The effect of crosslinking on free volume in rubbery networks can be complex. According to the classical Fox and Loshaek theoretical approach, the fractional free volume in networks ought to change linearly with the concentration of crosslinks or inversely with the molecular weight (Mc) between the crosslinks. However in reality, changing the crosslink concentration often results in simultaneous altering of the polymer polarity which itself affects the free volume of a polymer. It seems that in this work we uniquely managed to overcome this obstacle and by using the advantages of thiol-ene click chemistry designed and synthesized a family of model networks with largely different concentration of crosslinks but very similar polarity. The model polyethylene glycol (PEG) based thiol-ene networks, in which the concentration of crosslinks varied by one order of magnitude, was prepared by gradually decreasing the ratio of PEG containing trifunctional to bifunctional (telechelic) thiol monomers in the presence of unchanged molar amount of PEG containing telechelic ene monomer while maintaining 1:1 thiol:ene stoichiometry. Dielectric spectroscopy studies revealed very similar dielectric permittivity values for the networks at high frequencies which experimentally confirmed that their polarities were indeed very similar. High pressure dilatometry studies were performed to study the effect of temperature and pressure on specific volume of the networks (PVT analysis). Simha-Somcynsky equation of state (EOS) was used to fit the PVT data in order to extract the fractional free volume. Fractional free volume (FFV) of the networks changed linearly with the concentration of crosslinks or with 1/Mc, thus experimentally confirming the Fox and Loshaek model assumption. In addition to FFV assessment, average volume of the molecular holes (vh) in the network polymers was probed by positron annihilation lifetime spectroscopy and vh exhibited a similar linear trend with the concertation of crosslinks or 1/Mc. Gas diffusion was also studied in the network films and showed a vivid dependence with Mc. A free model based model was developed and explained well this behavior.

Free volume and gas transport studied in model elastomeric thiol-ene networks: Effect of crosslink density

255th National Meeting (2018)

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