Thin-film composite membranes for organophilic nanofiltration based on photo-cross-linkable polyimide

Relaxation Dynamics of Thin Matrimid 5218 Films in Organic Solvents

Polyimides are interesting polymer materials for organic solvent nanofiltration (OSN) applications because of their high excess free volume and high chemical and temperature resistance. However, an open challenge that remains for glassy polymer materials (i.e., polyimides) is their tendency to swell in organic solvents which can lead to a loss of performance. An understanding on how swelling influences the polymer properties and performance is then of crucial importance for assessing polyimide suitability in OSN applications. Here, the combination of in situ spectroscopic ellipsometry (iSE), broadband dielectric spectroscopy (BDS), and diffuse reflectance Fourier transform infrared spectroscopy (DRIFT-FTIR) is applied to study the molecular interaction of two organic penetrants, toluene and n-hexane, with Matrimid 5218 in detail. iSE shows that slightly cross-linked Matrimid 5218 swells approximately seven times more in toluene (swelling degree ≈ 28%) compared to in n-hexane (swelling degree ≈ 4%). Combined BDS and DRIFT-FTIR results indicate that toluene interacts with the benzene ring present in the diamine via π–π interactions, while n-hexane likely fills up the excess free volume and interacts via local van der Waals interactions. This work highlights the insights into the exact nature of the molecular interactions between the penetrant and polymer that can be gained from a combination of BDS and other techniques and how these insights can be used to estimate or understand solvent-induced swelling of polymers used in OSN applications.

The Selectivity Challenge in Organic Solvent Nanofiltration: Membrane and Process Solutions

Recent development of organic solvent nanofiltration (OSN) materials has been overwhelmingly directed toward tight membranes with ultrahigh permeance. However, emerging research into OSN applications is suggesting that improved separation selectivity is at least as important as further increases in membrane permeance. Membrane solutions are being proposed to improve selectivity, mostly by exploiting solute/solvent/membrane interactions and by fabricating tailored membranes. Because achieving a perfect separation with a single membrane stage is difficult, process engineering solutions, such as membrane cascades, are also being advocated. Here we review these approaches to the selectivity challenge, and to clarify our analysis, we propose a selectivity figure of merit that is based on the permselectivity between the two solutes undergoing separation as well as the ratio of their molecular weights.

Covalent Cross-Linking of Porous Poly(ionic liquid) Membrane via a Triazine Network

Porous poly(ionic liquid) membranes that were prepared via electrostatic cross-linking were subsequently covalently cross-linked via formation of a 1,3,5-triazine network. The additional covalent cross-links do not affect pore size and pore size distribution of the membranes and stabilize them toward salt solutions of high ionic strength, enabling the membranes to work in a broader environmental window.