The morphology characterisation and performance of dense PPO membranes for gas separation

Dense membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) using different solvents, that is, carbon disulphide, benzene, 1,1,2-trichloroethylene (TCE), toluene, chlorobenzene and bromobenzene. The membranes were characterised by atomic force microscope (AFM), electron spin resonance (ESR) and Raman scattering. The permeation data for N-2, O-2, CO2 and CH4 was taken by using a constant volume system. It was observed that the PPO membrane prepared by using carbon disulphide as a solvent (PPO-CS2) had a entirely different surface morphology when compared with the surface of the membranes prepared by using other solvents. The PPO-CS2 membrane's surface layer contained super nodular aggregates of size 1.199 mu m in diameter. These super nodular aggregates consisted of nodules of size about 43.0 nm. In contrast no super nodular aggregates were observed in the other membranes. Interestingly, both selectivity and permeability of the PPO-CS2 membrane were far superior to the other membranes. The physical properties, boiling point (BP), surface tension and viscosity of solvents used for the preparation, were correlated to the data from membrane characterisation as well as permeation experiments. Permeability increased and the selectivity (O-2/N-2 and CO2/CH4) decreased with an increase in the BP. A similar trend was observed for other properties. Regarding the roughness parameter of the membranes, it depended also on the physical properties of the solvents used. Generally, it increased with an increase in the BP of the solvents used. A similar trend was observed for other physical properties. The number of spins/g was less in the PPO-CS2 and PPO-TCE membranes compared with benzene or its derivatives. It was observed that in the presence of CH4 and N-2 the number of spins/g was always higher than CO2 and O-2 respectively.