Using Chemical Vapor Polymerization in Liquid Crystals to Synthesize Organized Arrays of End-Attached Polymeric Nanofibers and Interconnected Nanofibrous Membranes

Randomly organized mats of nanofibers are widely utilized as tissue scaffolds, membranes for separation processes, and as supports for biomolecular assays. Alternatively, ordered nanofiber arrays are found in nature to provide higher level functions such as transport of materials (cilia) and enhanced adhesion (Gecko feet). Whereas electrospinning, extrusion, and microdrawing provide ready access to random arrays of nanofibers, they offer limited opportunities for the synthesis of arrays of nanometer-scale fibers with controlled size, shape, and lateral organization. In this talk, we will present a new strategy that permits the synthesis of functional materials from nanofibers, including ordered arrays of end-attached nanofibers and interconnected nanofiber-based membranes. The approach is based on chemical vapor polymerization (CVP) into liquid crystal (LC) films.

CVP of paracyclophanes has been widely employed to form conformal polymeric films on solid surfaces (parylene). Here we will describe CVP into micrometer-thick films of LCs supported on solid surfaces, where the LC films function as templates to direct the formation of structure within the polymerized films.

First, the presentation will describe how CVP of unsubstituted and substituted monomers such as hydroxymethyl paracyclophane into thin LC films leads to the formation of ordered arrays of nanofibers with ends that are attached to the surfaces of solid substrates. The shapes and sizes of the nanofibers can be tailored by the properties of the LC template, including LC elastic moduli, LC interactions with confining surfaces and chirality. Such ordered nanofiber arrays of parylene have the potential to be used as biological transport systems, biosensors, guidance cues for cell growth and proliferation and to prolong the release of encapsulated drugs.

Second, we will describe a recent and surprising discovery that CVP into LC films can also lead to the formation of nanofiber-based structures with morphologies that are strikingly different from the end-attached nanofibers. Specifically, we have discovered that CVP of aminomethyl paracyclophane monomers into nematic films of LCs supported on solid surfaces can generate quasi-two-dimensional sheets of interconnected amine-functionalized nanofibers. We have established that the nanofiber networks form at the gas-LC interface with nanoscopic pores that can be tuned via the choice of LC and monomer loading. Structural analysis using electron microscopy reveals the nanofiber networks to possess quasi-2D morphologies ranging from open bicontinuous-like (thickness of 130±3 nm) to cellular foam-like structures (thickness of 260±14 nm) which, along with optical observations, supports a synthesis pathway involving an interface-confined phase separation during CVP. 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 nanofiber sheet synthesis can be integrated with existing membrane technology for separations and purification processes.

Because LC films can be coated on surfaces over wide areas, this synthesis technique has the potential to be scaled up to form nanofiber arrays and membranes over larger areas. The high surface-to-volume ratio of nanofibers as compared to conformal films, when coupled with their ability to covalently immobilize a range of biomolecules including polysaccharides, antibodies and enzymes, offers fresh approaches to achieving tailored interactions with cells, proteins, and drugs. Overall, our work demonstrates that CVP into LCs provides the basis of a general and facile methodology for synthesis of organized assemblies of polymeric nanofibers.