Reinforcement of Elastomers by The In-Situ Generation of Filler Particles
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REINFORCEMENT OF ELASTOMERS BY THE IN-SITU GENERATION OF FILLER PARTICLES JAMES E. MARK* AND DALE W. SCHAEFER** *University of Cincinnati, Dept. of Chemistry, Cincinnati, OH 45221 "**SandiaNational Laboratories, Albuquerque, NM 87185 ABSTRACT The goal of primary interest in these investigations was the development of novel methods for filling elastomeric networks. The techniques developed employ the in-situ generation of reinforcing fillers such as silica or a glassy polymer such as polystyrene either after, during, or before network formation. The reaction involves decomposition of organometallic compounds, using a variety of catalysts and precipitation conditions, or freeradical polymerization of a suitable monomer. The effectiveness of the technique is gauged by stress-strain measurements carried out on these elastomeric composites to yield values of the maximum extensibility, ultimate strength, and energy of rupture. Also of interest are calorimetric studies of the networks, to determine their crystallizability. Information on the filler particles themselves is obtained from density determinations, electron microscopy, and scattering measurements. INTRODUCTION There are a number of disadvantages to reinforcing an elastomer by the usual technique [1,2] of blending a finely divided filler (such as carbon black or silica) into a polymer before cross linking it [3]. A number of alternative techniques are therefore also under development. Examples of such techniques are presented here, with a strong emphasis on results beyond those described in three recent reviews which are at least partly on the same subject [4-6]. They include hydrolysis of organometallic compounds within a polymeric matrix to give ceramic particles such as silica and titania. The semi-inorganic polymer, poly(dimethylsiloxane) (PDMS) has been most studied in this regard. The case where the ceramic predominates and becomes the continuous phase is also mentioned. An alternative approach where monomers such as styrene or methyl methacrylate are polymerized in-situ to give glassy polymers is also described, as are some related systems in which there are magnetic particles or zeolites. Of primary interest here is the reinforcement provided by these fillers. It is easy to switch the focus, however, so that the elastomer is viewed as only a matrix in which the ceramic materials are being generated. In this "matrix isolation" approach [7], X-ray and neutron scattering techniques, for example, can be used to obtain information that transcends these particular systems. It should be useful in a variety of areas, including the new sol-gel technique for preparing ceramics of carefully controlled ultrastructure [8-11]. IN-SITU PRECIPITATIONS Typical Hydrolysis Reactions The most important reaction in this area is the acid or base catalyzed hydrolysis of tetraethoxysilane (TEOS) as described by the chemical equation [4-6], Si(OC 2H5 )4 + 2H 20 -+ SiO 2 + 4C H OH
(1)
Analogous reactions [8-11] can be carried out, however, on titanates [ 12-14], aluminates [ 15
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