(567e) Permselective Transport of Organic Molecules in Perfluorosulfonic Acid Polymer Membranes for Personal Protective Equipment

Personal protective equipment (PPE) typically employ air-permeable textiles that incorporate activated carbon or other microporous adsorbents and coated by a liquid-repellent outer shell. Alternately, full encapsulation by an air-impermeable material provides the greatest level of protection per OSHA Level A standard. Either system is heavy and bulky resulting in an experience of heat stress even under mild exertion. Permselective membranes and coatings are believed to be the logical next advance by eliminating activated carbon while improving breathability. Prior research with perfluorosulfonic acid (PSA) polymers and membranes such as Nafion attempted to exploit the heterophase morphology of these materials to achieve toxic agent-water permselectivity. In particular, water is known to diffuse through the hydrophilic subphase formed by the sulfonate side chains while, ideally, organic agents would be trapped or impeded by the hydrophobic subphase formed by the backbone. Such permselectivity would allow sweat from the wearer to escape while the transport of toxic agents would be impeded. Instead, water is found to significantly promote agent transport. Molecular dynamics (MD) simulations and indirect experimental evidence suggest that this is due to the “Janus” structure of toxic agents such as dimethyl methyl phosphonate (DMMP) and di-isopropyl methyl phosphonate (DIMP) which contain a hydrophilic “head” and a hydrophobic “tail”. This structure is hypothesized to permit the agents to persist at the interphase between water and the hydrophobic subphase and transport through the membrane. In this presentation, we provide diffusivity data for a variety of organic molecules with a “Janus” structure (e.g., ketones, phosphonates, phenols and organic acids) that exhibit Nafion permeability ranging from un-impeded transport in the case of ketones to near-immobilization in the case of organic acids. Such behavior demonstrates that, despite years of research and significant understanding of how PSA polymer morphology impacts the transport of ions and low-molecular weight solvents, substantial uncertainty remains concerning the permeation behavior of higher molecular weight organic molecules. Our efforts to develop mechanistic understanding in this area are discussed.