Tailoring nanospace

Low free volume liquid crystalline barrier polymers are compared and contrasted with ultrahigh free volume membrane polymers. The free volume cavities and hence the transport properties of the liquid crystalline polymers, based on p-hydroxybenzoic acid, isophthalic acid and hydroquinone in the ratio 40:30:30 mol% (HIQ-40), are tuned by thermal treatment. The mean size of the free volume cavities, or the average dimension of nanospace, in these polymers can be varied from 0.46 to 0.53 nm resulting in a systematic change in permeability dependent on penetrant size. In ultrahigh free volume poly(1-trimethylsilyl-1-propyne) PTMSP, the mean size of the large free volume cavities is varied from 1.40 to 1.44 nm via the addition of silica nanoparticles in order to alter the chain packing. This increase in the free volume cavity size results in a systematic increase in permeability. Remarkably, at the mean cavity size of 1.42 nm in PTMSP there is a crossover in transport mechanism from solution-diffusion to Knudsen transport, resulting in H₂/CH ₄ selectivity going from <1 to >1.