Polyethylene crystal lamella

According to the results of Figs. 4 and 7, dQ2o of crystalline polyethylene and of mica are very proximate. When the polyethylene crystal lamella grows on the mica (001) surface, the polyethylene and mica lattices are more proximate. Therefore moire fringes in the larger area can appear not only between do2o and but... 

In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

Abstract The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species. 

The existence of crystal lamellae in melt-crystallised polyethylene was independently shown by Fischer [28] and Kobayashi [39]. They observed stacks of almost parallel crystal lamellae with amorphous material sandwiched between adjacent crystals. At the time, another structure was well known, the spherulite (from Greek meaning small sphere ). Spherulites are readily observed by polarised light microscopy and they were first recognised for polymers in the study of Bunn and Alcock [40] on branched polyethylene. They found that the polyethylene spherulites had a lower refractive index along the spherulite radius than along the tangential direction. Polyethylene also shows other superstructures, e.g. structures which lack the full spherical symmetry referred to as axialites, a term coined by Basset et al. [41]. 

Very high molar mass polyethylene (M>2,000,000 g mol-1) crystallises without the, formation of a clear superstructure, sometimes referred to as the random lamellar structure [116]. The great many chain entanglements present in high molar mass polymers obstruct crystallisation and the crystals become small and their orientation less correlated with surrounding crystal lamellae. 

Fig. 21 Transmission electron micrograph of permanganic-etched linear polyethylene (Mn= 17,000 g mol-1, Mw=54,000 g mor1) crystallised at 130 °C for 13 h showing a lenticular crystal lamella. Courtesy of D.C. Bassett. From [54] with permission from Elsevier, UK...

The results show that above 240 °C the recrystallization occurs after complete melting of the crystal lamellae as in the case of polyethylene with rapid heating. In addition, some lamelae melt and do not recrystallize at the high temperature they crystallize however after cooling down to 120 °C with the same thickness as they had before heating. As long as the material is heated up only to temperatures below... 

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