Melting and zero growth rate temperatures of syndiotactic polystryrene

In polymer crystallization, the correct determination of the melting temperature is of crucial importance since crystallization kinetics mainly depends on the amount of undercooling. In this work, the thermodynamic melting temperature and the reference temperature for crystallization kinetics (also called "zero growth rate temperature") are calculated by means of alternative procedures with the objective of providing a general discussion on problems occurring in their determination and of identifying the relationship between them. Syndiotactic polystyrene is used as test material due to its very interesting polymorphic behavior and because many data have been recently reported in the literature concerning its melting temperature. It was found that the thermodynamic melting temperature resulting by differential scanning calorimetry heating analysis provides a poor description when applied to crystallization kinetic data. The reference temperature to be adopted in crystallization kinetic equations resulted to be significantly lower. This seems to agree with recent findings and somewhat to contradict conventional understanding.