The melting line, the crystallization line, and the equilibrium melting temperature of isotactic polystyrene

It is known that the determination of the equilibrium melting temperatures of polymers which show recrystallization during heating is a difficult problem. This is mainly due to the difficulty encountered in determining the melting temperature of the initial lamellae which exist before recrystallization takes place. We are reporting here the results of a study investigating this problem for a representative polymer, namely isotactic polystyrene. Both the crystallization line and the Gibbs-Thomson melting line were constructed on the basis of time- and temperature-dependent SAXS measurements, respectively. When the two lines are plotted on the same graph, they intersect each other at a finite crystal thickness and at temperature of about 243 degreesC. This temperature is very close to the commonly reported values of the equilibrium melting temperature of isotactic polystyrene obtained using the Hoffman-Weeks extrapolative method, namely 242 degreesC. However, the value obtained from the intercept of the Gibbs-Thomson melting line is found to lie in the range of 289 +/- 5 degreesC. This new, higher value of the equilibrium melting temperature of isotactic polystyrene might justify the high melting temperatures found for some isotactic polystyrene samples, which were as high as 255 degreesC for samples with a shish kebab morphology. Finally, DSC curves were recorded for samples crystallized at different crystallization temperatures in order to compare the Gibbs-Thomson melting line obtained by the SAXS measurements and that obtained by the DSC curves. The result of such comparison showed a good agreement.