Non-isothermal Cold Crystallization as a Tool for Analysis of Crystal Nucleation: The Example of Polytetrahydrofuran

Abstract Polytetrahydrofuran (PTHF) was employed for analysis of the effect of heating rate q on the peak temperature of cold crystallization Tp and enthalpy of cold crystallization Δhcc to follow crystal nucleation. The isothermal increase of the number of nuclei in the highly supercooled melt can be analyzed by heating at experimentally realizable rates between qmin and qmax, with slower and faster heating yielding maximum and zero cold-crystallization enthalpy, respectively. At intermediate fast heating, with q close to qmin, the cold-crystallization enthalpy first increases with the number of nuclei, reaches a plateau, and then decreases since crystals grow already at the nucleation temperature. The observation of a plateau is caused by completion of crystallization during slow heating if the number of nuclei exceeds a specific value. This limits the detection of a further increase of the number of nuclei, which, however, still causes a decrease of Tp, being a more sensitive measure of the number of nuclei than Δhcc, with the effect of the number of nuclei and q on Tp summarized with a continuous heating transformation (CHT) diagram. In contrast, fast heating, with q close to qmax, is unable to detect the initial stage of the nucleation process, caused by the limited time for growth to yield measurable crystallinity and the initial formation of nuclei of only sub-critical size at Tp. Besides the progress in understanding the interplay between nucleation and non-isothermal cold crystallization, the study also provides quantitative information about the kinetics of hot crystallization of PTHF as well as nucleation-pathway-dependent morphological features, requiring sophisticated experimentation to assess crystallization and melting below ambient temperature. Finally, a previously developed theoretical model of the effects of the heating rate and crystal nucleus density on Tp (Polymer 2026, 358, 130181) has successfully been tested, revealing agreement between morphological analyses of the time dependence of nucleation and theory: Homogeneous crystal nucleation of PTHF close to the glass transition temperature yields submicrometer-distant nuclei after sufficiently long annealing.