Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems

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1 Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems Chemistry (C4), University of Surrey, Guildford, Surrey, UK, GU 7XH Paper presented at Addcon World 00 SUMMARY The well-established sol-gel process permits the preparation of a wide range of organic-inorganic hybrid materials directly from molecular precursors under mild conditions. Over the past few years, the sol-gel has become of tremendous interest because of the remarkable opportunities it offers to produce hybrid nanocomposite particles with a number of potential uses, such as protection and modification of plastics and glasses. In this work, two approaches are described for the preparation of hybrid particles. A modified Stöber method is used to prepare a range of organically-modified silica nanoparticles with diameters in the range of nm. The synthesis of organically-modified silica particles has also been carried out using a non-hydrolytic sol-gel, which has been overlooked for the preparation of hybrid nanoparticles until very recently. The hydrolytic and non-hydrolytic s have produced hybrids with different structures and morphologies. The organicallymodified silica particles synthesised using these sol-gel methods have potential uses as nanofillers in polymer systems. 1. INTRODUCTION Over the past few years, the synthesis of organicinorganic hybrids has become the subject of extensive investigations because of the exceptional opportunities to tailor the properties of these materials for use in a range of applications, including their use as polymer additives. The sol-gel process has been widely used to provide convenient methods for the production of organic-inorganic hybrid materials. Although a range of hybrids, such as organically-modified silicas (ormosils), has been prepared in the form of monolithic products 1-, the use of this process as a to hybrid nanocomposite particles has been overlooked until quite recently. Such particles have real potential as speciality nanofillers, where the organic group can be tailored for compatibility with the matrix. The well-established hydrolytic sol-gel approach, which enables the preparation of nanoparticles by base-catalysed hydrolysis and condensation reactions of monomeric precursors in an aqueous solvent system, has begun to receive considerable attention. In recent years, a range of organicallymodified silica particles has been synthesised successfully by a modified Stöber method 4-1. This hydrolytic sol-gel approach was developed originally for the preparation of unmodified silica nanoparticles from tetraalkoxysilane precursors, such as tetraethoxysilane (TEOS) 11, and has been extended to the formation of silsesquioxane particles (with the empirical formula [R Si O ] n ) from mixtures of TEOS and organotrialkoxysilane precursors (RSi(OR ), with R = methyl, phenyl, octyl, aminopropyl, etc., and R = methyl or ethyl) 4-6 or directly from organically-modified precursors However, this type of synthesis very rarely permits the preparation of ormosil particles with diameters less than 500 nm 1. The less well-studied non-hydrolytic sol-gel offers a number of potential advantages over the hydrolytic method, such as reduction of the number of residual silanol groups in the product, the potential to avoid the use of solvents, as well as the possibility to avoid phase separation, which occurs during the hydrolytic synthesis of hydrophobic hybrids. The most common non-hydrolytic sol-gel to organically-modified silicas involves the reaction of silicon halides with oxygen donors, such as alkoxides or dialkyl ethers, in the presence Polymers & Polymer Composites, Vol. 1, No.,

2 of a Lewis acid catalyst,1,14. This approach can be used to produce silsesquioxanes, where each silicon atom is covalently bonded to one organic group, as well as ormosil structures containing only one organic group every two silicon atoms (where the general formula is [RSi 1.75 O ] n ). However, most of the work in this area has focused on the preparation of bulk or monolithic products, and the number of reports on the synthesis of organicinorganic hybrid particles has been very limited. The only relevant study on the preparation of organicallymodified silica particles seems to be that by Masters et al. 15 who have described the non-hydrolytic synthesis of methyl-modified silica particles under pseudo-dispersion polymerisation conditions. Very large aggregated, irregular particles with diameters in the range µm were obtained by reacting methyltrichlorosilane with tetraethoxysilane in the presence of surfactants, using a high shear stirrer. In this work, two recently developed s to hybrid nanoparticles are compared and contrasted. Phenyl-, methyl- and ethyl-modified silica nanoparticles have been synthesised using the Stöber method, as well as a non-hydrolytic sol-gel approach. The former is a simple method, which involves the use of an aqueous sodium silicate solution as a seed for subsequent formation of silsesquioxane particles from organotrimethoxysilane precursors in a mixture of water, ammonia and ethanol at room temperature 16. Well-defined, nearly monodisperse nanoparticles with diameters in the range nm were obtained in this way. The non-hydrolytic method has also been used to produce silsesquioxane particles with diameters in the range nm by reacting organotrichlorosilane precursors with dimethyl sulphoxide, used as an oxygen donor and as a solvent, in the absence of a catalyst 17. Scanning electron microscopy (SEM) was used to study particle size and morphology. IR spectroscopy and solid-state 1 C and 9 Si NMR spectroscopy were used to compare the structures obtained using the hydrolytic and non-hydrolytic solgel methods. Porosity and surface area data were obtained from nitrogen adsorption measurements using the Brunauer, Emmett and Teller (BET) method and the surface elemental composition was studied using X-ray photoelectron spectroscopy (XPS). The potential commercial applications of the ormosil materials include protective coatings 18, chromatographic and catalyst supports (bridged polysilsesquioxanes) 19, optical components 0, etc. One possible application for the materials obtained in this study is the use of these organic-inorganic hybrid nanocomposite particles as nanofillers to modify the properties of selected polymer systems, such as acrylic, styrenic and polyester polymers. The surface of the particles can be modified by various chemical reactions in order to improve compatibility between the filler and the host polymer. For example, functionalisation of the particles by reacting them with coupling agents may result in the formation of polymer chains that are chemically attached to the surface of the particles.. EXPERIMENTAL.1 Materials Reagents used for the Stöber synthesis. Phenyltrimethoxysilane (PhTMS) (97+%, Aldrich), methyltrimethoxysilane (MTMS) (98%, Aldrich), ethyltrimethoxysilane (ETMS) (97+%, Aldrich) and sodium silicate solution (Na O SiO, 7 wt% SiO, Riedel de Haën) were used as received. Absolute ethanol (99.86%, Hayman) and ammonium hydroxide (5%, Fisher) were of analytical reagent quality. Reagents used for the non-hydrolytic synthesis. Phenyltrichlorosilane (PhTCS) (97%, Aldrich), methyltrichlorosilane (MTCS) (97%, Aldrich) and ethyltrichlorosilane (ETCS) (99%, Aldrich) were used without further purification. Dimethyl sulphoxide (DMSO) (99.7%, Acros Organics) was dried over calcium hydride and then distilled from calcium hydride at reduced pressure at ca. 100 C.. Silsesquioxane Synthesis. The Stöber sol-gel to organically-modified silica particles has been studied over a wide range of experimental conditions (water, ammonia, organotrimethoxysilane concentrations, reaction time and the rate of addition of organotrimethoxysilane precursors) in order to establish the optimum conditions for the production of silsesquioxane nanoparticles. The exemplified procedure for molar ratios of H O/NH / organotrimethoxysilane/c H 5 OH/seed SiO of 85 / 40 / 1 / 160 / is shown below (note that the total water content was calculated by adding up the fractional amounts introduced by each component): Sodium silicate solution (. wt% SiO ) (0.1 ml) was mixed, under vigorous stirring, with absolute ethanol (6. ml; 105 mmol). This suspension was added to a mixture of ethanol (. ml; 55 mmol), ammonia (5% NH ) (. ml; 40 mmol) and an organotrimethoxysilane precursor (1 mmol). The mixture was stirred at room 10 Polymers & Polymer Composites, Vol. 1, No., 004

3 Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems temperature using a magnetic stirrer. After 0 minutes, a large excess of distilled water ( parts of water per 1 part of the dispersion) was added to the reaction mixture and the products were dried in air for approximately days and under vacuum for 4 h to yield fine white powders. Non-hydrolytic. In this work, DMSO was used as an oxygen donor, as well as a solvent. An organotrichlorosilane precursor (0 mmol) was slowly added to DMSO (0 ml; 80 mmol) under vigorous stirring at room temperature. The reaction temperature was gradually increased to 60 C and the reaction mixture was stirred under nitrogen using an overhead stirrer over a period of hours. The precipitates were washed with ethanol and centrifuged. The collected materials were dried in air for 4 hours and under vacuum for 48 hours to yield fine white powders. repetitions). The conventional Q n notation was used to denote tetrafunctional (unmodified) silica species, whereas the conventional T n notation was used to label trifunctional (due to the presence of a Si-C bond) silica species, where n is the number of siloxane groups bonded to the silicon atom. Surface area and porosity analysis was carried out on a Coulter SA 100 instrument. The data were obtained from nitrogen adsorption measurements at -196 C. Samples were outgassed for hours at 60 C prior to the analysis. The Brunauer, Emmett and Teller (BET) method was used to calculate total surface areas (S t ). Specific surface areas (S BET ) were obtained by dividing S t by the sample weight. Total pore volumes (V p ) were measured at a relative pressure of The % porosity (ϕ) and average pore diameters (D) were calculated using the following formulae:. Characterisation Techniques SEM micrographs were obtained on a Hitachi S- 00N scanning electron microscope at a voltage of 10 kv. A layer of dry powder was applied on a stub covered with a carbon film and then coated with a fine gold film in order to avoid charging effects. Micrographs were taken at a number of random locations on the stub. Ca particles were measured for each sample. Elemental analysis was performed on a Leeman Laboratories CE 440 Elemental Analyser. Transmission infrared (IR) spectra were recorded on a Perkin-Elmer 000 FT-IR spectrophotometer. A finely ground sample was mixed with potassium bromide and pressed to form a disc, which was used to record an IR spectrum. The following abbreviations are used to describe the absorptions: vs very strong, s strong, m medium, w weak, b broad, sp sharp and sh shoulder. Solid-state 1 C and 9 Si cross polarised (CP) nuclear magnetic resonance (NMR) spectroscopy was carried out at the University of Durham, UK. The spectra were recorded at room temperature on a Varian Unity Inova spectrometer with a 7 mm magic angle spinning (MAS) probe. The 9 Si CP spectra were obtained using an acquisition time of 0.0 ms, a contact time of 5.0 ms, a pulse delay of.0 s, a spinning speed of 400 Hz and a frequency of MHz (ca repetitions). The 1 C CP spectra were measured using an acquisition time of 0.0 ms, a contact time of 1.0 ms, a pulse delay of.0 s, a spinning speed of 400 Hz and a frequency of MHz (ca Vp ϕ= Vp Vp D = S BET X-ray photoelectron spectroscopy (XPS) was performed using a Thermo VG Scientific Sigma Probe spectrometer. A microfocused monochromated Al Kα source (E = ev * ) with a spot size of 500 µm operating at 140 W was used. For each sample a survey spectrum (0-150 ev) was recorded together with the high resolution spectra of C1s, O1s and Sip. The analysis was performed at a pass energy of 0 ev for the high resolution spectra and 100 ev for the survey spectra. Quantitative surface compositions were calculated from the integrated peak areas following the appropriate background subtraction using the Advantage data system. The CC/CH component was introduced at ca ev in order to correct the sample charging effects.. RESULTS AND DISCUSSION Silsesquioxane synthesis. The hydrolytic and nonhydrolytic sol-gel methods have been used to synthesise a chemically identical range of organicallymodified silica particles. It has been shown previously that monodisperse silica nanoparticles can be * The conversion factor to the appropriate SI unit is 1eV=1.595x10-19 J. Polymers & Polymer Composites, Vol. 1, No.,

4 prepared by the hydrolytic Stöber method using sodium silicate solution as a seed for further growth of the particles 1. In this work, this procedure has been adapted to produce silsesquioxane particles from organotrimethoxysilane precursors. The chemistry of the hydrolytic sol-gel process can be represented by the following general equation: Figure 1. Scanning electron micrograph of ethylsilsesquioxane particles obtained using the hydrolytic sol-gel method RSi(OCH ) + H O [R Si O ] n + 6 CH OH (1) The effects of water, ammonia, organotrimethoxysilane concentrations, reaction time and the rate of addition of organotrimethoxysilane precursors on the size of the particles have been studied. We have demonstrated that the size of the particles can be tailored by varying these experimental conditions 16. The results obtained in this study suggest that the average particle diameters decrease as follows: with increasing ammonia concentration, with increasing water concentration, with decreasing organotrimethoxysilane concentration, Figure. Scanning electron micrograph of ethylsilsesquioxane particles obtained using the nonhydrolytic sol-gel method with decreasing reaction time, and are more or less independent of the rate of addition of organotrimethoxysilane precursors. The conditions that are best suited to the formation of well-defined silsesquioxane nanoparticles (Figure 1) have been established. Typical results are presented here for a molar ratio of H O/NH /organotrimethoxysilane/ C H 5 OH/seed SiO of 85 / 40 / 1 / 160 / and a reaction time of 0 minutes, unless specified otherwise. A new non-hydrolytic method has also been used to prepare organically-modified silica particles using DMSO as an oxygen donor, as well as a solvent 17. The products are formally silsesquioxanes, with the empirical formula [R Si O ] n, and formally chemically identical to those synthesised by the Stöber. The chemistry of this non-hydrolytic sol-gel process is represented by the simplified equation shown below [note that the precise nature of the by-product has not been established]: RSiCl + (CH ) SO [R Si O ] n + (CH ) SCl () It is anticipated that the by-product is removed in the nitrogen flow during the reaction or by washing with ethanol. The presence of alkoxy groups is possible, since the residual Si-Cl groups might be converted to ethoxysilanes with ethanol used as a washing solvent. The possibility of the presence of some silanol species produced by Si-Cl hydrolysis should not be ruled out, since no specific care has been taken to avoid the presence of moisture during the washing and drying stages of the process. The particles obtained using the hydrolytic method (Figure 1) were more regularly shaped and exhibited less aggregation than those obtained via the nonhydrolytic (Figure ). Moreover, it can be seen that some monolithic domains, which are possibly 104 Polymers & Polymer Composites, Vol. 1, No., 004

5 Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems formed by coalesced particles, are embedded within the regions consisting of individual particles prepared using the non-hydrolytic process (Figure ). It can also be seen that the average diameters of the particles produced using the hydrolytic were consistently smaller than those of the materials synthesised using the non-hydrolytic sol-gel method (Table 1). Yields were calculated based on the empirical formulae of the products (Table 1). Elemental analysis. The empirical formulae of the products were used to calculate theoretical C and H percentages. The percentages obtained from the elemental analyses agreed reasonably well with the theoretical values for all the materials synthesised via the hydrolytic and non-hydrolytic sol-gel s (Table ). It is believed that the discrepancies between the predicted and experimental values were due to the presence of silanol species, together with some ethoxysilane groups in the case of the materials obtained using the non-hydrolytic method. IR spectroscopy. The data obtained from the IR spectra confirm the formation of Si-O-Si bonds and the retention of Si-C linkages during both the hydrolytic and non-hydrolytic reactions,. The spectra recorded for the materials synthesised using the Stöber method were very similar to those recorded for the silsesquioxanes obtained via the non-hydrolytic sol-gel (Figure ). The presence of Si-O-Si linkages is supported by sharp, intense absorptions at 110 cm -1 (vs, sp) (stretching) and cm -1 (s) (bending), and distinct vibrations at ca cm -1 (s) (bending). The existence of Si-C bonds was confirmed by deformation bands at ca cm -1 (m, sp) and 1410 cm -1 (m, sp) for methyl- and ethyl-modified materials, and 140 cm -1 (m, sp) for phenyl-modified silica. C-H and Si-C stretching vibrations were seen at ca cm -1 (w, b) and cm -1 (m, sh), respectively. The IR spectra of phenyl-modified silica exhibited a well-defined benzene ring absorption at 1591 cm -1 (m, sp). Relatively weak absorptions between ca. 100 and 600 cm -1 were assigned to the presence of some hydroxy groups. For the materials produced via the non-hydrolytic, it is believed that these groups were formed by the hydrolysis reactions of the residual Si-Cl groups during the washing and drying stages of the process. NMR spectroscopy. Solid-state NMR spectra of the organically-modified silicas synthesised using the hydrolytic and non-hydrolytic methods showed the presence of Si-O-Si linkages together with Si-C bonds (Figure 4). CP 9 Si spectra confirmed the presence of T (mono-substituted) silica species. For phenylsilsesquioxane, peaks at ca. 78 ppm and 71 ppm were assigned to T (fully condensed) Table 1. Average particle diameters and yields of silsesquioxane particles obtained using the hydrolytic and non-hydrolytic methods Material Average diameter/nm Yield (%) Average diameter/nm Non-hydrolytic Yield (%) Phenyl-modified 155± ± Methyl-modified 6± ± Ethyl-modified 80± ± Table. Elemental analysis of silsesquioxanes obtained via the hydrolytic and non-hydrolytic s M aterial C content (wt. %) H content (wt. %) Theor. Experimental Experimental Theor. H ydr. N on-hydr. H ydr. Non-hydr. Phenyl-modified Methyl-modified Ethyl-modified Polymers & Polymer Composites, Vol. 1, No.,

6 Figure. IR spectra of phenyl-modified silica obtained using the hydrolytic (a) and non-hydrolytic (b) sol-gel s Figure 4. 1 C and 9 Si CP-MAS NMR spectra of methyl-modified silica obtained using the hydrolytic and nonhydrolytic sol-gel s species and T species, respectively. The 9 Si spectra obtained for methyl- and ethylsilsesquioxanes showed the presence of T and T peaks at ca. 65 ppm and 58 ppm, respectively. T species were usually abundant, whereas the peaks characteristic of T silica species were less intense. Some T 1 species were seen at ca. 68 ppm for phenyl-modified and 54 ppm for methyl- and ethyl-modified silicas obtained via the non-hydrolytic, suggesting a less complete condensation in this case. The absence of T species suggests that although condensation was not complete, no unreacted organically-modified precursor (an organotrimethoxysilane for the hydrolytic synthesis and an organotrichlorosilane for the non-hydrolytic reactions) was present. Although Q 4 species at ca. 111 ppm were observed in all the spectra recorded for the hydrolytic sol-gel method, they were attributed to the presence of seed 106 Polymers & Polymer Composites, Vol. 1, No., 004

7 Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems SiO. For the non-hydrolytic synthesis, the presence of Q species was negligible, as indicated by the absence of any significant peaks below 90 ppm. The 1 C spectra exhibited intense carbon resonances, characteristic of the corresponding organic groups. For organically-modified silicas synthesised using the Stöber method, no intense peaks characteristic of residual methoxy groups were observed, confirming that these groups were almost completely hydrolysed during the reaction, and silanol groups are the functional groups retained without condensation in T species. The spectra recorded for the silsesquioxanes obtained via the non-hydrolytic showed the presence of ethoxysilane groups formed by reaction of ethanol with residual Si-Cl groups. Resonances characteristic of CH and CH O carbon atoms were seen at ca. 18 and 58 ppm, respectively. Surface area and porosity analysis. The BET method was used to obtain surface area and porosity data for the silsesquioxanes synthesised using both sol-gel methods (Table ). It is anticipated that the extent of agglomeration or even coalescence of the particles, as in the case of the materials produced using the nonhydrolytic approach, may affect the S BET values. The silsesquioxanes obtained via the hydrolytic exhibited type IV isotherms 4,5, characteristic of mesoporous solids ( nm < D < 50 nm), whereas the shape of the isotherms displayed by the materials synthesised using the non-hydrolytic method was fairly close to that of a type II isotherm 4,5, which is typical of macroporous solids (D > 50 nm) (Figure 5). The relatively low surface area values and the absence of any micropores were in agreement with these conclusions. However, the average pore diameters and % porosity values obtained for the silsesquioxanes prepared by the non-hydrolytic method were Table. Surface area analysis of silsesquioxanes synthesised using the hydrolytic and non-hydrolytic sol-gel s Material Phenyl-modified Methyl-modified Ethyl-modified BET surface area, SBET/m g -1 Total pore volume, Vp/cm g -1 Micropore volume, Vmp/cm g -1 Monolayer ϕ, % volume, Vm/cm porosity g -1 Average pore diameter, D/nm Non-hydr Non-hydr Non-hydr Figure 5. Nitrogen adsorption-desorption isotherms for ethyl-modified silica obtained using the hydrolytic (a) and non-hydrolytic (b) sol-gel methods Polymers & Polymer Composites, Vol. 1, No.,

8 consistently smaller than the values obtained for the materials produced via the hydrolytic (Table ). In both cases, the average pore sizes were in the lower mesoporous range of -18 nm diameter. It is believed that the calculated average pore diameter values were not strictly accurate, because of the assumptions of the method used. The isotherms recorded for the materials produced via the hydrolytic displayed type H1 hysteresis 5, which is characteristic of solids made by aggregates of spherical particles, whereas the isotherms obtained for the materials synthesised using the non-hydrolytic reactions usually exhibit no hysteresis at all, which is typical of blind cylindrical, cone-shaped and wedge-shaped pores 5 (Figure 5). Near-surface elemental composition. The use of XPS to evaluate the elemental compositions on the surface of the particles is justified by the sampling depth of this analysis of ca. -10 nm 6, since the size of the particles obtained in this study was in the range of nm for the hydrolytic sol-gel (note that the experimental results discussed below are given for a molar ratio of H O/NH / organotrimethoxysilane/c H 5 OH/seed SiO of 150 / 75 / / 160 / 0.044, with a range of nm for the non-hydrolytic sol-gel. The surface compositions were calculated from the C1s, O1s and Sip spectra and were compared with the values obtained from the molecular formulae with no hydrogen taken into consideration (Table 4). The near-surface elemental composition of unmodified silica (Nyasil) with a particle size of 0 nm was also studied, in order to estimate the extent of organic contamination on the surface of the particles. It is anticipated that the particles interact differently with the different media, i.e. water and non-polar solvents for the hydrolytic and non-hydrolytic solgel s, respectively. Thus, for the materials obtained using the Stöber method, it was expected that the more hydrophilic silanol groups would be concentrated on the surface, while the more hydrophobic organic groups would be embedded in the core of the particles, whereas for the non-hydrolytic synthesis, it was expected that more of the organic groups would protrude into the solvent, with the more polar siloxane matrix forming the core of the particles. The values of the oxygen surface content for the materials synthesised using the non-hydrolytic method were of the same order as the theoretical values, whereas those obtained for the hydrolytic were consistently higher. On the other hand, the carbon content, less ca. 10% presumably introduced by organic contamination, agreed reasonably well with the theoretical values for the particles prepared by the Stöber method, whereas the near-surface carbon concentration was somewhat higher for the materials synthesised using the nonhydrolytic reactions. In both cases, the experimental silicon content values were significantly smaller than those calculated from the molecular formulae. It is believed that, on the surface of the particles, silicon is partly shielded by hydroxy groups in the case of the Table 4. XPS analysis Material C content (%) O content (%) Si content (%) Calculated SiO Nyasil Methyl-modified Phenyl-modified Ethyl-modified Calculated Non-hydrolytic Calculated Non-hydrolytic Calculated Non-hydrolytic Polymers & Polymer Composites, Vol. 1, No., 004

9 Synthesis of Organically-Modified Silica Particles for Use as Nanofillers in Polymer Systems particles prepared by the hydrolytic method, and by organic groups in the case of the materials synthesised using the non-hydrolytic sol-gel. These results are consistent with the hypothesis about the effect of solvent polarity. 4. CONCLUSIONS The hydrolytic and non-hydrolytic sol-gel methods can both be used to produce phenyl-, methyl- and ethylsilsesquioxane particles, with diameters in the range nm and nm, respectively. The results obtained in this study suggest that the hydrolytic and non-hydrolytic methods produce particles with slightly different structures and morphologies. The particles obtained using the hydrolytic method are consistently smaller, more regularly shaped and exhibit a lesser degree of aggregation than those obtained via the non-hydrolytic. The results of IR analysis and NMR spectroscopy suggest that although no unreacted organicallymodified precursors are present, the condensations are not complete, especially in the case of the nonhydrolytic method. The presence of silanol species has been detected together with some ethoxysilane groups in the case of the materials obtained using the non-hydrolytic method. The adsorption-desorption isotherms exhibited by the silsesquioxanes obtained in this study using the hydrolytic and non-hydrolytic sol-gel methods were fairly typical of mesoporous (type IV isotherm) and macroporous (type II isotherm) solids, respectively. The relatively low surface area and average pore diameter values were typical of mesoporous materials. The calculated average pore diameters and % porosity values of the silsesquioxanes prepared by the nonhydrolytic method were consistently smaller than those obtained for the materials synthesised using the hydrolytic approach. The isotherms recorded for the materials prepared using the Stöber method displayed type H1 hysteresis (aggregates of spherical particles), whereas no hysteresis was observed for the materials synthesised via the non-hydrolytic. On the surface of the particles, Si was partly shielded by hydroxy groups in the case of the particles prepared by the hydrolytic method, and by organic groups in the case of the silsesquioxanes synthesised using the non-hydrolytic sol-gel. One possible application of these materials is as nanofillers for modification of the properties of polymeric systems. The different surface and morphological characteristics of the particles will influence the interaction and compatibility of the nanofillers with host polymer systems. ACKNOWLEDGEMENTS The authors would like to thank the UK Engineering & Physical Sciences Research Council and the MoD for supporting this work under the Joint Grants Scheme. We also thank Dr David Apperly (University of Durham) for provision of solid-state NMR services, Prof. John Watts and Mr Steve Greaves for their help in undertaking XPS analysis and Ms Nicola Walker for elemental analysis. The technical assistance in acquiring SEM data provided by the Microstructural Studies Unit (University of Surrey) is also greatly appreciated. REFERENCES 1. T. Iwamoto and J.D. Mackenzie, J. Sol-Gel Sci. Tech., 4 (1995), D.L. Ou and A.B. Seddon, J. Non-Cryst. Solids, 10 (1997), J.N. Hay, D. Porter and H.M. Raval, J. Mater. Chem., 10 (000), F. Hatakeyama and S. Kanzaki, J. Am. Ceram. Soc., 7 (1990), A. van Blaaderen and A. Vrij, J. Colloid. Interface Sci., 156 (199), C.R. Silva and C. Airoldi, J. Colloid. Interface Sci., 195 (1997) R. Yin and R.M. Ottenbrite, Polym. Prepr., 6 (1996) J.Y. Choi, C.H. Kim and D.K. Kim, J. Am. Ceram. Soc., 81 (1998) K. Katagiri, K. Hasegawa, A. Matsuda, M. Tatsumisago and T. Minami, J. Am. Ceram. Soc., 81 (1998), A. Matsuda, T. Sasaki, T. Tanaka, M. Tatsumisago and T. Minami, J. Sol-Gel Sci. Tech., (00), W. Stöber, A. Fink and E. Bohn, J. Colloid. Interface Sci., 6 (1968), R.M. Ottenbrite, J.S. Wall and J.A. Siddiqui, J. Am. Ceram. Soc., 8 (000), J.N. Hay and H.M. Raval, Chem. Mater., 1 (001), 96. Polymers & Polymer Composites, Vol. 1, No.,

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