Correlation between melting behavior and ringed spherulites in poly(trimethylene terephthalate)

Abstract

The lamellar types as revealed by the multiple melting peaks and possible mechanisms of ringed spherulites in poly(trimethylene terephthalate) (PTT) were analyzed with differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy. Several interesting correlations were found. If PTT is melt-crystallized in a certain temperature range, it shows multiple melting peaks and rings in PTT. Once rings are formed in the original melt-crystallized PTT, they do not disappear but persist and become even more apparent upon postcrystallization annealing at higher temperatures. Furthermore, for PTT that is capable of exhibiting ringed spherulites, a temperature range exists where rings do not form. This behavior can be interpreted in relation with the demonstrated thermal behavior in PTT. Reorganization took place upon postcrystallization scanning or annealing to or at higher temperatures. A postulation was proposed and rigorously tested with evidence to correlate the ringed spherulites and melting behavior. Rings in PTT may be related to multiple lamellae in the spherulites. Consequently, if a temperature of crystallization is selected so that there is only one type of lamella in the spherulites, then there should be no rings.

Interior Lamellar Assembly and Optical Birefringence in Poly(trimethylene terephthalate) Spherulites: Mechanisms from Past to Present

“…For example, Ho, et al [3], Wu, et al [4], Hong et al [12], and Chuang, et al [13] shared similar results in that the banded structure could only be obtained at Tc = 195-215 °C. Xue, et al [43] reported banded spherulites formation at Tc = 170-195 °C.…”

“…For example, Ho, et al [3], Wu, et al [4], Hong et al [12], and Chuang, et al [13] shared similar results in that the banded structure could only be obtained at T c = 195-215 • C. Xue, et al [43] reported banded spherulites formation at T c = 170-195 • C. Wang, et al [3] found the lower and upper limitations of T c for banded spherulite formation are 135 • C and 165 • C, respectively. Rosenthal, et al [9] reported that the achiral PTT lamellae inverted their chirality at a certain crystallization temperature, separating the banded spherulites at lower temperature region from those at higher temperature region.…”

“…Poly(trimethylene terephthalate) (PTT) is an achiral aromatic polyester that has three methylene groups arranged in gauche-gauche conformation in its alkylene moiety. Unlike PET and PBT, PTT exhibits large and well-defined banded spherulites at a relatively fast crystallization time when melt-crystallized in a film [2][3][4][5][6][7][8][9]. Furthermore, the terephthloyl moiety makes PTT a highly birefringent material, allowing its spherulites to appear colorful under a polarized optical microscope.…”

“…With all these characteristic features, PTT is a good candidate for the study of polymer crystallization and self-assembly. Thus, in this paper, the works on PTT crystallization, including its kinetics [10,11], intrinsic birefringence [12][13][14], as well as its spherulitic morphologies, particularly its banded spherulite structures and formations [2][3][4][5][6][7][8][9] are reviewed in order to provide insight into the real lamellar structures in banded spherulites crystallized from melting.…”

Structural variation of melt-crystallized PTT during the heating process revealed by FTIR and SAXS

“…Morphology and thermal behavior of a crystalline PTT are usually very complicated and only one crystal modification with a triclinic unit cell has been identified so far [4,5]. It has been reported that the multiple melting behaviors of PTT vary with their preparation conditions revealed by differential scanning calorimetry (DSC) [6][7][8][9]. The multiple melting peaks are mainly explained by (i) the melting of crystalline entities with different stability and/or (ii) the meltingrecrystallization-remelting of the imperfect crystals.…”