Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor‐phase monomers to form chemically well‐defined polymeric films directly on the surface of a substrate. CVD polymers are ...desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet‐chemical chain‐ and step‐growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high‐resolution (60 nm) patterning, even on flexible substrates. Utilizing only low‐energy input to drive selective chemistry, modest vacuum, and room‐temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large‐area and roll‐to‐roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.
Chemical vapor deposition (CVD) polymerization bridges all‐dry microfabrication technology with the chemistry of functional and responsive organic materials. In a single step, vapor‐phase monomers can be transformed through selective reaction for surface modification of micro‐ and nanostructured surfaces. Shown is conformal CVD polymer deposition, ∼350 nm thick, on trenches 7 μm deep and 2 μm wide etched on silicon.
Initiated chemical vapor deposition (iCVD) is able to synthesize linear and cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) thin films, in one step, from vapors of 2-hydroxyethyl methacrylate ...(HEMA), ethylene glycol diacrylate (EGDA), and tert-butyl peroxide (TBPO) without using any solvents. This all-dry technique also allows control of the cross-link density by adjusting the partial pressure of the cross-linking agent EGDA in the vapor phase. Films with specific cross-link densities and hence thermal, wetting, and swelling properties can be created in one single vacuum processing step. Through selective thermal decomposition of the initiator TBPO, films with well-defined chemical structures and full functionality retention can be deposited, which is evident in the Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. These spectroscopic methods also facilitate determination of EGDA incorporation in the cross-linked films based on the fact that HEMA contains a hydroxyl group but EGDA does not. For the linear PHEMA depositions, the growth rate was found to be nonlinear in the partial pressure of HEMA, possibly due to nonlinear multilayer adsorption and/or primary termination. The EGDA/HEMA ratio in the films systematically increased from 0.00 to 0.46 as the EGDA partial pressure was raised. The onset temperatures of decomposition were between 270 and 302 °C for the linear and the most cross-linked films, respectively. Thermal annealing at ∼430 °C resulted in minuscule amounts of residue for all films, linear or cross-linked. The most cross-linked film had ∼99.50% thickness removed after annealing. The contact angle was found to increase with increasing cross-link density. Significant contact-angle hysteresis was observed, indicating surface reconfiguration, and the lowest receding angle was 17° for the linear film. Swelling measurements using spectroscopic ellipsometry showed that the degree of swelling decreased with increasing EGDA incorporation. The water content decreased from 35% (v/v) for the linear film to below 10% (v/v) for the most cross-linked film. These results show that iCVD is able to produce PHEMA thin films that function as hydrogels when soaked in water. The spectroscopic results, the contact-angle results, and the swelling analysis altogether prove the retention of the hydrophilic pendant groups in the iCVD process.
The article reports a two-step method for the synthesis of antifouling films; wherein we first deposited the copolymer films of poly(4-vinylpyridine-co-ethylene glycol diacrylate) (p-4VP-co-EGDA) ...onto the surface of different substrates (e.g., delicate substrate such as reverse osmosis (RO) membranes, Si wafers and gold sensors) via an all-dry and substrate-independent technique known as initiated chemical vapor deposition (iCVD). The iCVD technique is based upon free radical polymerization where vapors of initiator and monomers are transported to the surface to be modified to initiate simultaneous polymerization and thin film formation. In a subsequent step, th as-deposited copolymer films were converted to zwitterions by a diffusion limited gas-phase reaction with vapors of 1,3-propane sultone to form polysulfobetaine (pSB) type of zwitterionic surface moieties. Bare and the surface modified substrates were characterized by AFM, FTIR and XPS for their topographical and compositional analysis. The presence of zwitterionic moieties considerably discourages the irreversible attachment/adhesion of proteins and bacterial cells as demonstrated by quartz crystal microbalance with dissipation monitoring (QCM-D) and SEM analysis respectively. The results of bacterial adhesion studies showed 99.6% lower attachment of Pseudomonas aeruginosa cells onto the surface of modified RO membranes as compared to bare membranes. The authors wish to mention that a new pyridine-based pSB type of antifouling zwitterionic films has been added to the library of iCVD films. We believe that the demonstrated attempt of membrane surface modification strategy holds promise for potentially controlling the biofouling of other polymeric membranes such as ultrafiltration, nanofiltration and RO membranes. Furthermore, salient features such as solvent less nature, the easy tunablity of film and the capability of retention of functional group make iCVD a unique method to modify the surface of virtually any substrate including, metals, polymeric membranes, Si and gold sensors etc. Thus, it is expected that the developed coating technique could possibly lead to prepare a wide spectrum of environmentally benign antifouling films for other applications such as biomedical devices, medical implants and ship hulls where inhibition of microbial adhesion and subsequent biofilm growth is a severe challenge.
•Surface modification of commercial RO desalination membranes.•p(4VP-co-EGDA) co-polymerization via initiated chemical vapor deposition.•Functionalization of the co-polymer to polysulfobetaine zwitterionic surface moieties.•Significant reduction of bacterial cells and protein attachment on modified membrane surface.
An oxidative chemical vapor deposition (CVD) process is presented as an alternative to conventional solution-based processing of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. This solventless ...technique yields PEDOT with higher conductivities and conformally coats fibers and other high area morphologies, important for enhancing efficiencies in some organic electronic devices. The CVD method eliminates corrosive poly(styrenesulfonate) that is used to disperse PEDOT in an aqueous suspension for solution-based processing. A mechanistic approach is presented that favors the deposition of the conjugated, conducting form of PEDOT. We achieved conductivities as high as 105 S/cm and demonstrated films about 100 nm thick that do not crack upon bending and are more than 84% transparent to visible light. The compatibility of oxidative CVD deposition of PEDOT is demonstrated on silicon, glass, plastic, and paper substrates.
In previous studies of the desalination technology membrane distillation (MD), superhydrophobicity of the membrane has been shown to dramatically decrease fouling in adverse conditions, but the ...mechanism for this is not well understood. Additionally, air layers present on submerged solid superhydrophobic surfaces have been shown to dramatically reduce biofouling, and air-bubbling has been used to reducing fouling and increase flux and efficiency in MD. The present work studies the effect of maintaining air layers on the membrane surface and superhydrophobicity as a new method for preventing fouling of MD membranes by salts, particulates, and organic particles. Superhydrophobic MD membranes were prepared using initiated chemical vapor deposition (iCVD) of perfluorodecyl acrylate (PFDA) on poly(vinyldene fluoride) PVDF membranes and used to study the effects of surface energy on fouling. A static MD setup with evaporation through an MD membrane but no condensing of permeate was used to examine the effect of air exposure on fouling, by measuring the increase in weight of the membrane caused by scale deposition. Theory was derived for the reduction of fouling on superhydrophobic surfaces, and a review of related theory was included. Air layers may displace fouling gels, reduce the area of feed in contact with the membrane, reduce foulant adhesion, and enhance superhydrophobicity in a Cassie–Baxter state. The study shows that the presence of air on the membrane surface significantly reduces biological fouling, but in some cases had mildly exacerbating effects by increasing crystal formation of salts, especially when the air was not saturated with water vapor. Air recharging combined with superhydrophobicity reduced fouling in several cases where hydrophobic membranes alone did little.
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•MD membrane fouling experiments with CaSO4, NaCl, Silica, and Alginate with CaCl2.•Adding air periodically greatly reduced biofouling, but the effect on salts varied.•An iCVD coating of PFDA on PVDF membranes enhanced contact angle from 125° to 157°.•Nucleation in bulk and on bubble surfaces likely caused the majority of the fouling.•Demonstrated theory linking membrane hydrophobicity to reduced nucleation/fouling.
A novel approach to render virtually any surface superoleophobic underwater is developed via forming an ultrathin hydrophilic zwitterionic coating by initiated chemical vapor deposition. The ...superoleophobicity does not rely on nano‐fabricated hierarchical surface structures that have been considered necessary for underwater superoleophobicity.
Bulk polytetrafluoroethylene (PTFE) possesses excellent chemical stability and dielectric properties. Indeed, thin films with these same characteristics would be ideal for electret applications. ...Previously, the electret properties of PTFE-like thin films produced by rf sputtering or plasma enhanced chemical vapor deposition were found to deteriorate due to structural changes and surface oxidation. In this article, the technique of initiated chemical vapor deposition (iCVD) is evaluated for electret applications for the first time. The iCVD method is known for its solvent-free deposition of conformal, pinhole-free polymer thin films in mild process conditions. It is shown that PTFE thin films prepared in this way, show excellent agreement to commercial bulk PTFE with regard to chemical properties and dielectric dissipation factors. After ion irradiation in a corona discharge the iCVD PTFE thin films exhibit stable electret properties, which can be tailored by the process parameters. Due to the mild deposition conditions, the iCVD technique is suitable for deposition on flexible organic substrates for the next-generation electret devices. It is also compatible with state-of-the-art microelectronic processing lines due to the characteristics of conformal growth and easy scaling up to larger size substrates.
A solvent-free initiated chemical vapor deposition (iCVD) process was used to create low surface energy poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) thin films at deposition rates as high as 375 ...nm/min. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy showed full retention of the fluorine moieties, and no measurable cross-linking was detected. Additionally, the FTIR studies support the hypothesis that film deposition results from vinyl polymerization. For all iCVD PPFDA films, the static contact angle was found to be 120.8 ± 1.2°. The roughness of the films was found to be between 14.9 and 19.8 nm RMS, and the refractive index of the films was found to be between 1.36 and 1.37. The deposition rate was studied as a function of the substrate temperature and the partial pressure of the monomer. It was found that the deposition rate increases with decreasing substrate temperature and increasing monomer partial pressure. It was also found that the molecular weight increases with decreasing substrate temperature and increases with increasing monomer partial pressure. The highest molecular weight measured was 177 300 with a polydispersity of 2.27. Quartz crystal microbalance (QCM) measurements showed that these effects correlated with an increased monomer concentration at the surface. The deposition rate data and the QCM data were quantitatively analyzed to find the rate constants of the process using a previously published model for the iCVD process involving nonfluorinated monomers. The determined values of the rate constants of the surface polymerization were found to be similar to the rate constants measured in liquid-phase free radical polymerization. The kinetic data found in this paper can now be used to study iCVD deposition onto substrates with more complex geometries.
A simple, inexpensive, efficient, and scalable method to create air‐stable organic surface passivation layers on silicon using a vapor‐phase treatment is demonstrated. A variant of initiated chemical ...vapor deposition is used to synthesize a thin film that acts as both a passivation layer and an antireflective coating. The lowest surface recombination velocity reported to date is achieved and maintained during prolonged exposure to air.
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