 = 0.996

R2 = 0.991

R2 = 0.988

R2 = 0.987

R2 = 0.993

R2 = 0.988

-0.030

-0.020

-0.010

0.000

0 30 60 90 120 150 180

Time (seconds)

log

co

nce

ntr

ati

on

(M

)

Fig. 58. First-order plots for the polymerization of HEMA in the presence of ST in


ethanol; (■) 1-propanol; (∗) 1-butanol.

197

The slopes of the plots of kobs for the polymerization of HEMA versus

dielectric constants of the solvents used in the presence of different photoinitiators are

in the order:

RF > CQ > ST

indicating the magnitude of the solvent effect on the reactivity of these

photoinitiators in conducting the polymerization reaction. Thus RF appears to be the

best photoinitiators for HEMA polymerization.

6.6. Effect of Viscosity

6.6.1. Riboflavin as Photoinitiator

Another important factor that may influence the rate of a chemical reaction is

the viscosity of the medium. This appears to control the solute diffusion and hence

the rate of a reaction. A pervious study has shown that the flavin triplet (3Fl)

quenching by a substrate in proportional to the inverse of solvent viscosity as

expected for a diffusional process (Ahamd and Tollin, 1981a).

Polymerization reactions of HEMA in water at 1, 2, and 3 M concentrations

have shown that the rates are decreased with an increase in the viscosity of the

medium (Section 5.1.6, 5.2.5, 5.3.5) as observed by other workers (Anderzejewska et

al., 2009). A study of polymerization of HEMA in various solvents has shown that

the rates are affected by Hildebrand solubility parameters and the viscosities of the

solvent. The rates are faster in water compared to those determined in acetonitrile

(Encinas et al., 1996). In order to confirm the effect of medium viscosity on the rates

198

of these reactions, the values of kobs in organic solvents were plotted as a function of

the inverse of solvent viscosity (Table 25), and a linear relationship was observed as

expected (Fig. 59). These observations are supported by the data of rate constants

reported by Valdebenito and Encinas (2003) who reported a decrease in fluorescence

quantum yields of the photoinitiator with an increase in viscosity in organic solvents

compared to that obtained in aqueous medium. The decrease in fluorescence intensity

of RF in organic solvents (Table 26) indicates the effect of solvent viscosity (Fig. 59)

on the polymerization reaction. This may be explained on the basis of flavin excited

singlet quenching in organic solvents as a result of change in solvent viscosity. The

radical-radical reactions, as in the case of polymerization of HEMA, are sensitive to

solvent viscosity (Turro et al., 2010). The decrease in the rate of polymerization has

also been ascribed to the combination of a monomer viscosity effect (Biasutti et al.,

2003).

6.6.2. Camphorquinone as Photoinitiator

The effect of viscosity on the rate of polymerization of HEMA using CQ as a

photoinitiator shows a similar behavior as observed in the case of RF. A plot of kobs

versus the inverse of solvent viscosity shows a linear relationship and the rates tend to

decrease with an increase in the viscosity of the medium. This appears to be due to a

decrease in solute diffusional processes with an increase in solvent viscosity. The

slope of the plot (Fig. 59) indicates that viscosity exerts a lower effect on the rates in

the presence of CQ compared to that of RF.

199

Table 25. Apparent first-order rate constants (kobs) for the polymerization of HEMA

in water and organic solvents. Effect of viscosity.

kobs × 104 (s

–1)a

Solvent Viscosity

(mPa.s)–1

RF CQ ST

Water 1.000 5.05 4.04 2.98

Acetonitrile 2.898 3.71 3.02 2.10

Methanol 1.838 3.53 2.90 1.93

Ethanol 0.931 3.12 2.62 1.71

1-Propanol 0.514 – 2.45 1.56

1-Butanol 0.393 – 2.39 1.45

a The values of rate constants are relative and depend on specific experimental

conditions including light intensity.

200

RF

CQ

ST

0.0

1.0

2.0

3.0

4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0


ko

bs

× 1

04

(s–

1)

Fig. 59. Plots of kobs for polymerization of HEMA against inverse of solvent

viscosity. (●) acetonitrile; (▲) methanol; (♦) ethanol; (■) 1-propanol; (∗)

1-butanol.

201

Table 26. Fluorescence intensity of 1×10–5

M RF in water and organic solvents.

Solvent Relative fluorescence intensity at 520 nm

Water 100.0

Ethanol 87.1

Methanol 86.7

Acetonitrile 84.1

202

6.6.3. Safranin T as Photoinitiator

The results obtained with ST as a photoinitiator in the polymerization of

HEMA are similar to those of RF and CQ. A plot of kobs versus inverse of solvent

viscosity shows a linear relationship (Fig. 59) and the rates are further lower than

those observed in the case of CQ. The effect of viscosity on the rates of

polymerization of HEMA using ST as a photoinitiator is lower than that observed in

the presence of RF and CQ. Thus viscosity appears to play a significant role in the

photopolymerization of HEMA. The results indicate that in the polymerization of

HEMA, the effect of solvent characteristics should be taken into consideration to

facilitate the reaction and to achieve greater yields of the polymer.

6.7. Spectral and Structural Characteristics of Photoinitiators

In order to provide further explanation of the reactivity of the three

photoinitiators (RF, CQ, ST) used in this study, a consideration of the structural

characteristics and ionization behavior of these compounds is necessary. The

chemical structures of these photoinitiators are shown in Fig. 10 and 12.

RF absorbs at 445 nm and undergoes π–π* transitions resulting in high molar

absorptivity. It is a highly conjugated polar compound (pKa 1.9, 10.2) and exists as a

dipolar molecule in aqueous solution. It compared with RF, CQ possesses weakly

ionizable C=O groups. The light absorption at 468 nm would lead to n– π* transition

of the dicarbonyl group in the molecule which has low molar absorptivity. Hence its

efficiency in water would be lower compared to that of RF as observed. ST (pKa 4.0)

is an ionizable compound and absorbs at 520 nm to undergo π–π* transition. All these

203

photoinitiators, on excitation, produce radicals which interact with the amine and thus

initiate the polymerization of HEMA. The degree of interaction of these

photoinitiators with TEOHA would depend on the yield of these radicals in aqueous

and organic solvents. The rate of the reaction would depend on the viability of the

radical ion pair in a specific medium leading to polymerization.

6.8. Mechanism of Polymerization

RF is known to produce a polar intermediate on light absorption which would

further proceed to form radicals (Ahmad and Tollin, 1981a), and then interact

efficiently with the amine to initiate polymerization. The efficiency of CQ and ST is

lower than that of RF as explained in this chapter. The mechanisms of photoinitiated

polymerization of HEMA using RF (Orellana et al., 1999), CQ (Jakubiak et al.,

2003), and ST (Valdebenito and Encinas, 2003) as initiators have been reported.

However, these mechanisms involve similar steps to produce the amine radicals that

interact with HEMA resulting in the polymerization as presented earlier in schemes

1_3. Based on these mechanisms, a general scheme for the polymerization of HEMA

in the presence of different photoinitiators and the amine in presented in Fig. 60.

The photoinitiator (PI) on the absorption of light is promoted to the excited

singlet state (1PI*), followed by intersystem crossing (isc) to the excited triplet state

(3PI*).

3PI* abstracts a hydrogen atom from the amine, mediated by photoinduced

electron transfer through the radical ion pair. The free radicals thus formed in the

reaction add to the double bonds of HEMA monomer and thus initiate the

polymerization process. The rates and the extent of polymerization would depend on

204

the polarity and viscosity of the solvent used. It would also depend on the reactivity

and the efficacy of the photoinitiator used since the radical produced and their degree

of interaction with HEMA would determine the yield of the polymer.

205

Fig. 60. A general scheme for the polymerization of HEMA in the presence

of photoinitiators and amine.

CONCLUSIONS

AND

SUGGESTIONS

207

CONCLUSIONS

The conclusions of the present study on the polymerization of HEMA may be

summarized as follows.

1. Identification of Photodegradation Products of Riboflavin (RF)

RF solution (1×10-5

M) undergoes minor photodegradation at pH 9.0 during

irradiation for 5 min with a low intensity lamp (15 W) emitting in the visible region.

The products detected by thin layer chromatography are formylmethylflavin,

lumichrome and lumiflavin. However, there is no detectable spectral change affecting

the absorbance of RF at the maximum at 445 nm (0.125) on irradiation for about 1

min, the time involved in polymerization reactions. Similarly no changes in the

intensity of absorption of camphorquinone (CQ) and safranin T (ST) solutions

(absorbance 0.125) have been observed at the maxima at 468 nm and 520 nm,

respectively, during irradiation for 1–2 min. These compounds have been used as

photoinitiators in the polymerizations reactions carried out in the present study.

2. Assay of HEMA in Polymerized Solutions

A UV spectrometric method has been developed for the assay of HEMA at

208 nm in polymerized solutions at various pH values. It has also been applied to the

determination of HEMA in organic solvents under the same conditions. The method

has been found to give satisfactory results within about 5% change in the monomer

concentration. This is necessary to avoid any changes in the volume / viscosity of

polymerized solutions.

208

3. Kinetics of Polymerization Reactions in Aqueous Solution

The polymerization of HEMA has been studied in aqueous solution in the

presence of RF, CQ and ST as photoinitiators and triethanolamine (TEOHA) as a co-

initiator and the apparent first-order rate constants (kobs) have been determined for the

initial stages of the reactions. Under the steady-state assumption, the rate of initiation

being equal to the rate of termination is considered valid only at a low conversion of

the monomer. It has been observed that during the initial stages (upto about 5%

change), the reaction follows an apparent first-order kinetics. The rates of the

reactions increase with pH in the range of 6.0-9.0 (kobs 5.44–7.63×10–4

s–1

) probably as

a result of the gradual deprotonation of the amine which may produce a higher yield

of radicals for interaction with HEMA to undergo polymerization. An increase in

TEOHA concentration in the range of 0.0025-0.01 M has been found to increase the

rate of polymerization reactions. The second-order rate constants for the interaction of

HEMA and the amine are of the order of 1.79–8.87×10–2

M–1

s–1

. These values are

also affected by pH and the rates of interaction increase with an increase in the pH of

the solutions. The rate of polymerization reactions is also affected by changes in the

viscosity of the solutions (1–3 M concentration) and is lowered with an increase in

the viscosity of the medium due to changes in the diffusional processes.

4. Kinetics of Polymerization Reactions in Organic Solvents

The polymerization reactions of HEMA carried out in organic solvents

(acetonitrile, methanol, ethanol, 1-propanol, 1-butanol) indicate the effect of solvent

dielectric constant and viscosity on the rate of the reactions. A linear relation has been

209

observed between kobs and the solvent dielectric constant suggesting the presence of a

polar intermediate in the reaction pathway to facilitate polymerization in polar

medium. Thus the polarity of the medium does influence the rate of polymerization

reactions. A linear relation has also been observed between kobs and the inverse of

solvent viscosity indicating that an increase in the viscosity of the medium results in a

decrease in the rate of the reaction. This could be due to a relatively low diffusion of

solute in the medium as well as the triplet state quenching of the photoinitiators. The

rates of polymerization reactions are lower in organic solvents compared to aqueous

solutions due to difference in the polarity and viscosity of the medium.

5. Mechanism of HEMA Polymerization

The kinetic data indicate a similarity in the mechanism of the polymerization

reactions in the presence of different photoinitiators (RF, CQ, ST). These reactions

basically involve the formation of photoinitiator and amine radicals by electron /

proton transfer followed by the interaction of amine radicals and HEMA to undergo

polymerization. Reactions scheme for the mechanism of the polymerization of

HEMA in the presence of RF, CQ, ST and amine have been presented by different

workers. In all these reactions the final step involves the interaction of amine radicals

and HEMA and the rate of reaction would depend on the formation and degree of

interaction of amine radicals with HEMA. However, the rate of the reaction and

radical yield would be affected by the medium characteristics. It has been found that

the reactivity of RF as a photoinitiator is greater than CQ and ST and water is the best

medium for the polymerization of HEMA.

210

SUGGESTIONS

The following suggestions can be made on the polymerization of HEMA on

the basis of the work presented in this study.

The present work has shown the reactivity of certain photoinitiators (RF, CQ,

ST) in the polymerization of HEMA in aqueous and organic solvents. Considering

water as the best medium for the polymerization of HEMA, further studies may be

conducted to evaluate the reactivity of these photoinitiators in the presence of other

amine derivatives as co-initiators to optimise the reaction. Similarly, it would be a

good idea to use other photoinitiators mentioned in the literature and ascertain

whether there reactivity and efficacy is greater than those used in this present study.

Attempts can be made to develop conditions to optimize the reactivity of selected

photoinitiators in the polymerization of HEMA. Under specified conditions the

maximum concentration of the amine needs to be established to achieve greater yields

in the polymerization reactions. Laser flash photolysis studies on these reactions may

throw light on the delicate mechanistic aspects and factors affecting radical formation

and interaction leading to polymerization.

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AUTHOR’S PUBLICATIONS

PAPERS PUBLISHED

INTERNATIONAL

1. Iqbal Ahmad, Kefi Iqbal, Sofia Ahmed and M.A. Sheraz, “Applications of Laser

Flash Photolysis, Spectroscopy and Electron Microscopy in

Photopolymerization and Development of Glass Ionomer Dental Cements”,

International Journal of Clinical Dentistry. 4, 1-17 (2011).

2. Muhammad Ali Sheraz, Sofia Ahmed, Iqbal Ahmad, Faiyaz H.M. Vaid and Kefi

Iqbal, “Formulation and Stability of Ascorbic Acid in Topical Preparations”,

Systematic Reviews in Pharmacy, 2, 93-97 (2011).

3. Iqbal Ahmad, Sofia Ahmed, M.A. Sheraz, Kefi Iqbal and Fayiaz H.M. Vaid,

“Pharmacological Aspects of Borates”, International Journal of Medical and

Biological Frontier. 16, 977-1004 (2010).

4. Sofia Ahmed, M. A. Sheraz, M. Aminuddin, Iqbal Ahmad, Karamat Mahmood

and Kefi Iqbal, “Analysis of Amino Acids By Paper Chromatography”,

International Journal of Chemical and Analytical Science. 1, 174-176 (2010).

5. Sofia Ahmed, Muhammad Ali Sheraz, Kefi Iqbal and Iqbal Ahmad, “Surgical

Trends in the Treatment of Parkinson’s Disease and their Outcome”, Journal

of Medical Sciences 3, 142-152 (2010).

255

NATIONAL

6. Sajid Hanif, M. Noman Sheikh, S. Fareed Mohsin, Kefi Iqbal, and S. Azhar

Ahmed, “The Expression of Erythropoietin and Erythropoietin Receptor in

Oral Squamous Cell Carcinoma”, Pakistan Journal of Pathology. 21, 48-54

(2010).

7. Kefi Iqbal, Sana Adeba Islam, Iqbal Ahmad, Maria Asmat and M. Aminuddin,

“Variation in Powder / Liquid Ratio of a Restorative and Luting Glass

Ionomer Cement in Dental Clinics”, Journal of the College of Physicians and

Surgeons Pakistan. 19, 464-465 (2009).

8. Kefi Iqbal, Iqbal Ahmad and M. Aminuddin, “Spectroscopic Techniques in the

Development of Glass ionomer Cements”, Journal of Pakistan Dental

Association. 18, 1-5 (2009).

9. S. Fareed Mohsin, Saad A. Khan, Nasreen Amanat, Kefi Iqbal, Alan Cruchley

and S. Azhar Ahmed, “The Levels of Cytokines- The effect of Betel

Quid/Areca Nut on Oral Epithelium”, Pakistan Journal of Pathology. 20, 85-90

(2009).

10. Kefi Iqbal, Iqbal Ahmad, M. A. Sheraz, Sofia Ahmed, M. Aminuddin and Tania

Mirza, “Use of Different Light-Curing Units in Setting of Dental Restorative

Materials”, Journal of Baqai Medical University. 12, 51-54 (2009).

11. Tania Mirza, Kiran Qadeer, Sadia Hafeez, Kefi Iqbal and Iqbal Ahmad, “Folic

Acid and Deficiency Diseases”, Journal of Baqai Medical University. 12, 47-50

(2009).

256

12. Syed Fareed Mohsin, Shahnaz Fayyaz, Shazia Nazar and Kefi Iqbal, “Prevalence

of Periodontal Disease in Gadap Region Karachi”, Journal of Baqai Medical

University 12, 19-24 (2009).

13. Kefi Iqbal, Afreen Mushtaque, Maria Asmat, Ifthakar Ahmed, Fareed Mohsin and

Adel Siddiqui, “Role of Abrasive in Dentistry”, Journal of Baqai Medical

University. 11, 17-22 (2008).

14. Kefi Iqbal, Iqbal Ahmad and M. Aminuddin, “Development of Glass ionomer

Cements as Dental Restorative Materials”, Journal of Baqai Medical

University. 10, 9-12 (2007).

15. Iftikhar Ahmed, M. Sarwar, Hafiz Sheikh and Kefi Iqbal, “Acne in Adolescent”,

Journal of Baqai Medical University. 10, 25-30 (2007).

16. Kefi Iqbal, Maria Asmat, Sana Jawed, Majid Zia, Fareed Mohsin, Sajid Hanif,

Nuaman Sheikh and Iftikhar Ahmed, “Uses and Effects of Mercury in Medicine

and Dentistry”, Journal of Ayub Medical College (in press).

17. Kefi Iqbal, Maria Asmat, Sana Jawed, Majid Zia, Afreen Mushtaque, Fareed

Mohsin, Nuaman Sheikh, Sajid Hanif, Adel Siddiqui, Iftikhar Ahmed, Yawer

Abdi and Feroze Ali, Study of Dental Amalgam: Influence of Alloy/Mercury

mixing ratio, Uses and Waste Management”, Journal of Ayub Medical College

(in press).

18. Kefi Iqbal, Maria Asmat, Sana Jawed, Afreen Mushtaque, Fareed Mohsin, Sajid

Hanif, Nuaman Sheikh, “Role of Different Ingredients of Tooth Pastes and

Mouthwashes in Oral Health”, Journal of Pakistan Dental Association (in

press).

257

19. Tania Mirza, Kefi Iqbal and Iqbal Ahmad, “Clinical Analysis, Metabolism and

Bioavalibility of Thiamine”, Journal of Baqai Medical University (in press).

20. Tania Mirza, Sadia Hafeez Kazi, Zufi Shad, Kefi Iqbal and Iqbal Ahmad,

“Biochemical Importance, Deficiency and Clinical Assay of Cynocobalamin”,

Journal of Baqai Medical University (in press).

PAPERS SUBMITTED

21. Muhammad Irfan, Sofia Ahmed, Muhammad Ali Sheraz, Muhammad

Aminuddin, Iqbal Ahmad, Karamat Mahmood and Kefi Iqbal, “Heavy Metals

Accumulation in Fruits and Vegetables Grown on Sewage Sludge and that of

River Water”, submitted to Pakistan Journal of Botany.

22. Iqbal Ahmad, Kefi Iqbal, Sofia Ahmed, Muhammad Ali Sheraz, Muhammad

Aminuddin and Ihtesham ur Rehman, “Photoinitiated Polymerization of 2-

Hydroxyethyl Methacrylate by Riboflavin/ Triethanolamine in Aqueous

Solution: A Kinetic Study”, submitted to Journal of Dentistry.

23. Iqbal Ahmad, Kefi Iqbal, Muhammad Ali Sheraz, Sofia Ahmed, Mohammad

Aminuddin, Tania Mirza, Syed Abid Ali and Ihtesham ur Rehman, “Solvent

Effect on Photoinitiator Reactivity in the Polymerization of 2-Hydroxyethyl

Methacrylate”, submitted to Polymer Degradation and Stability.

24. Iqbal Ahmad, Tania Mirza, Sofia Ahmed, Muhammad Ali Sheraz, Kefi Iqbal and

Faiyaz H.M. Vaid, “Effect of pH on the Photolysis of Formylmethylflavin in

Aqueous Solution: A Kinetic Study”, submitted to Photochemical

Photobiological Sciences.

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