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 phase transformation and primary and secondary crystallizations arise to generate the multiple melting peaks. The β‐crystal with high equilibrium melting temperature ( = 510 K) is a structurally stable phase while the α‐crystal with low equilibrium melting temperature ( = 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 transformation obtained by WAXS/DSC. The 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.

Conformational Identification and Phase Transition Behavior of Poly(trimethylene 2,6-naphthalate) α-Form Modification

“…The structure changes of PTN α-form observed by WAXS are consistent with that observed by DSC. Chuang et al observed some diffraction peaks of β-form crystal when the α-form crystal was heated at temperature above 170 °C. They explained that the part of α-form crystal may be transformed into the β-form crystal during the heating process.…”

“…They explained that the part of α-form crystal may be transformed into the β-form crystal during the heating process. Chuang et al noticed that the WAXS profile measured during the α-form to β-form crystal transformation is different from that of the coexistence of α- and β-crystals. In our work, however, we did not observe PTN β-crystal diffraction peaks when the PTN α-form crystal was heated or annealed at above 170 °C of temperature.…”

“…The crystallization and melting behaviors of PTN have been investigated by Jeong’s and Chuang’s groups; however, some of their results are incompatible. For example, Joeng et al reported that the equilibrium melting temperatures of PTN α- and β-form crystals were 197 and 223 °C, respectively.…”

“…They suggested that the crystal transition from α- to β-form may occur at 154 °C based on analysis of spherulitic growth rate of PTN using the second nucleation theory proposed by Laurizen and Hoffman. Whereas Chuang et al reported that the both equilibrium melting temperatures of α- and β-form crystals were 215 and 237 °C, respectively. They suggested the mechanism of α- to β-crystal transformation which is a typical melting–recrystallization process based on heating of PTN α-form crystal, i.e., first 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.…”