Polyethylene crystallization kinetics data

Evidence for this behaviour has indeed been found [98] for (100) faces of polyethylene. It is rather indirect, however, as it draws on a combination of kinetic data from single crystals and twins, so that the results must be treated with great caution, as noted in Sect. 3.4.3. 

Fig. 5. A graph showing the variation of the fold length (1) with supercooling (AT) for polyethylene crystallized from a variety of solvents and from the melt. In the case of solvent crystallization, supercooling is taken with respect to the so-called equilibrium dissolution temperature. For the melt-crystallized data set the equilibrium melting temperature is used. The remarkable coincidence between the curves, despite the wide range of absolute temperatures to which each supercooling corresponds, is strong evidence in favor of the kinetic origin of crystal thickness selection. Solvents xylene, hexyl acetate, 0 ethyl esters, O dodecanol, V dodecane, A octane, x tetradecanol, + hexadecane, melt crystallized. Reprinted from Ref. 44. Copsright (1985), with permission from Kluwer Academic Publishers.

Figure 11.20 shows crystallization rate data as a function of the crystallization temperature for both polyethylene copolymers and for the polyethylene component within the blends. The linear low-density-type PE-1 crystallizes at much lower supercoolings than the very low-density-type PE-3. In the case of the blends, a nucleation effect caused by the previously crystallized polyamide phases was reported (the PA phases crystallize at higher temperatures than the PE phases see Ref [69]). This nucleation effect accelerates the overall crystallization kinetics and therefore the polyethylene component in the blends crystallizes faster than the corresponding neat polyethylene material, as shown in Eigure 11.20. 

Since the solid-state structure formed in block copolymers with homogeneous melts is driven by crystallization, the kinetics of lamellar-scale structure formation might be expected to parallel those of crystallization at the unit-cell level. In the same poly(ethylene-6-(ethylene- / -propylene)) diblock copolymer system, the time evolution of the copolymer crystallinity calculated based on the observed SAXS peaks, as illustrated in Figure 11.16, overlaps with that calculated based on the WAXS data. Since SAXS measures the development of diblock copolymer microstructure on the tens-of-nanometers scale, while WAXS measures polyethylene crystallization on the angstrom scale, the observation that the SAXS data track the WAXS data indicates that the formation of the lamellar microstructure in these diblock copolymers is indeed driven by crystallization, rather than by microphase separation between chemically incompatible blocks [115]. 

Again, the influences of nucleants or additives on crystallization behavior have made considerable inroads in the scientific and patent literature (39), but the importance of shear rates (40,41) on crystallization kinetics is beginning to receive more attention in the literature (42). In the field of single crystal growth many new results have been added to older kinetic data (43,44) tabulated in previous editions of Polymer Handbook . The most recent are concerned with the crystallization of oligomers of polyethylene, polyethylene oxide and so on. Many aspects of crystallization are reported in some recent reviews (45,45a,45b) and later in this article. 

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