Improving Gas Selectivity in Membranes Using Polymer-Grafted Silica Nanoparticles

Abstract

Herein, we demonstrate that tuning of structural parameters and material chemistry enables control of the permeability and selectivity of gases (CO 2 , He, and CH 4 ) in membranes based on poly(butyl methacrylate)-grafted nanoparticles (PGNs). Our data show that the presence of nanoparticles and the overall dense packing of grafted chains noticeable in PGNs with low grafting density have an adverse effect on the diffusivity of gases. This effect is compensated by an improvement in the solubility of CO 2 gas promoted by the silica nanoparticle surface, yielding a substantial improvement in the permeability of CO 2 versus CH 4 . In membranes with high grafting density, changes in the structural arrangements and alterations in the membrane porosity, evident from small-angle X-ray scattering and positron annihilation lifetime spectroscopy, positively influence the permeability of He and CO 2 gases. In contrast, CH 4 permeability in the same membranes is significantly suppressed, suggesting the formation of a unique, highly selective environment for gas separation. As a result, an improvement of up to 50% in selectivity for gas pairs containing large CH 4 molecules is observed. Our studies provide fundamental insights into the role that structural parameters play in gas transport through polymer membranes, laying a foundation for the rational design of membranes with improved permeability and selectivity.

Understanding Gas Transport in Polymer-Grafted Nanoparticle Assemblies

“…We have attempted to use extended Maxwell models as suggested in the literature, separately above and below the maximum permeability: we assume that the dry region has significantly higher permeability than the interpenetrated zone, while the NP core is not permeable. This model does not capture the trends observed in Figure , in agreement with previous conclusions of Bocherova and co-workers …”

“…This model does not capture the trends observed in Figure 4, in agreement with previous conclusions of Bocherova and coworkers. 42 We thus resorted to a more empirical approach. For ϕ NP > ϕ NP,max , we used a variety of forms, including an exponential, a hyperbolic tangent, and a power law to see which one adequately describes the data.…”

Fundamentals and Applications of Polymer Brushes in Air

“… 141 Experiments by Jeong et al using poly(butyl methacrylate)-grafted particles show that high grafting densities are required for increased gas permeability. 144 This is attributed to the formation of polymer bridges between particles at low grafting densities, either during polymerization or by penetration of polymer chains from one particle to the surface of another, which can result in inhomogeneous dispersion of the particles. Xin et al found that brush functionalization with polystyrene-derived polymers improved the compatibility of various inclusions with a sulfonated poly(ether ether ketone) (SPEEK) matrix.…”

Local Structure of Polymer-Grafted Nanoparticle Melts

“…Pure GNP melts display significant improvements in their gas transport properties relative to the corresponding neat polymer, making them germane to direct air capture and natural gas separations. ,, For a fixed grafting density (we mostly employ σ ≈ 0.47 chains/nm 2 but have systematically examined σ ≈ 0.11–0.66 chains/nm 2 ), we demonstrated a nonmonotonic gas permeability enhancement as a function of graft chain molecular weight (MW g ), with a peak in the vicinity of MW g ≈ 100 kDa for σ ≈ 0.47 chains/nm 2 (see Supporting Information, Figure S1). ,, Moreover, gas penetrants with larger kinetic diameters appear to have increased enhancements . We seek to understand this unusual behavior, especially the gas transport maximumspecifically, here we explore whether the packing structure of the GNP layer (and hence of the NPs) could indirectly influence the diffusivity of light gases through GNP membranes.…”