Crystallization and melting of biodegradable poly(propylene suberate)

Thermal behavior of poly(propylene suberate) (PPSub) was studied. PPSub is a novel biodegradable polyester which always shows lower crystallinity, and thus faster biodegradation, than poly(ɛ-caprolactone) or poly(butylene succinate). Crystalline structure, multiple melting behavior and re-crystallization of PPSub on heating were studied by using Wide Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Step Scan DSC. The equilibrium melting point and the enthalpy of fusion of the pure crystalline polymer were found 79.7 °C and 28.9 kJ/mol, respectively. Polarized Light Microscopy (PLM) tests showed that PPSub forms banded spherulites. The Lauritzen–Hoffman theory was used. Values of spherulite growth rates and also overall crystallization rates after self-nucleation, from DSC data, were used. In the latter case the inverse of crystallization half times or the Avrami constant K values were supposed to be a measure of crystallization rates. Critical breakpoint in the Lauritzen–Hoffman plots appeared at 40 °C showing crystallization regime II to III transition. Non-isothermal crystallization kinetics was also analyzed. Interestingly, the Ozawa model seems to hold for PPSub. The iso-conversional method of Friedman was used to study the activation energy of PPSub crystallization on cooling and the Lauritzen–Hoffman parameters were also calculated by applying the Vyazovkin method.