Paper number 1203

CRYSTAL FORMS AND MELTING BEHAVIOR IN COLD-CRYSTALLIZED VS. MELT-CRYSTALLIZED SYNDIOTACTIC POLYSTYRENE

Ya Sen Sun and E. M. Woo

Department of Chemical Engineering, National Cheng Kung University
Tainan, 701-01, Taiwan

Summary

Syndiotactic polystyrene has been known to possess polymorphic crystals. Normally, alpha and beta forms are obtained with melt-processed s-PS. Combination of various fractions of alpha-type (hexagonal) and beta-type (orthorhombic) crystals exists in melt-crystallized syndiotactic polystyrene (s-PS) if crystallized at lower temperatures (240°C or lower), or majority of beta-type if at temperatures higher than 260°C. Slow-cooled s-PS samples in simulation of typical processing cycles were investigated in this study. The DSC results revealed that there are three (or fewer depending on the temperature of melt-crystallization) sharp melting peaks (labeled as Peak-1, -2, and -3, respectively) for the s-PS sample melt-crystallized. The first two peaks are relatively minor while the highest peak is prominent. With annealing at higher temperatures for various times, the first peak (Peak-1) was elevated to a higher temperature and increased in intensity rapidly while Peak-2 was also elevated to higher temperatures but its intensity decreased. Peak-3 remained at the same temperature regardless annealing but decreased in intensity rapidly. X-ray result suggested that beta-type crystal is largely associated with the melting entity of Peak-1, and alpha-type is associated with Peak-2 and Peak-3. Crystal forms in cold-crystallized sPS were compared to those in melt-crystallized one. sPS is commonly used in galss-reinforced forms. Investigation on possible effects of glass fiber on morphology, polymorphism, and properties of sPs is in progress and will be reported in the meeting.

Keywords

crystal structure, DSC, crystallization, s-PS, crystal forms.