Intraspherulitic melting-temperature distribution of poly(butylene 2,6-naphthalate) containing β′-crystals controlled by secondary crystallization

Abstract

The combination of low crystal-growth rate and low nuclei density, as evident, e.g., on hot-crystallization at low melt-supercooling, allows formation of rather large spherulites containing isothermally grown crystals subjected to different times of secondary crystallization, causing an intraspherulitic melting-temperature distribution. As demonstrated on example of the β′-high-temperature-crystal polymorph of poly(butylene 2,6-naphthalate) (PBN), crystals located in the spherulite centers, subjected to annealing during the slow growth of the spherulite, melt at distinctly higher temperature than non-annealed crystals near the spherulite boundary, causing spherulite inward melting. The melting-temperature gradient along the spherulite radius, however, diminishes if all parts of the spherulites are annealed, e.g., after a space-filled spherulitic morphology is achieved, yielding a radius-independent intraspherulitic melting temperature. Otherwise, the intraspherulitic melting-temperature distribution may be preserved/frozen-in by cooling, with implications on properties due to the presence of crystals of different stabilities. Assessing the intraspherulitic melting-temperature distribution required suppression of crystal reorganization on heating, which was achieved by analysis of the heating-rate dependence of melting. These experiments confirmed the initially lower stability of crystals near the spherulite periphery by their enhanced reorganization/stabilization on sufficiently slow heating compared to crystals located in the spherulite center, being less vulnerable for reorganization. In summary, the study highlights the importance of secondary crystallization/annealing on the thermodynamic stability/melting behavior of crystals arranged in a spherulitic semicrystalline superstructure. In addition, the performed study also provides new data about the growth of radial and tangential lamellae in PBN when crystallized at low melt-supercooling.