Gibbs–Thomson, Thermal Gibbs–Thomson, and Hoffman–Weeks Plots of Polyethylene Crystals Examined by Fast-Scan Calorimetry and Small-Angle X-ray Scattering

Abstract

Crystallization and melting behaviors of polyethylene (PE) crystals were examined by fast-scan chip sensor calorimetry (FSC) and small-angle X-ray scattering (SAXS). The melting point TM of chain-folded thin lamellar crystals of PE was determined by utilizing FSC and the crystalline lamellar thickness dc by SAXS. The range of crystallization temperature Tc from SAXS as well as from FSC was extended much more broadly than before by examining those samples prepared on a chip sensor of FSC and applying a deep temperature jump for crystallization at large supercooling ΔTc. While the equilibrium melting point TM0 was reconfirmed with linear relationship of the melting and crystallization lines in the Gibbs–Thomson (G-T) plots of TM and Tc against (dc)−1, respectively, and the Hoffman–Weeks (H-W) plot of TM against Tc at relatively small ΔTc, those G-T and H-W plots seriously deviated from linear straight lines at large ΔTc. The origin of the curved G-T plots was ascribed to the Tc-dependent folding surface free energy σe. The determined values of TM0- and Tc-dependent σe were supported by a newly proposed thermal G-T plot of TM against the inverse of specific heat of fusion Δhfs in terms of the increase during the secondary stage of crystallization on long-term isothermal annealing at Tc of chain-folded PE crystals transforming to more stable states by lamellar thickening and crystal perfecting. The deviation from a linear H-W plot at large ΔTc was then ascribed to the crystal reorganization on heating of those less stable crystals formed at large ΔTc, even with fast heating. Application of fast heating and cooling by FSC plays an essential role in the confirmation of those thermodynamic behaviors of metastable polymer crystals with chain folding.