Formation of Interconnected Nanofiber Sheets by Chemical Vapor Polymerization at the Free Surface of Liquid Crystalline Films

We report that chemical vapor polymerization (CVP) of aminomethyl[2.2]paracyclophane into nematic liquid crystal (LC) films (thicknesses of 18 µm) yields quasi-two-dimensional, sub-micron thick nanoporous polymer networks consisting of interconnected amine-functionalized nanofibers/nanowalls (widths of 30 ± 1 nm). We establish that the polymer networks form at the free surface of the LC films with thicknesses ranging from 79 ± 5 to 280 ± 14 nm and nanoscopic pores tunable via the choice of LC and monomer loading. Structural analysis using electron microscopy reveals the networks to possess morphologies ranging from open bicontinuous-like to cellular foam-like structures which, along with optical observations and molecular dynamics (MD) simulations, supports a synthesis pathway involving an interface-confined phase separation. MD simulations provide further insight into the atomic-scale processes determining the synthesis pathway, including the role of reactive precursor chemistry (e.g., hydroxymethyl[2.2]paracyclophane versus aminomethyl[2.2]paracyclophane versus [2.2]paracyclophane) in defining the nanostructure of the polymer product. Fluorescence and X-ray photoelectron spectroscopy confirm that the nanofiber sheets are decorated with primary amine groups, permitting covalent functionalization of the surfaces of the nanosheets. Finally, we show how the nanosheet synthesis can be integrated with existing membrane technology, illustrating the potential utility of the nanoporous sheets in a range of contexts, including filters, separators, and heat exchanger surfaces.