We studied the formation of microstructured films at liquid surfaces via vapor phase polymerization of cross-linked polymers. The films were composed of micron-sized coral-like structures that originate at the liquid—vapor interface and extend vertically. The growth mechanism of the microstructures was determined to be simultaneous aggregation of the polymer on the liquid surface and wetting of the liquid on the growing aggregates. We demonstrated that we can increase the height of the microstructures and increase the surface roughness of the films by either decreasing the liquid viscosity or decreasing the polymer deposition rate. Our vapor phase method can be extended to synthesize functional, free-standing copolymer microstructured thin films for potential applications in tissue engineering, electrolyte membranes, and separations.

•Porous polymer deposition was patterned using poly(dimethylsiloxane) masks.•The porosity and thickness was controlled by varying the substrate temperature.•Free-standing patterned hydrophilic membranes were fabricated.•The surface chemistry of the membrane was tuned.The deposition of porous polymer membranes was patterned using poly(dimethylsiloxane) (PDMS) masks. The porous polymer was deposited by introducing the monomer and initiator sequentially to allow for the temperature, pressure, and duration of each step to be controlled independently. The porosity and thickness of the membranes was controlled by varying the substrate temperature during monomer deposition. The addition of a cross-linker during polymerization allowed for the fabrication of robust free-standing shaped hydrophilic membranes that are insoluble in aqueous solutions. Our ability to control the shape, thickness, porosity, and functionality of the porous membranes allows for the design of new surfaces for a variety of applications in sensors, filtration, and microfluidics.Download high-res image (106KB)Download full-size image

•Ionic liquid droplets are placed onto paper coated with fluoropolymer.•Polymerization must be performed sequentially.•Monomer is absorbed in the first step.•The initiator is introduced in the second step.•Resulting gel beads can be easily lifted from the underlying substrate.In this paper, we demonstrate the fabrication of gel beads composed of ionic liquid (IL) and polymer. The IL droplets are kept spherical during the deposition process by placement onto chromatography paper coated with fluoropolymer. The deposition process then occurs in two steps. In the first step, the monomer is absorbed into the IL droplet. In the second step, the initiator radicals are introduced. This sequential deposition process allows polymerization to primarily occur within the liquid droplet and therefore the beads are not attached to the underlying substrate and can be easily removed.

In this article, we study the growth of polymer nanoparticles that are formed on the surface of silicone oils via initiated chemical vapor deposition. The average radius of the particles can be increased by decreasing the silicone oil viscosity, increasing the deposition time, or increasing the deposition rate. The time series data indicates that there are two stages for particle growth. Particle nucleation occurs in the first stage and the particle size is dependent on the liquid viscosity and deposition rate. Particle growth occurs in the second stage, during which the particle size is dependent only on the amount of deposited polymer. This two-step process allows us to make core–shell particles by sequentially depositing different polymers. The benefits of our nanoparticle synthesis process are that solvents and surfactants are not required and the size of the nanoparticles can be controlled over a wide range of radii with a relatively narrow distribution.

In this work, we study the use of initiated chemical vapor deposition in conjunction with liquid scaffolds to deposit polymer canopies onto structured surfaces. Liquid is applied to micropillar and microstructure surfaces to act as a scaffolding template such that the deposited polymer films take the shape of the liquid surface. Two methods for directing the location of the scaffolding liquid were examined. In the first method, high surface tension liquids rest in a Cassie–Baxter state over the structured surfaces, allowing for control over the canopy location and size by varying the position and volume of the liquid. In the second method, the structured surfaces are inverted onto a thin layer of low surface tension liquid, allowing the coverage and height of the canopy to be controlled by varying the area and thickness of the liquid layer. Although the canopies demonstrated in this study were fabricated using initiated chemical vapor deposition, the generality of our scaffolding method can easily be translated to other vapor deposition processes.Keywords: chemical vapor deposition; coatings; polymers; thin films

We studied the copolymerization of an ionic liquid (1-ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI])) with ethylene glycol diacrylate (EGDA) via initiated chemical vapor deposition to form polymerized ionic liquid (PIL) copolymer films. The copolymerization was carried out by placing droplets of [EVIm][TFSI] in the reactor and introducing EGDA and tert-butyl peroxide initiator in the vapor phase. The heterogeneous films that formed at the surface of the liquid droplets were composed of a homopolymer PEGDA top layer that formed by the polymerization of adsorbed EGDA at the liquid—vapor interface and a poly([EVIm][TFSI]-co-EGDA) copolymer bottom layer that formed by the copolymerization of [EVIm][TFSI] with absorbed EGDA within the liquid. The copolymer layer contained a gradient composition with decreasing concentration of [EVIm][TFSI]. We showed that the composition of the copolymer films can be controlled by tuning the reaction time and pressure. In addition, we demonstrated that the films can be formed on solid supports which could allow these materials to be used as separation membranes and catalyst supports.

Here we fabricate patterned porous polymer membranes on porous substrates by a combination of physical masking and chemical vapor deposition. This all-dry technique eliminates solvent-related issues and allows for the fabrication of hierarchical porous-on-porous structures with a wide range of chemical compositions and shapes. The porous polymer membranes are made by operating at unconventional processing conditions to simultaneously deposit and polymerize monomer. The solid monomer serves as a porogen and creates microstructures around which polymer forms. Membranes with thicknesses ranging from a few hundred micrometers to a millimeter are fabricated on porous paper substrates. The resolution of the patterning process and the structure of the resulting membranes are analyzed as a function of the deposition time. It was found that the patterned membranes exhibit a tapered structure and the dimensions are in good agreement with the dimensions of the mask. One potential application of these patterned polymer membranes is demonstrated for the selective separation of analytes for diagnostic applications on paper-based microfluidic devices. The ability to pattern porous-on-porous structures can be useful for the development of hierarchical membranes for water purification and gas separation, and for sensing, patterned tissue scaffolding, and other lab-on-a-chip applications.Keywords: chemical vapor deposition; functional polymers; membranes; polymer; porous polymer;

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.Keywords: coatings; fluoropolymer; microfluidics; patterning; polymers; separations;

In this paper, we studied the formation of heterogeneous polymer films on ionic liquid (IL) substrates via the simultaneous or sequential depositions of monomers that are either soluble or insoluble in the liquid. We found that the insoluble monomer 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) only polymerizes at the IL surface, while the soluble monomer ethylene glycol diacrylate (EGDA) can polymerize at both the IL surface and within the bulk liquid. The polymer chains that form within the bulk liquid entrap IL as they integrate into the polymer film formed at the IL surface, resulting in heterogeneous films that contain IL on the bottom side. Varying the order in which the soluble and insoluble monomers were introduced into the system led to different film structures. When the insoluble monomer was introduced first, a film formed at the surface and the soluble monomer then diffused through this film and polymerized within the bulk, leading to a sandwich structure. When the soluble monomer was introduced first, a layered film was formed whose structure followed the order in which the monomers were introduced. When the two monomers were introduced simultaneously, the soluble monomer polymerized in the bulk while a copolymer film formed at the surface. This study provides an understanding of how to control the composition of layered polymer films deposited onto IL substrates in order to develop new composite materials for separation and electrochemical applications.

We have observed that the vapor-phase deposition of polymers onto liquid substrates can result in the formation of polymer films or particles at the liquid–vapor interface. In this study, we demonstrate the relationship between the polymer morphology at the liquid–vapor interface and the surface tension interaction between the liquid and polymer, the liquid viscosity, the deposition rate, and the deposition time. We show that the thermodynamically stable morphology is determined by the surface tension interaction between the liquid and the polymer. Stable polymer films form when it is energetically favorable for the polymer to spread over the surface of the liquid, whereas polymer particles form when it is energetically favorable for the polymer to aggregate. For systems that do not strongly favor spreading or aggregation, we observe that the initial morphology depends on the deposition rate. Particles form at low deposition rates, whereas unstable films form at high deposition rates. We also observe a transition from particle formation to unstable film formation when we increase the viscosity of the liquid or increase the deposition time. Our results provide a fundamental understanding about polymer growth at the liquid–vapor interface and can offer insight into the growth of other materials on liquid surfaces. The ability to systematically tune morphology can enable the production of particles for applications in photonics, electronics, and drug delivery and films for applications in sensing and separations.

We present a simple solution casting technique to apply polymer welds to stabilize capillary-force directed self-assembled systems including arrays of pillars and microbeads. The strength of the polymer welds can be enhanced by increasing either the polymer concentration or molecular weight. The use of responsive polymers to form the welds allow for the fabrication of hierarchical structures that actuate in response to external stimuli. For example, temperature-responsive and pH-responsive microstructures can be formed by solution casting poly(vinyl methyl ether) and poly(methacrylic acid), respectively. We demonstrate that polymer welds formed using biocompatible alginate allows for controllable release of microbeads in microfluidic channels, which has potential applications in drug delivery.Keywords: adhesion; capillary forces; controlled release; polymers; self-assembly; soft lithography;

Paper-based microfluidic devices have recently received significant attention as a potential platform for low-cost diagnostic assays. However, the number of advanced unit operations, such as separation of analytes and fluid manipulation, that can be applied to these devices has been limited. Here, we use a vapor phase polymerization process to sequentially deposit functional polymer coatings onto paper-based microfluidic devices to integrate multiple advanced unit operations while retaining the fibrous morphology necessary to generate capillary-driven flow. A hybrid grafting process was used to apply hydrophilic polymer coatings with a high surface concentration of ionizable groups onto the surface of the paper fibers in order to passively separate analytes, which allowed a multicomponent mixture to be separated into its anionic and cationic components. Additionally, a UV-responsive polymer was sequentially deposited to act as a responsive switch to control the path of fluid within the devices. This work extends the advanced unit operations available for paper-based microfluidics and allows for more complex diagnostics. In addition, the vapor phase polymerization process is substrate independent, and therefore, these functional coatings can be applied to other textured materials such as membranes, filters, and fabrics.

We studied the vapor deposition of polymers onto the surfaces of silicone oil and imidazolium-based ionic liquids (ILs). We found that the deposition of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(N-isopropylacrylamide) (PNIPAAm) resulted in polymer particles on silicone oil whereas continuous polymer skins formed on 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]). The silicone oil and ILs were patterned onto a common substrate by exploiting their different wetting properties. Ultrathin free-standing PHEMA and PNIPAAm films of different shapes were produced by confining the shape of the IL within a wax barrier, surrounding it with silicone oil, and then depositing the polymer. The silicone oil prevented the polymer film from connecting to the underlying substrate and maintained the shape of the polymer film during deposition. Our process allows for multidimensional control over the resulting free-standing film: the area of the shape can be controlled by patterning the IL, and the thickness of the film can be controlled by adjusting the duration of polymer deposition. The films are highly pure and do not contain any residual monomer or solvent entrapment which extends their potential applications to include in vivo biomedical research.

We demonstrate the use of vapor phase deposition to completely encapsulate ionic liquid (IL) droplets within robust polymer shells. The IL droplets were first rolled into liquid marbles using poly(tetrafluoroethylene) (PTFE) particles because the marble structure facilitates polymerization onto the entire surface area of the IL. Polymer shells composed of 1H,1H,2H,2H-perfluorodecyl acrylate cross-linked with ethylene glycol diacrylate (P(PFDA-co-EGDA)) were found to be stronger than the respective homopolymers. Fourier transform infrared spectroscopy showed that the PTFE particles become incorporated into the polymer shells. The integration of the particles increased the rigidity of the polymer shells and enabled the pure IL to be recovered or replaced with other fluids. Our encapsulation technique can be used to form polymer shells onto dozens of droplets at once and can be extended to encapsulate any low vapor pressure liquid that is stable under vacuum conditions.

In this paper, we demonstrate that thin layers of polymer coatings can be used to self-assemble pillars into stable microstructures. Polymer coatings are deposited onto elastomeric pillars using solventless initiated chemical vapor deposition and capillary forces are used to collapse the coated pillars into microstructures. The location of pillar collapse can be controlled by patterning regions of hydrophilicity and hydrophobicity. Poly(hydroxyethyl methacrylate) and poly(methacrylic acid) coatings stabilize the self-assembled microstructures by providing an adhesive force through solvent bonding. These solvent bonds allow the response of the microstructures to be tuned by varying the thickness of the polymer coating and the solubility parameter of the solvent. The coating process described in this paper is substrate-independent and therefore can be applied to pillars composed of any material.Keywords: adhesion; capillary forces; coatings; polymers; self-assembly; soft lithography;

The interior surfaces of pre-assembled poly(dimethylsiloxane) (PDMS) microfluidic devices were modified with a cross-linked fluoropolymer barrier coating that significantly increased the chemical compatibility of the devices.

The ability to pattern porous materials with functional polymeric coatings is important for the fabrication of next-generation microfluidic platforms, membranes, tissue scaffolds, and optical devices. Here, we demonstrate for the first time that solventless initiated chemical vapor deposition (iCVD) can be used for three-dimensional patterning of porous substrates. The individual fibers of hydrophilic chromatography paper were uniformly coated with a thin layer of hydrophobic photoresponsive poly(o-nitrobenzyl methacrylate) (PoNBMA). X-Ray photoelectron spectroscopy and contact angle measurements confirmed that the PoNBMA coating penetrated the entire depth of the paper and scanning electron microscope images confirmed that the porosity and hierarchical structure of the paper were retained during the coating process. The PoNBMA coating was then patterned through the entire depth of the paper by exposure to ultraviolet light followed by rinsing in biologically compatible buffer. We demonstrated the utility of our patterning process by fabricating three-dimensional hydrophilic and hydrophobic regions into the chromatography paper for use as paper-based microfluidic devices. Our patterning process represents an environmentally friendly method to pattern three-dimensional materials since no organic solvents are used during the polymerization process or patterning step.

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 CuCl2 and Cu(NO3)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.

Here we demonstrate a novel technique for the fabrication of porous polymer membranes via vapor phase polymerization. Vapor phase processing allows for control over the chemical functionality of the membranes and eliminates solubility requirements and surface tension effects. Porous polymer membranes are formed by concurrent deposition of solid monomer and polymerization, which is achieved by increasing the partial pressure of the monomer above its saturation pressure and decreasing the substrate temperature below the freezing point of the monomer. The membranes exhibit dual-scale porosity, where the large-scale pores form during the deposition and the small-scale pores form upon sublimation of the solid monomer. We demonstrate that the growth rate and pore size of the membrane can be controlled by varying the reactor parameters, including deposition time, monomer partial pressure, and substrate temperature. Stimuli-responsive poly(methacrylic acid) and poly(N-isopropylacrylamide) membranes were fabricated to show the generality of the process. Furthermore, the ability to make copolymer membranes was demonstrated using ethylene glycol diacrylate as a cross-linking agent. Our ability to produce tailored polymer membranes with chemically diverse compositions has potential applications in separations and biosensing.

We studied a new method for preparing polymer–ionic liquid (IL) gels via deposition of vapor phase precursors onto thin layers of IL. The solubility of 2-hydroxyethyl methacrylate in 1-ethyl-3-methylimidazolium tetrafluoroborate enabled polymerization at both the IL–vapor interface and within the IL layer. We observed a transition from a viscous liquid to a gel with increasing polymer concentration. At short deposition times, there were two distinct molecular weights reflecting polymerization at the IL–vapor interface and within the IL layer, while at longer deposition times the molecular weight distribution within the IL layer broadened. The polymer chains within the IL were orders of magnitude larger than the polymer chains at the IL–vapor interface, and increasing the reactor pressure was shown to increase the molecular weight. Our ability to form high molecular weight polymer chains allows for the formation of gels for utilization as fuel-cell membranes and thin-film transistors.