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 DOI: 10.1177/0954008314528225

published online 8 April 2014High Performance PolymersD. Duraibabu, T. Ganeshbabu, P. Saravanan and S. Ananda Kumar

gel films−inorganic hybrid sol−Development and characterization of novel organic  

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Original Article

Development and characterizationof novel organic–inorganic hybridsol–gel films

D. Duraibabu1, T. Ganeshbabu1, P. Saravanan2

and S. Ananda Kumar1

AbstractOrganic–inorganic hybrid films were fabricated by reacting 3-glycidoxypropyltrimethoxysilane with 4,40-diaminodiphe-nylether by hydrolysis and condensation reaction with acid catalysis. The chemical bonding between the organic andinorganic phases provides reinforcement to the films, and tetraethoxysilane was added in such a way that silica contentsvaried from 1 wt% to 3 wt% in the films. Structural characterization of the hybrid films was performed using Fouriertransform infrared and nuclear magnetic resonance spectroscopic techniques. The thermal properties studied usingthermogravimetric analysis, indicate an improved thermal stability for the films according to the percentage concentra-tion of silica present in them. The water absorption was also found to be reduced for the films with increased silicacontent. The surface morphology was investigated by means of x-ray diffraction and scanning electron microscopictechniques. The films transparency and homogeneity seem to have been affected severely when the silica content kepton increasing and ultimately led to opaqueness in film.

KeywordsHybrid film, sol–gel process, thermogravimetric analysis, water absorption

Introduction

Sol–gel process is known to be one of the feasible meth-

ods for preparing organic–inorganic hybrid film from

metal alkoxides and/or organoalkoxysilanes. It is a conve-

nient route to get a uniformly dispersed inorganic compo-

nent in the organic polymer matrix. The main idea in

the development of hybrid materials was to take advan-

tage of the best properties of each component that forms

a hybrid, trying to decrease or eliminate their drawbacks

getting in an ideal way a synergic effect, which results

in the development of new materials with new properties.

The synthesis of a new class of materials, inorganic–

organic hybrid composites, has attracted growing interests

because of the possibility of combining the properties of

both inorganic and organic components. Organically

modified ceramics, in which organic groups are cova-

lently attached to siloxane networks, is a convenient

method to produce silica-based hybrids by sol–gel process

using organoalkoxysilanes (RnSi (OR0)4�n).1 The elabora-

tion of organic–inorganic composites and their usage as

structural and biomedical materials has become indis-

pensable due to their potential applications in catalysis,

photonics, and electronics. The multilayered organic–

inorganic thin films are one of the most important systems

because they are expected to exhibit unique properties

arising from structural anisotropy and compositional vari-

eties.2 Two general approaches have been proposed:

(1) Solution synthesis of inorganics in the presence of

molecular assembly, which usually relies on noncovalent

interactions such as hydrogen or ionic bonds between

inorganic and organic components and (2) design of

molecular building blocks capable of self-assembly and

subsequent inorganic cross-linking.3 The first interest in

the development of hybrid materials was mainly based

on the design of hybrid polymers with special emphasis

on structural hybrid materials. A variety of silicates,

1 Department of Chemistry, Anna University, Chennai, Tamil Nadu, India2 Department of Chemistry, St. Joseph’s College of Engineering, Chennai,

Tamil Nadu, India

Corresponding author:

S. Ananda Kumar, Department of Chemistry, Anna University, Chennai

600025, Tamil Nadu, India.

Email: sri_anand_72@yahoo.com

High Performance Polymers1–9ª The Author(s) 2014Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954008314528225hip.sagepub.com

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polysiloxanes, and so on modified with organic groups or

networks for improvement of mechanical properties

were the first type of hybrid materials investigated. The

expectations for hybrid materials go further than mechan-

ical strength, such as thermal and chemical stability.

Recently, the self-assembly of amphiphilic molecules has

been widely exploited to create nanostructured organic–

inorganic composites due to fundamental interest and a

wide range of potential applications. Organic–inorganic

nanocomposites have been extensively studied recently

since these materials exhibit the combined characteristics

of organic polymer (e.g. flexibility, ductility, and dielec-

tric property) and inorganic materials (e.g. rigidity, high

thermal stability, strength, hardness, and high refractive

index).4–10 The properties of hybrid materials could be

tailored through adjustment of functionality or segment

size of each component. In addition, some special or novel

properties can be acquired because of the effects of nano-

particles. Therefore, these materials could be widely

used in the applications of protective coatings,11 high

refractive index films,12–15 thin film transistors,16 light-

emitting diodes,17–19 solar cells,20 optical waveguide

materials,21,22 and photochromic materials.23 The hybrid

network obtained in this manner has an excellent

optical transparency and was characterized using different

spectroscopic and microscopic techniques. Here in this

work, we report the development of a new hybrid mem-

brane prepared by sol–gel method with covalent

bond between organic and inorganic segments using

3-glycidoxypropyltri-methoxysilane (GPTMS) with 4,40-diaminodiphenyl ether (DDE). The precursor was then

subjected to hydrolysis and condensation in the presence

of water and acid catalyst. The inorganic content of

the polymer was increased by adding tetraethoxysilane

(TEOS) to the reacting medium. Fourier transform infra-

red (FTIR), nuclear magnetic resonance (NMR), thermo-

gravimetric analysis (TGA), x-ray diffraction (XRD),

water absorption, and morphological studies of these

hybrid materials were investigated in detail to understand

the suitability of these films for gas separation and ultra-

filtration as well.

Experimental sections

Chemicals

GPTMS, DDE, and TEOS were purchased from Sigma

Aldrich (Bangalore, Karnataka, India) and used as received.

Tetrahydrofuran (THF) and hydrochloric acid (HCl; 35–38%)

purchased from SD fine Chemicals (Mumbai, Maharashtra,

India) were used after purification.

Preparation of organic–inorganic hybrid films

Stoichiometric amount (1:4) of DDE and GPTMS were

mixed for 24 h at room temperature in THF. The obtained

product after the completion of the reaction is shown in

Figure 2(a). TEOS and HCl (0.15 M) were added in stoi-

chiometric amounts (GPTMS:H2O ¼ 1:3 and TEOS:H2O ¼1:4) and mixed for 24 h at ambient temperature followed by

2 h at 80�C to complete the hydrolytic condensation reaction.

The final product obtained is illustrated in Figure 2(b).

The casting solution was dropped into petridishes, and

the solvent was slowly evaporated (about 2 days). The final

drying step was carried out in an oven under vacuum at

80�C for 1 day.12

Different percentage of silica content

TEOS was added in such a way that silica contents varied

from 1 wt% to 3 wt% in the films, and the chemical bond-

ing between the organic and inorganic phases provides

reinforcement to the hybrid films. The effect of increase

in silica content toward the change in property of films was

studied. Different weight percentages of silica were pre-

pared by dissolving respective grams of TEOS in 100 ml

of THF solution.

Test methods

UV-Vis spectroscopy. The diffuse reflectance ultraviolet–visi-

ble (UV-Vis) spectroscopy was used to obtain the spectra

for hybrid films.

The spectra were recorded between 250 and 400 nm on a

Shimadzu UV–Vis spectrophotometer (model 2450;

Tokyo, Japan).

FTIR spectra. To identify the functional groups of hybrid

films, FTIR spectra were recorded on a Perkin Elmer 781

infrared spectrometer (Waltham, Massachusetts, USA) using

potassium bromide pellets for solid sample. Sodium chloride

was used for taking IR spectra of viscous liquid samples. The

data are depicted in Figure 3(a) and (b) respectively.

250 300 350 400

0

1

2

3

4 bac

Abs

orba

nce

(a.u

.)

λ max (nm)

Figure 1. UV-Vis spectra of hybrid films having (a) 1 wt%, (b) 2wt%, and (c) 3 wt% silica content.

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Screw gauge. Mitutoyo 293-340 IP65 Digimatic micro-

meter (Mitutoyo South Asia Pvt Ltd, New Delhi, India)

was used to measure the thickness of the hybrid films.

29Si NMR. The 29Si (75.47 MHz) NMR with magic angle for

analysis of solid samples was performed on a Bruker DSX

300 (Billerica , Massachusetts, USA) NMR spectrometer.

Thermogravimetric analysis. The thermal degradation of

hybrid films was investigated using a thermogravimetric

analyzer (Perkin Elmer TGA 7, Waltham, Massachusetts,

USA) from room temperature to 800�C under nitrogen

atmosphere. The measurements were conducted using

6–10 mg of samples at a heating rate of 20�C min�1.

Weight-loss versus temperature curves were recorded and

illustrated in Figure 5.

X-Ray diffraction. A Carl Zeiss URD6 (Germany) diffract-

ometer, employing a copper K� radiation source (� ¼ 1.542

A) operating at 48 kV and 30 mA was used to analyze the

hybrid films samples having varied silica content. Figure 6

shows the diffractograms taken in 5�–80� angle range with a

rate of 2� min�1.

Scanning electron microscopy. The morphology of the frac-

tured surfaces of films was investigated using scanning

electron microscopy (SEM) to visualize and study the

O NH2H2N H2C CH

O

CH2 O (CH2)3 Si

OCH3

OCH3

OCH3

24 h

N

(OCH3)3Si(CH2)3OCH2CH(OH)CH2

(OCH3)3Si(CH2)3OCH2CH(OH)CH2

O N

CH2CH(OH)CH2O(CH2)3Si(OCH3)3

CH2CH(OH)CH2O(CH2)3Si(OCH3)3

THF

GPTMS

TEOS

HCl

N

Si(CH2)3OCH2CH(OH)CH2

Si(CH2)3OCH2CH(OH)CH2

O N

CH2CH(OH)CH2O(CH2)3Si

CH2CH(OH)CH2O(CH2)3SiO

O

O

O

OO

O

O

O

O

OO

(a) (Before condensation)

(b) (After condensation)

DDE

Figure 2. Organic–inorganic hybrid film.

34202876

2354 15041118

3442

2908

2348 16301086

–200

20406080

100(b)%

Tra

nsm

itta

nce

Wave number (cm–1)

4000 3500 3000 2500 2000 1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

–200

20406080

100 (a)

Figure 3. (a) FTIR spectra for organic–inorganic hybrid film (a)before condensation and (b) after condensation.

Duraibabu et al. 3

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distribution of silica in the hybrid films. For SEM analysis,

the films were fractured in liquid nitrogen and coated with

gold by sputtering. The SEM images of the hybrid films are

depicted in Figure 7.

Water absorption. The water absorption measurements of

hybrid films were carried out according to ASTM D570-

81 specifications. The films were placed in a vacuum oven

at 80�C until the films attained a constant weight and then

immediately weighed out to the nearest 0.001 g to get the

initial weight (W0). The films were immersed in a container

of distilled water and maintained at 25�C.

After 24 h, the films were removed from water and were

quickly placed between sheets of paper to remove the

excess water, and films were weighed immediately. The

films were again soaked in water for 24 h and then the films

were removed, dried, and weighed for any weight gain.

This process was repeated till the films almost attained a

constant weight.24 The total soaking time was 168 h, and

the samples were weighed at regular 24 h time intervals

to get the final weight (Wf). The percentage of increase in

weight of the samples was calculated to the nearest

0.01% as per the following equation. The result is shown

in Figure 9.

Water absorption ¼ Wf �W0

W0

� 100

where W0 is the initial weight of the films and Wf is the final

weight of the films.

Results and discussion

UV-Vis spectroscopy

Figure 1 shows the absorption of epoxy–silica hybrid films

in the wavelength range of 250–400 nm. The experimental

results revealed that the viscosity of these hybrid solutions

increased with increasing silica content and resulted in the

Figure 4. 29Silicon NMR spectra of hybrid films. NMR: nuclear magnetic resonance.

0 100 200 300 400 500 600 700 800

3

4

5

6

7

8cba

Wei

ght

(mg)

Temperature (°C)

Figure 5. TGA image of (a) 1 wt%, (b) 2 wt%, and (c) 3 wt% silicacontent. TGA: thermogravimetric analysis.

10 20 30 40 50 60 70 800

5

10

15

20

25

30 abc

Inte

nsit

y

2� (deg)

Figure 6. XRD images of hybrid films having (a) 1 wt%, (b) 2 wt%,and (c) 3 wt% silica content. XRD: x-ray diffraction.

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aggregation of silica particle in the hybrid films. The optical

transparency linearly decreases with increasing the silica

fraction in the hybrid thin films. At above 3% silica content,

the film transparency decreased with increasing silica con-

tent; the converse was true at above 3% silica content. These

results showed that the prepared hybrid films demonstrated

tunable transparency with the silica fraction in the films. Such

composite films could have important applications for optical

coating or refractive index tuning films.25 As shown in the

inset (Figure 1), the hybrid flims having 1, 2, and 3% silica

content are recorded in diffuse reflectance UV-Vis spectro-

meter. The hybrid films were observed at 250–400 nm due

to the presence of silicon materials. The increase in silicon

content increases absorbance intensity due to increasing inor-

ganic contents and is shown in Figure 1.

At low silica content (1%), the silica particle was iso-

lated as a ‘‘dispersive’’ heterogeneous phase in the hybrid

matrix, which leads to a full transparency thin film.25 At

high silica content (3%), the silica particles blended into

epoxy resin to form an aggregation phase in the hybrid

matrix. This led to a considerable decrease in transparency

of the hybrid film. These results suggest that the transpar-

ency of the prepared hybrid films could be tunable through

the adjustment of silica content. Through suitable adjust-

ment of silica content, some hybrid films have potential

applications as the materials for microlens inkjet

printing.25

FTIR spectroscopy analysis

Infrared spectroscopy was used to characterize the formation

of organic–inorganic hybrid through sol–gel reactions. The

complete reaction (Figure 2(a) and (b)) is depicted in Figure

3(a) and (b), the consumption of oxirane group by DDE lead-

ing to the formation of the secondary –OH group was moni-

tored by the disappearance of oxirane peak at 913 cm�1 and

subsequent appearance of secondary –OH peak between

3600 and 3000 cm�1 confirmed the reaction between GPTMS

and DDE containing ether segment. The Si–O–Si group in the

hybrid films was detected at 1181 cm�1. It indicates that the

condensation of TEOS in acid medium was accomplished

with a formation of Si–OH bond, which formed a connection

between the organic and inorganic phases.

29Si NMR29Si NMR spectrum of organic–inorganic hybrid films after

condensation is illustrated in Figure 2(b). Figure 4 shows the

signals of four siloxane bonds around at �59.2, �62.5,

Figure 7. SEM images of hybrid films having (a) 1 wt%, (b) 2 wt%, and (c) 3 wt% silica content. SEM: scanning electron microscopic.

Duraibabu et al. 5

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�85.5, and�98.9 ppm that correspond to (a), (b), (c), and (d)

peaks, respectively.

Thermogravimetric analysis

The thermal stability of films having varied silica content,

namely, 1, 2, and 3 wt%, was studied under nitrogen atmo-

sphere by thermogravimetric analyzer and the results are

presented in Figure 5. Thermal stability of this films as

indicated in Figure 5 increased with increased inorganic

silica content. The thermal degradation patterns exhibited

by different films were found to be quite interesting. For

example, the degradation pattern of films having 1 2 and

3 wt% of silica content followed a double degradation pat-

tern. The initial degradation took place at around 100�C in

the case of films having 1 and 2 wt% of silica content; this

may be attributed to the water absorbing nature of such film

with the formation of hydrogen bonding with oxygen of

DDE group. The data obtained from water absorption and

SEM analyses strongly supports this behavior. The second

degradation observed around 400–500�C may be mainly

due to their degradation. However, the film having 3 wt%of silica content started the initial degradation at 300�C

followed by a further degradation around 400–500�C,

which may be mainly due to its degradation without hydro-

gen bond formation and water absorption. As the silica con-

tent increases, the tendency for hydrogen bond formation

Figure 8. (a) Fully transparent, (b) semitransparent, and (c) opaque behavior exhibited by hybrid films with 1%, 2%, and 3% silica.

1% 2% 3%

0.0

0.2

0.4

0.6

0.8

Wat

er a

bsor

ptio

n

Silica content (%)

Figure 9. Water absorption studies of hybrid films at equilibrium.

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decreases because of the presence of silica, which forms a

self-healing cover over oxygen of DDE and making it una-

vailable for hydrogen bond formation. This may be the rea-

son why the film having 3 wt% of silica content exhibited

superior water resistance (Figure 9) than the other two

films. It is observed that the degradation temperature of

hybrid films was gradually enhanced with an increase in

silica content indicating that the thermal stability of the

films can be enhanced by increasing the silica content.1

However, film having 3 wt% of silica content exhibited the

maximum thermal stability among all the other weight per-

centages of silica containing films. This could be due to the

higher concentration of low surface energetic silica that

makes it to migrate to the surface of the DDE by forming

a self-healing coating and consequently prevents further

degradation. Similar observation was made by one of the

authors Anandakumar for phosphorous-containing silico-

nized epoxy materials.27

X-Ray diffraction

Crystallinity of the films was studied using XRD and is

illustrated in Figure 6. It is well known that the intricate and

highly specific geometry of tetrahedral crystal network of

silica takes several years to form under incredible terres-

trial pressure at great depth. This is, however, unlikely in

ideal laboratory conditions. Thus, amorphous silica appears

to be the major component in laboratory synthesis. The

same thing was observed here in XRD pattern as shown

in Figure 6. As we increase the silica content, the amor-

phous nature of hybrid films was also increased. This trend

is clearly illustrated in Figure 6 where 1 and 2 wt% of silica

content exhibiting some crystallinity in the films while the

crystallinity of the same film completely vanished when

the silica content is 3 wt% leading to amorphous nature.

As we increase the TEOS content, the silica molecules

formed by in situ, due to its surface active properties, get

adsorbed on the surface of the films and the amount of

silica adsorbed kept on increasing with increased silica

content (evident from SEM images) and thus makes the

sample from transparent (1% silca) to opaque (3% silica).

This crystalline to amorphous transformation exhibited by

films was clearly indicated by the XRD patterns (Figure 6)

of the corresponding hybrid films having 1, 2, and 3 wt%of silica content.26

Scanning electron microscopy

The morphology and the distribution of silica content in the

hybrid materials of the films having 1–3 wt% of silica con-

tent was investigated using SEM. We found that the distri-

bution of silica kept on increasing with respect to increase

in silica content, which is clearly indicated in Figure 7(a) to

(c). Figure 7(a) exhibited a slight homogenous morphology

having lesser silica content of 1 wt% while Figure 7(b) and

(c) exhibited an heterogeneous morphology that increased

with increase in silica content, namely, 2 and 3 wt%,

respectively. The micrographs clearly show a fine intercon-

nected or co-continuous morphology.27 It can be seen from

the figures that the distribution of silica content was less in

the case of Figure 7(a) and more in the case of Figure 7(c)

while it fell in between these two extremes in the case of

Figure 7(b).

Transparency of hybrid films

The percentage of silica content had an influencing effect

on the transparency of the hybrid films, which can be seen

from Figure 8(a) to (c). For example, hybrid film having

low silica content (1 wt%) exhibited very good transpar-

ency (Figure 8(a)). However at higher silica contents, the

trend was reversed leading to complete opaqueness (Figure

8(b) and (c)). An increase in silica content increases the dis-

tribution of silica in the resultant films, and this phenom-

enon also reveals that the silica network have rough

surfaces and diffused boundaries that result in increased

light scattering leading to opaqueness to the hybrid films.

The photographs of hybrid films having varied silica con-

tent severely affect their transparency and their thickness

as shown in Figure 8(a) to (c).

Water absorption

The water absorption measurement of hybrid films showed

that the water absorption by the film consisting of 1 wt% of

silica content was maximum, that is, 0.83%. Figure 9 shows

the increase in the weight of the samples may be due to

water absorption that gradually decreased as the silica con-

tents in the films increased. This may be due to the abun-

dant exposure of DDE groups to the surface of films,

where water molecules develop hydrogen bonding with

oxygen groups. However, as the concentration of silica is

increased in the hybrid materials, the extent of water

absorption at saturation point is considerably reduced. It

may be due to the mutual physical interaction between the

organic and inorganic phases. This interaction results in

lesser availability of oxygen groups to interact with water.

Similar observation was reported by Zulfiqar et al.24 This

observation clearly indicates the effect of increased silica

content toward reducing the water absorption of the

organic–inorganic hybrid films.

Optimizing silica content

Chemical bonding between the organic matrix and inor-

ganic silica network seems to have provided reinforcement

to the resultant hybrid films. The results of our present

study indicate that if silica content is increased beyond 3

wt%, the excess silica particles do not seem to offer any

beneficial properties to the resultant films as they may not

Duraibabu et al. 7

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be linked with polymer chains and also increases the ten-

dency toward particle growth. The growth of the particles

led to irregular and nonhomogeneous distribution within

the film leading to agglomeration, making the samples

more porous and brittle24 thus limiting their use for specific

end use application. Owing to the above reasons, the per-

centage of silica content in the hybrid film was optimized

to a maximum of 3 wt% of silica content in order to offer

better set of film properties ideally suitable for ultrafiltra-

tion and gas separation as well.

Conclusions

Thermally stable organic–inorganic hybrid films were suc-

cessfully developed through the sol–gel process using

DDE, GPTMS, and TEOS. The chemical bonding between

the organic and inorganic phases, which seems to have pro-

vided reinforcement in the resultant films, has been ascer-

tained by spectral data and the effect of chemical bonding is

reflected in the resultant films that possess superior thermal

properties. However, only appropriate amounts of both the

phases gave better interactions and desired properties to the

resultant films. The amount of water absorption is reduced

with the addition of inorganic silica network according to

its percentage. It may be due to the mutual physical inter-

action between the organic and inorganic phases. This

interaction results in lesser availability of oxygen groups

to interact with water. The morphological investigations

indicate a narrow size distribution of silica particles in the

hybrid films. An increase in silica content increases the dis-

tribution, and these large domains of silica resulted in

enhanced light scattering leading to opaqueness to the

hybrid films. However, beyond 3 wt% of silica content,

irregular and nonhomogeneous particles distribution

occurred leading to agglomeration; making the samples

more porous and brittle thus limiting their use for specific

end-use application. Hence, the percentage of silica content

in the hybrid films was optimized to a maximum of 3 wt%in order to obtain a better film possessing properties ideally

required for ultrafiltration and gas separation processes.

Funding

Instrumentation facility provided under FIST-DST and DRS-

UGC to Department of Chemistry, Anna University, Chennai,

Tamil Nadu, India, are gratefully acknowledged.

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