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

Multiple crystal forms and complex melting behavior of syndiotactic polysty rene (sPS) were thoroughly examined using wide-angle X-ray diffraction (WAX D) and differential scanning calorimetry (DSC). This study has provided cle ar evidence for interpretation of relationships between the multiple meltin g peaks and polymorphism crystals in sPS under isothermal melt-crystallizat ion, dynamic cooling, or other thermal histories. For the first time, assig nment of different crystal types to the numerous melting peaks in sPS was a ccomplished. Effects on the mechanism of chain packing were examined for co rrelating the relationships between the polymorphism and multiple melting p eaks. The alpha-crystal packing can be favored and becomes an alternative r oute in sPS crystallization under three conditions: (1) slow cooling from m olten state, (2) melt crystallization at low temperatures, or (3) cold crys tallization from quenched amorphous glass. By comparison, melt-crystallizat ion at most accessible temperatures produces solidified sPS containing both alpha-type and beta-type crystals of various fractions, but a higher tempe rature tends to favor greater fractions of beta-type crystal. The beta-type crystal becomes the only discernible species if sPS is melt-crystallized a t temperatures equal to or higher than 260 degrees C, which suggests that i n conditions approaching equilibrium the beta-crystal lamella is the favore d packing in sPS.