Structure of Microphase-Separated Silica/Siloxane Molecular Composites
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STRUCTURE OF MICROPHASE-SEPARATED SILICA/SILOXANE MOLECULAR COMPOSITES DALE W. SCHAEFER*, JAMES E. MARK**, DAVID McCARTHY1:, LI JIANt, C. -C. SUN•: AND BELA FARAGOt *Sandia National Laboratories, Albuquerque, NM 87185, USA :•Department of Chemistry and the Polymer Research Center, The University of Cincinnati, Cincinnati, OH 45221, USA tlnsititut Laue-Langevin, 38042 Grenoble, France ABSTRACT The structure of several classes of silica/siloxane molecular composites is investigated using small-angle x-ray and neutron scattering. These filled elastomers can be prepared through different synthethic protocols leading to a range of fillers including particulates with both rough and smooth surfaces, particulates with dispersed interfaces, and polymeric networks. We also find examples of bicontinuous filler phases that we attribute to phase separation via spinodal decomposition. In-situ kinetic studies of particulate fillers show that the precipitate does not develop by conventional nucleation-and-growth. We see no evidence of growth by ripening whereby large particles grow by consumption of small particles. Rather, there appears to be a limiting size set by the elastomer network itself. Phase separation develops by continuous nucleation of particles and subsequent growth to the limiting size. We also briefly report studies of polymer-toughened glasses. In this case, we find no obvious correlation between organic content and structure. INTIRODUCTION Historically, the development of specialty polymers has proceeded largely through the manipulation of polymer chain architecture. Glassy vs rubbery behavior, for example, can be adjusted with backbone stiffness. Strain-induced crystallization can be enhanced via stereoregularity. Flame retardancy is augmented by incorporation of chlorinated moieties. Silicon-based systems provide enhanced high-temperature stability. In all these cases, the enhancement of a targeted property usually implies the sacrifice of another. If backbone stiffness is increased to raise the glass transition temperature, for example, toughness is bound to suffer. Multicomponent systems that are homogeneous on length scales exceeding l.tm offer new promise to meet the competing requirements of high-performance polymers. The hope is that by appropriate manipulation of phase structure, it will be possible to simultaneously enhance multiple properties. Unfortunately, successful techniques for achieving these so-called molecular composites (MCs) are limited. In the absence of systematic relationships between synthetic protocol, structure and properties, it is difficult to optimize these materials. Even for conventional composites prepared by mixing, for example, the properties (ramification, stiffness, interfacial properties etc.) of the ideal filler are not well established. Two factors have limited the understanding of the microstructure of complex phase-separated materials: the absence of unambiguous methods for characterizing structure, and the lack of reasonable models to predict structure based on chemical
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