Fundamentals and Applications of Polymer Nanofiber Arrays Fabricated by Chemical Vapor Polymerization

Abstract

Chemical vapor deposition Chemical vapor deposition (CVD) has been widely used in industry and academia as a manufacturing tool to create high-quality films. The excellent step coverage and controllable process of CVD enable its applications in medical devices and semiconductor engineering. One of the promising materials compatible to CVD is poly(p-xylylene) (PPX) as it can be easily modified with various functional groups to immobilize biomolecules and growth factors on medical implants. Advancing from 2-dimensional (2D) film coating, templated CVD polymerization has been suggested to leverage the manufacturing advantages of CVD and the chemical diversity of PPX into topological structures. Recently, liquid crystal (LC) film was used as a template during CVD polymerization, and the synthesis of end-attached nanofiber arrays that resemble the configuration of LC orientations was presented. The first portion of the thesis proposes the growth mechanisms of LC-templated CVD polymerization. Within a certain range of synthesis conditions, we discovered hierarchically converging structures near the solid interface of nanofibers, suggesting that nanofibers grow through a process in which multiple sub-contacts merge into the nanofibers. To observe the intermediate phases of the growth process, we examined the morphological transition of nanostructures by controlling the polymerization time and synthesis pressure. We further used a computational tool to quantify the geometrical features of the hierarchical structures and ultimately derived a growth equation describing the development of the network over time. In the second portion of this dissertation, we designed an LC/fiber composite that exhibits shape-encoded actuation that mimics the behavior of biological systems. This composite takes advantage of the dynamic responses of LC molecules to the electric field, namely the Fréedericksz transition. To observe a clear response from the composite during the electric field application, we used a cyclohexyl-based LC mixture that assumes bent anchoring at the LC-air interface. After the polymerization of PPX, we placed conductive substrates on both the top and bottom sides of the LC/fiber composites. Upon applying an electric field across the specimen, the LC/fibers stretched vertically in the direction of the electric field due to the mechanical coupling of the nanofiber arrays and the surrounding LC template. The composite exhibited programmable responses to the electric field, mimicking the behavior of biological systems such as the tentacles of coral. Subsequently, we fabricated an array of chiral nanofibers made of poly(phenylene vinylene) (PPV) templated by cholesteric LC (C-LC) that emit circularly polarized luminescence (CPL). Our preliminary study reported the templated CVD polymerization of PPV nanofibers using the Gilch route. Our approach aimed to test whether the chiral conformation of the fluorescent polymer could polarize the emitted light into CPL. We prepared arrays of chiral PPV nanofibers with different pitches by varying the chiral dopants of C-LC. The CPL spectrometer confirmed the circular polarization of green light, matching the inherent emission profile of PPV. Our work provides both fundamental and practical insights into LC-templated CVD polymerization. The growth mechanism of templated nanofibers offers guidelines for optimal synthesis conditions and mathematical modeling for hierarchically merging network growth. The second part of this thesis explores the potential use of these materials as optical devices, such as optical tweezers or vortex light beams, based on the optical vortex properties of the composite. The final chapter introduces a new platform to engineer CPL-emitting surfaces, highlighting the manufacturing advantages of CVD.