Syntheses and evaluation of gas transport properties in polystyrene-POSS membranes

Copolymers based on styrene and a nanostructured polyhedral oligomeric silsesquioxane, PSPOSS, were synthesized and their gas permeability coefficients, P(i), to O2 and N2 were measured at 2.0265 ×105 Pa (2 atm) and 35 °C in a standard permeation cell. The syntheses of the copolymers were carried out using styrene and different loads of the macro initiator POSS-TEMPO which was also synthesized in this work. The gas transport results show that the gas permeability coefficients in the PSPOSS polymers increases with the amount of the POSS-TEMPO used in the synthesis, but interestingly, for relatively low concentrations, i.e. up to 10 wt.%, the selectivity for the gas pair O2/N2 does not show dramatic losses in the selectivity. The P(O2) for pure PS, which is 21.75145 × 10−18 m2 s−1 Pa−1 (2.9 Barrer) with 5.6 O2/N2 selectivity, increases to 37.5025 × 10−18 m2 s−1 Pa−1 (5.0 Barrer) and the selectivity still remains as high as 5.2 for a PSPOSS synthesized with a 10 wt.% load of the macro initiator. When the PSPOSS is synthesized with higher loads of the macro initiator, i.e. 20 wt.%, there are impressive increases in permeability with decreases in the O2/N2 selectivity. After factorization of the permeability coefficients into the diffusivity and solubility factors, it is observed that having POSS units in the copolymer affect the perm-selectivity of the PS membranes. This effect can be attributed to changes in the gas diffusivity rather than changes in the solubility coefficients for PS synthesized with up to 10 wt.% of POSS-TEMPO. Higher loads of the POSS-TEMPO in the PSPOSS membranes increase both the diffusivity and selectivity coefficients.