X‐ray studies on the double melting behavior of poly(butylene terephthalate)

Abstract

The double melting behavior of poly(butylene terephthalate) (PBT) was studied with differential scanning calorimetry (DSC) and wide-angle X-ray analysis. DSC melting curves of melt-crystallized PBT samples, which we prepared by cooling from the melt (250°C) at various cooling rates, showed two endothermic peaks and an exothermic peak located between these melting peaks. The cooling rate effect on these peaks was investigated. The melt-crystallized PBT sample cooled at 24 K min Ϫ1 was heated at a rate of 1 K min Ϫ1 , and its diffraction patterns were obtained successively at a rate of one pattern per minute with an X-ray measurement system equipped with a position-sensitive proportional counter. The diffraction pattern did not change in the melting process, except for the change in its peak height. This suggests that the double melting behavior does not originate from a change in the crystal structure. The temperature dependence of the diffraction intensity was obtained from the diffraction patterns. With increasing temperature, the intensity decreased gradually in the lowtemperature region and then increased distinctly before a steep decrease due to the final melting. In other words, the temperature-dependence curve of the diffraction intensity showed a peak that is interpreted as proof of the recrystallization in the melting process. The peak temperature was 216°C. The temperature-dependence curve of the enthalpy change obtained by the integration of the DSC curve almost coincided with that of the diffraction intensity. The double melting behavior in the heating process of PBT is concluded to originate from the increase of crystallinity, that is, recrystallization.

The effect of extrusion conditions and the use of a compatibilizer in the crystallization of PBT/ABS blends

“…On the other hand, as the PBT was further heat up the (010) intensity became higher again at 216 °C. According to the authors 33 , that increase in the X-ray diffraction intensity at that temperature would be concrete proof of recrystallization during the melting process. Figure 3 shows PBT/ABS blends melting behavior observed during the DSC experiments.…”

“…Due to the faster cooling that PBT was subjected to before the first heating step the temperature of the exothermic peak was reduced for approximately 12 °C, inhibiting the occurrence of the double melting peak in the first heating 2 . Several authors have explained the occurrence of the double melting peak for PBT through the melt-recrystallization model 2,[31][32][33] . That model suggests that the lower-temperature and higher-temperature for the double peak in the DSC curve are attributable to the melting of some amount of original crystals and the melting of crystals formed through the melt-recrystallization process during a heating scan, respectively.…”

“…On the other hand, the lower temperature endothermic peak happens when the rate of recrystallization overwhelms that of the melting. Accordingly, the melt-recrystallization behavior, especially an increase in crystallinity, is considered to be substantial in the melting process 33 . Studies 33 using wide-angle X-ray diffraction (WAXD) and DSC have shown that during cooling at a controlled rate in the DSC, only a single exothermic peak appeared on the DSC thermogram, suggesting a single modal distribution of the size of crystallites formed during the cooling process.…”

Comparative Studies on Thermal, Mechanical, and Flame Retardant Properties of PBT Nanocomposites with Functionalized Amino-Carbon Nanotubes Modified by Different Oxidation State Phosphorus-containing Agents

“…In the heating scan, the influence of DPP(Ox)-A-CNT on melting temperature (Tm) of PBT can be seen to be negligible. As can be seen from Figure 5 and stable crystals, which becomes melt at higher temperature [32][33][34]. Therefore, the first melting peak shifts to high temperature region, resulting in the reduction of multiple melt behavior.…”

Effect of Hydroxyapatite Nanoparticles on the Degradability of Random Poly(butylene terephthalate-co-aliphatic dicarboxylate)s Having a High Content of Terephthalic Units

“…In particular, strong (001), (0-11), (010), (1-11), (100), (1-11), (101), and (11-1) reflections of the α-form defined by a triclinic unit cell (a = 0.483 nm, b = 0.596 nm, c = 1.162 nm, α = 99.9˝, β = 115.2˝, and γ = 113.8˝) and a gauche-trans-gauche conformation for the glycol unit were observed ( Figure 6) [29,30]. It is clear that copolymers crystallized according to the structure of the major component and that aliphatic comonomers should be mainly excluded from the crystalline phase, being probably located in the amorphous lamellar folds.…”