Polyethylene, crystallites

Polyethylene, crystallites in, 1215 high-density, 1210 high-molecular-weight, 1210 kinds of, 1210 low-density, 1210 synthesis of, 240-241 ultrahigh-molecular-weight, 1210 uses of, 242... 

The difference between the energy minima in the trans and gauche staggered conformations is labeled Ae in Fig. 4-2. When this energy is less than the thermal energy RT jL provided by collisions of segments, none of the three possible staggered forms will be preferred. If this occurs, the overall conformation of an isolated macromolecule will be a random coil. When Ac > RT/L, there will be a preference for the trans state. We have seen that this is the only form in the polyethylene crystallite. 

On irradiation with the same radiation dose above and below the mdting temperature of polyethylene crystallites [54], a higher gel fraction is formed with samples irradiated above the melting point. 

Figure 1.10 Arrangement of molecules in polyethylene crystallites. (From Fibres from Synthetic Polymers (ed. R. Hill), Elsevier, Amsterdam, 1953)...

As a rule of thumb the crystalline-amorphous blend composition is soft (the behavior is more amorphous than crystalline) when the blend contains more than 70% of the amorphous component, but becomes rigid and crystalline when the weight fraction of the crystalline component exceeds 30% of the blend. Besides composition, aging of the blend also determines the compatibility of the two polymers. For instance, the crystallization rate and induction time for crystallization is critically dependent on the concentration of the components and aging (5 2). Crystalline interactions have been shown to exist when PCL is blended with polyethylene and polypropylene, with a-relaxation in polyethylene being affected in particular. However, because this effect is interrelated to motion in the polyethylene crystallites it was elucidated by assuming that the blend might be cocrystalline in nature. 

Polyethylene crystallites are generally not planar rather, they have a hollow-pyramidal shape [134, 135]. A pleat or a trapezoid-shaped structure is observed in the center of the lamellae. By precipitation of crystals from highly diluted solutions, mats are obtained which are very brittle because of the scarcity of interlamellar connections and high crystallinity. 

Loo Y.-L., Register R. A., and Adamson D. H. (2000b) Direct imaging of polyethylene crystallites within block copolymer microdomains. J. Polym. Sci. B Polym. Phys. 38 2564-2570. 

It does not necessarily follow that the crystalUte thickness can be identified with the length of the completely ordered chain.(16) Details of the structure and morphology of such polyethylene crystallites will be discussed in detail in Volume 3. 

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