Structure-dependent CO2 separation in PEBAX membranes incorporating polystyrene nanoparticles: freestanding versus thin-film composite configurations

Polystyrene (PS) nanoparticles are typically regarded as low-permeability fillers in gas separation membranes; however, this study demonstrates that their incorporation into PEBAX matrices can enhance both permeability and selectivity through configuration-dependent transport mechanisms. A systematic comparison between freestanding and thin-film composite (TFC) membranes reveals a clear shift in dominant gas transport behavior arising from structural differences. In ∼100 µm-thick freestanding membranes, PS nanoparticles act as bulk microstructural modifiers that partially disrupt crystalline packing, resulting in reduced crystallinity. This structural perturbation enhances chain mobility and increases the fractional free volume, enhancing gas diffusivity. At an optimal loading of 10 wt%, CO 2 permeability and CO 2 /N 2 selectivity simultaneously increased to 198.6 Barrer and 41.4 (54% and 31% improvements over pristine PEBAX), indicating partial mitigation of the permeability–selectivity trade-off through controlled free-volume enhancement. In contrast, ∼250 nm TFC membranes exhibit shear-assisted nanoparticle alignment and confinement within the ultrathin selective layer, producing an interfacial diffusion-controlled structure. While permeance remains relatively stable at moderate loadings, preferential suppression of N 2 transport enhances selectivity, increasing CO 2 /N 2 selectivity from 26 to 39 at 20 wt% PS with only minor permeance reduction. These findings demonstrate a mechanistic transition from bulk free-volume enhancement in freestanding membranes to interfacial diffusion control in TFC configurations, highlighting the critical role of nanoparticle organization in governing gas transport performance.