Crystallization and melting behaviors of poly(vinylidene fluoride) examined by fast-scan calorimetry: Hoffman-Weeks, Gibbs-Thomson and thermal Gibbs-Thomson plots

Abstract

Crystallization and melting behaviors of chain-folded polymer crystals have been examined by fast-scan calorimetry (FSC) combined with small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) for poly(vinylidene fluoride). The melting point of lamellar crystals formed isothermally at Tc was measured by FSC in terms of the heating rate dependence and calibrated on the basis of the modeling of melting kinetics for the determination of the melting point at zero heating rate TM. The Hoffman-Weeks (H-W) plot of TM against Tc was on a linear straight line over Tc range broader than 30 K, and suggested the equilibrium melting point TM0 ≅ 200 °C of chain-extended infinite-size crystals of the α form. On the other hand, utilizing crystalline lamellar thickness dc determined by SAXS, both of the Melting and Crystallization lines in the Gibbs-Thomson (G-T) plots of TM and Tc against (dc)−1, respectively, were found to seriously deviate from linear straight lines. By examining the secondary stage of crystallization on long time isothermal annealing at Tc in terms of the changes in TM and the specific heat of fusion Δhfs by FSC, a newly proposed thermal G-T plot of TM against (Δhfs)−1 has confirmed the reliability of TM0 of the α form determined by the H-Wplot and the Tc dependent folding surface free energy σe, which brings the curved Melting and Crystallization lines in the G-T plots. In addition, on long time isothermal annealing, the changes in dc by SAXS and in crystallite size by WAXD have confirmed the molecular origin of the increase in TM and Δhfs in the secondary stage with thickening and perfecting of metastable chain-folded polymer crystals.