Controlling molecular weight cut-off curves for highly solvent stable organic solvent nanofiltration (OSN) membranes

Integrally skinned asymmetric membranes made from Lenzing P84 polyimide were prepared by immersion precipitation from a casting solution containing dimethylformamide (DMF) as the solvent and 1,4-dioxane (dioxane) as the co-solvent. The variation of the ratio between DMF and dioxane in the dope solution used for membrane formation provided control over the molecular weight cut-off curves of the resultant membranes. Coupling this methodology with chemical crosslinking provided excellent solvent stability, enabling application of the resulting family of organic solvent nanofiltration (OSN) membranes to separations in DMF. An analysis of the separation performance of the membranes using a two parameter pore flow model showed an increase in the mean pore size and standard deviation of pore size as the membranes became “looser” with higher concentrations of DMF in the dope solution. It was not possible to increase the mean pore size whilst maintaining the standard deviation of pore size constant. Nevertheless, this work demonstrates that a series of highly solvent stable membranes with controlled molecular weight cut-offs could be formed from a single polymer/immersion precipitation platform.