Thermal properties and crystalline structure of syndiotactic polystyrene‐based miscible blends

Abstract

Abstract This work examined the effect of the pre‐melting temperature ( T max ) on the thermal properties and crystalline structure of four miscible syndiotactic polystyrene (sPS)‐based blends containing 80 wt % sPS. The counterparts for sPS included a high‐molecular‐weight atactic polystyrene [aPS(H)], a medium‐molecular‐weight atactic polystyrene [aPS(M)], a low‐molecular‐weight atactic polystyrene [aPS(L)], and a low‐molecular‐weight poly(styrene‐ co ‐α‐methyl styrene) [P(S‐ co ‐αMS)]. According to differential scanning calorimetry measurements, upon nonisothermal melt crystallization, the crystallization of sPS shifted to lower temperatures in the blends, and the shift followed this order of counterpart addition: P(S‐ co ‐αMS) > aPS(L) > aPS(M) > aPS(H). The change in T max (from 285 to 315 °C) influenced the crystallization of sPS in the blends to different degrees, depending on the counterpart's molecular weight and cooling rate. The change in T max also affected the complex melting behaviors of pure sPS and an sPS/aPS(H) blend, but it affected those of the other blends to a lesser extent. Microscopy investigations demonstrated that changing T max slightly affected the blends' crystalline morphology, but it apparently altered that of pure sPS. Furthermore, the X‐ray diffraction results revealed that the α‐form sPS crystal content in the blends generally decreased with an increase in T max , and it decreased with a decrease in the cooling rate as well. The blends showed a lower α‐form content than pure sPS; a counterpart of a lower molecular weight more effectively reduced the formation of α‐form crystals. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2798–2810, 2006