Effect of Block Copolymer Membrane Nanostructure on Ethanol-Water Transport Measured by Infrared Spectroscopy

Block copolymers (BCPs) are recognized for exhibiting unique assets, not achievable in homogeneous, single component materials, by combining the peculiarities of two different homopolymer blocks, in terms of physical, chemical and mechanical properties. Three polystyrene-b-poly(ethylene oxide) (SEO) BCPs with distinct morphologies—lamellar, cylindrical, and spherical—synthesized via anionic polymerization, were investigated for their possible use as membrane materials for ethanol-water transport, aiming to explore sustainable separation processes. Ethanol permeation experiments at 30 °C, using water/ethanol feeds with various concentrations, were conducted via in-situ Fourier transformed infrared spectroscopy (FTIR). Results revealed that ethanol permeability in SEO membranes depends on the feed mixture concentration and the PS-PEO mass ratio. The hydrophilic PEO fraction interacts strongly with both solvents, while the hydrophobic PS moiety, though not dissolvable, is more affine to ethanol. A mass balance model confirmed that at 8.9 mol/L ethanol, permeability increased from 2.7⋅10-7 to 8.5⋅10-7 cm2/s moving from spherical to lamellar structures, while water permeability at 50.1 mol/L rose from 2.0⋅10- 9 cm2/s to 2.1⋅10-8 cm2/s. Despite the moderate separation performance of the membranes, the study enhances understanding of swelling, morphology, and counter-diffusion effects, contributing to polymer design advancements.