Gas separation properties of aromatic polyamides containing hexafluoroisopropylidene groups

The synthesis and gas transport properties of aromatic polyisophthalamides (PIPAs), based on isophthaloyl chloride derivatives bearing pendent groups and hexafluoroisopropylidene (6F) linkages in the main chain, are reported and compared with properties of a similar series of PIPAs containing sulfonyl (SO₂) rather than 6F in the main chain. All of those polymers exhibit high glass transitions temperatures. The polymers containing 6F groups were markedly more permeable and somewhat less selective than their sulfonyl analogs. Polymers containing a t-butyl pendent group at the 5 position of the isophthaloyl linkage were much more permeable than those bearing only a hydrogen atom at this position, although a strong decrease in permselectivity accompanied the large increase in permeability. CO₂/CH₄ solubility selectivity values of the 6F-containing polymers were similar to values reported for other polymetric and non-polymeric organic materials with similar concentrations of polar carbonyl linkages. In contrast, the CO₂/CH₄ solubility selectivity in SO₂-containing variants of these polymers was substantially lower than expected based on total polar group concentration. The low CO₂/CH₄ solubility selectivity is believed to be related to the extremely efficient chain packing in the SO₂-containing polymers, which may lead to strong amide-amide linkage interaction, thereby inhibiting carbonyl groups in the amide linkage from interactions with CO₂ molecules to increase CO₂/CH₄ solubility selectivity.