Sorption, diffusion, and perm-selectivity of toluene vapor/nitrogen mixtures through polydimethylsiloxane membranes with two cross-linker densities

The sorption, diffusion, and permeation properties of toluene and nitrogen from binary gaseous mixtures were studied using polydimethylsiloxane (PDMS) membranes with various cross-linker levels. The prepared PDMS membranes were characterized using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and positron annihilation lifetime spectroscopy (PALS). The PDMS made with 10:1 pre-polymer/cross-linker ratio contained a negligible amount of unreacted cross-linker. The cross-linking reaction for curing PDMS with 10:2.5 pre-polymer/cross-linker ratio, however, only consumed 60% of the cross-linker and the actual cross-linker density was 15% (pre-polymer/cross-linker ratio). The resulting 10:1 membrane has a cross-linked network of dominant high-molecular-weight polymer chains linked by shorter cross-linker components. Although the PDMS made with 10:2.5 pre-polymer/cross-linker ratio has a higher cross-linking density and possibly a smaller fractional free volume, the excessive embedded short-chain cross-linker served as a plasticizer and counter-balanced the depressed free volume. The toluene sorption isotherms were similar for these two membranes and could be predicted using the Flory–Huggins model. The toluene diffusion coefficients were correlated with its concentration using the Long's equation for both PDMS samples. The toluene vapor permeability in the PDMS membrane made with a pre-polymer/cross-linker ratio of 10:1 was slightly higher than that of the film with a 10:2.5 ratio. However, the nitrogen permeability from gaseous mixtures was significantly lower in the sample made with more cross-linker (pre-polymer/cross-linker ratio of 10:2.5). The nitrogen permeabilities depended on the toluene activity, decreasing to several tens of Barrer when the toluene vapor activity increased from 0.2 to 1.0. The decreased nitrogen diffusion coefficient might be ascribed to a reduction in effective fractional free volume due to the free volume occupied by the adsorbed toluene molecules, especially at high toluene activity levels. The suppressed nitrogen permeability contributed to toluene/nitrogen selectivity being higher than the ideal selectivity value in the toluene-enriched systems. When the toluene vapor activity increased from 0.2 to 1.0, the selectivities were increased from 16 to 1480 (95-fold increase) and from 110 to 1170 (10-fold), respectively, using PDMS membranes made with 10:1 and 10:2.5 ratios. The selectivity dependence on vapor activity (or vapor concentration) suggests that the ideal selectivity obtained from single permanent (or saturated vapor) measurements may not truly reflect the separation efficiency in binary vapor/gas systems.