Fluoropolymer Surface Research Articles

Functionalization of fluoropolymer surfaces with nanopatterned polyelectrolyte brushes

We present a strategy to combine the excellent bulk properties of fluoropolymer substrates with the wide range of functionalities of surface-grafted polyelectrolyte brushes. Patterns of radicals serving as initiators were created by irradiation with extreme ultraviolet light (EUV) in an interference setup at the Swiss Light Source. From these initiators, brushes of poly(methacrylic acid) or poly(4-vinylpyridine) were grafted in one step by free-radical polymerization. Brushes carrying primary or secondary amines, i.e. poly(vinylamine), poly(allylamine) and poly(N-methyl-vinylamine), were obtained by grafting vinylformamide and acrylonitrile followed by hydrolysis or reduction. Periodic patterns with a resolution of 200 nm were achieved, while the thickness of the brushes in unpatterned areas could be controlled over a range of several hundred nanometers by variation of EUV dose and grafting parameters. The maximum dry brush thickness was used to estimate the average molecular weight of the polymer chains.

Electrowetting of Aqueous Solutions of Ionic Liquid in Solid−Liquid−Liquid Systems

The influence of water on the electrowetting performance of the ionic liquid (IL) [BMIM][BF4] in contact with an ambient phase of hexadecane was studied. Electrowetting experiments on a fluoropolymer surface with IL concentrations between 0 and 99% enabled investigation of various electrowetting parameters, including the maximum range of accessible contact angles, the saturation contact angle (and voltage), and spreading dynamics. Experiments carried out using dc and ac potentials were successfully described with the Young−Lippmann equation prior to contact angle saturation. The saturation contact angles differed for ac and dc electrowetting, indicating distinct saturation mechanisms. The dilution of [BMIM][BF4] by water, within the range of concentrations studied, had no direct impact on the electrowetting behavior, aside from indirect effects due to altered viscosity and interfacial tension.

Complementary Electrowetting Devices on Plasma-Treated Fluoropolymer Surfaces

A reversal of the normal two-fluid competitive (water vs oil) electrowetting (nEW) on dielectric has been achieved by plasma irradiation of the normally hydrophobic fluoropolymer followed by thermal annealing. This process first renders the surface hydrophilic and then returns it to its normal hydrophobic properties as measured by water droplet contact and rolling angles. In the plasma-irradiated and annealed EW device (pEW), the normal two-fluid EW action is reversed after an initial charging step, with the oil layer being displaced at zero voltage and being returned at high voltage. A possible explanation of this effect is a plasma-induced modification of the fluoropolymer, rendering it more susceptible to charge injection and trapping at high voltage. nEW and pEW devices exhibit complementary EW operation, as verified by oil movement and optical transmission. This method can lead to low-power operation of two-fluid EW devices.

Use of simple pharmacokinetic modeling to characterize exposure of Australians to perfluorooctanoic acid and perfluorooctane sulfonic acid

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) have been used for a variety of applications including fluoropolymer processing, fire-fighting foams and surface treatments since the 1950s. Both PFOS and PFOA are polyfluoroalkyl chemicals (PFCs), man-made compounds that are persistent in the environment and humans; some PFCs have shown adverse effects in laboratory animals. Here we describe the application of a simple one compartment pharmacokinetic model to estimate total intakes of PFOA and PFOS for the general population of urban areas on the east coast of Australia. Key parameters for this model include the elimination rate constants and the volume of distribution within the body. A volume of distribution was calibrated for PFOA to a value of 170ml/kgbw using data from two communities in the United States where the residents' serum concentrations could be assumed to result primarily from a known and characterized source, drinking water contaminated with PFOA by a single fluoropolymer manufacturing facility. For PFOS, a value of 230ml/kgbw was used, based on adjustment of the PFOA value. Applying measured Australian serum data to the model gave mean±standard deviation intake estimates of PFOA of 1.6±0.3ng/kgbw/day for males and females >12years of age combined based on samples collected in 2002–2003 and 1.3±0.2ng/kg bw/day based on samples collected in 2006–2007. Mean intakes of PFOS were 2.7±0.5ng/kgbw/day for males and females >12years of age combined based on samples collected in 2002–2003, and 2.4±0.5ng/kgbw/day for the 2006–2007 samples. ANOVA analysis was run for PFOA intake and demonstrated significant differences by age group (p=0.03), sex (p=0.001) and date of collection (p<0.001). Estimated intake rates were highest in those aged >60years, higher in males compared to females, and higher in 2002–2003 compared to 2006–2007. The same results were seen for PFOS intake with significant differences by age group (p<0.001), sex (p=0.001) and date of collection (p=0.016).

Addressable Cell Microarrays via Switchable Superhydrophobic Surfaces

Here we describe an approach to fabricate addressable cell microarrays, which are based on the patterned switchable superhydrophobic surfaces. The switchable superhydrophobic surfaces were prepared by roughening the surfaces of fluoropolymers on the electrodes. Upon the application of 150 V to the underneath electrodes, the water contact angle on the roughened fluoropolymer surfaces could be decreased from 163° to less than 10° allowing the deposition of fibronectin, which could guide the growth of the cells. Our result indicated that it was possible to control the spatial distribution of two different cells on the cell microarrays.

Drop size dependence of contact angles on two fluoropolymers

Young’s equation predicts that the contact angle of a liquid drop is independent of its size. Nevertheless, large drop size dependences of contact angles have been observed, especially for millimetre-sized drops, on a variety of solid surfaces. We report new measurements of drop size dependence of contact angles for several liquids on two fluoropolymer surfaces, Teflon AF 1600 and EGC-1700. We demonstrate a new strategy for contact angle measurement that allows detection of approximately 0.1° changes in the contact angle during the growth of a drop. We find that on the surfaces examined, drop size dependence of contact angles is ten times smaller than on all previously studied fluoropolymers at the millimetre scale. The data are insensitive to various attempted surface modifications. We discuss the interpretation of the data and possible physical sources.

Fabrication of planar polymer waveguides for evanescent-wave sensing in aqueous environments

We present a detailed account of processing issues related to fabrication of optical waveguide sensors intended for evanescent-wave sensing in aqueous solutions or surface-bound fluorescence excitation in biological samples. The waveguides consist of a polymer layer on top of a fluoropolymer (Cytop ™) cladding. The fluoropolymer is closely index-matched to water, providing a symmetric cladding environment which simplifies optical excitation and provides tunability in penetration depth not available with other evanescent-wave techniques. We present methods of controlling the wettability of the fluoropolymer surface and improving adhesion to the core waveguide layer. Furthermore, we demonstrate the application of the waveguide structure to fluorescence imaging of cultured cells.

Fabrication of high-mobility organic single-crystal field-effect transistors with amorphous fluoropolymer gate insulators

High-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable electrodes. Before constructing interfaces to crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize carrier mobility. Mobility values estimated in the saturation region reproducibly exceeded 15 cm 2/V s for all the 10 devices, reaching 30 cm 2/V s for the best two devices. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable for the excellent transistor performance.

The influence of hydrophobicity, roughness and charge upon ultrafiltration membranes for black tea liquor clarification

This paper investigates the clarification of black tea using UF membranes. The efficiency of separation and the final product quality were investigated using regenerated cellulose (RC) and fluoropolymer (FP) membranes of different nominal molar mass cut-off values. Changes during filtration in the hydrophobicity, surface charge on the pore walls, surface roughness and chemical properties were investigated. Results indicate that both of the membrane materials studied produced clarified tea liquors with polyphenol transmission rates of ca. 90%. Increased fouling was present on rougher, more hydrophobic FP surfaces. When roughness, charge and hydrophobicity were similar (i.e. for the RC membrane), variation in pore size was not found to affect the filtration properties significantly over the range investigated. Porous structures with isoelectric points at the pH of the liquor being filtered were linked to an increased fouling tendency. All RC and FP100 kg mol −1 membranes were cleaned relatively easily. FP10 kg mol −1 and FP30 kg mol −1 membranes were found to have undergone surface modification, including increased negative charge and changes in hydrophobicity. The dominant fouling mechanism during all ultrafiltration experiments was found to be cake filtration. The cake formed on RC membranes is thought to dominate filtration properties due to similar transmissions detected through varied pore sizes.

High-mobility Organic Single-crystal Transistors with Amorphous Fluoropolymer Gate Insulators

AbstractHigh-mobility rubrene single-crystal field-effect transistors are built on highly water- and oil-repellent fluoropolymer gate insulators. Roughness is intentionally introduced at the surface once to provide good adhesion to metal films and photoresist polymers for stable bottom electrodes. Before constructing interfaces with rubrene crystals, smoothness of the fluoropolymer surface is recovered by annealing at a moderate temperature to maximize mobility of the carriers induced near the interfaces. The estimated mobilities in the saturation region reproducibly exceeded 15 cm2/Vs for all the ten devices fabricated in this method and reach 30 cm2/Vs for the best two samples among them. The results demonstrate that the water-repellency and smoothness of the dielectric polymers are favorable in the excellent transistor performance.

Study of the properties and structure of polyimide and bilayer fluoropolyimide films after space exposure on the Mir space station

Results of examination of bilayer polyimide and fluorinated polyimide films after their exposure to outer space on the Mir space station are reported. The polyimide films were screened by quartz plates during exposure. An increase in the surface energy and the work of adhesion indicate substantial hydrophilization of the open surface, as well as slight hydrophilization of the screened film surface. A difference in values of the surface energy and work of adhesion depending on the orientation of a sample relative to the direction of motion of the space station indicates that the surface properties of the outer surface of the unscreened film became anisotropic in character, with the anisotropy axis being aligned with the direction of motion of the spacecraft. The fact that the outer surface of the film has acquired anisotropic properties is corroborated by the results of study of scanning electron microscopic images and polar plots of brightness. The hydrophilization of the films indicates an increase in the concentration of polar groups. In the space environment, the degradation of the fluoropolymer surface layer and its removal under high-vacuum conditions take place. For the quartz-shielded polyimide films, the surface energy and the work of adhesion remained practically the same as for the unexposed films. It was concluded that solar radiation (μ > 200 nm) does not cause a significant decline in the properties of the polyimide film.

Functionalization of Saturated Fluorocarbons with and Without Light.

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Effect of coating on cleanability of glazed surfaces

The aim of this study was to establish the effects of different coatings on the cleanability of glazed ceramics. The surface properties were examined with a contact angle meter, a contact profilometer and a confocal microscope. The surfaces were soiled with three radiochemical model soils: inorganic particle soil, organic particle soil and oil soil. Soil adhesion on surfaces was measured with a quantitative radiochemical procedure. Generally, cleanability of the particles present in the model soil was found to be affected by the roughness of the surfaces; however, the cleanability of the oil in the model soils correlated with the contact angle of water on the surfaces. Coating of glazes, especially with fluoropolymer film, generally increased the contact angle values. The coatings affected the cleanability of ceramics somewhat: particle soils were removed most efficiently from glazes coated with TiO 2 and Zr. By contrast the oil soil residues of the fluoropolymer surfaces were the lowest. The cleanability results of the three model soils based on inorganic or organic particles or oil were different indicating differences between the cleanabilities of these main components of the soils.

In vivo biostability of polyether polyurethanes with fluoropolymer and polyethylene oxide surface modifying endgroups; resistance to metal ion oxidation

Polyether polyurethanes are subject to oxidation catalyzed by, and through direct (redox) reaction with transition metal ions (metal ion oxidation, MIO). The source of the ions is corrosion of metallic parts within an implanted device. A Shore 80A polyether polyurethane was modified with fluoropolymer (E80AF) or polyethylene oxide (E80AP) surface modifying end groups (SME). The SME migrates to the surface to form a covalently bonded monolayer, while maintaining the bulk properties of the polyurethane. In vitro tests in H(2)O(2) solution indicated that both SME's accelerated MIO. Tubing samples containing cobalt mandrels were implanted in the subcutis of rabbits for up to 2 years. In vivo, E80AF significantly slowed the rate of visible degradation, but did not prevent MIO. E80AP had virtually identical visual performance to the unmodified control in vivo. Infrared spectroscopy and molecular weight correlated well with visual appearance. When cracks were seen, polyether soft segment oxidation was occurring. Both E80AP and the control developed severe loss of molecular weight in vivo. The changes were much less severe for E80AF. Thus, contrary to in vitro test results, the PEO SME had no effect at all on MIO resistance, while the fluoropolymer SME produced a significant improvement in biostability.

In vivobiostability of shore 55D polyether polyurethanes with and without fluoropolymer surface modifying endgroups

Polyether polyurethanes are subject to autooxidation and environmental stress cracking (ESC) because of interactions with lysosomal oxygen-free radicals. Oxidation can also be catalyzed by and caused by direct (redox) reaction with transition metal ions (metal ion oxidation, MIO). The source of the ions is corrosion of metallic parts within an implanted device. A previous study on a Shore 80A polyether polyurethane modified with fluoropolymer surface modifying end groups demonstrated improved biostability over unmodified controls. We predicted that this could be extrapolated to the inherently more biostable Shore 55D version (E55DF). While it is difficult to demonstrate significant biodegradation in the harder polymers within a reasonable time frame, we did see excellent biologic autooxidation and ESC resistance for both E55DF and its unmodified Shore 55D (P55D) control. E55DF was slightly, but significantly more resistant to MIO than P55D. This was particularly evident in molecular weight distributions with P55D exhibiting a large decrease in number average molecular weight compared to no change for E55DF. Both were markedly superior to the softer Shore 80A control. It does appear that one can extrapolate accelerated in vivo biostability results with the softer polymers to their harder analogues.

In vivo biostability of polyether polyurethanes with fluoropolymer surface modifying endgroups: Resistance to biologic oxidation and stress cracking

A series of Shore 80A polyether polyurethanes were synthesized with from 0 to 6% fluoropolymer surface modifying endgroups (SME) to provide the bulk properties of the polyurethane with the surface properties of the fluoropolymer. It was theorized that the fluoropolymer would migrate to the surface, forming a monolayer barrier to the oxidants and crack-driving agents released by macrophages and foreign body giant cells in vivo. In a 12-week biostability screening test, samples strained to 400% elongation appeared to be highly stable. In a longer-term study, the fluoropolymer SME significantly delayed, but did not completely prevent the onset of microcracking and the development of environmental stress cracking in strained samples. Even so, the 4 and 6% SME polymers explanted at 2 years performed significantly better than the control. FTIR analysis did not correlate with SME concentration, but increased hydrogen-bonding index and loss of aliphatic ether (autoxidation) did correlate with the visual appearance and density of microcracks. Significant molecular weight reductions were seen for the SME-free control, but were small (within instrumental error) for the polymers with SME. The use of fluoropolymer as a SME does appear to be warranted as a means to improve polyether polyurethane biostability.

Functionalization of saturated fluorocarbons with and without light

Photochemical transformation of saturated fluorocarbons into tetrabutylammonium enolates has been improved, and a method employing ketyls as reductants has been developed that accomplishes the same chemistry without light. Enolates have been isolated as enol methyl ethers, from which they can be efficiently regenerated with tetrabutylammonium iodide. In other cases, enolates have been isolated as the corresponding ketone or stable enol. Fluorocarbon LUMO energies correlate with their reactivity and serve as a guide to the choice of ketyl. Use of this chemistry for fluoropolymer surface modification is discussed.

Engineering the Electrocapillary Behavior of Electrolyte Droplets on Thin Fluoropolymer Films

This study presents methods for engineering the electrocapillary behavior of fluoropolymer surfaces through the use of surfactants and an external insulating liquid. By the scaling of the appropriate surface energies, electrocapillary behavior is obtained at a record low voltage, with contact angle changes in excess of 100 degrees at 4 V. A consistent description of electrocapillary saturation is presented, identifying three separate regimes: breakdown, thermodynamic instability, and relaxation. Methods for identifying and mitigating some of the saturation behaviors are discussed. Finally, the parameters influencing the observed voltage of zero charge are summarized.

Electrowetting of Ionic Liquids

We have successfully demonstrated that imidazolium- and pyrrolidinium-based commercial room-temperature ionic liquids can electrowet (with a dc voltage) a smooth fluoropolymer (Teflon AF1600) surface. Qualitatively, the process is analogous to the electrowetting of aqueous electrolyte solutions: the contact angle versus voltage curve has a parabolic shape which saturates at larger voltages (positive or negative). On the other hand we observed several peculiarities: (i) the efficiency is significantly lower (by about an order of magnitude); (ii) the influence of the bulky cation is larger and the importance of the smaller anion is lesser, especially with respect to electrowetting saturation; (iii) there is an asymmetry in the saturation contact angles found for positive and negative voltages. The asymmetry may be correlated with the cation-anion asymmetry of the ionic liquids. The low efficiency may be caused by the presence of water and other impurities in these commercial materials.

Adhesion of microwave‐plasma‐treated fluoropolymers to thermoset vinylester

AbstractPoly(tetrafluoroethylene) and a fluoroethylene copolymer were surface treated with a 2.45‐GHz microwave plasma to enhance their adhesion to a vinylester thermoset. The plasmas were generated with an inert gas (Ar) and with reactive gases (H2, O2, and N2). The lap‐joint shear stress was measured on fluoropolymer samples glued with the vinylester. In general, the stress at failure increased with increasing plasma‐energy dose. The H2 plasma yielded the best adhesion, and X‐ray photoelectron spectroscopy revealed that it yielded the highest degree of defluorination of the fluoropolymer surface. The defluorination efficiency declined in the order H2, Ar, O2, and N2. Contact angle measurements and scanning electron microscopy revealed that the surface roughness of the fluoropolymer depended on the rate of achieving the target energy dose. High power led to a smoother surface, probably because of a greater increase in temperature and partial melting. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 838–842, 2005