Improving the Interface in Thin-Film Nanocomposite Membranes for Organic Solvent Separation

Chemical separations and solute purification from hydrocarbon solvents are the most energy-intensive and expensive steps involved in the production of fuels, pharmaceuticals, and chemicals. As a promising alternative, membrane-based separations have emerged over thermal-based separations since they can consume up to 90% less energy than traditional separation technologies, such as distillation. However, one of the primary challenges in achieving efficient polymer-based membrane solvent separations is overcoming plasticization to enable size-based differentiation of molecules. Although porous inorganic additives may, in principle, enhance selectivity, via their uniform pore-size distribution, they introduce compatibility issues that prejudice selectivity. To mitigate the latter issue, attention shifted to porous organic fillers. In actuality, the best performing membranes for organic solvent separation contain a variety of organic or hybrid fillers. In this study, we applied fundamental design principles by tuning the crosslinking chemistry and interfacial compatibility between the polymer and filler matrix. Carboxylated PIM-1 (CPIM) was utilized for its rigid monomer design and acid groups to provide a facile route to crosslinking by thermal treatment at 200 °C. Triptycene-isatin porous polymer networks (PPNs) were chosen as the organic filler due to their inexpensive and facile synthesis, rigidity, and linkage chemistry to enhance size-sieving capabilities and solvent uptake. Estimates of the free volume element (FVE) size distributions at various filler loading and crosslinking degree were measured using X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS). Organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO) tests in 1 mol% triisopropylbenzene (TIPB) in toluene were conducted to evaluate the material platform's performance, both initially and after extended solvent exposure. Moreover, the crosslinked interface between the CPIM selective layer material and the surface functionalities of the Matrimid support was thoroughly studied as a novel approach to improve plasticization resistance, revealing its potential to enable the development of ultrathin, highly selective membranes. In general, crosslinking helps maintain membrane performance by preventing the formation of large defects and mitigating long-term relaxation. Inter-crosslinking offers the added advantage of preserving performance within the OSRO range. Cutoffs in this range indicate that such membranes are well-suited for more demanding hydrocarbon-based separations, such as isomer separations or fractionation of light crude oil. These membranes exhibit the long-term stability necessary for minimizing energy consumption in thermal-based chemical separation processes.