Arrays of anchored fibers are widely used in nature and consumer products such as sensors, thermal insulators, and adhesives, among other applications. As reported in a recent issue of Science, a collaboration led by researchers from the University of Michigan and Cornell University has introduced a potentially scalable approach to synthesizing nanometer-scale polymer fiber arrays. This process offers unprecedented control over the fiber properties.
Polymer nanofibers with (a) counterclockwise helicity and (b) clockwise helicity, created using the new approach. Credit: Kenneth Chang, University of Michigan
Nanofiber mats are usually created by extrusion, electrospinning, or microdrawing, but these techniques produce fibers with a limited range of physical properties. The new method can produce polymer nanofibers with various shapes, lengths, diameters, and functionalities, and could point the way to advanced manufacturing processes for nanostructured surfaces.
The development of this approach began with a surprising discovery. The research team was attempting to encapsulate a liquid crystal layer within a polymer thin film by chemical vapor polymerization (CVP), a technique in which monomers are evaporated, activated, and then deposited onto a substrate. While characterizing a sample, the researchers observed nanofibers in the liquid crystal layer. Removing the liquid crystal layer revealed an array of ordered nanofibers emanating from the substrate.
In follow-up experiments, the team observed that nanofiber arrays were created whenever monomers were diffused onto a liquid crystal-covered substrate through CVP.
Researchers have attempted to template chemical reactions with liquid crystals before, because the long-range order and fluidity of liquid crystals can orient molecules. However, the interactions between the chemical reactants and the templating compounds can change the structure of the liquid crystal, limiting nanoscale control and leaving residual reactants in the liquid crystal.
“The level of control over nanofiber length, diameter, and shape demonstrated in the paper is made possible because the chemical vapor polymerization process does not require preloading of the monomer in the liquid crystal—this minimizes perturbation to the liquid crystal organization,” says Nicholas Abbott of Cornell University, who co-led the research team with Joerg Lahann of the University of Michigan.
Furthermore, this research showed that both the long-range order and fluidity of liquid crystals are necessary to create nanofiber mats. When the team performed CVP on a layer lacking either long-range order or fluidity, no fibers were created.
In a series of systematic experiments that varied the type of nematic liquid crystal used, the film thickness, and the interaction between the liquid crystal layer and the substrate, the researchers created arrays of nanofibers that were straight, helical, and banana-shaped, and fibers with well-defined but different diameters and lengths. The results show that the liquid crystal phase templates the fiber shape and diameter, while the liquid crystal film thickness influences fiber length.
By adding functionalized chemical groups to the monomers, the researchers created fibers that were water-repelling, semiconducting, and photoluminescent. They also demonstrated the ability to grow nanofiber arrays on curved surfaces and on micrometer-sized liquid crystal droplets sprayed onto surfaces, providing evidence of versatility and scalability.
“This is an entirely new approach to creating these nanofiber arrays,” says Lahann. “It is a very versatile way of creating all kinds of nanofibers with all kinds of different chemical properties on pretty much any solid substance that is stable under the conditions of CVP,” he says.
“The ability to template polymer nanofibers using liquids crystals is clever,” says Malancha Gupta, an expert in chemical vapor deposition of polymer thin films at the University of Southern California who was not affiliated with this research. “This patterning technique is exciting because it can potentially be extended to and combined with other vapor deposition processes such as sputtering and atomic layer deposition to fabricate nanofibers of different shapes, materials, and function,” she says.
CVP is already widely used in industrial applications, so this new approach could open the door to large-scale manufacturing of nanostructured surfaces. “What I think is most interesting about this approach is that it appears scalable. There are many examples of nanostructures formed on surfaces in the literature, but an unresolved challenge in doing something useful with them is often scale-up,” says Abbott.
By Kendra Redmond December 10, 2018
Read the abstract in Science.