Melting behavior of polymorphic crystals of poly(trimethylene 2,6-naphthalate) studied by simultaneous synchrotron X-ray scattering and thermal analysis

Abstract

The polymorphic crystallization and melting behavior of poly(trimethylene 2,6-naphthalate) (PTN) have been investigated using small-angle X-ray scattering and simultaneous wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The α-crystal, the β-crystal and the coexistence of both crystal forms of PTN develop at an isothermal temperature below 393 K, above 453 K and between these two temperatures, respectively. The simultaneous WAXS/DSC measurement provides a good way to identify the origin of multiple melting peaks and to get equilibrium melting temperatures. During the PTN melting process, the thermal evolutions of crystallinities, Bragg diffraction intensities and DSC thermograms reveal that the \alpha \rightarrow \beta phase transformation and primary and secondary crystallizations arise to generate the multiple melting peaks. The β-crystal with high equilibrium melting temperature (T^{0}_{{\rm m},\beta}} = 510 K) is a structurally stable phase while the α-crystal with low equilibrium melting temperature (T^{0}_{{\rm m},\alpha}} = 488 K) is a metastable phase. The temperature-dependent structural parameters such as the long period, lamellar thickness and amorphous thickness were extracted from the interface distribution function. Two-step changes in the lamellar thickness and the invariant during the subsequent melting of PTN crystallized at 383 K are consistent with the \alpha \rightarrow \beta transformation obtained by WAXS/DSC. The \alpha \rightarrow \beta transformation, a typical melting–recrystallization, proceeds firstly via surface melting of α-lamellae, and then the PTN chains near the boundaries of surviving α-lamellae modify their conformation to form the β-crystal resulting in thickening lamellae.