Morphological properties of membranes fabricated by VIPS process using PEI/NMP/water system: SEM analysis and mass transfer modelling

A morphological study of polymeric membranes elaborated by VIPS process with the poly(etherimide) (PEI)/1-methyl-2-pyrrolidone (NMP)/water system was conducted using both experimental and numerical approaches. The mass transfer model is non-isothermal and integrated multicomponent diffusivities. The self-diffusion coefficients were estimated from the Vrentas and Duda free-volume theory and various formalisms that make a link between the self- and mutual-diffusion coefficients were tested. At different initial polymer concentration, the Dabral formalism also leaded to the best agreement between membrane morphology (determined by SEM analysis) and the predicted concentration profiles. The influence of the process parameters (RH and temperature) was then investigated on the membrane morphology. Reducing the relative humidity induced a longer delay in term of demixing time, leading to reduce the polymer concentration gradient near the air/solution interface, and therefore to prevent the cell size gradient in this region. Between 25 °C and 40 °C, the influence of the temperature on the concentration profiles was shown to be weaker. Lastly, the asymmetric membrane structures characterized by cell size gradients observed at the lowest polymer concentrations were prevented when doping the initial solution with a small amount of non-solvent. In such cases, the model predictions exhibited that the demixing time and the polymer concentration gradient were reduced, leading to more symmetric membranes.