Effects of supercritical carbon dioxide on phase homogeneity, morphology, and mechanical properties of poly(styrene-blend-ethylene-stat-vinyl acetate)

Significant enhancement of the mechanical properties in PS/EVA blends is achieved via sc CO ₂ exposure in the pressure/temperature domain where the amplitude of the density fluctuations is maximal. Before exposure, bulk samples of the blends elongate an average of 134% under uniaxial tension, whereas the average elongation is 682% after exposure. SEM micrographs of the exposed blends indicate that foams with pores roughly 5-15 μm in diameter, separated by splines 1-2 μm wide, are formed at all pressures higher than 800 psi. We attribute the enhancement of the mechanical properties in PS/EVA to an increased degree of miscibilization of the immiscible polymers along the spline surfaces. When foaming occurs, the surface-to-volume ratio is increased, thereby increasing the regions where surface miscibilization can occur. Since tensile stresses in the material are sustained by the splines, reinforcement of the mechanical properties in this specific area results in an overall enhancement of the tensile properties of the material. The increased miscibility was further confirmed by contact angle and DSC measurements of the blends before and after exposure. The results showed a decrease in the contact angle from 18° to 3° and the appearance of a second glass transition, which was intermediate between the T _g values of the PS and EVA homopolymers.