Analysis of Polymorphism and Dual Crystalline Morphologies in Poly(hexamethylene terephthalate)

Abstract

Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.

Amorphous-Phase Miscibility and Crystal Phases in Blends of Polymorphic Poly(hexamethylene terephthalate) with Monomorphic Poly(pentamethylene terephthalate)

“…In previous work, melt-crystallized PHT is already known to exhibit complex melting peaks with various states of polymorphism depending on T c . [28][29][30] Here, the objective in this study was to further clarify whether post-crystallization thermal annealing/scan-ning at temperatures higher than the original T c might induce polymorphism transition or lamellar thickening. Earlier work [28][29][30] has discussed and concluded that melting of P 1 leads to immediate re-packing into P 2 (or later P 3þ4 ).…”

“…[28][29][30] Here, the objective in this study was to further clarify whether post-crystallization thermal annealing/scan-ning at temperatures higher than the original T c might induce polymorphism transition or lamellar thickening. Earlier work [28][29][30] has discussed and concluded that melting of P 1 leads to immediate re-packing into P 2 (or later P 3þ4 ). Only a merged P 3þ4 peak is seen with melting/re-crystallization of P 3 at 140 C, whose total enthalpy of melting (48 J/g) does not change much in comparison to un-annealed PHT (DSC Trace-a).…”

“…The DSC results show that the heating scan in DSC induces almost immediate re-packing of melted species into new lamellar crystals, and scan-induced thickening of lamellae takes place in multiple (as many as four) steps for PHT within a narrow temperature range. It would be of interest to compare the melting peak of a 140 C-melt-crystallized PHT 30 with that of 110 C-melt-crystallized PHT then post-scanned to 140 C, as shown in DSC Trace-b of this figure. DSC melting peaks of the former (140 C-melt crystallized PHT) is sharper and narrower 30 than the latter (110 C-melt-crystallized then heat-scanned to 140 C).…”

“…It would be of interest to compare the melting peak of a 140 C-melt-crystallized PHT 30 with that of 110 C-melt-crystallized PHT then post-scanned to 140 C, as shown in DSC Trace-b of this figure. DSC melting peaks of the former (140 C-melt crystallized PHT) is sharper and narrower 30 than the latter (110 C-melt-crystallized then heat-scanned to 140 C). Thus, the DSC result seems to suggest that post-scanning or annealing (up to 140 C) on the 110 C-crystallized PHT sample does not completely and uniformly transform all thin lamella into thick lamella.…”

“…They differ only by one methylene in the main-chain repeat unit; nevertheless, this difference may result in different crystallization rates, polymorphism, and spherulite morphology, or even constitute a critical factor in determining the phase behavior in blends of these two polyesters. PHT is a polymorphic polyester that has been characterized to display as many as three different crystal cells (, , ) depending on thermal and/or solution treatments; [28][29][30] however, by contrast, PPT is packed with only a single type of crystal cell. PPT exhibits distinct banded spherulites; 31 but the PHT forms radial spherulites with Maltese-cross.…”