Relationships between polymorphic crystals and multiple melting peaks in crystalline syndiotactic polystyrene

Multiple crystal forms and complex melting behavior of syndiotactic polystyrene (sPS) were thoroughly examined using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). This study has provided clear evidence for interpretation of relationships between the multiple melting peaks and polymorphism crystals in sPS under isothermal melt-crystallization, dynamic cooling, or other thermal histories. For the first time, assignment of different crystal types to the numerous melting peaks in sPS was accomplished. Effects on the mechanism of chain packing were examined for correlating the relationships between the polymorphism and multiple melting peaks. The α-crystal packing can be favored and becomes an alternative route in sPS crystallization under three conditions: (1) slow cooling from molten state, (2) melt crystallization at low temperatures, or (3) cold crystallization from quenched amorphous glass. By comparison, melt-crystallization at most accessible temperatures produces solidified sPS containing both α-type and β-type crystals of various fractions, but a higher temperature tends to favor greater fractions of β-type crystal. The β-type crystal becomes the only discernible species if sPS is melt-crystallized at temperatures equal to or higher than 260 °C, which suggests that in conditions approaching equilibrium the β-crystal lamella is the favored packing in sPS.