The dependence of the β-to-α phase transition behavior of poly(1,4-butylene adipate) on phase separated morphology in its blends with poly(vinylidene fluoride)

Synchrotron wide-angle X-ray diffraction was used to monitor the melting and β-to-α transition behavior of β-poly(1,4-butylene adipate) (β-PBA) and its blends with poly(vinylidene fluoride) (PVDF). Two kinds of typical phase separated morphologies are prepared: interfabrillar (70/30 PBA/PVDF) and interlamellar (50/50 PBA/PVDF) morphologies. After melt recrystallization at 10 °C, β-PBA crystals are obtained. The thermal expansion, phase transition and melting behavior of β crystals highly depend on the phase separated morphology. The β crystals show the highest melting temperature and β-to-α phase transition temperature in 70% PBA, which is caused by the thick lamellae of PBA and/or the lower surface energy of chain folding, while the β crystals show the lowest melting temperature and β-to-α phase transition temperature in 50% PBA, which is caused by the thinner lamellae of PBA. An unexpectedly high melting temperature of α crystals transited from β crystals observed in 50% PBA due to the lamellar thickening occuring in the heating process. The different phase separated morphologies lead to different lamellae thicknesses and surface energies of chain folding which consequently alter the stability of the crystals. The phase transition and melting behavior of β-PBA in PBA/PVDF is also compared with its blends with poly(vinyl phenol) (PVPh). Completely different factors influence the phase transition behavior of β crystals in these two systems.