Impact of thermal ageing on sorption and diffusion properties of PTMSP

Gas and vapour permeability in both freshly cast and aged poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes were investigated in terms of solubility and diffusion coefficients for two probe molecules, a permanent gas (nitrogen) and an organic vapour (dichloromethane). To get reliable data for this study, we set up a fast and reproducible ageing procedure consisting of thermal treatment of the polymer films (100 °C during 24 h under vacuum). As expected, measurements recorded from time-lag experiments and isothermal sorption showed strong variations of the PTMSP transport properties before and after the thermal ageing procedure. Freshly cast membranes exhibited high permeability, whereas after ageing a 40–45% decrease of the permeability was recorded for both probes. The results demonstrated that only the glassy physical microstructure of PTMSP was affected by the ageing procedure, while the chemical structure was unchanged. Based on a dual-mode model for sorption and a Long's model for diffusion, the analysis of the data showed that the solubility and diffusion coefficients of the gas and the vapour were not affected in the same way. For nitrogen, only the diffusion coefficient decreased, whereas for dichloromethane, the thermal treatment mainly influenced the sorption coefficient. The lower permeability due to the combination of sorption and diffusion parameters could be attributed to a change of the PTMSP hole geometry or the hole connections.