Switching on/switching off solubility controlled permeation of hydrocarbons through glassy polynorbornenes by the length of side alkyl groups

A series of vinyl-addition poly(5-alkyl-2-norbornenes) with varied alkyl side group from methyl to tetradecyl was synthesized and systematically studied in terms of pure and mixed gas permeability. It was found that the length of the side group plays a key role in the gas separation performance. For instance, the permeability of permanent gases through these polymers changed non-linearly with the size of alkyl groups and the highest gas permeability was achieved for the polymers with short side-alkyl groups. Interestingly, a traditional trade-off between gas permeability and selectivity was not observed: vinyl-addition polynorbornene with methyl groups displayed both higher CO2 permeability and higher CO2/CH4 selectivity. The solubility-controlled permeability of hydrocarbons is another feature of these polymers, which is unusual for low free-volume and silicon-free glassy polymers. According to the results of the pure gas permeability study, this feature was realized for all investigated polymers with short and long alkyl-side groups. However, the study of mixed gas permeability through these polymers showed that truly solubility-controlled permeability of hydrocarbons appeared only for the polymers bearing alkyl-substituents larger than ethyl-groups and therefore it is possible to switch on/off this feature for vinyl-addition poly(5-alkyl-2-norbornenes) by the choice of the size of alkyl groups. The obtained results are considered and discussed along with WAXD data and free volume fraction in these polymers.