Influence of Oblique Angle Deposition on Porous Polymer Film Formation.

In this study, we applied oblique angle deposition to a modified initiated chemical vapor deposition (iCVD) process to synthesize porous poly(methacrylic acid) (PMAA) films. During the modified iCVD process, frozen monomer molecules are first captured on a cooled substrate, then polymerization occurs via a free radical polymerization mechanism, and finally, the excess monomer is sublimated, resulting in a porous polymer film. We found that delivering the monomer through an extension at an oblique angle resulted in porous films with three morphological regions. Region 1 is located nearest to the monomer extension outlet and consists of porous polymer pillars; region 2 consists of densified pillars, which occur due to the recapturing and polymerization of the sublimated monomer; and region 3 is located furthest from the monomer extension outlet and consists of dendritic structures, which occur due to low monomer concentration. We investigated the role of substrate temperature and monomer deposition time on the growth process. We found that changing the extension angle influenced the location of the regions and the film coverage across the substrate. Our results provide useful guidelines for tuning the structures within porous polymer films by varying the angle of monomer delivery.

Vapor Deposition of Silicon-Containing Microstructured Polymer Films onto Silicone Oil Substrates.

In this study, a silicon-containing cross-linked polymer, poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane-co-ethylene glycol diacrylate) (p(V4D4-co-EGDA)), was deposited onto high-viscosity silicone oil using initiated chemical vapor deposition (iCVD). The ratio of the feed flow rate of V4D4 to EGDA was systematically studied, and the chemical composition and morphology of the top and bottom surfaces of the films were analyzed. The films were microstructured, and the porosity and thickness of the films increased with increasing V4D4 content. The top of the film was composed of densely packed and loosely packed microstructured regions. X-ray photoelectron spectroscopy on the top and bottom surfaces of the films showed a heterogeneous chemical composition along the thickness of the film, with higher silicon content on the top surface compared to that on the bottom surface. To the best of our knowledge, this is the first study of iCVD deposition of a silicon-containing polymer film onto silicone oil. The results of this study can be used for the synthesis of polymer precursor films for the fabrication, via pyrolysis, of silicon-based inorganic membranes for use in hydrogen production using silicone oil to prevent infiltration of monomer into the underneath membrane support structure during vapor deposition.

Modular microfluidics for double emulsion formation.

For many engineering applications such as manipulating two phase flows, generating single and double emulsions, and passively propelling liquids through channels, control over the surface energy of microfluidic channels is essential. In particular, double emulsion formation, which benefits from alternating hydrophobic and hydrophilic sections of channel, represents a challenge in fabricating controlled microfluidic channel surface properties. As double emulsions find further applications in single-cell handling and analysis, straightforward methods for generating them increase in value. Here, we present a method for generating double emulsions in microfluidic channels fabricated from modular fluidic blocks. By using a vapor-phase polymer coating technology-initiated chemical vapor deposition-we are able to fabricate blocks with varying surface properties. Assembling these blocks together then creates step-like changes in surface energy within a microchannel.

Giant Lipid Vesicle Formation Using Vapor-Deposited Charged Porous Polymers.

In this study, we prepare giant lipid vesicles using vapor-deposited charged microporous poly(methacrylic acid- co-ethylene glycol diacrylate) polymer membranes with different morphologies and thicknesses. Our results suggest that vesicle formation is favored by thinner, more structured porous hydrogel substrates. Electrostatic interactions between the polymer and the lipid head groups affect vesicle yield and size distribution. Repulsive electrostatic interactions between the hydrogel and the lipid head groups promote vesicle formation; attractive electrostatic interactions suppress vesicle formation. Ionic strength and sugar concentration are also major parameters affecting the yield and size of giant vesicles. The presence of both ions and sugars in the hydration buffer results in increased vesicle yields. These results indicate that lipid-polymer interactions and osmotic effects in addition to the substrate morphology and surface charge are key factors affecting vesicle formation. Our data suggest that surface chemistry should be designed to tune electrostatic interactions with the lipid mixture of interest to promote vesicle formation. This vapor-deposited hydrogel fabrication technique offers tunability over the physicochemical properties of the hydrogel substrate for the production of giant vesicles with different sizes and compositions.

Surface functionalization of 3D-printed plastics via initiated chemical vapor deposition.

3D printing is a useful fabrication technique because it offers design flexibility and rapid prototyping. The ability to functionalize the surfaces of 3D-printed objects allows the bulk properties, such as material strength or printability, to be chosen separately from surface properties, which is critical to expanding the breadth of 3D printing applications. In this work, we studied the ability of the initiated chemical vapor deposition (iCVD) process to coat 3D-printed shapes composed of poly(lactic acid) and acrylonitrile butadiene styrene. The thermally insulating properties of 3D-printed plastics pose a challenge to the iCVD process due to large thermal gradients along the structures during processing. In this study, processing parameters such as the substrate temperature and the filament temperature were systematically varied to understand how these parameters affect the uniformity of the coatings along the 3D-printed objects. The 3D-printed objects were coated with both hydrophobic and hydrophilic polymers. Contact angle goniometry and X-ray photoelectron spectroscopy were used to characterize the functionalized surfaces. Our results can enable the use of iCVD to functionalize 3D-printed materials for a range of applications such as tissue scaffolds and microfluidics.

Synthesis of Functional Particles by Condensation and Polymerization of Monomer Droplets in Silicone Oils.

We studied the synthesis of poly(4-vinylpyridine) and poly(2-hydroxyethyl methacrylate) polymer particles in silicone oil using a sequential vapor phase polymerization method in which monomer droplets were first condensed onto a layer of silicone oil and subsequently polymerized via a free radical mechanism. The viscosity of the silicone oil was systematically varied. At lower viscosities, a heterogeneous particle size distribution was produced where small particles were formed by engulfment of the monomer droplets at the liquid surface and large particles were formed by coalescence of the monomer droplets inside the liquid layer. Coalescence could be inhibited by increasing the viscosity of the silicone oil leading to a decreased average radius and a narrower size distribution of the polymer particles. The advantages of our method for the fabrication of polymer particles are that it does not require surfactants or organic solvents.

Two-Stage Growth of Polymer Nanoparticles at the Liquid-Vapor Interface by Vapor-Phase Polymerization.

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.

Flow invariant droplet formation for stable parallel microreactors.

The translation of batch chemistries onto continuous flow platforms requires addressing the issues of consistent fluidic behaviour, channel fouling and high-throughput processing. Droplet microfluidic technologies reduce channel fouling and provide an improved level of control over heat and mass transfer to control reaction kinetics. However, in conventional geometries, the droplet size is sensitive to changes in flow rates. Here we report a three-dimensional droplet generating device that exhibits flow invariant behaviour and is robust to fluctuations in flow rate. In addition, the droplet generator is capable of producing droplet volumes spanning four orders of magnitude. We apply this device in a parallel network to synthesize platinum nanoparticles using an ionic liquid solvent, demonstrate reproducible synthesis after recycling the ionic liquid, and double the reaction yield compared with an analogous batch synthesis.

Fabricating Polymer Canopies onto Structured Surfaces Using Liquid Scaffolds.

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.

Microstructured Films Formed on Liquid Substrates via Initiated Chemical Vapor Deposition of Cross-Linked Polymers.

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.

Synthesis of polymer nanoparticles via vapor phase deposition onto liquid substrates.

In this article, the growth of polymer nanoparticles formed at the liquid-vapor interface via vapor phase polymerization is studied. The particles grow by polymer aggregation, which is driven by the surface tension interaction between the liquid and polymer. It is demonstrated that the mechanism of particle growth is determined by whether polymer particles remain at the liquid-vapor interface or submerge into the liquid. The position of the particles depends on the interaction between the polymer and the liquid. For example, the deposition of poly(n-butyl acrylate) onto poly(dimethyl siloxane) and Krytox liquids leads to the formation of nanoparticles that remain at the liquid-vapor interface. The size of these particles increases as a function of deposition time. The deposition of poly(4-vinylpyridine) onto poly(dimethyl siloxane) and Krytox leads to the formation of nanoparticles that submerge into the liquid. The size of these particles does not significantly change with deposition time. Our study offers a new rapid, one-step synthetic approach for fabricating functional polymer nanoparticles for applications in catalysis, photonics, and drug delivery.

Fluoropolymer surface coatings to control droplets in microfluidic devices.

We have demonstrated the application of low surface energy fluoropolymer coatings onto poly(dimethylsiloxane) (PDMS) microfluidic devices for droplet formation and extraction-induced merger of droplets. Initiated chemical vapor deposition (iCVD) was used to pattern fluoropolymer coatings within microchannels based on geometrical constraints. In a two-phase flow system, the range of accessible flow rates for droplet formation was greatly enhanced in the coated devices. The ability to controllably apply the coating only at the inlet facilitated a method for merging droplets. An organic spacer droplet was extracted from between a pair of aqueous droplets. The size of the organic droplet and the flow rate controlled the time to merge the aqueous droplets; the process of merging was independent of the droplet sizes. Extraction-induced droplet merging is a robust method for manipulating droplets that could be applied in translating multi-step reactions to microfluidic platforms.

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.

Solventless fabrication of porous-on-porous materials.

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.

Effect of surface tension, viscosity, and process conditions on polymer morphology deposited at the liquid-vapor interface.

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.

Formation of heterogeneous polymer films via simultaneous or sequential depositions of soluble and insoluble monomers onto ionic liquids.

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.

Responsive polymer welds via solution casting for stabilized self-assembly.

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.

Vapor phase deposition of functional polymers onto paper-based microfluidic devices for advanced unit operations.

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.

Encapsulation of ionic liquids within polymer shells via vapor phase deposition.

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.

Two-phase microfluidic droplet flows of ionic liquids for the synthesis of gold and silver nanoparticles.

Droplet-based microfluidic platforms have the potential to provide superior control over mixing as compared to traditional batch reactions. Ionic liquids have advantageous properties for metal nanoparticle synthesis as a result of their low interfacial tension and complexing ability; however, droplet formation of ionic liquids within microfluidic channels in a two-phase system has not yet been attained because of their complex interfacial properties and high viscosities. Here, breakup of an imidazolium-based ionic liquid into droplets in a simple two-phase system has for the first time been achieved and characterized by using a microchannel modified with a thin film fluoropolymer. This microfluidic/ionic liquid droplet system was used to produce small, spherical gold (4.28 ± 0.84 nm) and silver (3.73 ± 0.77 nm) nanoparticles.