Substrate Regulation And Its Influence On Composite Membrane Structure And Performance

Substrates with different composition and structure have been used to prepare reverse osmosis membranes and CO2 separation membranes, and the influences of the composition and structure of sustrates on the stucture and performance of the composite membrans have been investigated. Influence of nonsolvent intrusion through the casting film bottom surface on the macrovoid formation during immersion precipitation phase inversion process has been investigated to prepare sbstrates with pressure resistance.Three kinds of commercial substrates with different pore size were selected and the sustrates soaked with surfactant sodium dodecyl sulfate(SDS) solution were used to prepare reverse osmosis membranes. The pore size of the substrates and soaking substrates with SDS solution influenced the skin layer cross-linking extent, skin layer thickness, surface roughness and deformation of the skin layer, and then influencd the performance of the reverse osmosis membranes. The water flux and salt rejection of the reverse osmosis first increased and then decreased with the increase of the substrate pore size. When the pore size of the substrate was relatively small(9.8 nm), the water flux of the reverse osmosis membranes decreased and the salt rejection increased with soaking substrates with SDS solution. When the pore size of the substrate was relatively large(14.2 or 20.3 nm), both the water flux and salt rejection increased with soaking substrates with SDS solution.Emeraldine base polyaniline(PANI) was added to the casting solution to control the composition and structure of the substrates for reverse osmosis membrane preparation. The content of amine group on the substrate surface increased, the pore size decreased and the porosity increased with the increase of the PANI content in the casting solution. Water flux increased with the increase of PANI content in the casting solution, which indicated that the resistance of the substrates decreased. The cross-linking extent of the reverse osmosis membrane skin layer and surface roughness first increased and then decreased, and the skin layer thickness first decreased and then increased as a result of the combined effect of the amine group and pore size. The resistance of the skin layer to deformation increased with the decrease of pore size. The combined effect of the cross-linking extent of skin layer, skin layer thickness, surface roughness and the substrate resistance led to the result that the water flux first increased and then decreased with the increase of PANI content in the casting solution for preparing substrates. The combined effect of the cross-linking extent and resistance of the skin layer to deformation led to the result that the salt rejection of the reverse osmosis membranes first incresed and then decreased with the increase of PANI content in the casting solution for preparing substrates.Polyvinylpyrrolidone(PVP) was added to the casting solution to control the composition and structure of the substrates and the CO2 separation membranes were prepared. The content of carbonyl group increased and the pore size, pore density and surface porosity first increased and then decreased with the increase of PVP content in the casting solution. Hydrogen bond cross-linking formed between carbonyl of PVP and amine of skin layer material polyvinylamine(PVAm), which resulted in a more tight state of molecular aggregates on the interface of substrate and skin layer, and thus resulted in lower permeability and higher selectivity. The extent of skin layer intrusion into the substrate pores decreased with the decrease of pore size, which was beneficial for permeability enhancement. The diffusion path of the gases on the interface of substrate and skin layer was shorter when the substrate with larger surface pore density and porosity was used, which was beneficial for the permeability of the composite membrane. The combined effect of hydrogen bond, surface pore density and porosity led to the result that CO2 permeance and CO2/N2 selectivity of the composite membranes first increased and then decreased with the increase of PVP content in the casting solution of substrates. In addtion, the permeance of the composite membrane increased with the pretreatmente of soaking substrates with ethanol solutionThe influence of the nonsolvent intrusion through the casting film bottom surface on macrovoid formation was investigated. When the casting film supported by glass plate was inmmersed into coagulation bath, the edge of the casting film separated from the glass plate earlier than the center area. The nonsolvent flowed into the gap between the casting film bottom surface and the glass plate and intrued into the casting film bottom surface, which resulted in the formation of macrovoids. The mass transfer of solvent and nonsolvent preceeded through the gap of the non-woven fabrics, resulting in the formation of macrovoids in non-woven fabrics supported membrans. Macrovoids could be surpressed and substrates with pressure resistance could be obtained by adding solvent into coagulation bath.