Effect of Polymorphism on Isothermal Crystallization Kinetics of Long-Spaced Precision Polyacetals

Under isothermal crystallization (T_c) from the melt, polyacetals spaced by 12, 18, 19 or 23 methylenes develop two or three layered polymorphs. Crystals formed in the lowest T_c range are kinetically favored (hexagonal and Form I) and characterized by highly nucleated small axialites. In the higher range of T_c, thermodynamically more stable spherulitic Form II crystals develop. While the overall crystallization kinetics of Form I display the usual negative temperature coefficient, an inversion of the dependence of the rate of Form II with temperature occurs when approaching from above the narrow T_c range where Form I and Form II coexist. The inversion is attributed to a competition in nucleation between Forms I and II. Just before inception of Form II, the crystallization rate is so low that it becomes basically extinguished. The degree of crystallinity recovers when pure Form II develops with a small increase in T_c. From analysis of linear growth rate data, the 2-6 times higher energy barrier found for nucleation of Form II explains the large differences in primary nucleation. The differences in energy barriers, and a possible preferential self-poisoning during nucleation of Form II explain the extinguished crystallization at the overlap T_c between two polymorphs.