Role of mobile phases in the crystallization of polyethylene. Part 1. Metastability and lateral growth

Abstract

As part of a more comprehensive investigation of following crystallization of polyethylene isobarically and isothermally at preselected portions of the pressure (P ) and temperature (T ) phase diagram within the P range of 2-5 kbar and supercoolings (AT ) up to 10 "C, the present work is centered on formation, lateral growth (including measurement of growth rates), and melting of crystals. In the course of it the salient observation was made that, within the above specified P and AT range at least, all crystal growth occurs in the hexagonal phase, and only in this phase, irrespective of whether in the hexagonal (h) or orthorhombic (0) stability regime of the P-T phase diagram. In the latter case the h crystals represent a metastable form and transform into the stable o phase at some stage of growth, when, as now observed, all growth stops. The observed lower melting temperature of the h phase in the appropriate portion of the P-T phase diagram ("below" the triplet point) is consistent with all the above and introduces a newly recognized "nongrowth" region in the phase diagram. The lateral growth measurement of h crystal (the only crystals which are seen to grow) is readily interpretable by an activated growth mechanism when referred to the supercooling in the h form (as opposed to the o form, even when the latter is the stable one). Here the barrier attributable to nucleation is largely uneffected by P, but the preexponential (including transport) is retarded by increasing pressure. The implications of these findings for polymer crystallization are discussed, at this stage, in a provisional manner.

Harnessing the Melting Peculiarities of Ultra‐High Molecular Weight Polyethylene Fibers for the Processing of Compacted Fiber Composites

“…[13] There have been reports of a metastable hexagonal phase at relatively low pressures in a very narrow temperaturetime region. In constrained PE samples at atmospheric pressure, metastability was reported at temperatures of about 148-153 8C for only 30 s. [14] Enhanced metastability is only reported at sufficiently high pressures (more than 300 MPa) for extended-chain PE crystals. [15,16] This phase was also observed in cross-linked UHMWPE fiber compacts and UHMWPE fiber compacts that were not crosslinked, both of which showed a mixture of constrained and unconstrained PE.…”

New trends in polymer crystallization studies: Part II ‐ the role of transient mesophases in polymer crystallization

“…In that case the "no growth" Region III could be operative even at atmospheric pressures, and it is achallenging thought how far our present data on Region III (Ref. [7][8] would extrapolate to ambient pressure conditions.…”

Temperature window effect and its application in extrusion of ultrahigh molecular weight polyethylene

“…There are three types of polyethylene crystal phases, i.e., orthorhombic, hexagonal, and monoclinic [30]. The hexagonal crystal phase has a chain-extended structure and a large laminar thickness, which demonstrates a higher chain mobility than in the other two phases [22]. However, the hexagonal crystal phase exists stably only under high temperature (>230°C) and pressure (>360 MPa) during the equilibrium process [30].…”

“…Keller and his associates called this phenomenon 'temperature window effect' and systematically investigated it in a series of papers [11][12][13] and in a review [14]. Keller and coworkers [15][16][17][18][19][20][21][22], Cheng [23] and Somani et al [25] independently or collaboratively studied the mechanisms of unexpected melt flow behavior, and attributed reduced flow resistance to the transient mobile hexagonal mesophase by flow-induced chain alignment critical to strain rate and molecular weight. This hypothesis was confirmed by the observation of the orthorhombic transformation to the hexagonal phase through in situ wide angle X-ray diffraction [25][26][27].…”