Polymer Degradation and Stability 37 (1992) 41-49

Ultrasonic detection of photo cross-linking in some acrylate copolymers

S. H. EI-Hamouly, ° W. Aziz , ~ E. H. El-Shamy °

~Department of Chemistry, Faculty of Science, Menoufia University, Shebin EI-Kom, Egypt bNational Institute for Standards, Dokki, Cairo, Egypt

&

K. N. Abd-EI-Nour Department of Physics, National Research Centre, Dokki, Cairo, Egypt

(Received 16 April 1991; accepted 3 May 1991)

The copolymerization of glycidyl methacrylate (GMA) with methyl methacry- late (MMA) at 65°C with 2,2'-azobisisobutyronitrile as radical initiator has been investigated. Monomer reactivity ratios for GMA and MMA are found to be rl = 0-90 + 0.001 and ra = 0.83 i 0.002, respectively. One of the copoly- mers obtained (GMA-MMA) containing 30% epoxide, was modified by reaction with acrylic acid. The polymerization of the unsaturated polyacrylic ester obtained has been photoinitiated in a solid state matrix.

The ultrasonic pulse-echo technique was used, in the range of frequencies between 2.5 and 6 MHz and temperatures between 270 and 360 K, to detect the secondary relaxations in GMA-MMA. The activation energies for such relaxations were calculated and interpreted according to the flexibility of those unsaturated polyacrylate ester copolymers, both before and after exposure to ultraviolet irradiation, and also in the absence and presence of polyfunctional acrylate ester monomer.

INTRODUCTION

The copolymerization of glycidyl methacrylate- methyl methacrylate (GMA-MMA) has been studied previously and reactivity ratios have been reported. 1-3 The photoinitiation of radical chain polymerization has become increasingly impor- tant because of its use in applications such as relief printing plates, printing coating and printing inks. In all these processes the final polymeric products are usually cross-linked resins. 4-s Many attempts have been made to systematize the approach to the naming of the many observed secondary relaxation processes which occur below the glass transition tempera- ture in polymeric materials. 6,7 These processes are designated by letters of the Greek alphabet. o: representing the highest temperature transi- tion, with r, y, 6, etc., representing other

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd.

dispersion regions in decreasing order of temperature. Such processes can be studied over a wide frequency range by dynamic mechanical techniques of various kinds, by dielectric methods or by nuclear magnetic resonance s and recently by an ultrasonic technique2

The aim of this work is to use the ultrasonic technique to investigate the secondary relaxa- tions of copolymers containing various con- centrations of GMA and MMA. It is also aimed to study the effect of UV irradiation on the secondary relaxation of the acrylic ester copoly- mer in the presence of photoinitiator and cross-linking material.

41

EXPERIMENTAL

Materials

Most reagents were purchased except methyl methacrylate

from Aldrich, and 2,2'-

42 S. H. El-Hamouly et al.

azobisisobutyronitrile (AIBN), which were ob- tained from Merck, Darmstadt.

MMA was first washed with 10% aqueous sodium hydroxide to remove the inhibitor, then with distilled water to neutral pH. After drying over anhydrous sodium sulphate, the monomer was distilled twice under nitrogen.

GMA and acrylic acid were distilled under reduced pressure.

AIBN was purified by recrystallization from absolute ethanol (m.p. 105°C).

Triethylamine (catalyst), 2,4,6-tri-t-butyl phe- nol (inhibitor) and benzoin methyl ether (photoinitiator) were used without further purification.

Pentaerythritol triacrylate (cross-linking agent) was washed before use with aqueous sodium hydroxide to remove the inhibitor.

Methods

Copolymerization Ampoules of Pyrex glass were charged with the monomer mixtures, solvent and initiator. Copolymerizations were carried out in a thermo- stat at 65°C to conversions below 10%. The contents of the ampoule were then poured into a large excess of cold methanol. The copolymers obtained were purified by reprecipitation from chloroform by petroleum ether (60°-80°). The copolymers were finally dried to constant weight in a vacuum oven at 40°C.

Reaction with acrylic acids Copolymer (GMA-MMA) (30:70) was reacted in benzene solution with a threefold excess of acrylic acid, an equivalent amount of triethyl- amine as catalyst and 1.5% of 2,4,6-tri-t-butyl phenol as polymerization inhibitor. The mixtures were refluxed for 8 h at 80°C, then cooled to 0°C and decantated. The precipitate was dissolved in chloroform, reprecipitated in n-hexane at room temperature, and finally dried in vacuum at room temperature.

Dry films Dry films of G M A - M M A polymers were obtained by casting chloroform solutions on a mercury surface. Dry films of MMA-acrylate ester copolymer were obtained by casting chloroform solutions containing 3% wt benzoin methyl ether (photo initiator) on a mercury surface in the dark. Films of MMA-acrylate

ester copolymer containing cross-linking agents were similarly obtained by adding 1% wt pentaerythritol triacrylate before casting.

Spectral measurements

Nuclear magnetic resonance (NMR ) The compositions of copolymers were deter- mined, using a proton magnetic resonance (1H-NMR) Varian E.M. 390 spectrometer operating at 90 MHz.

Determination of glass transition temperature (Tg) The glass transition temperature (Tg) was determined using 'Shimadzu' differential thermal analysis (sample size 15 mg, sensitivity 250/~V, nitrogen atmosphere, heating rate 15°C/min).

Ultraviolet irradiation Irradiation of MMA-acrylate ester copolymer matrices in the presence of photoinitiator and in the absence and presence of cross-linking agents was carried out using an UV lamp with filter, model V.L. 30L.C. (1 × 15W. 365nm tube, 1350 ~w/cm 2, long wavelength) from M. Vilber Lourmat (Marine La Vall6e, France). The irradiation assembly included a lamp, collecting lens and shutter. The sample was set at a distance of 20 cm. Dry film of 2 mm thickness was used. The temperature of irradiation was 40°C and the time of irradiation was 30 min.

Ultrasonic measurements The ultrasonic absorption by the samples was measured using a conventional pulse-echo technique. 9 An ultrasonic flow detector (USM2), produced by Kraut Kramer, (Cologne- Klettenberg, FRG) was used. The apparatus is capable of producing high frequency pulses in the range 0.5 to 12MHz and usually operates with the same transducer as the transmitter and receiver simultaneously. Since the ultrasonic absorption (cQ is much higher than in metals, usually only one or two echoes are observed in the oscilloscope.

Accordingly, measurements were taken on a comparative basis where the dependence of L -1 (L is the height of a particular echo on the oscilloscope) on temperature is considered to represent the dependence of a~ on temperature, since they are related by the equation L = Lo e -"a and have the same dimensions, 1°-" where d is the distance traversed and L0 is the amplitude of the

Photo cross-linking in acrylate copolymers 43

pulse at d = 0. Measurements of the height of a particular echo (L), on the face of the oscilloscope were made by means of a calibrated scale capable of measuring to an accuracy of +0 .2mm. The ultrasonic measurements were carried out in the frequency range 2 .5 -6MHz and at temperatures between 270 and 360K achieved using an electric furnace.

RESULTS AND DISCUSSION

Copolymerization of GMA with many acrylic monomers offers an economical means of introducing into a polymer molecule reactive functional groups which exhibit many of the reaction characteristics of the oxirane ring.

Copolymers of GMA and MMA with six different compositions were prepared in benzene solution at 65°C in the presence of AIBN as a free radical initiator. The compositions of these copolymers were determined quantitatively by ~H-NMR on the basis of the characteristic bands, shown in Table 1 and Fig. l(a). The molar compositions of these copolymers are given in Table 2 and the monomer reactivity ratios rl and r2 of the copolymers were calculated from the data in Table 2 by application of the Kelen and T/Jdos ~2 method as represented in Fig. 2. The values obtained, namely, rl ( G M A ) = 0 . 9 0 + 0.001 and r2 (MMA) = 0.83 + 0-002, indicate that the two radicals have more or less the same preference for adding the two monomers, and monomer insertion into the copolymer chain is random.

Figure 3 shows the relationship between F~ and f~, the mole fractions of GMA in the copolymer and in the monomer feed, respectively. The curve crosses the line representing F1 =f~ at GMA:MMA = 0.7:0.3. This point of intersection corresponds to the azeotropic composition which

Table 1. Chemical shifts of glycidyi methacrylate (GMA)- methyl methacrylate (MMA) copolymers

Resonance signal (ppm)

Proton Assignment Remarks

0"8-2"3 10H CH3 5H MMA I 5H GMA

2(CH:--C)

i[ Y m l / c=o I I c=o I / i ~ /

l ° ,I . 1 ° I _ CH 3 ~^ L_ CH2 ~Y I -// / ~

C H ~ , , / J

CH2-O

H3

/

l [ ",IF

/ ?°11 I : ° / L i.~J,L ,!H2 JY

CH-OH I CH2-O - C - C H : CH 2

O

(b)

1 1 I I 7 6 5 4

/ /

/ / OCH3 /

/ .

I I I 3 2 1

p p m

Fig. 1. 1H-NMR spectra of G M A - M M A copolymer (a) and the acrylate copolymer (b).

Table 2. Molar composition of G M A / M M A Copolymers

Copoly- Initial Peak area Copolymer mer conc. integ- compos- No. (mol/litre) ration ition

(tool %)

GMA MMA GMA MMA GMA MMA (M 0 (M_~) (3H) (3H) (m) (mz)

1 0.1 0.9 4 30 11.74 88.26 2 0.3 0.7 16 35 31.37 68.63 3 0.4 0-6 25 36 40.97 59.03 4 0.5 0.5 25 24 51.00 49-00 5 0-6 0.4 63 38 62-38 37.61 6 0.7 0.3 60 26 69-78 30.22

yields homogenous copolymer regardless of conversion.

Unsaturated polyacrylic ester

2-4-3-6 3H - - C H ~ - - C H - - 3 GMA

\ o /

3"6-3"9 3H O---CH3 MMA

Unsaturated polyacrylic ester was synthesized by reaction of the epoxide groups in the copolymer G M A - M M A (30:70) with acrylic acid in the presence of tertiary amine (triethylamine) as a

44 S. H. EI-Hamouly et al.

1,o

0.8

0.6

0.4

0.2

o

-0.2

-o.t,

-o.6

-o.8

-1.o

-1.2

-1.4

-1.6

-1.8

/ ~ . . . o . .

/ o

/ Z Z

0,6

0.6 F / / / * /

g/ 0.4 /

• /

// /

0.2 / /

// /.i" I 1 I I

0.2 0.4 0.6 0.8 t

Fig. 3. Composition curve for the copolymerization of GMA with MMA. The line represent calculated values, [0] represents experimental values and f~ and F~ are the molar fractions of M~ in the feed and in the copolymer

respectively.

catalyst. This synthesis is represented by the reaction shown in Scheme 1. The composition of the unsaturated polyacrylic ester was determined quantitatively by 1H-NMR spectrometry on the basis of the characteristic bands, as in Table 3 and Fig. l(b), and values are presented in Table 4.

The photoinitiated polymerization was studied in a solid state matrix in the presence of benzoin

Fig. 2. Kelen-Tfid6s plot; determination of copolymeriza- tion parameters;

a 2 a(b + 1) ~ = o:b + a2 and r/ trb + a2 ,

where a and b are the molar ratios (MJM2) of the comonomer in the feed and copolymer, respectively, and

aminamax Ol = (bminbmax) l /2

Table 3. Chemical shifts of MMA-acrylate ester copolymer

Resonance Proton Assign- signal ment (ppm)

Remarks

CH3 0"8-2"3 10H I 5H. MMA

2(CH2--C) 5H. GMA

3.6-3.9 3H OCH3 MMA

5-6-7.6 3H C H ~ C H ABX system

CH3

? CH2

I CH

r> CH2

X - Co (GMA-MMA) polymer

CH3

CH2__ f I

C = O I

O I

CH3

O II CH~CH---C~OH )

(C2Hs)3N

Y

~ H3

? C H - - O H

S c h e m e

X Unsaturated polyacrylate ester

CH3 I

-CH2---C I

c ~ o I

O J

CH3

Y

P h o t o cross-linking in acrylate c o p o l y m e r s 45

Table 4. Molar composition of Co(MMA-(mD-l-aeroyl- oxy-2-hydroxypropyl methacrylate)-polymer m3 °

Copolymer Copolymer before Peak area Molar (GMA/MMA) reaction with integration composition

acrylic acid (tool%)

PGMA PMMA MMA Acrylate (m2) (m3) (ml) (m2) (3H) (3H)

30:70 31.37 68-63 43 18 70.49 29-51

a

CH3 0 0 I il II

CH 2 - - C - - C - - O - - - C H ~ - - C H - - C H 2 - - O - - - C - - C H = C H 2 I OH

methyl ether (3%wt) as photolabile initiator (;~max 340 nm).

O OCH3 H f

C H hv 6 c - - C - - C H - - C 6 H 5 ) - " 340 nm

O II

C6H~--C'+ C6H:,--;CH--OCH~

t l chain propagation and cross-linking

The kinetics of polymerization was followed by an ultrasonic technique. The changes in attenua- tion of the longitudinal ultrasonic waves with temperature for the copolymer samples contain- ing 10: 90, 30 : 70, 50 : 50 and 70: 30 (GMA/MMA) are presented in Fig. 4. The measurements were carried out on a thin film

0,28

0.26

0.24 E v

-- 0,22 I , _ l

0.20

0.32

0.3C

o.2e

E ,#

0.26 %

0,24

0,22

0.20

0.18

C1) 0.26

..o. • '.. 0.24

olV t / ~ ~ ...

/ O ~ "; ~;~,,, , o2~

":" "" : 0.20

036

i I i i

(3)

2.5 MH z 0.50

- - x - - 4.0 MH z

5.0 I.IH z 0.56

- - , ~ - - 6.0 HH z 0.52~

0.48!

0.44

0.40

0.36

0.28

0.24

I I 1 r

280 300 320 340 360 X K

(2) r

~ ' , , : , i,

I : 1 ; xf~ I - i "

j l~l: • " L ",

..~ " ~ t . e "

I I I I i

(4) f.,

i i

f . , i l l r, L , I! '

r 'l ~l "

i!I~ i . i " 'r" ~'

i , k

i L.. i i

280 300 320 340 360

Fig. 4. Relationship between L -] (cm -~) and T(K) at various frequencies: (1) G M A - M M A copolymer (10:90); (2) G M A - M M A copolymer (30: 70); (3) G M A - M M A copolymer (50 : 50); (4) G M A - M M A copolymer (70 : 30).

S. H. El-Hamouly et al.

7,C

I 6.6

3.0

J

o 7.0

Table 5. The maximum absolute temperature T= at which the peaks occur in relation to the operating frequencies for

the various copolymers

f TIn(K) (MHz)

10:90 30:70 50:50 70:30

2.5 314 306 294 320 4.0 322 314 303 328 5.0 327 321 313 334

(2 mm thickness) at frequencies 2.5, 4 and 5 or 6MHz. From Fig. 4, it is clear that the absorption of the ultrasonic waves increases with increasing temperature to a maximum and then decreases giving rise to series of peaks, which indicate some sort of relaxational processes. These peaks are observed in the temperature regions given in Table 5. With increasing frequency, the peaks are shifted to higher temperature and become greater in intensity. This trend is similar to that found previously in the case of nylon 6 and nylon 66.13

On the other hand, it is to be noted that the peaks are shifted to lower temperature by increasing the percentage of GMA in the copolymer. This shift could be attributed to the low glass transition temperature (T~) of poly- GMA (63°C) compared with that of poly-MMA (105°C). TM The apparent activation energies (W) of the relaxational processes of the copolymers were calculated, using the Arrhenius equation: ~5

f = fo e x p ( - W/gTmax)

where f is the operating frequency, fo is the natural frequency of the sample, Tmax is the maxi- mum absolute temperature at which the relaxa- tional processes occur, and K is the Boltzmann constant. The relationship between l n f and 1/Tmax is represented by a straight line in the form:

W 1 l n f = lnfo - ~ × Tmax

The slopes of these lines gives the values of W/K from which the apparent activation energies of the relaxational processes of the copolymers are calculated. Plots are shown in Fig. 5 and the apparent activation energies are presented in Table 6. These values are comparable with those to be found in the literature. 16-19

For the fl-relaxation process, whether in crystalline or amorphous polymer (28-100 k J), it

6.6

6.2 31

C o ( I )

(10 : 90)

' i 3.1 312 3.3

(50:50)

(30:70)

3.1 ~'2 ~'3 ,i,

12 ,:3 31, 1000 ITm

(70:30)

\~-....°

! I i

2.9 3,0 3.1 3.2

Fig. 5. Relationship between logf and 100/Tm(K -~) for GMA-MMA polymers.

Table 6. The apparent activation energy W in kJ for the GMA-MMA copolymers of various compositions

10: 90 30: 70 50: 50 70: 30

W 40.5 31-4 28-5 45.0

.93"C

Co( 2)"

~- Co t 3 ) <~

Co ( 4 )

46

40 60 100 120 Temperature ( ~ )

Fig. 6. Glass transition temperatures of the copolymers: (1) GMA-MMA copolymer 10:90, (2) GMA-MMA copoly- mer 30:70; (3) GMA-MMA copolymer 50:50; (4)

GMA-MMA copolymer 70: 30.

Photo cross-linking in acrylate copolymers 47

0.22

0.20

O,1B

E

u 0.16

0.14

0.12

- - ' - - 2.5 (A) - - x - -

- - o - - /°\ (\ o/ \ \

B e f o r e i r r a d i a t i o n

I I I I I I

M H z

4 . 0 M H z

6 . 0 M H z

E 0 .16 u

LJ

0.22

O. 20

0.18

0.14

0.12

I 28O

/o\ /°\

,7 /,, \\ ': \ \ / 7 ' ° / \, x o

o ~ \e x \

% o

A f t e r i r r a d i a t i o n

I I I I I I 300 32O 240

Tk

Fig, 7. Relationship between L -'(cm -~) and T(K) at various frequencies for MMA-acrylate ester i (70:30) copolymer in solid matrix-3 % wt benzoin methyl ether.

can be concluded that the relaxation of the different compositions of the copolymer in the given range of temperature is of secondary or /~-type, which could be attributed to side chain rotations. From Table 6, it may be seen that the activation energies of different copolymer com- positions decrease on increasing the percentage of GMA in the copolymer (from 10% up to 50%). This result could be attributed to the decrease in the glass transition temperature, which results in an increase in the flexibility of the copolymer. This result is comparable with

that found previously in the case of thermoplastic elastomers where the activation energy for this type of relaxation was found to decrease on decreasing the hard segment content in the copolymer. 2° The measured values of glass transition temperature, ~ , of these copolymers are presented in Fig. 6, from which it is clear that the value of Tg decreases on increasing the GMA content in the copolymers.

On the other hand, the increase in the activation energy noticed in case of the copolymer containing 70% of GMA could be

48 S.H. EI-Hamouly et al.

0.17

0.16

0.15

0.14

0.13

0.12

0.21

0.2O

0.19

0.18

0.17

0.16

0.1.-,

0.14

0.13

(B) / ° \ . . . . . 3 M H z , / - , - 4 M . z

/\o - -6M.z

'~ xxx

B e f o r e i r r a d i a t i o n " " ° - " e'--°

I I I I I I I

/°\

/ ? ! / /"

/i(\ i . t o . ~ J /,/ , /'1",')\

X eO \\ X \

A f t e r i r r a d i a t i o n '~x~X ~ \

\ o I I I I I I

280 300 320 240 Tk

Fig. 8. Relationship between L -t (cm 1) and T(K) at various frequencies for MMA-acrylate ester (70:30) copolymer in the presence of pentaerythritol triacrylate resin in the solid matrix with 3 % wt benzoin methyl ether.

attributed to the fact that the copolymer has an azeotropic composition and should, therefore, be homogenous regardless of conversion.

Table 7. The maximum absolute temperature T. at which the peaks occur in relation to the operating frequency for

matrices A and B before and after UV irradiation

f TIn(K) (MHz)

Before irradiation After irradiation

A B A B

2.5 289 294 301 301 4-0 298 303 307 308 6.0 307 313 315 317

Table 8. The apparent activation energy W in kJ for mat- rices A and B before and after irradiation

Before irradiation After irradiation

A B A B

W 37.2 24-4 54.9 43.9

The change in attenuation of the ultrasonic waves with temperature was also studied for MMA-acrylate ester (70:30) copolymer both in the presence of benzoin methyl ether as photoinitiator (3% wt), in the solid matrix (A) and in the presence of photoinitiator (3% wt) and 1%wt cross-linking agent, pentaerythritol triacrylate monomer, in the solid matrix (B). Both matrices were subjected to UV irradiation and the results before and after irradiation are shown in Figs 7 and 8. The peak temperatures are given in Table 7. These peaks can also be attributed to the movements associated with a secondary relaxation process (fl-type). By expos- ing the matrices (A) and (B) to the UV

o ~ .J

7.0

6. e

6.6

6.4

(A) 6.2

3,0

1 -

2- B e f o r e i r r a d i a t i o n A f t e r i r r a d i a t i o n

(2 )X~"

\ \ I I I I I

3.1 3.2 3.3 3.4 3.5 1000 I T m

(B) I

0 3.1

(1)

' J 3 ' 4 ' 3.2 3.3 . 3.5 1000 I T m

Fig. 9. Relationship between logfand lO00/Tm (K 1) for: (A) MMA-acrylate ester (70: 30) copolymer in the solid matrix; (B) MMA-acrylate ester (70 : 30) copolymer in the presence of pentaerythritol triacrylate resin in the solid matrix.

Photo cross-linking in acrylate copolymers 49

irradiation, the peaks are found to shift to higher temperature and also to become higher. The increase in height is more pronounced in the case of the matrix B, which could be attributed to the increase of cross-linking in that matrix.

To obtain more details about these matrices it is interesting to determine the apparent activa- tion energies associated with the relaxation processes before and after irradiation. Straight line Arrhenius plots are obtained as shown in Fig. 9, from which activation energies were calculated, as in Table 8. From these values it is clear that for both matrices the activation energy increases after exposure to UV irradiation. This increase can be attributed to the increase in the rate of cross-linking, which results in an increase in the rigidity of the matrix. The increases in activation energy were found to be 48 and 80%, respectively. The higher increase for matrix B can be attributed to the addition of the polyfunctional acrylate ester monomer (pen- taerythritol triacrylate), which causes an increase in the cross-linking of the copolymer.

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