Pore preserving crosslinkers for polyimide OSN membranes

Integrally skinned asymmetric organic solvent nanofiltration (OSN) membranes of P84 polyimide (PI) are usually prepared via a multi-step process consisting of phase inversion, crosslinking and pore preservation, each step performed separately. Posttreatment of membranes with a pore preserving agent is a very important and necessary step to avoid pore collapse in the membrane after drying. The aim of the present work is to investigate if the crosslinking and pore preservation might be combined in one step with the help of polymeric crosslinkers (jeffamine 400 and amine terminated PDMS (Polydimethylsiloxane)). The crosslinked membranes were dried at room temperature and at an elevated temperature, with and without posttreatment with polyethylene glycol 400 (PEG) (the conventional pore preserving agent). Membranes crosslinked with jeffamine 400 were easy to handle without PEG, while membranes crosslinked with amine terminated PDMS and control membrane crosslinked with HDA were brittle. Nanofiltration performance of these membranes was evaluated in solvents such as acetone, and toluene. For both crosslinkers the rejection performance was found to be the same (±3% difference in % rejection) with and without the PEG posttreatment. In case of amine terminated PDMS crosslinker and HDA there was inconsistency (flux and rejection) observed between different batches for the same solvent, while flux for all the solvents dropped by ≈50% from the initial value when the membranes were dried without treatment with PEG. In the case of jeffamine 400 fluxes were the same (±5% difference in steady state flux) after drying with and without posttreatment with PEG. Membranes crosslinked with jeffamine and heated to 75 °C without adding PEG also showed nanofiltration performance with ±5% variation (flux and rejection). Membrane morphology was investigated by scanning electron microscopy and no significant change in morphology was observed after crosslinking with polymeric crosslinkers.