Robust, Hyper-Permeable Nanomembrane Composites of Poly(dimethylsiloxane) and Cellulose Nanofibers

Robust, nanometer-thick, permselective membranes were developed by composite formation from poly(dimethylsiloxane) (PDMS) and cellulose nanofibers (CNF). Their unique behavior is discussed in relation to that of a single-component PDMS nanomembrane. In the absence of the CNF component, the PDMS nanomembrane with a thickness of 34 nm displays ultrahigh permeability of CO₂ gas, which is only ca. one order of magnitude smaller than that of free-flowing gases through a porous poly(acrylonitrile) support film (PAN, thickness 150 μm). The constant CO₂/N₂ selectivity observed for the whole range of membrane thickness (34 nm-10 μm) suggests that in the single-component membrane, the kinetic process at the membrane surface determines the permselective behavior. Multilayered composite membranes are obtainable by repeated spin coating. The mechanical weakness of the single-component PDMS membrane is improved by complexation with CNF, as confirmed by the bulge test and the ease of macroscopic handling. Such a robust PDMS-CNF nanomembrane gives superior permeation of 50,000 GPU with a defect-free PDMS layer of ca. 17 nm thickness. Interestingly, the permeation characteristics of the composite membrane are strongly affected by the asymmetric arrangement of PDMS and CNF layers, and the gas permeation from the side of the CNF layer is drastically reduced. The PDMS composite membrane is expected to provide practically useful systems as a means of direct air capture.