Gradient blending of poly(dimethylsiloxane) with polystyrene and polyethylene in supercritical carbon dioxide

Abstract

Direct impregnation and blending of poly(dimethysiloxane) (PDMS) with polystyrene and polyethylene has been investigated. Blends of polystyrene were prepared by first swelling and impregnation of polystyrene pellets in 0.1, 3 and 5 wt% solutions of PDMS in CO 2 at 80 °C and 45.7 MPa and then depressurization either directly at the impregnation temperature or after cooling to 50 or 27 °C. Blends of polyethylene were formed by impregnation at 130 °C and 57 MPa with 0.1 wt% PDMS solutions in CO 2 and depressurization at either 130 or 50 °C. The microstructural features of the blends were investigated by scanning electron microscopy (SEM). PDMS distribution across the pellet cross sections was investigated in terms of silicon atom distribution using energy dispersive X-ray (EDX) analysis. It is shown that the blends display pore-wise and compositionally gradient features. They typically show a steep compositional gradient, with a high PDMS concentration near the edge of the pellets which decrease and reach a plateau value a short distance away from the outer surface. In blends with polystyrene, when decompressions were carried out at temperatures lower than the impregnation temperature, the PDMS distributions were found to be significantly different, displaying a characteristic local maximum in the 20–100 μm range from the outer surface. In these blends, the SEM results show the formation of a dense non-porous layer with a thickness that corresponds closely with the location of the observed PDMS maximum, pointing to a competition between the vitrification of the host matrix and the escaping ability of PDMS dissolved in carbon dioxide before its entrapment during depressurization. At decompressions at the impregnation temperature, or with impregnations with PDMS solutions of higher concentrations, polystyrene pellets were found to expand more, and display a pore-gradient feature with layers of progressively increasing pore sizes from the outer edges towards the central region of the pellets. In blends of polyethylene, in contrast to polystyrene, the local maximum in PDMS concentration was not observed. The overall PDMS levels in the central region away from the outer surface were lower. The central region of the pellet showed a nanoporous morphology.