PEG Containing Thiol-Ene Network Membranes: A Model System to Study the Effect of Cross-link Density on Free Volume and Gas Diffusivity

摘要

Poly(ethylene glycol) (PEG) based networks were prepared using a trithiol cross-linker and a PEG diene, and gradually replacing trithiol cross-linker with a PEG dithiol, while maintaining 1∶1 thiol:ene stoichiometry, to obtain a family of networks of decreasing cross-link densities. An ideal, lattice-like, network structure was assumed to calculate the average molecular weight between crosslinks Mc (inverse of crosslink density), based on monomer stoichiometry. Change in specific volume of the networks with temperature and Mc was studied using pressure-volume-temperature (PVT) dilatometry. Simha-Somcynsky analysis was applied on the specific volume data to obtain the occupied volume and the free volume of the networks. Change in occupied volume and free volume with temperature and Mc was studied. Change in free volume with temperature and Mc was also studied using Positron Annihilation Lifetime Spectroscopy (PALS). Free volume calculated using different methods were modeled as a function of Me and showed a good fit to the Fox-Loshaek equation, showing that a model system was developed to study the effect of crosslink density on free volume. Change in gas permeability, diffusivity and solubility of different light gases with Mc, of the networks, was studied. Diffusivity of different gases were modeled as function of free volume and showed a good fit to the Cohen-Turnbull relation. Thus combining the Cohen-Turnbull relation and Fox-Loshaek relation, it was shown that this is also a model system to study the effect of crosslink density on gas diffusivity.