Characteristic features of isothermal growth, thickening, and melting behavior of melt grown single crystals of low molecular weight poly (ethylene oxide) fractions are presented and discussed in terms of their relevance to basic concepts of polymer crystal growth. Measurements of growth (G) and thickening rates of once folded chain crystals were extended (up to the melting point) into a temperature range where usually only extended chain crystals grow. Monolayer extended and folded chain crystals melt by lateral shrinkage, the rate of which (G−) depends not only on temperature, but also on thermal history. G− increases exponentially with the degree of superheating, and the value of the temperature coefficient d log G−/dT is proportional to the lamellar thickness. In the narrow temperature interval where transition from once folded to extended chain growth occurs, the crystal morphology displays some spectacular features. These may be accounted for in terms of the crystal habit and the rate of completion, g. of a molecular layer in the particular (hk0) prism face considered. Morphological analysis shows that g is of the same order of magnitude as G(hkO), leading to the conclusion that the growth of these PEO crystals is controlled by multiple surface nucleation rather than by deposition of a single surface nucleus.
Characteristic features of isothermal growth, thickening, and melting of melt‐grown single crystals of poly(ethylene oxide) fractions, as reported in a recent paper, are complemented and further discussed. A newly discovered phenomenon–isothermal superheating–is described and its implications are discussed. Growth and thickening rate data are analyzed in terms of the two independent molecular parameters which determine the lamellar thickness: the number, n, of folds per molecule and the total chain length, λ Current formulations of kinetic theories are critically analyzed in view of their applicability to the present growth rate data. The analysis reveals that, for the low‐molecular‐weight fractions, the theory only applies to fast growth obtained at high undercooling. At low undercooling, where molecules deposit in extended or n‐times folded‐chain conformation, however, growth proceeds by a nucleation mechanism which appears to be quite different from that implied by the theory.
Bicomponent fibers were melt spun to investigate the effect of individual component viscosity on the cross section morphology. Two couples were tested, polypropylene MFI 18 (PP 18 ) with polyamide 6 (PA6) and, polypropylene MFI 25 (PP 25 ) with polyamide 6 (PA6). Viscosity versus shear rates was evaluated by both rotational and capillary rheometers to cover a large range of shear rates. Higher viscosity of PA6 compared to PP 25 and PP 18 at the theoretical shear rate resulted in the encapsulation of PA6 by PP 25 and PP 18 at the periphery of the bicomponent fiber cross section. Interfacial instability from the center to the periphery fiber was explained as a result of non-uniformity of shear rates across the fiber cross section. Interface shape dependency on the viscosity ratio of the two polymers has been clearly demonstrated: lower viscosity component wrapping around the higher viscosity one whatever the initial configuration, that is, side-by-side or segmented pie structure.
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