Thermal transitions in cold-crystallized blends of isotactic and static polystyrene

Abstract

Solution-cast blends of isotactic and atactic polystyrene were stripped of casting solvent and heated above the glass transition temperature to temperatures below the equilibrium melting point of the respective blends to induce the development of crystallinity of the isotactic polystyrene. The samples, maintained at these crystallization temperatures for various times, were subsequently characterized by differential scanning calorimetry. The amount of crystallinity, the crystalline melting point, and the rate of crystallization were determined for each of the blends from the d.s.c. scans. The development of crystallinity, characterized by increased melting points, resulted from annealing at progressively higher temperatures. The maximum rate of crystallization for each of the blends was observed at a temperature corresponding closely to 8 9 of the absolute equilibrium melting temperature. The decrease in the melting point of the isotactic polymer, crystallized in the presence of atactic diluent, resulted primarily from weak enthalpic interactions between isotactic and atactic polystyrene homopolymers. Scanning electron photomicroscopy revealed that atactic-rich blends, immersed in n-hexane at 40°C, developed a non-intercommunicating network of microvoids. Similar solvent treatment of isotactic rich blends had little effect on the samples. D.s.c. measurements confirmed recently published results which indicated that n-hexane, at modest temperatures, does not induce the development of crystallinity in isotactic polystyrene.