Crystal polymorphism of poly(butylene-2,6-naphthalate) prepared by thermal treatments

The crystal polymorphisms of the poly(butylene-2,6-naphthalate) (PBN) prepared by various thermal treatments in bulk state were investigated. Crystalline forms obtained from different thermal treatments were characterized using wide-angle X-ray diffraction (WAXD) measurements. The α-form crystal is obtained by annealing the quenched PBN sample in the solid state or by crystallizing from the static melt at a relatively lower temperature (TC<205 °C). On the other hand, an exclusive β-form crystal is generated through unique thermal treatment by non-isothermal crystallization at a cooling rate of 0.1 °C min−1 from 280 to 25 °C. Additionally, both α- and β-form crystals appear in the sample melt-crystallized at a relatively higher temperature, and the relative ratio of the α- and β-forms depends on the predetermined crystallization temperature. In this study, it was found that the crystalline form transition never occurs in the solid state. Optical microscopic observations indicate that the α-form crystal exhibits a typical spherulite morphology, while the β-form crystal displays a dendritic spherulite morphology due to the extremely slow spherulite growth rate. Fourier transform infrared spectroscopy was used to examine the crystal structures of both α- and β-forms of the PBN. It is proposed that the main difference between these two crystal structures of PBN prepared by different thermal treatments lies in the packing efficiency of the crystal chains, rather than in the conformation change of the glycol residue, with the β-form possessing tighter packing efficiency than the α-form.