On the time for fold surfaces to order during the crystallization of polyethylene from the melt and its dependence on molecular parameters

New experiments underpin the interpretation of the basic division in crystallization behaviour of polyethylene in terms of whether or not there is time for the fold surface to order before the next molecular layer is added at the growth front. For typical growth rates, in Regime 11, polyethylene lamellae form with disordered {001} fold surfaces then transform, with lamellar thickening and twisting, towards the more-ordered condition found for slower crystallization in Regime 1, in which lamellae form with and retain {201} fold surfaces. Several linear and linear-low-density polyethylenes have been used to show that, for the same polymer crystallized alone or in a blend, the growth rate at which the change in initial lamellar condition occurs is reasonably constant thereby supporting the concept of a specific time for surfaces to attain the ordered {201}) state. This specific time, in the range from milliseconds to seconds, increases with molecular length, and in linear-low-density polymer, for higher branch contents. (c) 2006 Elsevier Ltd. All rights reserved.

The structure of crystallisable copolymers of L-lactide, epsilon-caprolactone and glycolide

We apply a new X-ray scattering approach to the study of melt-spun filaments of tri-block and random terpolymers prepared from lactide, caprolactone and glycolide. Both terpolymers contain random sequences, in both cases the overall fraction of lactide units is similar to 0.7 and C-13 and H-1 NMR shows the lactide sequence length to be similar to 9-10. A novel representation of the X-ray fibre pattern as series of spherical harmonic functions considerably facilitates the comparison of the scattering from the minority crystalline phase with hot drawn fibres prepared from the poly(L-lactide) homopolymer. Although the fibres exhibit rather disordered structures we show that the crystal structure is equivalent to that displayed by poly(L-lactide) for both the block and random terpolymers. There are variations in the development of a two-phase structure which reflect the differences in the chain architectures. There is evidence that the random terpolymer includes non-lactide units in to the crystal interfaces to achieve a well defined two-phase structure. (c) 2005 Published by Elsevier Ltd.

Linear nucleation of polymers

The use of high-melting fibres as linear nuclei for quiescent polymeric melts is instrumental in providing the superior mechanical properties of polymeric self-composites. It also has inherent advantages in the elucidation of fundamental aspects of polymeric crystallization and self-organization, not least in allowing systematic microscopic studies of polymeric crystallization from nucleation through to the growth interface. This has demonstrated explicitly that lamellae develop in two distinct ways, for slower and faster growth, depending on whether fold packing has or has not time to order before the next molecular layer is added with only the former leading to banded growth in linear polyethylene. Other gains in understanding concern cellulation and morphological instability, internuclear interference, isothermal lamellar thickening and banded growth being a consequence of the partial relief of initial surface stress. (c) 2006 Elsevier Ltd. All rights reserved.