SORPTION AND TRANSPORT OF CO2 AND CH4 IN CHEMICALLY MODIFIED POLY(PHENYLENE OXIDE)

The sorption and permeability of CO2 and CH4 in poly(phenylene oxide) (PPO) and several of its derivatives are reported. Three modified PPO's were synthesized, containing carboxyl, methyl esterified carboxyl, and bromine substituents, referred to as CPPO, MeCPPO, and BPPO, respectively. The carbonyl-containing derivatives, substituted at the methyl side chain site of the PPO backbone, were chosen to increase the CO2/CH4 solubility selectivity of PPO. The brominated PPO, substituted at the phenylene ring position, was prepared in order to study the effect of substitution position on permselective properties. The modified PPO's were characterized via FTIR, FTNMR, DSC, density, and wide angle X-ray measurements. The differences in permeability and CO2/CH4 selectivity of the modified polymers are explained in terms of the effect of each substituent on the solubility and diffusivity factors which govern the transport of gases in glassy polymers. The solubility selectivity of the carbonyl-containing PPO derivatives increases, with the greatest increase (41%) observed for CPPO with 1.0 carboxyl group per repeat unit. The methyl-substituted polymers are significantly less permeable, but more selective, than PPO. The observed decreases in gas sorption, diffusivity, and permeability in these materials are attributed to decreases in free volume and decreased segmental rotation in the polymer. Simultaneous decreases in glass transition temperature suggest a reduction in sub-T(g) motions at low concentrations of methyl substituents which are analogous to diluent-induced antiplasticization effects. Internal plasticization is observed at higher levels of carboxylation. Brominated PPO shows a 150% increase in CO2 permeability, with no change in CO2/CH4 selectivity. These changes are explained by bromination-induced stiffening of the polymer molecules. An increase in chain stiffness results, but is offset by an increase in the average diffusional jump length of dissolved gas molecules.