Thermodynamic and kinetic thermal analyses on dual crystal forms in polymorphic poly(heptamethylene terephthalate)

Abstract

Thermal analyses were performed for determining the equilibrium melting temperatures T of the respective α‐ and β‐crystal in melt‐crystallized polymorphic poly(heptamethylene terephthalate) (PHepT) using both linear and nonlinear Hoffman‐Weeks (H‐W) methods for comparison of validity. These two crystals in PHepT do not differ much in their melting temperatures. The equilibrium melting temperatures of the α‐ and β‐crystal as determined by the linear H‐W method are 98 °C and 100.1 °C, respectively; but the nonlinear H‐W method yielded higher values for both crystals. The equilibrium melting temperatures of the α‐ and β‐crystal according to the nonlinear H‐W method are 121 °C and 122.5 °C, respectively. Both methods consistently indicate that T of the β‐crystal is only slightly higher than that of the α‐crystal. Such small difference in T between the α‐ and the β‐crystal causes difficulties in judging the relative thermodynamic stability of these two crystals. Thus, kinetics of these two crystals was compared using the Avrami and Ozawa theory. The crystallization produced by quenching from Tmax = 110 °C and 150 °C shows a heterogeneous and homogeneous nucleation mechanism, respectively. The lower Tmax = 110 °C leads to heterogeneous nucleation and only α‐crystal in PHepT, whose crystallization rates at same Tc are much higher than crystallization rates by quenching from Tmax = 150 °C leading to either α‐ or β‐crystal with homogeneous nucleation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1839–1851, 2009

Microscopic Fourier Transform Infrared Characterization on Two Types of Spherulite with Polymorphic Crystals in Poly(heptamethylene terephthalate)

“…There has been speculation that these two types of spherulite, of apparently different lamellar patterns (ringless vs. ring-banded), might be packed with different crystal cells (a vs b). [1][2][3][4] However, direct experimental evidence has yet to be offered. Techniques of Fourier FTIR microspectrometry with in situ temperature variation and IR-peak-mapping capability, and POM characterization were used to study the crystal distribution in dual spherulites in poly(heptamethylene terephthalate).…”

“…[1][2][3][4] Recent studies [3,4] have revealed that solution-grown or electrospun PHepT fibers can take an orthorhombic crystal form, which is related to the a-crystal and the unit-cell parameters are a ¼ 1.409 nm, b ¼ 1.480 nm, c ¼ 3.392 nm, and a ¼ b ¼ g ¼ 908. However, the unit-cell structure of the b-crystals in PHePT has not yet been studied.…”

Determination of the quaternary phase diagram of the water–ethylene glycol–sucrose–NaCl system and a comparison between two theoretical methods for synthetic phase diagrams

“…Accordingly, the direct use of the peak temperatures of the exothermic melting curves of solutions as their melting points may result in a difference as high as several °C, depending on warming rates, from the results measured using other methods [9]. To overcome this disadvantage, a redemption method based on the sample masses can be applied to minimize the influence of the “thermal lag” [10,15,19,21,26]. In this study, combining these two techniques, we determined the quaternary phase diagram of the water-ethylene glycol (EG)-sucrose-NaCl system, which is the first fully developed quaternary phase diagram of multi-solute solutions.…”