Inversion in the Temperature Gradient of the Crystallization Kinetics of Long-Spaced Polyesters at the Transition Between Two Crystalline Structures

We investigate the sharp range of isothermal crystallization temperatures for which a transition between non-integer (thinner) and integer (thicker) layered crystals occurs in long-spaced aliphatic polyesters, and furthermore demonstrate a discrete minimum in the overall crystallization and linear growth rates near the melting point of the non-integer layered crystals. We explain the presence of these minima on the basis of self-poisoning, arising from unstable depositions of non-integer layered chains on the growth surface of integer layered crystals, which dramatically slow the crystallization process. This self-poisoning phenomenon was observed originally in long-chain n-alkanes and low molar mass PEO fractions and has been found more recently in bromine-substituted precision polyethylenes. In this work, we demonstrate that self-poisoning is ubiquitous to high molar mass polymers when transitioning between structures that are not energetically equivalent, and consequently display sufficiently different nucleation and growth kinetics. We thus contribute further evidence to support a polymer crystallization process controlled by segmental deposition events taking place at the crystal growth front.