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

摘要

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 T-M of chain-folded thin lamellar crystals of PE was determined by utilizing FSC and the crystalline lamellar thickness d, by SAXS. The range of crystallization temperature T-c 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 Delta T-c. While the equilibrium melting point T-M(0) was reconfirmed with linear relationship of the melting and crystallization lines in the Gibbs Thomson (G-T) plots of T-M and T-c against (d(c))(-1), respectively, and the Hoffman Weeks (H-W) plot of T-M against T-c at relatively small Delta T-c, those G-T and H-W plots seriously deviated from linear straight lines at large Delta T-c. The origin of the curved G-T plots was ascribed to the T-c-dependent folding surface free energy sigma(e). The determined values of T-M(0)- and T-c-dependent sigma(e), were supported by a newly proposed thermal G-T plot of T-M against the inverse of specific heat of fusion Delta h(fs) in terms of the increase during the secondary stage of crystallization on long-term isothermal annealing at T-c 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 Delta T-c was then ascribed to the crystal reorganization on heating of those less stable crystals formed at large Delta T-c 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.