Size-Dependent Permeability Deviations from Maxwell’s Model in Hybrid Cross-Linked Poly(ethylene glycol)/Silica Nanoparticle Membranes

Currently, separation of gaseous mixtures largely relies on energy intensive and expensive processes, like chemical looping of amines. This has driven research into less energy-intensive, passive methods of performing separations such as the use of polymer membranes. Although pure polymer membranes have demonstrated appealing separation performance, they suffer from an inherent trade-off between permeability and selectivity, which limits overall performance. Recent research efforts have shown that the introduction of a secondary phase, often an inorganic species, is added to selectively boost permeability or selectivity. However, these hybrid organic/inorganic systems have not seen widespread adoption because synthetic control over the size, shape, and dispersion of the inorganic species is poor and understanding of transport in these membranes is largely empirical. Thus, understanding and optimizing hybrid membranes requires development of well-controlled model systems in which size, shape, and surface chemistry of the inorganic species are precisely controlled, leading to homogeneous membranes amenable to careful study. Here, we report on the synthesis, characterization, and gas transport properties of tailored hybrid membranes composed of cross-linked poly­(ethylene glycol) and silica nanoparticles. We show excellent control of nanoparticle size, loading, and dispersibility. We find that permeability deviations from Maxwell’s model increases as the size of silica nanoparticle decreases and loading increases. These size-dependent deviations from Maxwell’s model are attributed to interfacial interactions, which scale with surface area and act to decrease segmental chain mobility

Role of Grafting Density and Nitrile Functionalization on Gas Transport in Polymers with Side-Chain Porosity

This study details the enhancement of CO2 selectivity in ring-opening metathesis polymerization (ROMP) polymers that contain nitrile moieties and micropore-generating ladder side chains. A material, CN-ROMP homopolymer, with nitriles in the ladder side chains was originally targeted and synthesized; however, its low molecular weight and backbone rigidity precluded film formation. As a result, an alternative method was pursued wherein copolymers were synthesized using norbornene (N) and nitrile norbornene (NN). Herein, we report an investigation of the structure–property relationships of backbone functionalization and grafting density on the CO2 transport properties in these ROMP polymers. Nitrile-containing copolymers showed an increase in CO2/CH4 sorption selectivity and a concomitant increase in CO2/CH4 permselectivity when compared to the unfunctionalized (nitrile-free) analogues. The stability in CO2-rich environments is enhanced as grafting density of the rigid, pore-generating side chains increases and an apparent tunability of CO2 plasticization pressure was observed as a function of norbornene content. Lower loadings of norbornene resulted in higher plasticization pressure points. Gas permeability in the ROMP copolymers was found to correlate most strongly with the concentration of the ladder macromonomers in the polymer chain

Free Volume Manipulation and <i>In Situ</i> Oxidative Crosslinking of Amine-Functionalized Microporous Polymer Membranes

Membranes for gas separations are limited by the trade-off relationship between permeability and selectivity. In this study, we demonstrate an in situ thermal oxidative crosslinking strategy for amine-functionalized polymers using tert-butoxycarbonyl (tBOC) groups to improve separation performance. The use of labile tBOC groups offers two major benefits for inducing thermal oxidative crosslinks: (1) they trigger free radical chain reactions at more moderate temperatures, preventing polymer backbone degradation pathways that otherwise occur at elevated temperatures, and (2) they enable free volume manipulation (FVM) conditions that yield increased free volume and narrower free volume element size distribution. This thermal oxidative crosslinking strategy is demonstrated using an amine-functionalized polymer of intrinsic microporosity (PIM-NH2). The resulting crosslinked polymer yielded up to a 22-fold increase in H2/CH4 selectivity while retaining 96% of H2 permeability from pristine PIM-NH2 films. By combining thermal oxidative crosslinks and FVM, we demonstrate an effective approach to overcome the traditional permeability–selectivity trade-off and offer a greater resistance to major performance stability issues like plasticization and physical aging, making membranes better suited for industrial applications