Structure transfer between a polymer melt and the solid state. Investigation of the nanostructure evolution in oriented polyethylene by means of continuous X-ray scattering

Abstract

During the melting and crystallization of uniaxially oriented polyethylene small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) patterns are recorded simultaneously and continuously, i.e. with high signal-to-noise ratio and a time resolution of 7 s. The multidimensional chord distribution function (CDF) is computed from each of the SAXS patterns and visualises the nanodomain structure of the material in physical space. Thus without application of a model a detailed and continuous multidimensional stream of data is obtained that reveals the mechanisms which govern melting and crystallization in the material studied. Finally the CDF is semi-quantitatively analysed as a function of the temperature programme.

We find that swarms of small crystalline blocks which are observed during the heating of the injection moulded rods are melting earlier than the extended lamellae. Directly after the last lamellae have vanished we quench to a crystallization temperature. No formation of blocky crystals is observed. Instead, before the beginning of crystallization, a mesophase separation into disentangled and entangled regions is indicated. Moreover, row structures of nuclei are observed coming and going. As long as these rows are oriented in fibre direction, the orientational memory of the melt is not erased. Crystallization starts in disentangled bundles of chain segments at the tips of knots (entanglement strands). Fast and continuous lamellar growth is observed. Most of the lamellae are positioned at random. Correlations between lamellae are limited to twins which are formed when lamellae grow at both ends of the same entanglement strand. Thereafter slow thickness growth of lamellae is observed, and in twins this growth is directed outwardly, away from the entanglement-rich amorphous zone between them. Only during this period the wide-angle X-ray scattering (WAXS) exhibits an increasing orientation of the crystals. At a high undercooling or after there is no space for lamellae any more, secondary crystals (blocks) are formed that are unoriented but placed in such a manner that correlations among them are high both in longitudinal and transversal direction.

Dedicated to Prof. Dr. Wilhelm Ruland on the occasion of his 80th birthday