Radiation Effects and Crystallinity in Polyethylene and Paraffins
A survey is presented of a series of studies on the influence of crystallinity on the radiation-induced effects, cross-linking in particular, in polyethylene and paraffins. The principal theme is that the usual conception of random introduction of cross-links into a random assembly of chains needs to be modified in the presence of crystallinity in general and chain folding in particular. A long series of varied investigations on polyethylene have indeed demonstrated through a series of conspicuous effects that not only the ordering intrinsic to crystals and the increased intrachain contacts due to chain folding, but the higher level morphology, the nature and mutual arrangement of lamellae in particular, have a major influence on the effectivity of the radiation leading to networks. Extension of the studies to paraffins identified unsuspected mobility of both the radiation precursor species and the paraffin molecule itself (cross-linked and uncross-linked) within the crystal lattice, leading to phase segregation of the cross-linked species into microscopically identifiable domains together with identifying a trend for the cross-links themselves to form non-randomly in groups. The latter phenomenon, observed also in the molten state, indicates that the departure from randomness in the cross-linking process is much more deep-rooted than originally anticipated, and calls for a general reassessment of our knowledge of cross-linking.
Other topics included as part of the general enquiry are the destruction of crystallinity and radiation effects beyond it as influencing electron microscopy, the promotion of hexagonal phase through radiation, the effect of morphology on chain scission and the general still unsolved issue of how to assess cross-links by a direct analytical method (involving NMR).
The hope is expressed that the bringing together of these varied pieces of work will serve the unification of presently widely diverse areas of experience and might influence developments in the radiation studies of paraffinoid substances.
KeywordsEntropy Migration Manifold Enthalpy Foam
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