|Brian Leonard Chaloux
Initiated and oxidative chemical vapor deposition (iCVD / oCVD) are two recently developed, transformative processes for preparing thin (10 nm – 1 µm), conformal, and pin-hole free polymer films on a variety of substrates. Vapor-deposited polymers have been explored for a variety of applications including: anti-biofouling coatings, solar photovoltaics, flexible thermoelectrics, and battery separator materials. It is not only the ability to conformally coat non-traditional materials (like textiles) that makes iCVD and oCVD attractive polymerization methods, but also that monomers not amenable to solution processing (e.g. thiophene, EDOT) can often be processed directly by these techniques.[1,2] Of particular interest to my group are ionically, thermally, and electrically conductive materials (for solid state battery and thin film polymer thermoelectric applications, respectively).
Despite the surge of interest in iCVD and oCVD over the last decade, the structure–processing–property relations of CVD-polymerized materials are still poorly understood compared to the huge body of literature on traditionally-prepared (e.g., bulk-, solution-, emulsion-polymerized) polymers. Some of this gap in understanding arises from the novelty of these methods. However, much of it is due to challenges intrinsic to characterizing these materials: Many are crosslinked or otherwise insoluble; thin films are often anisotropic, and anisotropy is typically substrate- and growth condition–dependent; polymerization mechanisms and kinetics are fundamentally different from even bulk polymerization; and the volume of deposited material is low (~100 µg / cm² of surface area covered at 1 µm thickness), restricting the physical and chemical techniques available for analysis of polymer structure and properties.
These challenges present opportunities both to develop robust processing–structure–properties relations for CVD polymers and to improve the methodologies of CVD polymer synthesis. Robust spectroscopic techniques must be developed for determining thin film chemistry (e.g. residual monomer / comonomer content, side reactions) and microstructure (e.g. molecular weight, branching, crosslink density, tacticity) and for comparing these to equivalent 'bulk' polymers. Synthetically, several topics ripe for investigation are:
- Is controlled / 'living' CVD polymerization feasible? How might one convert a typical iCVD or oCVD process into a controlled one?
- Can better initiators / oxidants be designed to increase the rate of film deposition / reduce the amount of monomer lost to dynamic vacuum?
- Is post-deposition film annealing required to create anisotropic films, or can such films be prepared as-is by tuning surface chemistry and deposition conditions.
- Is it possible to generate compositionally-graded thin films by changing comonomer feed rates over the course of a deposition, and if so, how tunable are these compositional gradients?
The two overarching goals of this opportunity are to improve our understanding and control of polymer CVD processes and to utilize iCVD/oCVD to prepare conformal, functional polymer films for application geometries that are difficult, if not impossible, to prepare by traditional polymerization / polymer deposition methods.
 Lee, S.; Borrelli, D.C.; Jo, W.J.; Reed, A.S.; Gleason, K.K. "Nanostructured Unsubstituted Polythiophene Films Deposited Using Oxidative Chemical Vapor Deposition: Hopping Conduction and Thermal Stability." Adv. Mater. Interfaces 2018, 5, 1701513.
 Kaviani, S.; Ghaleni, M.M.; Tavakoli, E.; Nejati, S. "Electroactive and Conformal Coatings of Oxidative Chemical Vapor Deposition Polymers for Oxygen Electroreduction." ACS Appl. Polym. Mater. 2019, 1, 552–560.
 Moni, P.; Mohr, A.C.; Gleason, K.K. "Growth Rate and Cross-Linking Kinetics of Poly(divinyl benzene) Thin Films Formed via Initiated Chemical Vapor Deposition." Langmuir 2018, 34, 6687–6696.
polymer; chemical vapor deposition; batteries; thermoelectric; power; thin films;