Miscibility, thermal properties and polymorphism of syndiotactic polystyrene/poly(styrene-co-α-methyl styrene) blends

This work examined the miscibility, crystallization kinetics, melting behavior and crystal structure of syndiotactic polystyrene (sPS)/poly(styrene-co-α-methyl styrene) blends. Differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction technique were used to approach the goals. The single composition-dependent T_gs of the blends and the melting temperature (T_m) depression of sPS in the blends indicated the miscible characteristic of the blend system at all compositions. Furthermore, the T_gs of the blends could be predicted by either of the Gordon–Taylor equation (with K=0.99) or the Fox equation with a slightly higher deviation. The dynamic and isothermal crystallization abilities of sPS were hindered with the incorporation of the miscible copolymer. Complex melting behavior was observed for melt-crystallized pure sPS and its blends as well. Nevertheless, the blends showed relatively simpler melting curves. Comparing with melt-crystallized samples, the cold-crystallized samples exhibited simpler melting behavior. The equilibrium melting temperature (T_m⁰) of β form sPS crystal determined from the conventional extrapolative method is 295.2 °C. The Flory–Huggins interaction parameter, χ, of the blends was estimated to be −0.27. The crystal morphology of sPS was disturbed in the blends. Only underdeveloped granular-like crystalline superstructure of sPS exhibited in cold-crystallized blends. Moreover, the existence of the copolymer in the blends apparently reduced the possibility of forming the less stable α form sPS crystals.