Effect of radiation on the crystals of polyethylene and paraffins: 3. Irradiation in the electron microscope

Paraffins were irradiated with electrons in the electron microscope. The electron microscopic image and the electron diffraction patterns were followed as a function of dose. The objectives were: (a) to establish a connection between the ‘polyethylene-type’ and ‘paraffin-like’ behaviour identified in Parts 1 and 2, respectively; and (b) to identify the phase segregation, which has emerged in Part 2, by visual means. (a) Increasing chain length, increasing dose rate (at levels such as realisable only with electrons) and decreasing temperature individually and in combination, were found to favour the ‘polyethylene-type’ behaviour (crystal destruction through increasing lattice defects) while the reverse trend of the above three variables favoured the ‘paraffin-like’ behaviour (phase-segregated damaged and undamaged species). (b) Segregated phases could under some circumstances be identified as non-diffracting ‘droplets’ within a crystalline matrix, with the lattice hardly affected, in the electron microscopic image. These droplets remain constant in number but increase in size as the irradiation progresses, the number of droplets depending on the chain length of the paraffin, on the irradiation temperature and on the dose rate. This behaviour, together with some further observations, reveals that the radiation-induced active species do not form crosslinks in situ but migrate over distances which can amount to μm. In contrast to the above, in the case of the lowest paraffin investigated, (C₂₃H₄₈), the lattice became uniformly distorted as judged from the diffraction pattern, but the damage was observed to ‘heal-out’ with time. Considering this evidence, phase segregation may thus occur in two ways: (i) the crosslinks form in the liquid phase subsequent to the nucleation of such a phase; (ii) the crosslinks are produced originally within the lattice but subsequently become excluded from it. Both cases are examples of the extreme inhomogeneity of the lattice damage (also supported on a molecular level, by an analysis of the crosslinked products by g.p.c.^20,21) involving migration of the active precursor species and of the actual crosslinked material respectively. All this has implications for the underlying radiation chemistry, for molecular mobility and compatibility of the systems.