(350d) Organic Solvent Nanofiltration and Reverse Osmosis Performance of Carboxylated PIM-1 in Neat and Inter-Crosslinked Mixed Membrane Configurations

Organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO) are promising alternatives to reduce the energy burden of thermally-based separation processes, such as recovery of pharmaceuticals or hydrocarbons fractionating. In general, while highly permeable materials have been reported, highly selective performance remains difficult to achieve due to the chemically challenging operating conditions. Additionally, the long-term stability of OSN-OSRO membranes remains an essentially unsolved issue.

Carboxylated PIM-1 (CPIM) is a more highly selective derivative of the widely studied PIM-1 precursor, but most studies on CPIM and its composites have revolved around gas separations. To fill this gap, in this work, we systematically investigate the OSN and OSRO potential of CPIM and composites based on CPIM and triptycene-isatin porous polymer networks (PPNs) to determine if their high selectivity and stability in gas separations translates to organic liquid separations. PPNs are highly crosslinked, organic 3-D networks that mix favorably with polymers to form defect-free mixed-matrix membranes (MMMs). However, their potential to improve selectivity, stability, or permeability in liquid separations remains essentially unexplored.

We show that CPIM undergoes a radical-induced crosslinking reaction as low as 200 C, occurring not only between CPIM chains, but also between CPIM and PPN, leading to the formation of inter-crosslinked, defect-free mixed matrix membranes (IcMMMs) exhibiting remarkable stability and high selectivity in gas and organic solvent separations. In an OSN test with polystyrene oligomers in toluene, neat CPIM has a toluene permeance of 0.042 L m^-2 h^-1 bar^-1 with a MWCO of 200 g/mol, while thermally crosslinked CPIM has a toluene permeance of 0.046 L m^-2 h^-1 bar^-1 with a MWCO as low as 162 g/mol. Finally, thermally treated inter-crosslinked mixed matrices based on CPIM and 5 wt.% PPN exhibits an impressive OSN performance with a MWCO <215 g/mol with a toluene permeance of 0.237 L m^-2 h^-1 bar^-1.

The effects of polymer-polymer, polymer-PPN crosslinking, PPN loading, solvent polarity, and solvent exposure time on the OSN and OSRO performance and its long-term stability are thoroughly investigated and discussed. Fundamental structure-property correlations are proposed, to serve as a useful tool to design next-generation materials for molecular separations in aggressive conditions.