Enhancing the Kinetics of Vapor-based Polymerization by Pulsed Filament Approach

Initiated chemical vapor deposition is a versatile technique for synthesizing conformal polymer films on both planar and porous surfaces. It can retain functional groups and avoid undesired cross-linking. However, there is still room for enhancing its performance without altering the feed parameters. Here, we investigate a pulsed iCVD approach to improve the deposition process, achieved by switching on and off the resistively heated filament periodically. By strategically switching off the filament, a shortage of thermally activated primary radicals was created, which allowed uninterrupted chain propagation with fewer termination reactions and potentially increased monomer conversion rates. This has caused significantly faster deposition kinetics with a higher molecular weight and longer chain length for poly(glycidyl methacrylate) compared to continuous deposition. Spectra analyses confirmed that the functionality and stoichiometry ratios remained intact throughout the pulsed deposition process. The pulsed iCVD method is therefore a competitive and sustainable tool, demonstrating fast deposition kinetics and a well-preserved functionality.

Nanotopographical control of surfaces using chemical vapor deposition processes

In recent years much work has been conducted in order to create patterned and structured polymer coatings using vapor deposition techniques - not only via post-deposition treatment, but also directly during the deposition process. Two-dimensional and three-dimensional structures can be achieved via various vapor deposition strategies, for instance, using masks, exploiting surface properties that lead to spatially selective deposition, via the use of additional porogens or by employing oblique angle polymerization deposition. Here, we provide a concise review of these studies.

Patterned fluoropolymer barriers for containment of organic solvents within paper-based microfluidic devices

In this study, we demonstrate for the first time the ability to pattern lipophobic fluoropolymer barriers for the incorporation of pure organic solvents as operating liquids within paper-based microfluidic devices. Our fabrication method involves replacing traditional wax barriers with fluoropolymer coatings by combining initiated chemical vapor deposition with inhibiting transition metal salt to pattern the polymer. Multiple techniques for patterning the transition metal salt are tested including painting, spray coating, and selective wetting through the use of a photoresist. The efficacy of the barrier coatings to contain organic solvents is found to be dependent on the conformality of the polymer deposited around the paper fibers. We demonstrate examples of the benefits provided by the containment of organic solvents in paper-based microfluidic applications including the ability to tune the separation of analytes by varying the operating solvent and by modifying the channel region of the devices with additional polymer coatings. The work exhibited in this paper has the potential to significantly expand the applications of paper-based microfluidics to include detection of water insoluble analytes. Additionally, the generality of the patterning process allows this technique to be extended to other applications that may require the use of patterned hydrophobic and lipophobic regions, such as biosensing, chemical detection, and optics.

25th anniversary article: CVD polymers: a new paradigm for surface modification and device fabrication

Well-adhered, conformal, thin (<100 nm) coatings can easily be obtained by chemical vapor deposition (CVD) for a variety of technological applications. Room temperature modification with functional polymers can be achieved on virtually any substrate: organic, inorganic, rigid, flexible, planar, three-dimensional, dense, or porous. In CVD polymerization, the monomer(s) are delivered to the surface through the vapor phase and then undergo simultaneous polymerization and thin film formation. By eliminating the need to dissolve macromolecules, CVD enables insoluble polymers to be coated and prevents solvent damage to the substrate. CVD film growth proceeds from the substrate up, allowing for interfacial engineering, real-time monitoring, and thickness control. Initiated-CVD shows successful results in terms of rationally designed micro- and nanoengineered materials to control molecular interactions at material surfaces. The success of oxidative-CVD is mainly demonstrated for the deposition of organic conducting and semiconducting polymers.

CVD of polymeric thin films: applications in sensors, biotechnology, microelectronics/organic electronics, microfluidics, MEMS, composites and membranes

Polymers with their tunable functionalities offer the ability to rationally design micro- and nano-engineered materials. Their synthesis as thin films have significant advantages due to the reduced amounts of materials used, faster processing times and the ability to modify the surface while preserving the structural properties of the bulk. Furthermore, their low cost, ease of fabrication and the ability to be easily integrated into processing lines, make them attractive alternatives to their inorganic thin film counterparts. Chemical vapor deposition (CVD) as a polymer thin-film deposition technique offers a versatile platform for fabrication of a wide range of polymer thin films preserving all the functionalities. Solventless, vapor-phase deposition enable the integration of polymer thin films or nanostructures into micro- and nanodevices for improved performance. In this review, CVD of functional polymer thin films and the polymerization mechanisms are introduced. The properties of the polymer thin films that determine their behavior are discussed and their technological advances and applications are reviewed.

Directed deposition of functional polymers onto porous substrates using metal salt inhibitors

This paper demonstrates the ability to control the location of polymer deposition onto porous substrates using vapor phase polymerization in combination with metal salt inhibitors. Functional polymers such as hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate), click-active poly(pentafluorophenyl methacrylate), and light-responsive poly(ortho-nitrobenzyl methacrylate) were patterned onto porous hydrophilic substrates using metal salts. A combinatorial screening approach was used to determine the effects of different transition metal salts and reaction parameters on the patterning process. It was found that CuCl(2) and Cu(NO(3))(2) were effective at uniformly inhibiting the deposition of all three polymers through the depth of the porous substrate and along the entire cross section. This study offers a new and convenient method to selectively deposit a wide variety of functional polymers onto porous materials and will enable the production of next-generation multifunctional paper-based microfluidic devices, polymeric photonic crystals, and filtration membranes.

Chemical vapor deposition of conformal, functional, and responsive polymer films

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.

Positive-tone nanopatterning of chemical vapor deposited polyacrylic thin films

Terpolymers of methyl alpha-chloroacrylate (MCA), methacrylic acid (MAA), and methacrylic anhydride (MAH) were synthesized by initiated chemical vapor deposition (iCVD) of MCA and MAA followed by low-temperature annealing that partially converts MAA into MAH. The MAA composition in the iCVD copolymer can be systematically varied between 37 and 85 mol % by adjusting the gas feed fractions of monomers. Study of the monomer reactivity ratios and the copolymer molecular weights supports the hypothesis of a surface propagation mechanism during the iCVD copolymerization. The carboxylic dehydration reaction at the annealing temperature of 160 degrees C is dominated by a mechanism of intramolecular cyclization, resulting in intramolecular MAH anhydride formation while preventing crosslink formation. The incorporation of highly electron-withdrawing anhydride functionality enhances chain scission susceptibility under electron-beam irradiation. P(MCA-MAA-MAH) terpolymer thin films can be completely developed at dosages as low as 20 microC/cm2 at 50 kV. High-quality positive-tone patterns were created with 60 nm feature size achieved in the vapor deposited films.

Photoinitiated chemical vapor deposition of polymeric thin films using a volatile photoinitiator

Photoinitiated chemical vapor deposition (piCVD) is an evolutionary CVD technique for depositing polymeric thin films in one step without using any solvents. The technique requires no pre- or post-treatment and uses a volatile photoinitiator to initiate free-radical polymerization of gaseous monomers under UV irradiation. Glycidyl methacrylate (GMA) was used as a test monomer for its ability to undergo free-radical polymerization, and 2,2'-azobis(2-methylpropane) (ABMP) was used as the photoinitiator, as it is known to produce radicals when excited by photons. GMA and ABMP vapors were fed into a vacuum chamber in which film growth was observed on a substrate exposed to UV irradiation. The resulting poly(glycidyl methacrylate) (PGMA) thin films were comprised of linear chains and had high structural resemblance to conventionally polymerized PGMA, as shown by the high solubility in tetrahydrofuran and the infrared and X-ray photoelectron spectroscopy measurements. The introduction of ABMP into the vacuum chamber significantly increased growth rates. The maximum growth rate achieved was approximately 140 nm/min and represents a 7-fold enhancement over the case without ABMP. The molecular weight was found to increase with increasing monomer-to-initiator (M/I) feed ratio, and the polydispersity indexes (PDIs) of the samples were between 1.8 and 2.2, lower than the values obtained in conventional batch polymerization but in agreement with the theoretical expressions developed for low-conversion solution-phase polymerization, which are applicable to continuous processes such as piCVD. Molecular-weight distributions can be narrowed by filtering out wavelengths shorter than 300 nm, which induce branching and/or cross-linking. The strong dependence of the molecular weight on the M/I ratio, the rate enhancement due to the use of a radical photoinitiator, the good agreement between the experimental, and the theoretical PDIs provide evidence of a free-radical mechanism in piCVD. The clear films obtained in this work had number-average molecular weights between 12 500 and 97 000 g/mol. The similarities in growth conditions, growth rates, and molecular weights between the initiated CVD, a previously reported thermal process able to synthesize a wide range of polymers, and the piCVD of PGMA suggest that piCVD can also be used to produce those polymers and potentially others whose monomers undergo free-radical mechanisms. This paper serves as an introduction to the technique by demonstrating piCVD's ability in synthesizing high-molecular-weight PGMA thin films with narrow molecular-weight distributions from vapors of GMA and ABMP in a single, dry step under UV irradiation.