Superhydrophobic Fabrics Produced by Electrospinning and Chemical Vapor Deposition

Fluorophobic effect for building up the surface morphology of electrodeposited substituted conductive polymers

During the past decade, several works display the electrochemical polymerization of fluorinated monomers as a highly efficient method (one-pot method, mild conditions, various morphologies) to obtain superhydrophobic or superoleophobic surfaces. Here, we point out the fluorinated tails not only are useful to increase both hydrophobicity and oleophobicity but also are involved in the formation of surface structurations: two necessary conditions for liquid dewetting. To support this assertion, a series of fluorinated pyrrole derivatives and their hydrocarbon homologues were synthesized and electrochemically deposited in the same conditions. Whereas hydrocarbon pyrroles give rise to smooth films, structured films are achieved from fluorinated pyrroles. The abundance of the surface structurations depends on the length of the fluorinated tail.

Superhydrophobic conductive carbon nanotube coatings for steel

We report the synthesis of superhydrophobic coatings for steel using carbon nanotube (CNT)-mesh structures. The CNT coating maintains its structural integrity and superhydrophobicity even after exposure to extreme thermal stresses and has excellent thermal and electrical properties. The coating can also be reinforced by optimally impregnating the CNT-mesh structure with cross-linked polymers without significantly compromising on superhydrophobicity and electrical conductivity. These superhydrophobic conductive coatings on steel, which is an important structural material, open up possibilities for many new applications in the areas of heat transfer, solar panels, transport of fluids, nonwetting and nonfouling surfaces, temperature resilient coatings, composites, water-walking robots, and naval applications.

Superhydrophobic and self-cleaning bio-fiber surfaces via ATRP and subsequent postfunctionalization

Superhydrophobic and self-cleaning cellulose surfaces have been obtained via surface-confined grafting of glycidyl methacrylate using atom transfer radical polymerization combined with postmodification reactions. Both linear and branched graft-on-graft architectures were used for the postmodification reactions to obtain highly hydrophobic bio-fiber surfaces by functionalization of the grafts with either poly(dimethylsiloxane), perfluorinated chains, or alkyl chains, respectively. Postfunctionalization using alkyl chains yielded results similar to those of surfaces modified by perfluorination, in terms of superhydrophobicity, self-cleaning properties, and the stability of these properties over time. In addition, highly oleophobic surfaces have been obtained when modification with perfluorinated chains was performed.

A conformal nano-adhesive via initiated chemical vapor deposition for microfluidic devices

A novel high-strength nano-adhesive is demonstrated for fabricating nano- and microfluidic devices. While the traditional plasma sealing methods are specific for sealing glass to poly(dimethylsiloxane) (PDMS), the new method is compatible with a wide variety of polymeric and inorganic materials, including flexible substrates. Additionally, the traditional method requires that sealing occur within minutes after the plasma treatment. In contrast, the individual parts treated with the nano-adhesive could be aged for at least three months prior to joining with no measurable deterioration of post-cure adhesive strength. The nano-adhesive is comprised of a complementary pair of polymeric nanolayers. An epoxy-containing polymer, poly(glycidyl methacrylate) (PGMA) was grown via initiated chemical vapor deposition (iCVD) on the substrate containing the channels. A plasma polymerized polyallylamine (PAAm) layer was grown on the opposing flat surface. Both CVD monomers are commercially available. The PGMA nano-adhesive layer displayed conformal coverage over the channels and was firmly tethered to the substrate. Contacting the complementary PGMA and PAAm surfaces, followed by curing at 70 degrees C, resulted in nano- and micro-channel structures. The formation of the covalent tethers between the complementary surfaces produces no gaseous by-products which would need to outgas. The nano-adhesive layers did not flow significantly as a result of curing, allowing the cross-sectional profile of the channel to be maintained. This enabled fabrication of channels with widths as small as 200 nm. Seals able to withstand > 50 psia were fabricated employing many types of substrates, including silicon wafer, glass, quartz, PDMS, polystyrene petri dishes, poly(ethylene terephthalate) (PET), polycarbonate (PC), and poly(tetrafluoro ethylene) (PTFE).

Superhydrophobic polyimide films with a hierarchical topography: combined replica molding and layer-by-layer assembly

Artificial superhydrophobic surfaces with a hierarchical topography were fabricated by using layer-by-layer assembly of polyelectrolytes and silica nanoparticles on microsphere-patterned polyimide precursor substrates followed with thermal and fluoroalkylsilane treatment. In this special hierarchical topography, micrometer-scale structures were provided by replica molding of polyamic acid using two-dimensional arrays of polystyrene latex spheres as templates, and nanosized silica particles were then assembled on these microspheres to construct finer structures at the nanoscale. Heat treatment was conducted to induce chemical cross-linking between polyelectrolytes and simultaneously convert polyamic acid to polyimide. After surface modification with fluoroalkylsilane, the as-prepared highly hydrophilic surface was endowed with superhydrophobicity due to the bioinspired combination of low surface energy materials and hierarchical surface structures. A superhydrophobic surface with a static water contact angle of 160 degrees and sliding angle of less than 10 degrees was obtained. Notably, the polyimide microspheres were integrated with the substrate and were mechanically stable. In addition, the chemical and mechanical stability of the polyelectrolyte/silica nanoparticle multilayers could be increased by heat-induced cross-linking between polyelectrolytes to form nylon-like films, as well as the formation of interfacial chemical bonds.

Conformal coverage of poly(3,4-ethylenedioxythiophene) films with tunable nanoporosity via oxidative chemical vapor deposition

Novel nanoporous poly(3,4-ethylenedioxythiophene) (PEDOT) films with basalt-like surface morphology are successfully obtained via a one-step, vapor phase process of oxidative chemical vapor deposition (oCVD) by introducing a new oxidant, CuCl(2). The substrate temperature of the oCVD process is a crucial process parameter for controlling electrical conductivity and conjugation length. Moreover, the surface morphology is also systemically tunable through variations in substrate temperature, a unique advantage of the oCVD process. By increasing the substrate temperature, the surface morphology becomes more porous, with the textured structure on the nanometer scale. The size of nanopores and fibrils appears uniformly over 25 mm x 25 mm areas on the Si wafer substrates. Conformal coverage of PEDOT films grown with the CuCl(2) oxidant (C-PEDOT) is observed on both standard trench structures with high aspect ratio and fragile surfaces with complex topology, such as paper, results which are extremely difficult to achieve with liquid phase based processes. The tunable nanoporosity and its conformal coverage on various complex geometries are highly desirable for many device applications requiring controlled, high interfacial area, such as supercapacitors, Li ion battery electrodes, and sensors. For example, a highly hydrophilic surface with the static water contact angle down to less than 10 degrees is obtained solely by changing surface morphology. By applying fluorinated polymer film onto the nanoporous C-PEDOT via initiative chemical vapor deposition (iCVD), the C-PEDOT surface also shows the contact angle higher than 150 degrees . The hierarchical porous structure of fluorinated polymer coated C-PEDOT on a paper mat shows superhydrophobicity and oil repellency.

Towards bioinspired superhydrophobic poly(L-lactic acid) surfaces using phase inversion-based methods

The water repellency and self-cleaning ability of many biological surfaces has inspired many fundamental and practical studies related to the development of synthetic superhydrophobic surfaces. However, the investigation of such substrates made of biodegradable polymers has been scarce. Simple approaches based on a single step, performed at room temperature (and pressure), were implemented to obtain superhydrophobic poly(L-lactic acid) (PLLA) surfaces via phase inversion-based methods, without addition of low-surface-energy compounds. Water contact angles above 150 degrees were obtained using some processing conditions. In such cases scanning electronic microscopy micrographs of such surfaces revealed a clear rough texture composed by leafy clusters with micro-nano binary structures. Such materials could be used in specific environmental and biomedical applications, namely in implantable materials or in antibacterial or antithrombogenic surfaces.

Aligned silicon carbide nanowire crossed nets with high superhydrophobicity

Aligned silicon carbide nanowire crossed nets (a-SiCNWNs) were directly synthesized by using a vapor-solid reaction at 1100 degrees C. Zinc sulfide was used as catalyst to assist the growth of a-SiCNWNs with small size and crystal structure. After functionalization with perfluoroalkysilane, a-SiCNWNs showed excellent superhydrophobic property with a high water contact angle more than 156 +/- 2 degrees , compared to random nanowires (147 +/- 2 degrees ) and pure silicon wafers (101 +/- 2 degrees ). The topographic roughness and chemical modification with CF 2/CF 3 groups contributed the better superhydrophobicity. Furthermore, the as-grown SiCNWNs can be scraped off and coated on other substrates such as pure silicon wafers. The novel nanowire coating with good superhydrophobicity displays extensive applications in silicon-related fields such as solar cells, radar, etc.

Formation of fibers by electrospinning

Electrostatic fiber formation, also known as "electrospinning", has emerged in recent years as the popular choice for producing continuous threads, fiber arrays and nonwoven fabrics with fiber diameters below 1 microm for a wide range of materials, from biopolymers to ceramics. It benefits from ease of implementation and generality of use. Here, we review some of the basic aspects of the electrospinning process, as it is widely practiced in academic laboratories. For purposes of organization, the process is decomposed into five operational components: fluid charging, formation of the cone-jet, thinning of the steady jet, onset and growth of jet instabilities that give rise to diameter reduction into the submicron regime, and collection of the fibers into useful forms. Dependence of the jetting phenomenon on operating variables is discussed. Continuum level models of the jet thinning and jet instability are also summarized and put in some context.

All-dry synthesis and coating of methacrylic acid copolymers for controlled release

Initiated chemical vapor deposition (iCVD) is presented as an all-dry synthesis and coating method for applying methacrylic acid copolymers as pH-responsive controlled release layers. iCVD combines the strengths of liquid-phase chemical synthesis with a precision solvent-free chemical vapor deposition environment. Copolymers of methacrylic acid and ethyl acrylate were confirmed by a systematic shift in the carbonyl bond stretching mode with a shift in the comonomer ratio within the copolymer and by the ability to apply the Fineman-Ross copolymerization equation to describe copolymerization kinetics. Copolymers of methacrylic acid and ethylene dimethacrylate showed pH-dependent swelling behavior that was applied to the enteric release of fluorescein and ibuprofen.

Carbon nanotubes by electrospinning with a polyelectrolyte and vapor deposition polymerization

Electrospinning of a polyelectrolyte and vapor deposition polymerization were combined to fabricate nanotubes of oxidatively stabilized poly(acrylonitrile) (PANDelta) with an outer diameter of 100 nm, a wall thickness of 14 nm, and centimeter-scale length. Poly(styrene sulfonate) sodium (PSSNa) nanofibers serves as sacrificial cores while vapor deposition polymerization was used to form smooth PAN sheaths of even thickness. After the PAN sheaths had been oxidatively stabilized, the PSSNa cores were etched away with water to form nanotubes of PANDelta. High-temperature carbonization of these nanotubes at 900 degrees C under Ar flow yielded carbon nanotubes with an outer diameter of 80 nm and wall thickness of 10 nm. Raman spectroscopy confirms that the carbon nanotubes were composed of highly disordered graphene sheets, consistent with the carbonization of PAN under similar conditions. These carbon nanotubes have many promising applications as catalyst supports, gas absorbents, and as encapsulants for controlled release of active compounds.

Superhydrophobic films of electrospun fibers with multiple-scale surface morphology

Superhydrophobic nanofiber films were created from electrospun nanofibers with undulated surfaces at multiple scales in micrometers and nanometers. The electrospun nanofibers were produced out of aqueous solutions which contained water-soluble polymers and different colloids: monodisperse silica or polystyrene microspheres for larger particles and monodisperse silica nanoparticles for smaller particles. Various types of fibrous films were produced depending on the properties of the dispersing medium, the effects of additives, and the compositions of the bidisperse colloids. When polystyrene microspheres were used as sacrificial templates, macropores were left behind in the nanofibers during the removal of polystyrene microspheres by calcination. The nonwoven films of electrospun nanofibers, which were decorated with silica microspheres or macropores, could be continuously produced with considerable ease under a relatively wide range of operating conditions. The surface properties of the films were characterized by contact angle measurement and an X-ray photoelectron spectrometer. Through the surface modification of the electrospun nanofibers with fluorinated silane coupling agents, superhydrophobic surfaces with low sliding angles were successfully prepared.

Simultaneous Tuning of Chemical Composition and Topography of Copolymer Surfaces: Micelles as Building Blocks

A simple method is described for controlling the surface chemical composition and topography of the diblock copolymer poly(styrene)-b-poly(dimethylsiloxane)(PS-b-PDMS) by casting the copolymer solutions from solvents with different selectivities. The surface morphology and chemical composition were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively, and the wetting behavior was studied by water contact angle (CA) and sliding angle (SA) and by CA hysteresis. Chemical composition and morphology of the surface depend on solvent properties, humidity of the air, solution concentration, and block lengths. If the copolymer is cast from a common solvent, the resultant surface is hydrophobic, with a flat morphology, and dominated by PDMS on the air side. From a PDMS-selective solvent, the surface topography depends on the morphology of the micelles. Starlike micelles give rise to a featureless surface nearly completely covered by PDMS, while crew-cut-like micelles lead to a rough surface with a hierarchical structure that consists partly of PDMS. From a PS-selective solvent, however, surface segregation of PDMS was restricted, and the surface morphology can be controlled by vapor-induced phase separation. On the basis of the tunable surface roughness and PDMS concentration on the air side, water repellency of the copolymer surface could be tailored from hydrophobic to superhydrophobic. In addition, reversible switching behavior between hydrophobic and superhydrophobic can be achieved by exposing the surface to solvents with different selectivities.

Initiated chemical vapor deposition of antimicrobial polymer coatings

The vapor phase deposition of polymeric antimicrobial coatings is reported. Initiated chemical vapor deposition (iCVD), a solventless low-temperature process, is used to form thin films of polymers on fragile substrates. For this work, finished nylon fabric is coated by iCVD with no affect on the color or feel of the fabric. Infrared characterization confirms the polymer structure. Coatings of poly(dimethylaminomethyl styrene) of up to 540 microg/cm2 were deposited on the fabric. The antimicrobial properties were tested using standard method ASTM E2149-01. A coating of 40 microg/cm2 of fabric was found to be very effective against gram-negative Escherichia coli, with over a 99.99%, or 4 log, kill in just 2 min continuing to over a 99.9999%, or 6 log, reduction in viable bacteria in 60 min. A coating of 120 microg/cm2 was most effective against the gram-positive Bacillus subtilis. Further tests confirmed that the iCVD polymer did not leach off the fabric.

To adjust wetting properties of organic surface by in situ photoreaction of aromatic azide

This article describes development of a simple and convenient method to provide stable low-surface-energy coatings on organic surfaces, by designing and synthesizing a surface-reactive molecule 4-azido-N-dodecylbenzamide, which bears an azide group as the reactive surface anchor and an alkyl chain as the hydrophobic tail. After the hydrophobic modification, rough organic surfaces with contact angle of about 0 degrees can change their surface wetting properties from superhydrophilicity to superhydrophobicity, whose contact angles are above 152 degrees and tilt angles lower than 5 degrees. Moreover, by changing the alkyl chain to a PEO segment, a similar concept can be used to adjust the surface wetting properties from hydrophobic (contact angle approximately 130 degrees) to superhydrophilic (contact angle approximately 0 degrees).

What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces

Superhydrophobic surfaces have drawn a lot of interest both in academia and in industry because of the self-cleaning properties. This critical review focuses on the recent progress (within the last three years) in the preparation, theoretical modeling, and applications of superhydrophobic surfaces. The preparation approaches are reviewed according to categorized approaches such as bottom-up, top-down, and combination approaches. The advantages and limitations of each strategy are summarized and compared. Progress in theoretical modeling of surface design and wettability behavior focuses on the transition state of superhydrophobic surfaces and the role of the roughness factor. Finally, the problems/obstacles related to applicability of superhydrophobic surfaces in real life are addressed. This review should be of interest to students and scientists interested specifically in superhydrophobic surfaces but also to scientists and industries focused in material chemistry in general.

Melt coaxial electrospinning: a versatile method for the encapsulation of solid materials and fabrication of phase change nanofibers

We have developed a method based on melt coaxial electrospinning for fabricating phase change nanofibers consisting of long-chain hydrocarbon cores and composite sheaths. This method combines melt electrospinning with a coaxial spinneret and allows for nonpolar solids such as paraffins to be electrospun and encapsulated in one step. Shape-stabilized, phase change nanofibers have many potential applications as they are able to absorb, hold, and release large amounts of thermal energy over a certain temperature range by taking advantage of the large heat of fusion of long-chain hydrocarbons. We have focused on compounds with melting points near room temperature (octadecane) and body temperature (eicosane) as these temperature ranges are most valuable in practice. We have produced thermally stable, phase change materials up to 45 wt % octadecane, as measured by differential scanning calorimetry. In addition, the resultant fibers display novel segmented morphologies for the cores due to the rapid solidification of the hydrocarbons driven by evaporative cooling of the carrier solution. Aside from the fabrication of phase change nanofibers, the melt coaxial method is promising for applications related to microencapsulation and controlled release of drugs.

Direct catalytic route to superhydrophobic polyethylene films

Polyethylene films grow on a flat silica surface modified by the bis(imino)pyridyl iron(II) catalyst during ethylene polymerization in toluene solvent. The resulting films show superhydrophobic properties. Advancing water contact angle as high as 169 degrees and sliding angles as low as 2 degrees are obtained on these films. SEM images reveal special surface structures of these films containing micrometer-sized islands, submicrometer particles on the islands, and stress nanofibers between the islands, which render superhydrophobicity to the polyethylene surfaces. After the submicrometer particles and stress nanofibers are removed by annealing, the superhydrophobic properties of the polymer films disappear.

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.