Conformal Polymer Thin Films on Structurally Complex Surfaces by Initiated Chemical Vapor Deposition
Authors:   , Hsieh Chia-Yun (Sharon) , Lau Kenneth K. S. 
Published: July 2021

Initiated chemical vapor deposition (iCVD) is a novel CVD technique for forming polymer thin films. Compared to traditional thermal and plasma CVD methods, iCVD operates at low substrate temperature and low power conditions. This has the benefit of enabling well-defined reaction pathways for polymerization that lead to stoichiometric polymers. The iCVD approach has been investigated for many polymer chemistries and the resulting iCVD polymers have been shown to possess analogous structures and properties as bulk polymers from liquid phase synthesis. Among iCVD reactions, free radical polymerization is the most common, where vinyl monomers can be polymerized with peroxide free radical initiators. Recently, cationic ring opening polymerization via iCVD was demonstrated by applying boron trifluoride diethyl etherate as a cationic initiator for the polymerization of ethylene oxide. This work will demonstrate for the first time the iCVD synthesis of polyglycidol (PGL) via cationic ring opening polymerization of glycidol. iCVD PGL shows similar structure and properties as liquid-synthesized PGL reported in literature based on spectroscopic analysis. Furthermore, the iCVD deposition behavior under different modes of iCVD polymerization environment - surface-driven, gas-driven, and supersaturation - will be discussed for forming polyglycidol (PGL), poly(2-hydroxyethyl methacrylate) (PHEMA), poly(tetrafluoroethylene) (PTFE) and polyvinylpyrrolidone (PVP) coatings on structurally complex substrates, including nanopores, nanorods, and microstructures. Two major parameters Pm/Psat that represents the ratio of the partial pressure of the monomer to its saturation pressure, and Knudsen number (Kn) will be evaluated and related with the observed deposition behavior. Surface-driven iCVD of PGL and PHEMA have been found to conformally deposit in nanoporous TiO2 and microcatheters by carefully controlling Pm/Psat over a wide range of Kn. However, with gas-driven iCVD of PTFE, although conformal coatings have been achieved on micropillars and nanorods, coating within nanoporous networks at very large Kn was difficult even with careful control of Pm/Psat. It is believed that the PTFE polymerization is significantly driven by gas phase reactions that are not well controlled with a surface Pm/Psat parameter and, by moving to smaller and more confined features, the gas phase chemistries dominate and interfere with surface polymerization. By controlling Pm/Psat > 1, i.e. in a supersaturated monomer state, a recent iCVD processing discovery was made. Under supersaturation conditions, PVP was found to selectively grow on certain material surfaces and not others. This is believed to be due to differences in wettability of the monomer that dictates where the polymer grows, and enables directed patterning through iCVD. With the ability to deposit polymer coatings on different substrates, this work will illustrate a number of applications that highlight iCVD as an enabling technology. iCVD of PHEMA on ventricular catheters is found to be an effective coating for reducing undesired cell attachment in vitro by 77% after 17 days in cultured media compared to bare catheters, and so has the potential for improving catheter viability and reliability. iCVD of PTFE on silicon micropillars and nickel nanorod arrays is able to produce effective non-wetting (superhydrophobic) surface structures for enhancing latent heat transfer. iCVD of PGL in mesoporous TiO2 nanoparticle networks produces polymer nanocomposites with ultrahigh nanofiller loading (>80 wt%), offering a valuable platform for studying polymer nanocomposites with uniform and ultrahigh loading that exceed conventional processing limits (10-15 wt%) due to filler particle aggregation. As a result, the PGL glass transition temperature is found to increase significantly by 50-60 °C compared to bulk PGL films without TiO2 nanofiller. The enhanced glass transition is attributed to appreciable hydrogen bonding interactions between PGL and TiO2.