Spatial nonuniformity of cross-linking in crystalline alkanes of different chain length

Regarding their irradiation behaviour, crystalline n-alkanes can be devided into three groups according to their chain length: (i) very long alkanes (polyethylene, degraded polyethylene of approximately 100 carbon atoms, alkane C₉₄), (ii) moderately long alkanes (C₄₀, C₂₈), and (iii) shorter alkanes (C₂₃). Within the crystal lattice of very long alkanes (group i) there are no large-scale heterogeneities in the distribution of cross-links; extensive distortion of the lattice is thus gradually introduced at high irradiation doses. In contrast, in crystals of moderately long alkanes (group ii) cross-links form almost entirely in the fully amorphous domains which appear in early stages of irradiation. In order to explain this non-uniformity a novel migration mechanism of cross-link precursors over micron distances is invoked. Going on to still shorter alkanes (iii) the evidence for large-scale heterogeneity of cross-link formation is again absent, at least in the case of high dose-rate irradiation (>0.3 Mrad s). This is due to the tendency of shorter alkane crystals towards a radiation-induced phase transition. In the resulting plastic crystal modification the newly postulated migration mechanism appears to be ineffective. Some possible factors influencing the operativity of this mechanism in hydrocarbon radiolysis are suggested.