Free Volume, Transport, and Physical Properties of n-Alkyl Derivatized Thiol-Ene Networks: Chain Length Effect

The free volume, transport, and physical properties of a series of n-alkyl derivatized thiol-ene networks are reported. Derivatized thiol monomers were prepared via a nucleophile-catalyzed thio-Michael addition reaction of multifunctional thiols to n-alkyl acrylates ranging from n = 1 to n = 16 in length. Using UV-initiated photopolymerization, cross-linked networks were fabricated from these systematically modified thiol monomers. Each network consisted of the same molar concentration of alkyl chains. Both the thio-Michael reactions and the network photopolymerizations reached high conversion regardless of n-alkyl length, which is typical of the thiol-ene click reaction. The incorporation of alkyl chains led to the formation of new networks with markedly loose packing as probed by density measurements. The density decreased by 11% as n increased from 1 to 16. It is believed that the alkyl chains acted as spacers or pillars which expanded the cross-linked network scaffold increasing the free volume. The free volume behavior of these expanded networks was probed by positron annihilation lifetime spectroscopy (PALS). The hole volume size as measured by PALS doubled with an increase in alkyl chain length from 1 to 16. Oxygen transport measurements indicated an exponential increase in oxygen permeability across 2 orders of magnitude which was related to the increase in free volume. Glass transition temperatures were interestingly comparable for all derivatized networks regardless of n-alkyl length. Water contact angle was additionally evaluated for these derivatized networks. As expected, contact angle increased with increasing n-alkyl length, demonstrating that the surfaces were altered (becoming more hydrophobic) due to an increased concentration of methylene groups in the bulk.