A scalable crosslinking method for PVDF-based nanofiltration membranes for use under extreme pH conditions

Crosslinked PVDF is a versatile, thermally and chemically ultrastable material for membrane applications. However, widespread use of this material is hampered by 3 major drawbacks: (1) long crosslinking times, (2) harsh crosslinking conditions and (3) expensive chemicals. In this work, a tremendous increase in the PVDF crosslinking efficiency was achieved by using a simplified procedure in which the crosslinker/base/solvent-roles were combined into a single compound, enabled by the use of an aliphatic diamine. The membranes were characterized by membrane filtrations combined with a detailed physico-chemical analysis of the crosslinked material using FTIR, XRD and TGA. The new crosslinking procedure was found to be between 17 (based on comparable N-content) and 70 (based on comparable degree of swelling in DMF) times faster. Additional benefits of this new crosslinking procedure are the significantly reduced cost of the required chemicals as well as the potential to recycle the crosslinking solution. Recycling of the crosslinking solution is possible since the crosslinking strategy theoretically allows for 100% conversion of the diamine. No significant negative changes in composition nor swelling of the membranes were observed even after 4 times recycling of the crosslinking solution while NMR showed that no significant degradation of the remaining diamine had occurred. An integrally skinned asymmetric as well as thin-film composite membrane based on a polyamine toplayer supported by a crosslinked PVDF ultrafiltration membrane were applied in extreme pH nanofiltration. For both membrane types, no significant change in retention was found after soaking for 5 days in either 5 M HCl or NaOH.