Correlation Between Thermal Behavior and Crystalline Morphology in β-Form Syndiotactic Polystyrene

Abstract

Syndiotactic polystyrene (sPS) of a low molecular weight (M̄w = 63 000 g/mol) was found to develop only a β-type unit cell upon melt crystallization at all temperatures. Correlation between the lamellar morphology and melting behavior in the sPS has been demonstrated in this study. When crystallized at lower temperatures (250°C or lower), sPS displays three melting peaks (i. e. P-I, -II, -III) and the morphology is composed of two main crystalline domains, which are initially thin and straight lamellar bundles (P-I) and an eye-like lamellae platelet crystal (P-II). The P-III crystal was not initially present in significant extents in sPS crystallized at low temperatures. However, upon scanning to higher temperatures, the initially thinner flat-on lamellae (P-I) are melted and then re-packed to more thickened and branched lamellae (P-III), whose orientation is also twisted from the original flat-onto an edge-on (perpendicular) pattern. Detailed mechanisms of transformation between P-I and P-III crystals via melting/re-packing are discussed in this paper. In addition, P-III lamellae could also be directly developed in sPS by melt-crystallization at higher temperatures (260°C) while P-I and P-II crystals were absent. Only the P-III crystal (branched, thick, axialite) was found in the sPS melt-crystallized at 260°C or above.