Investigations on the crystallization of polyethylene under high pressure: Role of mobile phases, lamellar thickening growth, phase transformations, and morphology

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Abstract
The outlines of a comprehensive study on crystallization of polyethylene under elevated pressure (P) are being reported. Through in situ viewing of formation and growth of the lamellar crystals, complemented by x-ray diffraction information, it could be established that within the range of experiments (P = 1.8–5.5 kbar and up to supercoolings (ΔT) of about 10°C) formation and growth of identifiable single crystals only proceeded in the mobile hexagonal (h) phase, irrespective whether this was the stable or metastable phase in the appropriate portion of the pressure-temperature (T) phase diagram. In the latter case (when h metastable) all growth stopped on transformation into the stable orthorhombic (o) phase. All this applies both to lateral growth and to growth into the thickness direction which proceeded from chain folded to extended chain thickness and to thicknesses considerably beyond. The foregoing identifies a primary thickening growth mechanism which, as here recognized, is distinct from the traditionally considered lamellar thickening of contiguous layers, where such thickening corresponds to rearrangement as opposed to the present primary growth. It follows from these studies that two distinct crystallization mechanisms are operative simultaneously, the so far considered lateral and the newly found thickening growth. Both lateral and (primary) thickening growth could be followed quantitatively, the thickening growth in the rather unique manner of mapping the cross-sectional taper obtained through electron microscopic investigation. The latter gives, for the first time, quantitative information on chain sliding, allowing interpretation of crystal growth by the sliding diffusion theory by one of us (M.H.). Other aspects of the work include extension of phase diagrams to embracing virtual and metastable phase lines which have a previously unsuspected influence on the crystallization. These, together with the preceding recognitions, are opening some new perspectives on crystallization studies, extending as here illustrated by three examples-poly(trichlorofluoroethylene), (l-4-trans-butadiene), and poly(vinyl-idenefluoride/trifluoroethylene)—beyond the specific case of polyethylene. A variety of morphological observations were made throughout of potential consequence for multilayer formation but also raising some still unanswered questions relating to lateral habits.