Polyethylene crystal thickening

Polyethylene crystals thicken at temperatures greater than 110°C. Polymer crystals grown under isothermal conditions thicken gradually with time. This has numerous important consequences. For... 

Polyethylene crystals change shape with time at temperatures between the crystallisation temperature and the final melting point [42]. The process is commonly referred to as crystal thickening because the major effect is that the crystals increase their dimensions along the c axis. These changes may occur without any change in crystal volume, i.e. crystal thickening is associated with a lateral contraction of the crystals (Fig. 9). A second option is that... 

The old work on crystal thickening was confined to medium to high molar mass polyethylenes with molar mass polydispersity, and the results obtained were in some respects different from the more recent findings obtained on strictly monodisperse w-alkanes. 

In Sect. 3 a set of experimental data on cyclic molecules is described supporting the basic discussions of Sects. I and 2. Central are the cycloalkanes that ultimately serve as a model for adjacent reentry, sharply folded polyethylene crystals. Chain-folded polyethylene was shown in the early 1960 s to thicken in the crystalline state by straightening as many as 100 to 1000 folds when brought to elevated pressure and temperature. This surprising observation found its explanation in the fast reptation possible in the condis-crystal state. 

In the following figures, further experiments are linked to the annealing and recrystallization of a number of additional polymers. In Fig. 6.81 the increase of lamellar thickness is illustrated on the example of solution-grown polyethylene. The crystals were collected in dried mats. The graph on the right side allows a comparison with data on melt-crystallized and annealed polyethylene (see also Sect. 5.2). Sufficiently mobile and flexible molecules, such as polyethylene, can thicken by chain... 

Crystal thickening has also been observed on polyoxymethylene (267), in polyethylene oxide (269,270), and polyesters (269). 

Abo el Maaty MI, Bassett DC (2005) Evidence for isothermal lamellar thickening at and behind the growth front as polyethylene crystallizes from the melt. Polymer 46 8682-8688 Abo el Maaty MI, Bassett DC (2006) On the time for fold surfaces to order during the crystallization of polyethylene from the melt and its dependence on molecular parameters. Polymer 47 7469-7476... 

Abo El Maaty MI, Bassett MD (2005) Evidence for isothermal lamellar thickening at and behind the growth front as polyethylene crystallizes from the melt. Polymer 46(20) 8682-8688... 

Keller et al. (1994) proposed a new scheme for polymer crystallization, also most probably applicable to melt-crystallization of polyethylene at normal pressure. They proposed that crystallization starts with the formation of thin hexagonal phase crystals, which in this crystal size range are more stable than the orthorhombic crystals. The high mobility of the chains in the hexagonal phase leads to rapid crystal thickening which proceeds until the orthorhombic phase becomes more stable than the hexagonal phase. 

The isothermal thickening (avoiding very short times) of mature crystals of conventional polyethylenes follows the equation [55] ... 

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