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

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(D], 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.