Effect of penetrant size, shape, and chemical nature on its transport through a thermoset adhesive. II. Esters

The transport properties of a series of ester penetrants through a polyamide-based elastomeric adhesive are reported. The results obtained are compared with those from a similar study on a series of n-alkane penetrants reported previously. It was established that the diffusion of the esters, as in the case of n-alkanes, is Fickian and follows a Henryʹs law-type mechanism. Furthermore, it was concluded from a combination of activation energy calculations and molecular simulations that the ester penetrants, similar to n-alkanes, diffuse linearly along their long molecular axes. The chemical composition influence of the penetrants was investigated by means of their Flory–Huggins interaction parameter with the elastomer, χ12. The χ12 values for the esters were found to be lower than those of the corresponding n-alkanes suggesting the role of favorable contributions from the polymer–ester interactions. Differences in the diffusion coefficients between the n-alkane and ester series were explained on the basis of these χ12 values. Finally, correlations between (a) the diffusion parameters, D and Ed, (b) penetrant size (molar volume, Vm) and (c) chemical nature of the penetrant (χ) were established. These correlations account for both penetrant size and chemical effects simultaneously, thereby providing a more general description of the transport properties of the polymer–n-alkane and polymer–ester systems.