The influence has been studied of systematic alterations in the amount of cross linking in elastomers upon diffusion coefficients and permeability constants in elastomers of homologous paraffin hydrocarbons. In a number of such media the influence of chain length of the paraffins upon the transport phenomena has also been investigated, using CH4, C2H6, C2H4, C3H8, and n-C4H10.
Apparent activation energies for permeation (Ep) and for diffusion (ED) for degrees of vulcanization up to 11.3% combined sulfur are in the decreasing order:
For low vulcanizates, EP is not very sensitive to the per cent sulfur. However for higher percentages of sulfur, EP tends to rise, especially when the migrating molecule is a large one. ED is rather more sensitive both to molecular size and to per cent combined sulfur.
Both EP and ED for natural rubber vulcanizates show definite temperature coefficients, manifested as curvature when log10D or log 10P is plotted against 1/T. Such curvature was absent, however, in butyl rubber.
The results have been interpreted in terms of the molecular nature of elastic networks and in terms of the kinetic-statistical (zone) theory and the transitionstate theory. All treatments indicate the need for a considerable zone of activation for each unit diffusion process, confirming an earlier investigation. When, in the Arrhenius equation, D = D0e, one plots log10D0 against ED/T, the clear functional relationship:
emerges. A simple interpretation of this and of similar functional relationships for other “activated” physical and chemical rate processes has been given.
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