Melting and β to α transition behavior of β-PBA and the β-PBA/PVPh blend investigated by synchrotron SAXS and WAXD

Abstract

Synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) are used to monitor the melting and βα transition behavior of β-PBA and the β-PBA/PVPh blend during the heating process. After melt recrystallized at 10 °C, the β crystals are obtained. The lamellar thickness of β crystals in the neat PBA is similar to that in its PBA/PVPh blend and the long period for the former one is shorter than the latter one. At lower heating rate, α crystals start to appear at 39 °C suggesting the βα phase transition occurs. Compared with the neat PBA, the β-PBA in the blend transits to α crystals at a higher rate. At a higher heating rate, β crystals melt directly. Both at the lower and higher heating rate, the crystal structure along the a-axis is much more stable than that along b-axis in the β crystals. Moreover, the heating rate does not play a role in the melting mechanism of the β crystals. The melting of the β crystals in the neat PBA and its PBA/PVPh blend chose different mechanisms. The melting mechanism of the β crystals in the PBA/PVPh blend is attributed to the sequential melting. On the contrary, the melting of the β phase in the neat PBA is complex. Besides the peak of q = 0.54 nm⁻¹, a new peak appears at q = 0.32 nm⁻¹ in the SAXS profiles in the heating process suggesting the appearance of a population with a larger long period. The presence of two peaks is caused by the coexistence of two kinds of lamellar stacks. And the partial melting of thinner β-PBA lamellae occurs firstly in one kind of lamellar stacks.