Polyhedral Oligomeric Silsesquioxane (POSS) particles in a polysiloxane melt and elastomer. Dependence of the dispersion of the POSS on its dissolution and the constraining effects of a network structure

Polysiloxane composites containing particles of polyhedral oligomeric silsesquioxane (POSS) were prepared by melt blending, and investigated with regard to their morphologies. Unexpectedly, the POSS crystallites were found to dissolve in the polysiloxane at temperatures that were elevated (but far below the POSS melting points). X-ray diffraction, differential scanning calorimetry, and polarizing optical microscopy were used to document changes in the dispersions from this dissolution and from the subsequent recrystallization precipitation occurring upon cooling. Quenching was found to give finer POSS dispersions. Cross linking the polysiloxane caused changes in POSS solubility that enhanced the phase separation, but the cross links caused constraints that decreased the domain sizes of the precipitated phases. These decreases in POSS domain size provide an interesting parallel to the decreases in solvent crystallite sizes in thermoporosimetry, and the decreases in ceramic particle sizes in sol-gel technologies.