Effects of solvent on the pore formation in asymmetric 6FDA-4,4 ' ODA polyimide membrane: terms of thermodynamics, precipitation kinetics, and physical factors

The pore-forming behavior in polyimide membranes was investigated by varyin g the amounts of N,N'-dimethylacetamide (DMAc, wt.% versus water as a nonso lvent) in coagulation baths. The pore-forming behavior was explained by pha se diagrams, coagulation values, diffusion coefficients of nonsolvents, and solubility parameters. The nonsolvent induced phase inversion (NIPI) proce ss prepared the asymmetric polyimide membranes. Increasing the amount of DMAc delayed the demixing phase inversion, increas ed the stability of the polymer ternary system, and reduced the pore specif ic volume of sub-layers and the pore size in the skin layers. The diffusion coefficient, D-e, of the nonsolvents into the PAA solutions were in the ra nge of 0.39-2.97 x 10(-5) cm(2)/s. Adding DMAc in a coagulation bath result ed in a small difference of the solubility parameter, Delta delta(S),(NS), and also making the delayed demixing easier. A less porous structure is for med by strong interactions between solvent and nonsolvent; nonsolvent penet ration is delayed in the polymer solution. The sub-pore shape in polyimide membranes, which were a finger-like structure, did not change as the amount of DMAc increased. Overall, the DMAc in the pore-forming process had signi ficant effects on the membrane morphology. The choice of solvent as the rol e of counter additive to a nonsolvent could be an important factor in the m embrane preparation. (C) 2000 Elsevier Science B.V. All rights reserved.