|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 thermally, electrically, and ionically conductivity materials (for polymer thermoelectric and battery separator applications, respectively).
Despite the surge of interest in iCVD and oCVD over the last 5 years, 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. Regarding characterization, 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. From a synthetic perspective, several related topics remain open to investigation, including:
- How do individual monomer properties (e.g. vapor pressure, reduction potential / radical stability, steric hindrance) affect film growth? What makes a monomer particularly amenable to iCVD / oCVD?
- 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 minimize side reactions (e.g. crosslinking) or change the purity of the final polymer (e.g. initiator / oxidant doping, atmospherically sensitive side products, trapped / unreacted monomer)?
Although iCVD and oCVD have already been demonstrated as useful techniques, much remains to be learned regarding both the synthesis of the resulting polymers and their physical properties, making this field ripe for basic polymer research.
 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.