Composites of poly(methylvinylsiloxane) ("silicone") elastomers with polyhedral oligomeric silsesquioxane (POSS) were prepared by melt blending. One goal was to establish conditions that would lead to morphologies different from that of a simple filler dispersed in a polymer matrix for the purposes of reinforcement. To this end, the Study focused on the dispersion and state of POSS in silicone rubber blends as determined by X-ray diffraction (XRD), polarizing optical microscopy (POM), scanning electron microscopy (SEM), and analysis using a rubber processing analyzer (RPA). Of particular interest were the thermal stability of POSS macromers, and the effects of mixing temperature and subsequent vulcanization of the polysiloxane. The results showed that highly crystalline POSS macromers could undergo condensation reactions at 230 degrees C in air, leading to partially amorphous structures. Also, POSS crystals apparently dissolved in the polysiloxane at high temperatures and POSS crystals with hexahedral or flake-like structures recrystallized out upon cooling. Both crystallites and POSS molecules co-existed in these blends, with the amount of dispersed molecular POSS being increased at higher temperatures. The POSS molecules exhibited some physical interactions with the polysiloxane uncross-linked chains, but phase separation was induced by the process of cross-linking. In this curing process, POSS molecules could react with the polysiloxane, resulting in decreases in cross-link density. The original POSS crystals could also be dissolvcd in the polysiloxane during the initial curing stages, but recrystallization upon cooling gave regenerated crystals that were roughly spherical. (c) 2007 Elsevier Ltd. All rights reserved.