Polyethylene dimensional changes

Mandelkem L, Roberts DE, Diorio AF, Posner AS (1959) Dimensional changes in system of fibrous macromolecules polyethylene. J Am Chem Soc 81(16) 4148-4157... 

The PE-3 is a very low density material with short linear methylene sequences t. Very low density polyethylenes crystallize with ill-defined morphologies frequently exhibiting fringe-like lamellar aggregates [69,116], They usually display Avrami index values lower than or equal to 2, as shown by PE-3 in Figure 11.21. An inverse dependence of the Avrami index values with could be due to dimensionality changes induced by temperature while the nucleation rate remains constant. 

In situ X-ray examination of crystallizing polyethylene, at high temperature and pressure, then confirmed this proposal in detail, showing that the wide-angle diffraction pattern changed abruptly with the optical texture [ 10]. That corresponding to the spherulitic texture was of the usual orthorhombic form while the new intermediate phase had two-dimensional hexagonal symmetry, with an increased cross-sectional area per chain, but without... 

Fig. 5.16. Imaging polymer morphology by spin-diffusion contrast on a sample of electrically aged polyethylene, (a) Sample for electrical aging in needle-plate geometry and region cut out for spin diffusion imaging with one-dimensional spatial resolution, (b) Spatially resolved distribution of the domain sizes derived from fitting theoretical diffusion curves of a sandwich layer model to the experimental data. Pronounced changes in the thickness for crystalline, interfacial and amorphous layers are obtained [58].

The data strongly suggest that crosslinking reactions are the major contributor to changes of NMR line shapes under these experimental conditions. This appears to be consistent with the finding that crosslinking reactions of peroxide polyethylene at 140 C are known to occur and produce three dimensional networks at approximately the same rate (3-minute reaction time) as observed in the current study. (17)... 

The change in scattering patterns upon deforming polyethylene was observed by Stein and Rhodes. The analysis of these patterns was carried out for two-dimensional spherulites by Gough et a/." and subsequently extended to three dimensions." The theory involves an affine deformation of the spherical spherulite to an ellipsoid of revolution, accompanied by the rotation of the optic axes which are assumed to be initially oriented at an angle Po to the radius, and at a twist angle oj... 

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