Formation of hollow fiber membranes from poly(ether imide) at wet phase inversion using binary mixtures of solvents for the preparation of the dope

Macrovoidal morphologies are commonly generated when forming membranes by immersion precipitation at an instantaneous phase demixing. In this investigation, hollow fiber membranes from poly(ether imide) were prepared using instantaneous phase separation in dependence on the dope solvent composition allowing a defined and wide variation of the thermodynamical state of the spinning solution from a good to a metastable state. The results show that by a systematic variation of the dope solvent composition, the morphology of the membranes can be varied from macrovoidal to sponge-like structures despite instantaneous phase demixing. Two locations were observed at which the macrovoids were started. First beneath the primary coagulation front (type 1) and second far away of the primary coagulation front in the cross-section (type 2). The first type could correlate to the cloud points and is preferable induced by the thermodynamic state of the dope. Basing on the kinetic data here reported it was hypothesized that the second type starting far away from the primary coagulation layer is preferable induced by the kinetics of the phase inversion.