Size-dependent permeability deviations from maxwells 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 Maxwells model increases as the size of silica nanoparticle decreases and loading increases. These size-dependent deviations from Maxwells model are attributed to interfacial interactions, which scale with surface area and act to decrease segmental chain mobility.