Multiple melting endotherms

The melting behavior of binary crystallizable blends often reveals multiple melting endotherms, that can be ascribed to recrystallization, secondary crystallization effects, phase separation, etc. 

It should be mentioned that several homopolymers (of which polyethylene is probably the best known sample) also exhibit a complex melting behavior. Branched polyethylene s (LDPE, LLDPE and VLDPE) show multiple melting endotherms, due to the presence of fractions with different branching content [Schouterden et al, 1985 Defoor et al, 1993]. This was clearly illustrated by Defoor et al. who fractionated LLDPE with respect to the short-chain branching content and blended the fractions with the highest and the lowest branching content. It was shown that they both crystallized and melted separately. Both fractions determined the sphemlitic morphology in a co-operative way. 

Saiani, A., Daunch, W.A., Verbeke, H., Leenslag, J.-W., and Higgins, J.S. (2001) Origin of multiple melting endotherms in a high hard block content polyurethane. 1. Thermodynamic investigation. Macromolecules, 34, 9059-9068. 

Examination on a hot-stage polarized microscope, as well as SALS and DSC data let us to the conclusion, that all polymers derived from the mixture of DHBP-A and DHBP with suberyl dichloride (Series I) or sebacyl dichloride (Series II) behave as thermotropic liquid crystals. For most of them DSC scans revealed two endo-therms, which are believed to correspond to solid-to-liquid crystalline (Tm) and mesophasic-to-isotropic (Ti) transitions. In few cases, i.e. for polycimidoester derived from DHBP-A with sebacyl dichloride and the copoly-amidoesters of Series II in which DHBP-A content exceeded 40 mol%, the multiple melting endotherms were observed. Though the similar phenomena were demonstrated by several authors (11,12) their origin is still not clear. 

Schematic representation of DSC curves in the region of a phase transition and the characteristic temperatures. (A) Melting endotherm. (B) Melting endotherm with multiple peaks. (C) Crystallization exotherm with multiple peaks...

At lower HS percentages (26-33%), a broad hard domain transition was observed. Increasing the HS content from 33% to 43% produced a multiple melting endotherm. A further increase in the HS composition (47%) was proved to lead to HS melting transitions... 

M. E. Nichols and R. E. Robertson, The origin of multiple melting endotherms in the thermal analysis of polymers, J. Polym. Sci., Polym. Phys. Ed.,30 305-307,1992. M. Yokouchi, Y. Sakakibara, Y. Chatani, T. Tadokoro, T. Tanaka and K. Yoda, Structures of two crystalline forms of polyfbutylene terephthalate) and reversible transition between them by mechanical deformation. Macromolecules, 9 266, 1976. 

Supaphol et al. [216-218] reviewed a number of hypotheses that have been proposed to explain the occurrence of multiple melting endotherm phenomena. In the studies on isothermal crystallization under quiescent conditions (i.e., crystallization is only a function of temperature), the multiple melting behavior of these semicrystalline polymers have been attributed to the following four phenomena (1) the presence of two (or more) crystal modifications in the sample (2) the presence of two (or more) crystalline morphologies (3) the presence of two populations of crystal lamellae of different thicknesses and (4) the simultaneous melting, recrystallization, and re-melting of the lamellae initially formed at the crystallization conditions. 

Figure 15 Typical DSC scan of PET presenting multiple melting behavior (endotherms l-lll).

Two main mechanisms have been commonly debated and proposed to explain the phenomenon of multiple melting endotherms dual/multiple modifications and reorganization via melting/recrystallization/remelting [76]. 

DSC measurements for these copolymers showed multiple melting endotherms and a broad composition distribution. A... 

Other effects of heating rate on the melting process include multiple melting endotherms of homopolymers. 

Zhang R, Luo X and Ma D (1995) Multiple melting endotherms from ethylene terephthalate-caprolactone copolyesters, PoZj/mer 36 4361-4364. 

No.7, 2000, p.560-6 CALORIMETRIC AND INFRARED SPECTROSCOPIC ANALYSIS OF MULTIPLE MELTING ENDOTHERMS OF POLY(ETHYLENE TEREPHTHALATE)... 

Tm determined to be the peak of the second, higher temperature endotherm in DSC cycles with multiple melting peaks. 

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