Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry

Abstract

Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history.

Insights into crystallization and melting of high density polyethylene/graphene nanocomposites studied by fast scanning calorimetry

“…Several melting events for neat HDPE and the graphene nanocomposites were observed, since the melting peaks were broad and overlapping. Multiple endotherm phenomena for HDPE under various crystallization conditions have been observed in the literature [43][44][45]. In particular, G. Amarasinghe et al [44] reported that the process of melting, recrystallization, and remelting occurs in HDPE sample; A. Toda et al [25] reported that the crystals formed at low temperatures are unstable and easily re-organize during heating.…”

Effects of blown film process on initial properties of HPDE geomembranes of different thicknesses

“…In method-1, the test specimens were punched from the GMB sheets as cylinders and were placed in DSC pans without encapsulation as shown in Figure 4a. The formation of double peaks in the first melting curve could be due to: (a) not encapsulating the specimen, or (b) the different physical characteristic of the specimens (i.e., specimen thicknesses perpendicular to pan base and the contact area between the specimen and the pans, Figure 4) that were different for specimens taken from 0.5, 1.0, 1.5, 2.0 and 2.4 mm GMBs, which affected the heat transfer from the specimens to the pans and, in turn, affected the recording of heat flow by DSC sensors (Hill et al 1999;Gabbott 2007), or (c) a different population of lamellae thicknesses (Wunderlich and Czornyj 1977;Wtochowicz and Eder 1984;Cebe and Chung 1990;Amarasinghe et al 2003) in the GMBs having different melting points resulting from different thermal and stress histories experienced by the GMBs of different thickness during processing (despite being made from the same resin and during the same production run as discussed earlier). The latter explanation is supported by several lines of evidence.…”

Numerical simulation of thermo-stimulated depolarization currents in polyethylene films

“…It is still necessary to further develop the understanding of the conduction mechanisms linked with the physical and chemical structure of the polymer. Numerous studies have dealt with the effects of additives and temperature in relation to the electrical properties and morphology of polyethylene [1,2]. The additives have a direct influence on the crystallinity rate, the spherulite diameter, the density and the nature of the traps [3].…”

“…The additives have a direct influence on the crystallinity rate, the spherulite diameter, the density and the nature of the traps [3]. Polyethylene morphology, and particularly the percent crystallinity, is mostly dependent on cooling rate from the melt, as well as time and temperature of annealing [2]. In this paper PEA and TSDC experimental measurements are compared to the numeric simulations obtained from a bipolar model developed for polyethylene [4] with the objective of a better understanding of the effect of trapped space charge on classical TSDC measurements.…”