Visualization of Nonequilibrium Properties of a Crystalline Polymer: Formation of Ring-Lite Due to the Gibbs–Thomson Effect and Dark-Ring Due to the Melting Point Inversion

A polymer crystallite having a ring-like morphology was obtained by combining the nonequilibrium nature of a polymeric material and a rapid temperature-jump technique. Termed “ring-lite” here, it consists of the concentrically arranged crystalline and molten regions of a single-component polymer. During the growing process of a spherulite, crystallization temperature T_c was, in turn, changed between low and high temperatures at certain intervals. In this way, bimodal melting temperatures T_m due to the Gibbs–Thomson effect were alternately imprinted within an identical spherulite. Subsequently, the sample was heated so rapidly that only the region having higher T_m remained in a crystalline state. Importantly, inversion of the melting point was also microscopically visualized as a “dark-ring”. Contrary to the appearance of ring-lites due to the “normal” Gibbs–Thomson effect, we found that a specific heating rate caused the region having originally a lower T_m to remain in a crystalline state and that with originally a higher T_m to melt, i.e., display inversion of the melting point. The occurrence of the melting point inversion was confirmed by heating rate variation measurements of fast-scan chip calorimetry (FSC).