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  • Chapter 18

    SolGel-Derived SilicaSiloxane Composite Materials

    Effect of Reaction Conditions in Polymer-Rich Systems

    E. P. Black1, T. A. Ulibarri1,3, G. Beaucage1, D. W. Schaefer1, Roger A. Assink1, D. F. Bergstrom2, P. A. Giwa-Agbomeirele2,

    and G. T. Burns2

    1Sandia National Laboratories, Albuquerque, NM 87185 2Dow Corning Corporation, Midland, MI 48686

    In situ sol-gel techniques were used to prepare silica/siloxane composite materials. Tetraethylorthosilicate (TEOS) was reacted to simultaneously produce the silica-filler phase and crosslink the silanol end-capped polydimethylsiloxane (PDMS). By systematically changing the reaction conditions, the phase separation within the materials was varied. The resulting materials were characterized by scanning electron microscopy (SEM), Small-Angle Neutron Scattering (SANS), solid state 29Si NMR and mechanical testing. A correlation between the molecular weight of the matrix, M, and the size of the resulting silica domains (varies with 0.4) was found. It was also shown that the catalyst amount and activity affects the phase separation characteristics, with evidence for chain extension at low catalyst amounts. Increasing the catalyst activity increased the number of micron-size particles within the system. 29Si NMR indicates that the observed changes in phase separation were not due to changes in the molecular environments present.

    Although silicones are inherently weak at room temperature due to their low glass transition temperature and lack of stress crystallization, methods have been developed to reinforce these materials and they now represent a multibillion dollar industry. Current reinforcement techniques center around blending pyrogenic silicas with polydimethylsiloxane (PDMS) polymers (1-4). While these conventional filling/blending techniques have proven successful, further advances in silicone-based materials production require the development of new methods of reinforcement.

    It has been determined that silica particles with high surface areas and ramified structures are the most effective at providing reinforcement. While silica particles of this type are not necessarily the only means of reinforcement, a reasonable approach to obtaining new high performance silicone-based materials lies in developing new methods for producing inorganic particles of this nature. One possible method of producing highly dispersed inorganic fillers which interact strongly with the PDMS matrix is the use of in situ sol-gel techniques.

    3Corresponding author

    0097-6156/95/0585-0237$12.00/0 1995 American Chemical Society

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    In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

  • 238 HYBRID ORGANIC-INORGANIC COMPOSITES

    Currently, the production of in situ reinforcement in polymeric systems by sol-gel methods is undergoing rapid development (5-20). However, despite this activity, a detailed understanding of synthesis/structure/property relationships is still lacking. In order to produce sol-gel derived composite materials with sufficient mechanical properties for commercial applications, this deficit of information must be addressed. Utilizing solid state 2 9 S i NMR, scanning electron microscopy (SEM), mechanical testing and Small-Angle Neutron Scattering (SANS), we have completed a detailed investigation on the effect of a number of reaction conditions on in situ silica growth in polydimethylsiloxane (PDMS)Aetraethylorthosilicate (TEOS) systems. In this chapter, we summarize our findings on some of the factors which affect silica domain formation, such as polymer molecular weight, catalyst activity and silica loading.

    Experimental Details.

    Materials and Instrumentation. Hydroxyl terminated polydimethylsiloxane was obtained from Dow Corning Corporation and Petrarch/Huls America. The number average molecular weight of the PDMS for the molecular weight study were 2,000; 4,200; 18,000; 36,000; 77,000; 150,000; 310,000 g mol 1 . TEOS (Reagent Grade) was obtained from Fischer Scientific and Aldrich Chemical Company and was used without further purification. Tin octoate (TO), [ C H a i C ^ C H t Q i y C O i L S n , was obtained from Huis as a 50% solution in PDMS or in neat form from Pfaltz & Bauer. Dibutyltin dilaurate (DBTDL), [CHsiCH^oCOz^SnKCH^CHaL, was obtained from Hiils as a 25% solution in PDMS.

    The 2 9 S i N M R spectra were recorded at 39.6 MHz on a Chemagnetics console interfaced to a General Electric 1280 data station and pulse programmer. The samples were spun about the magic angle at 2.5 kHz. A direct polarization sequence was used with pulse delay times of 120s (a factor of 4 times the longest observed Ti) so the resonance areas are expected to be quantitative. Neutron scattering was performed at the High Flux Isotope Reactor at Oak Ridge National Laboratory, the Manuel Lujan Neutron Scattering Center at Los Alamos National Laboratory and at the Cold Neutron Research Facility at the National Institute of Standards and Technology. Data from different q ranges were matched by an arbitrary vertical shift factor. Scanning Electron Micrographs (SEM) were obtained on Au/Pd coated samples using a JEOL 6400 scanning microscope.

    Ultimate tensile strength, 50% and 100% stress values, and elongations were obtained using an Instron Model 1122 Tester. Dogbone samples of dimensions 1 3/4 in (4.5 cm) long by 3/8 in (1 cm) wide were pulled to rupture at a speed of 500 mm/min using the procedure defined by A S T M Standard D412-87. Values reported are the average of five tensile samples. Hardness values were obtained on a Type A Durometer according to A S T M Standard D2290. Sample pieces were stacked to a thickness of about 1 cm and hardness measurements were done at three different positions on each side of the sample. The reported values are the average of the six numbers.

    General Procedure. TEOS and a tin catalyst (TO or DBTDL) were added to the hydroxyl terminated polydimethylsiloxane in amounts sufficient to both cross-link and fill the matrix with silica to the mole percent desired. The solution was stirred until it became homogeneous. The mixture was then poured into a glass petri dish which was previously treated with a mold release agent. The sample was degassed, placed into a humidity cabinet and cured at 60% relative humidity (RJJ) for 1 week.

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    In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

  • 18. BLACK ET AL. Sol-Gel-Derived Silica-Siloxane Composite Materials 239

    Results and Discussion

    Molecular Weight Dependence. In order to investigate the relationship between the molecular weight of the PDMS matrix and the resulting size of the silica domains generated, we prepared two series of silica/siloxane composite materials (19). Within each series, the molecular weight of the PDMS matrix was varied from 2,000 to 310,000 average molecular weight. The two series of materials differed only in the amount of catalyst used with one series containing TO in a catalyst/TEOS ratio of 1/300 and the other a 1/70 ratio.

    Neutron scattering data (Figure 1) shows a distinct shift to smaller q with increasing polymer molecular weight. This shift is consistent with the idea that the elasticity of the network controls the domain size since the molecular weight between crosslinks determines the elastic modulus. For both of the catalyst amounts, the domain size, R, was calculated from least-square fits to a five parameter model for interacting particles (e.g., Figure 2) and plotted versus the elastomer molecular weight (Figure 3). The observed slopes of 0.43 and 0.45 0.10 are consistent with de Gennes' prediction of 0.5 for domain growth limited by network elasticity (21,

    Figure 1. SANS data for high catalyst series (reproduced with permission from ref. 19. Copyright 1992 Materials Research Society.).

    22). I I I I I I I j a

    High Catalyst |

    P I I I I I j I I I I I 19

    Data Fit

    : Low Catalyst, M = 2,000 I I I I I I I I l l l l

    0.1

    Figure 2. Fit to the SANS data (reproduced from ref. 19. Copyright 1992 Materials Research Society.).

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    In Hybrid Organic-Inorganic Composites; Mark, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

  • 240 HYBRID ORGANIC-INORGANIC COMPOSITES

    By SANS, the high and low catalyst ratios show identical behavior except for a prefactor shift to larger length scales for the low-catalyst system (Figure 3). If network elasticity limits the domain growth as proposed by de Gennes, this shift indicates that the lower catalyst amount results in a

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