Interconnected pore networks as a unifying framework for transport in dense polymer membranes
Transport in many dense polymer membranes used for reverse osmosis, organic solvent nanofiltration, pervaporation, and gas separation has traditionally been interpreted through the solution–diffusion model, which treats the membrane as a dense, nonporous polymer phase where molecules dissolve and diffuse under concentration gradients. Although this framework has been useful for correlating membrane performance, its mechanistic foundation for water and solvent transport is contradicted by current experimental and molecular simulation evidence. Observations from positron annihilation lifetime spectroscopy, scattering methods, spectroscopic probes, and molecular dynamics simulations demonstrate that polymer membranes, long considered dense and nonporous, contain interconnected (sub)nanometer free-volume networks rather than isolated transient cavities. These molecular pathways provide a more physically grounded basis for interpreting transport across diverse membrane processes. Here, we synthesize recent experimental and molecular simulation evidence to establish interconnected pore networks as a unifying mechanistic framework for transport across major classes of dense polymer membranes. In pressure-driven liquid separations, solvent transport is governed by pressure-driven viscous flow through interconnected, solvent-filled pore networks. In pervaporation, liquid-like and vapor-like domains coexist within the membrane, indicating an internal phase transition rather than evaporation only at the downstream surface. In glassy gas-separation polymer membranes, permeability and selectivity are governed by pore connectivity, molecular sieving, and gas–wall interactions. Together, these insights shift membrane design from optimizing empirical solubility and diffusivity parameters toward engineering pore size, connectivity, tortuosity, and pore-wall chemistry.