sol–gel-derived organic–inorganic hybrid materials

www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 353 (2007) 313–320

Sol–gel-derived organic–inorganic hybrid materials

Yung-Hoe Han a,*, Alan Taylor b, Mick D. Mantle c, Kevin M. Knowles a

a Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UKb TWI, Granta Park, Great Abington, CB1 6AL, UK

c Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK

Received 5 September 2005; received in revised form 26 May 2006Available online 29 December 2006

Abstract

Optically transparent organic–inorganic hybrid coating materials have been prepared by a sol–gel process. Four different types of thecoating material produced by TWI in Cambridge, UK using the patented Vitresyn� method, all identical in terms of the starting mate-rials, but differing in terms of their relative proportions, have been examined. Tetraethoxysilane was used as the primary inorganic pre-cursor and urethane acrylate was used as the source of the organic component. 3-(Trimethoxysilyl)propyl methacrylate was used as botha secondary inorganic source and a silane coupling agent to improve the compatibility of the organic and inorganic phases. The degree ofchemical interaction of the organic and inorganic phases after processing was determined by 29Si and 13C nuclear magnetic resonance andFourier transform infrared spectroscopy. The effect of the relative amount of inorganic starting component in these hybrid materials ontheir thermal properties was investigated through differential scanning calorimetry and thermogravimetric analysis. Similar degrees ofchemical interaction between the organic and inorganic phases were found in all four samples. T3, Q3 and Q4 are the main cross-linkingnetwork structures in these hybrid systems, the relative proportions of which are determined by the relative proportions of the startingmaterials.� 2006 Elsevier B.V. All rights reserved.

PACS: 81.20.Fw; 76.60.�k; 81.70.Pg; 78.40.Ha

Keywords: Nuclear magnetic (and quadrupole) resonance; Organic–inorganic hybrids

1. Introduction

Organic–inorganic hybrid materials, usually known asceramers [1] or ormocers [2], have received considerableattention as a new class of composite materials throughthe novel properties that can arise from the combinationof organic polymer and inorganic material. The purposeof such hybrid materials is to achieve properties that a sin-gle phase material cannot provide. In general, the organicpolymer components of such materials have good elastic-ity, toughness, formability and low density, while the inor-ganic ceramic components are hard, stiff and thermallystable. Together, these components can produce hybrid

0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2006.05.042

* Corresponding author. Tel.: +44 1223 334342; fax: +44 1223 334567.E-mail address: [email protected] (Y.-H. Han).

materials which adhere well to both metallic and polymericsubstrates, are chemically stable, and have good abrasionresistance. These properties make these materials veryattractive as coatings for items such as automotive compo-nents, face shields and aircraft canopies. To achieve theoptimum balance of properties, phase separation betweenorganic and inorganic components in the hybrid must notoccur. Therefore, the nature of the chemical interactionbetween the organic polymer and inorganic ceramic duringthe processing of these materials plays an important role inavoiding such phase separation.

Sol–gel techniques have been widely used for the prepa-ration of organic–inorganic hybrid materials. The inor-ganic phase is mostly obtained from metal alkoxides viahydrolysis and condensation reactions in the sol–gel pro-cess. Tetraethoxysilane (TEOS) is commonly used as a

mailto:[email protected]

Table 1Designation of the various structural units of silicon atoms in TEOS and MPTMA

T species Q species

T1 T2 T3 Q1 Q2 Q3 Q4

Structural units of silicon atoms

R is an H or an alkyl group and R 0 is an alkyl or a (CH2)X group [7,14,19–21].

+

TEOS(during hydrolysis and condensation reactions) (during hydrolysis and condensation reactions)

OH

RO-Si-OR

O

RO-Si-OR

OX

OH

R'-Si-OR

O

R'-Si-OR

OX

OR OR R' OR

HO-Si-O-Si-O-Si-O-Si-OH

R' OR OR OR

-Si-

OR O R' OR


O O O OR

-Si-O-Si-O-Si-O-Si-OR

OR O R'

-Si-

polymerresin

-Si-

OR O R' OR


O O O OR

-Si-O-Si-O-Si-O-Si-OR

OR O R'-

-Si-

polymerresin

Q1

T1

Q2T2

Q4

T3

Q3

gelation

add organic resin and UV curing

MPTMA

Fig. 1. Schematic reaction scheme for the production of the organic–inorganic hybrid materials using TEOS, MPTMA and the urethane acrylate organicresin. In this scheme, R = alkyl groups, R 0 = methacryloxy group and X = R or H. The Ti (i = 1,3) and Qj (j = 1,4) species in this reaction scheme areclassified in Table 1.

314 Y.-H. Han et al. / Journal of Non-Crystalline Solids 353 (2007) 313–320

precursor of silica in this process [3–12]. In the study wereport here, four different hybrid coating materials of silicaand urethane acrylate with various organic–inorganicratios were prepared using sol–gel methods following theVitresyn� process [13] in order to determine how alteringthe proportions of the organic and inorganic materials in

the starting composition affects the thermal propertiesand the chemical interaction mechanisms of the resultanthybrid systems. TEOS and an aliphatic urethane acrylatewere used as the inorganic precursor and the source ofthe organic resin respectively to make these hybridmaterials. Urethane acrylate is UV-curable and has good

Y.-H. Han et al. / Journal of Non-Crystalline Solids 353 (2007) 313–320 315

flexibility and abrasion resistance. 3-(Trimethoxysilyl)pro-pyl methacrylate (MPTMA), also referred to in abbrevi-ated form in the literature as c-MPS [5], TPM [14],MSMA [10,15,16] and MEMO [17,18], was used both asa secondary inorganic source and a silane coupling agentto act as a compatibilizer to improve adhesion betweenthe inorganic phase and organic resin [5,18].

A schematic reaction sequence for these organic–inor-ganic hybrid materials using TEOS and MPTMA as inor-ganic precursors is shown in Fig. 1. The Ti (i = 1,3) and Qj

(j = 1,4) species in this reaction scheme are classified inTable 1. During hydrolysis and condensation reactions,siloxane networks (Si–O–Si) are formed by TEOS andMPTMA and provide the structural backbone of thesehybrid systems.

2. Experimental

All raw materials were used as received. The supplier forthe TEOS and MPTMA was Silanes and Silicones Ltd. Theurethane acrylate used was grade 260GP25, supplied byAkcros Chemicals. The hydrochloric acid (HCl) used as acatalyst to initiate hydrolysis of the inorganic monomerprecursors in the Vitresyn� process was 37% AR gradesupplied by Aldrich.

The procedure for synthesizing the organic–inorganichybrid materials is shown in Fig. 2. The three primary com-ponents used were TEOS, MPTMA and an aliphatic ure-thane acrylate. In addition, deionized water, hydrochloricacid and industrial methylated spirit (alcohol) were usedin the process. The purpose of the water was to hydrolysethe inorganic monomer precursors, while the alcohol wasused as an organic solvent.

The primary inorganic source (TEOS) and secondaryinorganic source and coupling agent (MPTMA) were madeseparately, then combined, mixed and aged. They weremixed together for 1 h at room temperature, and thensealed and aged at 50 �C for 5 h. After ageing, urethane

Alcohol + waterSolvent

preparati

Add TEOS

Combine

Add polymer resin: ureth

Organic-inorganic hybr

Primary inorganic source

Fig. 2. Flow chart of the various stages in the produ

acrylate was added with extra water and photoinitiatorinto the combined material. The addition of extra wateris required to allow TEOS to hydrolyse as completely aspossible. Finally, the prepared hybrid materials were eachcured for 10 min under a UV lamp. The intensity of theUV lamp was 46.3 mW cm�2.

Four different types of these organic–inorganic hybridsystems were made, as detailed in Table 2. Sample 1 hadthe largest component of inorganic source materialand the least amount of polymer resin, while sample 4had the smallest amount of inorganic source material andthe largest amount of polymer resin.

Infrared absorption spectra of the four samples in therange 4000–600 cm�1 were analysed by Fourier transforminfrared spectroscopy (FTIR) (Bruker Tensor 27) usingthe attenuated total reflectance method with a resolutionof 4 cm�1. 29Si and 13C nuclear magnetic resonance(NMR) experiments were run on a Bruker AV 400 spec-trometer operating at 29Si and 13C frequencies of 79.51and 100.64 MHz, respectively. Magic angle spinning(MAS) and radiofrequency excitation/detection wasachieved using a Bruker double-bearing 4.0 mm dual chan-nel MAS probe capable of producing a maximum spinningfrequency of 18.0 kHz. All samples were packed into4.0 mm diameter zirconia rotors and sealed using Kel-Fturbine rotor caps before placement into the MAS probe.The aim of the 29Si and 13C NMR experiments was to iden-tify the local environments of the silicon atoms and the car-bon bridging structures respectively in the hybrid materials.

Differential scanning calorimetry (DSC) scans and ther-mogravimetric analysis (TGA) curves were obtained on aTA Q1000 differential scanning calorimeter (TA Instru-ments) and a TA Q500 thermogravimetric analyser (TAInstruments), respectively. The DSC measurements wererun in a temperature range of 30–600 �C under nitrogengas at a heating rate of 10 �C/min. The TGA measurementswere run in a temperature range of room temperature to1000 �C under either nitrogen gas flow or air flow at a heat-ing rate of 10 �C/min.

Alcohol + wateron

Add MPTMA

ane acrylate

id material

Secondary inorganic source andcompatibilising agent

ction of the organic–inorganic hybrid material.

Table 2Precursors used and sample compositions of the organic–inorganic hybrid materials

Sample TEOS (mol) MPTMA (mol) R(A)a Relative amountb (wt%) Ceramicc loading (wt%) Product appearance

TEOS MPTMA Resin

1 0.96 0.24 0.8 75.3 22.6 2.1 95 Transparent2 0.40 0.40 0.5 44.6 52.4 3.0 95 Transparent3 0.72 0.18 0.8 54.5 16.3 29.2 50 Transparent4 0.40 0.40 0.5 29.1 34.2 36.7 50 Transparent

a R(A) is the relative measure of the amount of TEOS in molar terms: {TEOS/(TEOS + MPTMA)}.b Relative amount (wt%) is the amount of each source in the hybrid composites: {each source/(TEOS + MPTMA + Resin)}.c Ceramic loading (wt%) is the weight of TEOS and MPTMA (ceramic hybrid components) when fully cross-linked.


3. Results

The FTIR spectra of the hybrid coating materials shownin Fig. 3 are broadly similar. The very strong absorptionpeak between 1100 and 1000 cm�1 can be assigned to thesiloxane bond (Si–O–Si) which is the structural backboneof each hybrid material. The Si–OH peak at 940 cm�1

appears in all the hybrid materials, decreasing in intensityfrom samples 1 to 4. A peak observed at 780 cm�1 is likelyto be from the SiOCH2CH3 group due to the incompletehydrolysis of TEOS [22]. The weak broad Si–OH bandbetween 3600 and 3100 cm�1 is formed in the hydrolysisreaction of the alkoxy groups of either TEOS or MPTMA.The N–H peak from urethane acrylate overlaps with thisSi–OH peak. The C@C peaks of the methacrylate groupsand acrylate groups from the MPTMA precursor and theorganic resin respectively are also detectable at1624 cm�1. It is noticeable that the 1624 cm�1 peak is notpresent in the FTIR spectra from samples 3 and 4, both

2500300035004000

Wavenumb

Tra

nsm

issi

on

(%

)

1

2

3

4

3600-3100

Si-OH & N-H

Fig. 3. FTIR spectra of the four organic–inorganic hybrid materials specifiednumbers are shown at the left-hand side of each spectrum.

of which contain relatively large amounts of organic resinin the hybrid systems. The C@O absorption from boththe MPTMA precursor and urethane acrylate appearsstrongly at 1700 cm�1. A summary of the peak assignmentsof the FTIR peaks in Fig. 3 is given in Table 3.

The 29Si high power decoupling (HP/DEC) NMR spec-tra of organic–inorganic hybrid systems UV-cured for10 min are shown in Fig. 4. In general, these each have fivesignals at approximately �59, �65, �92, �102, and�109 ppm, corresponding to T2, T3, Q2, Q3 and Q4 species,respectively. The positions of these peaks are consistentwith those described elsewhere [7,14,16,28,29]. A Q speciesis one in which the Si atom is capable of producing foursiloxane bonds, whereas a T species is one which can onlyachieve three siloxane bonds, such as a silane which has asingle R 0 group directly bonded to a Si atom. Therefore,if there were complete cross-linking in the hybrid materialsas a result of the Vitresyn� process, only T3 and Q4 specieswould be detected in the 29Si HP/DEC NMR spectra.

500100015002000

er (cm-1)

1700 1440 1295 1000780 1624 1394 1100

1700 1440 1295 1000

C=O

C=CC-H

Si-O-Si

Si-OH

Si-OCH2CH3

940

in Table 2 measured in the wavelength range 4000–600 cm�1. The sample

-90-70-50-30

1234

ppm

Q

T3

T2

Fig. 4. 29Si high power decoupling NMR spectra of the four organic–inorganicinset.

Table 3Assignment of the FTIR peaks in Fig. 3

Peak position(cm�1)

Peakintensitya

Peakassignments

References

3600–3100 w Si–OH [5,19,23,24]2920 m C–H [16,17,19]2350 m CO2

(atmosphere)1700 s C@O [17–19,25–27]1624 m C@C [16,18,20,27]1440 m C–H [17]1394 m C–H [17,19]1295 m C–H [28]1100–1000 vs Si–O–Si [13,16–19,24–

26,29,30]940 s Si–OH [16,18,28,30]780 s Si–OCH2CH3 [19]

a Peak intensity: ‘vs’ is very strong, ‘s’ is strong, ‘m’ is medium and ‘w’ isweak.

Table 4Relative proportions of T and Q species in the organic–inorganic hybrid mate

Sample Proportionsa (%) Relativ

T2 T3 Q2 Q3 Q4 T2

1 5 15 4 43 33 252 18 37 – 23 22 273 3 20 – 45 32 134 20 33 – 28 19 38

a Proportions (%): these were calculated by the deconvolution technique. Erb Relative proportions (%): (each T species/total T species) · 100%.c Relative proportions (%): (each Q species/total Q species) · 100%.d Ratio (%): Ti = {total T species/(T species + Q species)} · 100%, Qj = {tot


The proportions of the T species and Q species in eachhybrid system quoted in Table 4 were obtained fromdeconvolution of the 29Si HP/DEC NMR spectra. Thespectra were deconvolved into individual Gaussian lineshapes, thus allowing a quantitative analysis of the spectrabased on the peak areas of each species [16].

13C cross-polarization magic angle spinning (CP/MAS)NMR spectra from �20 ppm to 200 ppm are shown inFig. 5. In comparison with the 29Si NMR spectra, the pat-terns of the 13C NMR peak positions are significantly morecomplex. This suggests that there are several chemical com-pounds within the hybrid systems containing carbon-bond-ing structures. Peak assignments for carbon-bonding typeswhich can be attributed to the TEOS and MPTMA precur-sors are given in Fig. 5.

The peaks from TEOS and MPTMA in the 13C NMRspectra are in good agreement with earlier reports [7].The peaks at 18 and 60 ppm (peaks 6 and 8 respectively

-150-130-110

Q3

Q4

2

hybrid materials specified in Table 2 and indicated by the numbers in the

rials from the 29Si HP/DEC NMR spectra in Fig. 4

eb proportions (%) Relativec proportions (%) Ratiod (%)

T3 Q2 Q3 Q4 Ti Qj

75 5 54 41 20 8073 – 51 49 55 4587 – 58 42 23 7762 – 60 40 53 47

ror value assumed is ±1%.

al Q species/(T species + Q species)} · 100%.

-20020406080100120140160180200

1

2

3

4

ppm

16

2

8

3

754

H2C = C − C − O − CH2 − CH2 − CH2 − Si − O − CH3

CH3

− Si − O − CH2 − CH3

O

8 6

7 5 4 3 2 1

6

Fig. 5. 13C CP/MAS NMR spectra of the four organic–inorganic hybrid materials. The sample numbers are shown at the left-hand side of each spectrum.

0

3

0 100 200 300 400 500 600Temperature (˚C)

Exo

ther

mic

Hea

t Flo

w (

W/g

)

1

2

4

urethaneacrylate

3

Fig. 6. DSC curves of the four organic–inorganic hybrid materials heatedat 10 �C/min in nitrogen. The sample numbers are shown at the left-handside of each curve.


in Fig. 5) are from ethoxy groups (CH3CH2O) in the pri-mary inorganic source, TEOS. As the relative amount ofTEOS in the hybrid systems decreases, these two peaksbecome less intense. In particular, peak 8 at 60 ppm disap-pears in sample 4, which has the least amount of TEOS inthe starting composition. The peaks at 127 and 136 ppm,peaks 7 and 5, respectively, attributable to CH2 andC@C bonds, are both weaker in samples 3 and 4 whichhave relatively large amounts of organic resin. The peakat 168 ppm and the peak at 176 ppm are from an unreactedcarbonyl group (C@O) and the result of polymerization ofthe C@C bonds in MPTMA and urethane acrylate, respec-tively. The peak at 47 ppm can be interpreted in terms of a–CH2– group in close proximity to the C@O group in thepolymerized material. Other unlabelled peaks can be attrib-uted to urethane acrylate. The peaks from urethane acry-late appear mainly on the spectra from samples 3 and 4,which, as we have already noted, have relatively largeamounts of the organic resin.

DSC curves of the four hybrid materials in a nitrogenflow are shown in Fig. 6. The curves from samples 1 and2 are distinctly different from those of samples 3 and 4.This difference correlates with the silica content of the var-ious hybrid systems. Within the temperature range of 30–600 �C used for the DSC measurements none of the hybridsystems showed evidence for a glass transition temperature.Endothermic peaks between 50 and 120 �C in each of theDSC curves arose from evaporation of volatile species.

TGA curves obtained under air flow are shown in Fig. 7.Small weight losses occur between room temperature and100 �C, while significant weight losses occur between270 �C and 500 �C. The final weights at 1000 �C of the

TGA samples heated in nitrogen were 67%, 54%, 33%,27% and 0% of their initial weights corresponding to sam-ples 1–4 and pure urethane acrylate, respectively. A com-parison of TGA curves under an air flow and under anitrogen flow for sample 1 is shown in Fig. 8. If all theweight loss were due to volatile species in both cases, suchas water, alcohols and polymer resin, the final weights afterthe two TGA runs would be same. However, the sampleheated in the air flow had a lower final weight. This sug-gests that oxidation reactions can occur during heating inair, so that carbon-based species in the sample are removedin the form of carbon dioxide or carbon monoxide.

Wei

ght (

%)

Temperature (˚C)

N2

air

0 200 400 600 800 100060

70

80

90

100

Fig. 8. TGA graphs from sample 1 comparing the weight loss as afunction of temperature under air flow and under nitrogen flow. Bothexperiments were undertaken at a heating rate of 10 �C/min.

Wei

ght (

%)

Temperature (˚C)

4

3

2

1

urethaneacrylate

0 200 400 600 800 10000

20

40

60

80

100

Fig. 7. TGA graphs of the four organic–inorganic hybrid materials heatedat 10 �C/min under air flow. The sample numbers are shown at the right-hand side of each graph.


4. Discussion

In agreement with previous reports [16,29,30], the FTIRspectra in Fig. 3 show that the intensity of the absorptionpeak attributable to Si–O–Si bonds increases with anincrease in the silica content of the starting materials.The presence of the Si–OH peak at 940 cm�1 is due toincomplete condensation of the Si–OH bonds. The inten-sity of this peak also increases with an increase in the silicacontent of the starting materials, being most strong in sam-ple 1 and of negligible intensity in sample 4. This is consis-tent with the trends seen at lower temperatures in the TGAruns (Fig. 7): sample 1 lost the most weight between roomtemperature and 100 �C. The reduced intensity of the C@Cpeaks in samples 3 and 4 in comparison with samples 1 and2 evident in the 13C NMR spectra in Fig. 5 suggests thatalmost all the C@C bonds in these two samples were fullypolymerized during the UV curing process for 10 min, butthat both sample 1 and sample 2 may need more than

10 min UV curing time for complete polymerization ofthe C@C bonds.

As the 29Si NMR spectra in Fig. 4 show, the relativeintensity of the T and Q species both increase with anincrease in the silica content. As might also be expectedfrom the schematics of TEOS and MPTMA in Fig. 1, therelative intensity of the Q species increases as the amountof TEOS in the starting compositions increases (samples1 and 3), while the relative intensity of the T speciesincreases as the amount of MPTMA in the starting compo-sitions increases (samples 2 and 4). These trends confirmthat the Q and T species are from the TEOS and MPTMAprecursors, respectively.

According to the 29Si HP/DEC NMR spectra, the finalcured hybrid materials are mainly cross-linked by T3, Q3

and Q4 species. No T1 peaks were found in the NMR spec-tra, indicating that all the T species in the starting compo-sitions from MPTMA have participated in thecondensation reactions. The lack of completeness of thecondensation reactions can be gauged from the significantamounts of T2 and Q3 species present in the samples. Sam-ples 1 and 2, both of which have relatively small amountsof organic resin in the starting compositions, also have ahigher degree of cross-linkage in the final hybrid materials,judging from the total area under the T3, Q3 and Q4 peaksin Fig. 4.

In terms of wt% of inorganic precursor, the relativeamounts of TEOS in samples 1–4 material calculated fromthe data in Table 2 are 76.9 wt%, 46.0 wt%, 77.0 wt% and46.0 wt%, respectively. These figures are very similar tothe percentages of (Q2 + Q3 + Q4) species quoted in Table 4as a total of 100% (Q + T) species calculated throughdeconvolution of the spectra in Fig. 4. The calculationusing the deconvolution procedure is clearly quite reliablebecause the ratio values of the T and Q species shown inthe two far right-hand columns in Table 4 agree well withthe relative amounts of inorganic precursors (TEOS andMPTMA) shown in Table 2. The differences between theratios of the precursors used and those calculated throughthe deconvolution technique are only ±1%.

The two significant features of the 13C CP/MAS NMRspectra in Fig. 5 are the ethoxy group peaks at 18 and60 ppm, showing that the hydrolysis and condensationreactions for TEOS do not go to completion in the Vitre-syn� in sol–gel process, and the disappearance in the spec-tra from samples 3 and 4 of the C@C bond peaks present inthe spectra from samples 1 and 2. This latter observation isalso consistent with the disappearance of the C@C FTIRpeak at 1624 cm�1 in samples 3 and 4 (Fig. 3) and the open-ing of the carbon double bonds present in both theMPTMA precursor and the organic resin during the10 min UV curing process. The C@O peak shift from 168to 176 ppm is likely to be due to the loss in conjugationof the ‘C@C–C@O’ group as a result of polymerization.

The purpose of the DSC and TGA analyses was to ascer-tain the thermal stability of the polymeric chains in the sil-ica network. There were no glass transition temperatures


detected in the 30–600 �C temperature range used for theDSC measurements. As others have noted, this is consis-tent with one or more of a number of possibilities: the silicacontent in the hybrid materials can enhance thermal stabil-ity [16], the polymer chains are uniformly distributed in theinorganic glass network [31], and/or there is strong interac-tion between the organic and inorganic phases [26]. Thebroad endothermic peaks arising in Fig. 6 in the tempera-ture range 50–120 �C are from evaporation of water andthe decomposition of the remnant organic solvents. Thesecond endothermic peaks around 410 �C from samples 1and 2 and the two endothermic peaks in the temperaturerange of 330–410 �C from samples 3 and 4 can be attrib-uted to the decomposition of urethane acrylate in thehybrid materials. This temperature range agrees well withthe endothermic peak of pure urethane acrylate. Further-more, agreement can be seen between the DSC results inFig. 6 and the results in Fig. 7 from TGA which showsminor initial weight loss of the samples below 100 �C andsignificant weight loss in the range of 270–500 �C.

The minor initial weight losses of the samples in the TGAruns below 100 �C is likely to be due either to the removal ofthe residual water and/or removal of water from remnantsilanol groups, the presence of which are detected in theFTIR spectra of the different samples. Significantly, sam-ple 1, which has the most intense Si–OH peak at 940 cm�1

in its FTIR spectrum (Fig. 3) loses the most weight of thefour samples below 100 �C. The significant weight lossbetween 270 �C and 500 �C from each of the four samplescan be attributed to the thermal degradation of the organicspecies in the four hybrid systems. In each case, the percent-age weight loss between 270 �C and 500 �C agrees well withthe relative amount of organic resin in the starting compo-sitions. The difference in Fig. 8 between the TGA curves ofsample 1 heated in air and in nitrogen and the interpretationof this difference in terms of the removal of carbon-basedspecies through oxidation when heated in air is consistentwith the appearance of the color of the residual materialfrom the samples after the TGA runs – the residual materialfrom the samples heated in air were white in color, indicat-ing removal of the carbon content of the hybrid materials,whereas the samples heated in nitrogen were black in color,indicating trapping of the organic content in the inorganicnetworks [16,32].

5. Conclusions

Hybrid materials containing TEOS as the primary inor-ganic source, MPTMA as a secondary inorganic sourceand coupling agent and urethane acrylate as an organicresin have been prepared by a sol–gel process and studiedby a variety of chemical and physical characterization tech-niques. The hybrid composites all have a network structurewith a siloxane backbone and are mainly cross-linked byT3, Q3 and Q4 species. Even though all the Q and T speciesfrom TEOS and MPTMA respectively participate in thecondensation reactions, the resultant hybrid systems are

not fully hydrolysed and condensed. When TEOS andMPTMA are mixed together, inorganic cross-links formthrough the reaction of the Si–OH bonds which producewater as a by-product. The C@C double bonds in boththe MPTMA compatibilizer agent and urethane acrylateopen to enable the inorganic network and organic resinto be connected in the hybrid systems. The thermal stabilityand the amount of the siloxane bond network both increasewith increasing silica content in the starting precursors ofthe hybrid systems.

Acknowledgement

We thank Professor L.F. Gladden, Department ofChemical Engineering, Nuclear Magnetic Resonance Cen-tre, University of Cambridge, for the use of the NMRequipment.

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