Characterization of thermal behavior and kinetics of crystallization of a thermotropic rigid‐chain copolymer

Abstract

Thermal analysis was employed to estimate the heat of transition, equilibrium melting temperature and surface free energy of a thermotropic liquid crystal copolyester resin (70 mol% p-hydroxybenzoic acid and 30 mol% 2,6-hydroxynaphthoic acid). Specific heats and melting and crystallization behavior were evaluated using differential scanning calorimetry (DSC). Experiments over a wide range of cooling rates indicate two different crystallization processes: a rapid crystallization process and a slow crystallization process. The kinetics of the rapid crystallization process were evaluated using a non-isothermal method. The analysis indicates that the rapid crystallization process can be suitably described by the non-isothermal Avrami equation. The Avrami exponent is found to be equal to 2, which is in agreement with the nucleation and growth mechanism of the rigid rodlike molecules. The kinetics of the slow crystallization process, which were studied using a n isothermal method, indicate that the degree of crystallinity varies linearly with log(time). The equilibrium melting temperature for this process is found to be 372°C. The Arrhenius plot yields a linear relationship in the lower temperature range.

The effect of vitamin D supplementation on peripheral regulatory T cells and β cell function in healthy humans: a randomized controlled trial

“…Figure 4 shows DSC thermograms obtained during heating after the crystallization process, where an endothermic peak corresponding to the crystal–mesophase transition of Vectra can be observed with a maximum at 274 °C and an associated enthalpy of about 2.2 J g −1 , neither of which are significantly affected by the cooling rate. A similar behaviour has been found by Kamal et al 36 for a copolyester with small variations in the composition of HBA (70%) and HNA (30%) at high cooling rates. However, at lower cooling rates, they found an increase in the peak temperature and enthalpy of the endotherm on heating that they explained as a secondary slow crystallization process that occurred in the samples that were allowed to crystallize for longer times.…”

Thermal properties, structure and morphology of PEEK/thermotropic liquid crystalline polymer blends

“…Figure 9 shows the DSC thermograms of the blends obtained during heating at 10 °C min −1 after dynamic crystallization at 10 °C min −1 . The presence of a double endothermic peak of PEEK can be seen over the whole composition range and a broad endotherm of Vectra associated to the crystal‐nematic phase transition33, 36 is also observed, controlled by the cooling rate as previously noted. The thermal parameters obtained from the heating scans as a function of the cooling rate and composition are reported in Table 5.…”

Melting Behavior of a Wholly Aromatic Liquid Crystalline Random Copolyester

“…[4,8,12,15] From the data reported in the literature and in this work, it is obvious that the fast crystallization process observed upon rapid cooling bears a close relationship to Wunderlich's "cold crystallization". [31] It corresponds to fast lateral crystallization in relatively ordered domains of the supercooled nematic melt and occurs by matching short identical sequences between adjacent chains.…”

“…In the latter case, we have calculated the area under the endotherm II by substracting the area under the four peaks (estimated to be 5.8 kJ N kg -1 ) from the total area. As observed for HBA/HNA copolymers [4,12,15] both the endotherms I and II are characterized by a linear growth of the magnitude of the heat of transition with the logarithm of the crystallization time (Figure 13). However, as observed for the temperature of transition, the increase in the heat of transition of the crystals formed via the process II is again more rapid than that of the crystals formed via the process I.…”