Nanoconfinement and Chemical Structure Effects on Permeation Selectivity of Self-Assembling Graft Copolymers

Abstract

Permeation of small molecule solutes through thin films is typically described by the solution-diffusion model, but this model cannot predict the effects of nanostructure due to self-assembly or additives. Other models focusing on diffusion through isolated nanopores indicate that confining permeation to channels slightly larger than the size of the solute can lead to an increased influence of solute−pore wall interactions on permeation rate. In this study, we analyze how differences in polymer nanostructure affect the relative contributions of solute size and polymer−solute interactions on transport rate. We compared the diffusion rates of several small molecules through two polymer thin films: A cross-linked, homogeneous film of poly(ethylene glycol phenyl ether acrylate) (PEGPEA) and a graft copolymer with a poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) backbone and PEGPEA side chains that self-assemble into continuous ∼1−3 nm PEGPEA domains through which transport occurs. We correlated these rates with the size of each solute and its chemical affinity to PEGPEA, as measured by the difference between their solubility parameters. Diffusion rate through the homogeneous polymer film was controlled by solute size, whereas diffusion rate through the copolymer was strongly controlled by the difference between the solubility parameters. Furthermore, permeation selectivity between two selected molecules was 2.5× higher for the nanostructured copolymer, likely enhanced by the nanoconfinement effects. These initial results indicate that polymer self-assembly is a promising tool for designing polymeric membranes that can differentiate between solutes of similar size but differing chemical structures. R egulating permeation through materials is crucial for many applications, including selective membranes, controlled drug delivery, and packaging. 1−7 Permeation selectivity is especially critical in membrane separations. Membranes that are capable of separating small molecules of similar size by their chemical structure can potentially have great impact in drug manufacture 8−10 and the petrochemical industry. A better understanding of how to modulate the permeation of small molecules through polymers by manipulating chemical interactions and nanostructure can open up new avenues to such membranes.Most current membrane selective layers are homogeneous polymers 11−13 where permeation is described by the solutiondiffusion model. The solute dissolves in the polymer on the feed side, diffuses, and desorbs into the permeate. The molar flux J A of solute A through the polymer film is given bywhere S A , D A , and P A are the solubility, diffusivity, and permeability of A in the polymer, respectively, z is the polymer film thickness, and ΔC A is the concentration difference between the feed and permeate. The permeability of a solute increases with increasing solubility in the polymer and decreasing solute size. This model works well for most gas separation membranes 11−14 and has been adapted for desalinatio...

Selective Transport through Membranes with Charged Nanochannels Formed by Scalable Self-Assembly of Random Copolymer Micelles

“…For these experiments, we selected a set of organic small molecules with varying electrostatic charges: Basic Blue 3 (BB3, cationic), Acid Blue 45 (AB45, anionic), and riboflavin (RIB, neutral). All three solutes were selected to have similar calculated sizes (8.3–8.6 Å, calculated using Molecular Modeling Pro , ) (Figure S12, Supporting Information). The pH of the solutions used in diffusion and filtration experiments varied between 5.4 and 7.5 (Figure S12, Supporting Information).…”

“…This makes BCP membranes especially suitable for ultrafiltration applications and protein separation and purification . Random and comb-shaped copolymers can form smaller and bicontinuous domains down to 1 nm − and have shown size-selective screening of small molecules. With further functionalization, these copolymers can exhibit selectivity that goes beyond size screening.…”

Environmental Applications of Engineered Materials with Nanoconfinement

“…In contrast to conventional polymeric membranes with poorly defined pore structures, recent studies have explored selective membranes with precisely controlled pore morphology and surface chemistry to create molecule/ion-specific transport behavior under spatial nanoconfinement . Examples of membranes exploiting confinement for enhancing selectivity include (i) a Janus copolymer membrane with an asymmetric structure and confined pores that exhibits high anion selectivity (Figure c) and (ii) a nanostructured copolymer that achieves a molecular permeation selectivity that is 2.5 times higher than the corresponding homogeneous film . Adding functional groups to the nanoconfinement structure can further enhance the selectivity of ion transport through modifying the ability of ionic species to pass through a pore entrance or their distribution within the pore .…”

“…50 Examples of membranes exploiting confinement for enhancing selectivity include (i) a Janus copolymer membrane with an asymmetric structure and confined pores that exhibits high anion selectivity (Figure 2c) 117 and (ii) a nanostructured copolymer that achieves a molecular permeation selectivity that is 2.5 times higher than the corresponding homogeneous film. 127 Adding functional groups to the nanoconfinement structure can further enhance the selectivity of ion transport through modifying the ability of ionic species to pass through a pore entrance or their distribution within the pore. 128 Adding well-ordered reactive ionic liquids onto the surfaces confined inside boron nitride nanochannels was also found to promote selective ethylene transport.…”

Nanostructured Virus Filtration Membranes Based on Two-Component Columnar Liquid Crystals

“…Membrane separation is prevalent in water purification and biopharmaceutical production processes because it is inexpensive and simple. , Development of membranes with high separation performance is important for future applications. ,, Many commercially available membranes use cross-linked aromatic polyamides and cellulose acetate derivatives. , However, it is difficult to obtain well-defined pores with uniform diameters in these conventional membranes. Molecular self-organization methods such as thermotropic or lyotropic liquid crystals, − copolymers, − and carbon-based materials , have emerged as solutions for fabricating membranes with well-controlled pore sizes.…”