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

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 T-c 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 T-M. The Hoffman-Weeks (H-W) plot of T-M against T-c was on a linear straight line over T-c range broader than 30 K, and suggested the equilibrium melting point T-M(0) congruent to 200 degrees C of chain-extended infinite-size crystals of the a form. On the other hand, utilizing crystalline lamellar thickness d(c) determined by SAXS, both of the Melting and Crystallization lines in the Gibbs-Thomson (G-T) plots of T-M and T-c against (d(c))(-1), respectively, were found to seriously deviate from linear straight lines. By examining the secondary stage of crystallization on long time isothermal annealing at T-c in terms of the changes in T-M and the specific heat of fusion Delta h(fs) by FSC, a newly proposed thermal G-T plot of T-M against (Delta h(fs))(-1) has confirmed the reliability of T-M(0) of the a form determined by the H-Wplot and the T-c dependent folding surface free energy sigma(e), which brings the curved Melting and Crystallization lines in the G-T plots. In addition, on long time isothermal annealing, the changes in d(c) by SAXS and in crystallite size by WAXD have confirmed the molecular origin of the increase in T-M and Delta h(fs) in the secondary stage with thickening and perfecting of metastable chain-folded polymer crystals.