Long-range active site transfer in irradiated hydrocarbon crystals

New evidence is presented in support of the existence of unusually rapid and long-range migration of radiation-produced active species across n-alkane crystals. Amorphous (liquid) droplets, separated by micrometer distances, are seen in thin crystals of n-alkanes of between 27 and 40 carbon atoms when irradiated by an electron microscope beam with doses above ca 100 Mrad and at temperatures within 40°C of the melting point. An almost damaged lattice is maintained in the remaining parts of the crystal. The amorphous domains serve as effective sinks for the active species migrating within their capture radius, thus protecting the surrounding lattice from further damage. The continuing destruction of crystallinity proceeds by spreading of the droplets. The absence of liquid domains in electron-irradiated alkanes shorter than C₂₇ is explained in terms of the intervening rotator phase transition taking preference over local melting in the damaged crystal. Thus, in the low-temperature crystal form, the migration is probably equally efficient in 'short' as it is in 'medium' length alkanes with n ≥ 27. However, the disorder in the rotator phase seems to inhibit the migration. By analogy, the equilibrium orientational disorder in crystals of polyethylene and long alkanes (C₉₄H₁₉₀) at comparatively high temperatures near their melting point, may be responsible for the fact that no evidence of long-range migration is observed in these latter materials. Substituted alkanes, 1-triacontanol and 1-triacontanoic acid, follow a similar pattern as unsubstituted ones, regarding the presence or absence of long-range active site migration. The nature of the active species remains unclear.