Crystallization and melting of poly(butylene terephthalate) and poly (ethylene terephthalate) investigated by fast-scan chip calorimetry and small angle X-ray scattering

The crystallization and melting behaviors of poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) were examined by Hoffman-Weeks (H-W), Gibbs-Thomson (G-T), and thermal Gibbs Thomson (t-G-T) plots constructed by using fast-scan calorimetry and small-angle X ray scattering. With PBT and PET, neither an H-W nor a G-T plot could be utilized for the determination of the equilibrium melting point (T-M(0)) of chain-extended infinite-size crystals. The thermal Gibbs-Thomson plot utilizes the change in the melting point and the heat of fusion of chain-folded crystals during the secondary stage of isothermal crystallization. T-M(0) was determined from a t-G-T plot, and the results were in good agreement with the literature values for PBT and PET. G-T and t-G-T plots suggested a temperature-dependent folding surface free energy (sigma(e)), as has been proposed by Hoffman et al. The sigma(e) values obtained with G-T and t-G-T plots support the consistency of the analysis.