Memory effect of low-density polyethylene crystallized in a stepwise manner

Abstract

Stepwise crystallization and successive melting of Tipolen low density polyethylene was studied by DSC. As a result of the stepwise crystallization of the polyethylene, there were as many peaks on the melting curve as the number of steps used for the crystallization of the sample. Similar results were obtained on Tipolen lowdensity polyethylene fractionated by gel permeation chromatography, and on Celene low-density polyethylene, too.Crystallization and melting processes of polymers show many features different from those of materials of low molecular weight [1,2]. It is well known that the crystallinity of polymers depends on the temperature, and their melting processes take place in wide temperature intervals. Experimentally determined melting processes and melting point of polymers depend on conditions of previous crystallization. These features are attributed to the macro-and polymolecular nature of they polymers, to the significant role of kinetic factors in their crystallization process, as well as to morphological characteristics of the polymer crystals [3].It is assumed that a great number of crystallites with different melting points are formed during the crystallization process. Richardson et al. [4] studied in detail melting characteristics of polymethylene copolymers containing short branches. The samples in their investigation were made by stepwise crystallization. In order to be able to reach thermodynamic equilibrium conditions, the samples were held at each temperature for a long time (some days). Mentioned authors have established that the melting point is decreased and the melting interval is increased as a consequence of introducing comonomer units. This paper deals with the influence of the crystallization conditions and thermal prehistory of polymers on their melting. It is experimentally proved that the separation of crystals with different melting points is possible in their melting processes. ExperimentalIn most of our experiments non-fractionated Tipolen FA 2210 low density polyethylene (produced by Chemical Works of Leninvfiros, Hungary) was used (m. p. 388 K; CHa/1000 C = 15.3; M w = 148 000). Some experiments were made using fractions of Tipolen PE produced by gel permeation chromatography

Multiple shape-memory behavior and thermal-mechanical properties of peroxide cross-linked blends of linear and short-chain branched polyethylenes

“…By means of blending of nearly linear high density polyethylene (HDPE) and EOCs with different 1-octene content and consequently different degree of branching the materials containing several crystalline phases with different T m.i and T res.i can be created. The multiple crystal populations can be created in polymer owing to multistep crystallization as it was shown by Varga et al [14] for longchain branched low-density polyethylene. The significant difference between the systems presented in this work and the network systems described by Bellin et al [11,12] are firstly the relatively chemical homogeneity of the links between network nodes which are predominantly ethylene sequences by reason that tertiary carbon atoms connected with branches are extra susceptible by peroxidic cross-linking [15] and secondly the low costs of polyethylenes and their processing.…”

Direct correlation between modulus and the crystalline structure in isotactic polypropylene

“…Of course the holding time should be long enough, because the development of crystalline phase at high temperatures is slow. Based on this idea the crystalline structure of the polymer can be fractioned by using appropriate temperature program [29,30]. We have to notice here that the accurate experimental condition would be if the sample could be held for endless time at higher temperatures.…”

Chain regularity of isotactic polypropylene determined by different thermal fractionation methods

“…The history and literature background of thermal fractionation of polymers is collected thoroughly in the review of Müller and Arnal [3]. The pioneering works in this area were done by Grey et al [4] and Varga et al [1,5] on polyethylene. According to these early works, two classes of thermal fractionation methods can be distinguished.…”

“…The possible number of fractions must be defined first to partition the melting curve into sections. It was demonstrated during the pioneering works [1,5], that the number of fractions formed during stepwise crystallization equals the number of steps plus one. Therefore, nine sections should be defined in the present methods (Figure 10a).…”