FEP Surface Research Articles

Changes in optical properties of polymeric materials due to atomic oxygen in very low Earth orbit

Atomic oxygen (AO) is the dominant constituent of the residual atmosphere in low Earth orbit (LEO). AO collides with spacecraft at a velocity of 8 km/s, causing oxidation and erosion of polymeric materials. Particularly in very low Earth orbit (VLEO) at altitudes below 300 km, the AO density is at least ten times that at 500–700 km, and the concentration of molecular nitrogen (N2) is 10% or more. Such high-flux AO and N2 can cause nonlinear processes and mass losses for polymers. This study aims to clarify the optical property changes for polymeric materials exposed to LEO and VLEO in the Material Degradation Monitor (MDM) mission onboard the Super-Low-Altitude Test Satellite (SLATS). The AO and N2 fluences and the UV irradiances were estimated using the NRLMSISE-00 atmospheric and solar radiation models. The relationships were investigated between the changes in CCD images and the estimated fluences of the samples. For silsesquioxane (SQ)-coated polyimide film, the diffuse reflection of visible light increased noticeably with an AO fluence above 1 × 1021 atoms/cm2. The formation of a silica layer on the coating induces cracks. For silver-coated FEP films, the diffuse reflection increased entirely at AO fluences above 3 × 1021 atoms/cm2. The AO and N2 collisions would erode and roughen the FEP surfaces. Visible transmission through silver-coated FEP films also increased locally with increasing AO fluence, indicating that some AO oxidized the silver layer despite the presence of an Inconel (Ni alloy) layer. The visible reflection from Beta Cloth was decreased by its reaction with UV. Further, the competitive reactions with UV and AO resulted in two-step changes in the visible reflection from the ETFE polymer (cable covering).

Insight into direct phosphorus release from simulated wastewater ferric sludge: Influence of physiochemical factors

Phosphorous recovery from chemical phosphorus removal (CPR) plants is limited. CPR plants that use iron salts for phosphorous removal generate iron phosphate (Fe-P) rich sludge. This study investigates the influence of pH and competing anions (chloride), on phosphorus and soluble iron release from lab simulated Fe-P sludge. Factors influencing recovery, such as sludge age and Fe/P molar ratio are also investigated. Results showed that alkaline treatment of Fe-P sludge was not effective in releasing Fe (<3%) but effective in releasing P, particularly at controlled pH value of 10 where the %P release was (90 ± 2%). Sludge ageing time did not affect P release from Fe-P sludge at ages less than 5 days. However, a remarkable decrease in %P release (i.e., 50% reduction) was observed at ages of 9 and 11 days for pH values of 9–10. Arsenic extraction of exchangeable surface bound P from Fe-P sludge and surface area determination of hydrous ferric oxide (HFO) under different aging times supported qualitatively the internalization of surface bound P during aging, which is a proposed mechanism responsible for the decrease in P release for older sludge. Chloride effect in releasing P from Fe-P sludge was negligible. The reduction in P release by aging was best described by a zero-order kinetic model. Modeling with the PHREEQC geochemical software did not always agree with the measured release of phosphorus from the Fe-P sludge. PHREEQC assumes all surface sites for phosphorus are exchangeable and has no mechanism to represent phosphorus trapped within HFO particles.

Highly Selective Electrochemical Reduction of CO2 to Alcohols on an FeP Nanoarray.

Electrochemical reduction of CO2 into various chemicals and fuels provides an attractive pathway for environmental and energy sustainability. It is now shown that a FeP nanoarray on Ti mesh (FeP NA/TM) acts as an efficient 3D catalyst electrode for the CO2 reduction reaction to convert CO2 into alcohols with high selectivity. In 0.5 m KHCO3 , such FeP NA/TM is capable of achieving a high Faradaic efficiency (FE ) up to 80.2 %, with a total FE of 94.3 % at -0.20 V vs. reversible hydrogen electrode. Density functional theory calculations reveal that the FeP(211) surface significantly promotes the adsorption and reduction of CO2 toward CH3 OH owing to the synergistic effect of two adjacent Fe atoms, and the potential-determining step is the hydrogenation process of *CO.

Highly Selective Electrochemical Reduction of CO2 to Alcohols on an FeP Nanoarray

AbstractElectrochemical reduction of CO2 into various chemicals and fuels provides an attractive pathway for environmental and energy sustainability. It is now shown that a FeP nanoarray on Ti mesh (FeP NA/TM) acts as an efficient 3D catalyst electrode for the CO2 reduction reaction to convert CO2 into alcohols with high selectivity. In 0.5 m KHCO3, such FeP NA/TM is capable of achieving a high Faradaic efficiency (FE ) up to 80.2 %, with a total FE of 94.3 % at −0.20 V vs. reversible hydrogen electrode. Density functional theory calculations reveal that the FeP(211) surface significantly promotes the adsorption and reduction of CO2 toward CH3OH owing to the synergistic effect of two adjacent Fe atoms, and the potential‐determining step is the hydrogenation process of *CO.

Fabrication of Wettability-Patterned Surface for Cellular Micropatterning Using Step-Wise Ion Beam Processing.

In this study, the fabrication of a wettability patterned surface for cellular micropatterning was investigated using step-wise ion beam processing. A perfluorinated poly(ethylene-co-propylene) (FEP) film was first irradiated using accelerated Xe+ ions with 100 keV of energy at the low current density of 1 μA/cm² over the entire surface. Second, its confined regions were irradiated at the higher current density of 15 μA/cm² at various ion fluences through the pattern mask to generate patterns with big differences in wettability. From the analytic results, it was clearly verified that the step-wise irradiation induced effective chemical and morphological changes on the FEP surface, resulting in the successful formation of well-defined micropatterns with relatively hydrophilic and superhydrophobic surfaces. Moreover, the results of in-vitro cell culture showed well-resolved formation of 200 μm cell micropatterns on the wettability patterned FEP surface due to the individual effects of the relatively hydrophilic and superhydrophobic properties on the cell adhesiveness and proliferation.

Surface Performance and Cytocompatibility Evaluation of Acrylic Acid-Mediated Carboxymethyl Chitosan Coating on Poly(tetrafluoroethylene-co-hexafluoropropylene)

To improve the biocompatibility of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) film, a technique based on Ar plasma pretreatment and UV-induced grafting polymerization was used to immobilize carboxymethyl chitosan (CMCS) on the FEP film surfaces. Initially Ar plasma was used to treat FEP film. Then, plasma treated FEP film was modified via UV-induced grafting polymerization with hydrophilic acrylic acid (AAc) monomer. The following immobilization of CMCS on the FEP-pAAc surface was carried out via an amidation reaction. The change of chemical composition and surface morphology of FEP film were characterized by attenuated total reflectance Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Results of water contact angles measurement showed that the hydrophilicity of the surface has improved significantly after surface modification. Furthermore, methyl thiazolyl tetrazolium (MTT) assay and cell morphology analysis confirmed that mouse fibroblasts (L929 cells) attachment and proliferation were improved remarkably on the modified FEP surface. These results suggest that CMCS were successfully employed to surface engineering FEP film, and enhanced its cell biocompatibility. The approach presented here may be exploited for surface modification of biomaterials.

Poly(acrylic acid)-Grafted Fluoropolymer Films for Highly Sensitive Fluorescent Bioassays

In this study, a facile and effective method for the surface functionalization of inert fluoropolymer substrates using surface grafting was demonstrated for the preparation of a new platform for fluorescence-based bioassays. The surface of perfluorinated poly(ethylene-co-propylene) (FEP) films was functionalized using a 150 keV ion implantation, followed by the graft polymerization of acrylic acid, to generate a high density of carboxylic acid groups on the implanted surface. The resulting functionalized surface was investigated in terms of the surface density of carboxylic acid, wettability, chemical structure, surface morphology, and surface chemical composition. These results revealed that poly(acrylic acid) (PAA) was successfully grafted onto the implanted FEP surface and its relative amount depended on the fluence. To demonstrate the usefulness of this method for the fabrication of bioassays, the PAA-grafted FEP films were utilized for the immobilization of probe DNA for anthrax toxin, followed by hybridization with Cy3-labeled target DNA. Liver cancer-specific α-feto-protein (AFP) antigen was also immobilized on the PAA-grafted FEP films. Texas Red-labeled secondary antibody was reacted with AFP-specific primary antibody prebound to the AFP antigen using an immunoassay method. The results revealed that the fluorescence intensity clearly depended on the concentration of the target DNA hybridized to the probe DNA and the AFP antigen immobilized on the FEP films. The lowest detectable concentrations of the target DNA and the AFP antigen were 10 fg/mL and 10 pg/mL, respectively, with the FEP films prepared at a fluence of 3 × 10(14) ions/cm(2).

Heparin-grafted poly(tetrafluoroethylene-co-hexafluoropropylene) film with highly effective blood compatibility via an esterification reaction

The aim of this work was to modify poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) film surface in order to improve its hydrophilicity and blood compatibility. Acrylic acid (AAc) was introduced onto Ar plasma-pretreated FEP surface by UV-induced grafted copolymerization. Subsequently, heparin (Hp) was also immobilized covalently with the carboxylic group of PAAc grafted onto the FEP surface via an esterification reaction. The chemical composition and the morphological change of the modified films were characterized by attenuated total reflectance Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM), atomic force microscopy (AFM) and water contact angel measurements. The root–mean–square average surface roughness (Ra) changed from 5.82nm (FEP) to 13.60nm (FEP-PAAc) and 11.74nm (FEP-PAAc-Hp). Water contact angles changed from 115.0o (FEP) to 32.4o (FEP-PAAc) and 44.8o (FEP-PAAc-Hp). The blood compatibility of the modified films was evaluated by platelet adhesion experiments. All results suggested that the FEP-PAAc-Hp surface presented excellent antithrombotic properties and good hydrophilicity. Therefore, this method will provide a potential and effective solution for the surface modification of medical polymer materials.

Antibacterial coatings on poly(fluoroethylenepropylene) films via grafting of 3-hexadecyl-1-vinylimidazolium bromide

Surface coatings of pretreated poly(fluoroethylene propylene) (FEP) films by UV-induced graft copolymerization with 2-hydroxyethyl methacrylate (HEMA) and an antimicrobial agent, 3-hexadecyl-1-vinylimidazolium bromide (VIB), have been carried out. Surface chemical and morphological modifications were investigated with spectroscopic analysis using attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR), scanning electron microscopy (SEM) and X-ray diffraction distribution map of bromide atoms. Fluorescein dye absorption studies were performed in order to quantify the amount of reacted VIB. Antibacterial activities were assayed with Staphylococcus aureus (S. aureus, a Gram positive bacterium) culture on HEMA-VIB modified FEP specimens, prepared with different amounts of the antimicrobial agent. In addition to bactericidal properties, the functionalized FEP surface also prevented the adhesion of microorganisms to the film.

Fabrication of fine micro protrusions on fluoropolymer surface using ion beam irradiation

Tissue reconstruction is the aim of regeneration medicine and tissue engineering. One of the techniques is cell sheet engineering, which requires creating a contiguous cell sheet and harvesting it safely. To create a favorable substrate for cultivation, we attempted to fabricate the microstructure at polytetrafluoroethylene (PTFE) surfaces using ion beam irradiation. PTFE is a typical fluoropolymer and it possesses chemical and bioinertness. When the surface is irradiated with an ion beam, it is covered with a large number of microprotrusions, which have an aspect ratio of about 100. We previously reported that fibroblast cells grow and spread between the tops of the protrusions. In this study, we fabricated fine protrusions at both the PTFE and the poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) surfaces. The composition of FEP and PTFE is similar but FEP melts at a lower temperature than PTFE does. These samples were irradiated under controlled temperature and rubbing preparation. Their surface morphology and chemical bonding were observed. When the PTFE surface was irradiated at 350 °C, the protrusions were formed at the lower fluence, with an improved uniformity of shape. This was because the morphological change and evaporation of the thin parts were enhanced during high-temperature irradiation. In the FEP sample, very fine protrusions formed at much lower fluence by irradiation at room temperature. On the surface, the cells spread closely, like a monolayer. Further, the FEP samples were still optically transparent after the irradiation. They have an advantage in that we can observe the living cells attaching to the surface in their natural state without them being killed, fixed, and stained in an optical microscope. We found that the irradiated FEP surface is suitable for making a cell sheet.

PMMA and FEP surface modifications induced with EUV pulses in two selected wavelength ranges

Surface modification of PMMA and FEP polymers using extreme ultraviolet (EUV) in two spectral ranges was investigated. A laser-plasma EUV source based on a double stream gas puff target equipped with an Au coated ellipsoidal collector was used for the experiment. The spectrum of the focused radiation from Kr plasma consisted of a narrow feature with a maximum at 10 nm and a long-wavelength tail up to 70 nm. Al and Zr filters were employed for the selection of radiation from these two spectral regions. The radiation fluences in the two cases were comparable. Polymer samples were mounted in the focal plane of the EUV collector and irradiated for 1–2 min with a 10 Hz repetition rate. Weak ablation accompanied by creation of micro- and nanostructures of different kinds was obtained in both cases. Significant differences in the surface structures after irradiation of PMMA and FEP in these two spectral regions were revealed.

Cell Micro-Patterning by Atom Beam Exposure

This study aimed to develop a new cell micro-patterning method by controlling material surface affinity of the cell using atomic oxygen beam exposure. Surfaces of low-density polyethylene (LDPE) and tetrafluoroethylene-hexafluoropropylene (FEP) were exposed to the atomic oxygen beam. On the LDPE surface, the roughness measured by atomic force microscopy (AFM) did not change much, however, the oxygen concentration on the surface measured by X-ray photoelectron spectroscopy (XPS) significantly increased that resulted in increase in wettability. Contrary to this, on the FEP surface, the oxygen concentration showed no significant change, but roughness of the surface remarkably increased and the wettability decreased. As a result of the surface modification, affinity of the osteoblastic cells on the FEP surface increased, which was also confirmed by increase in the cell area. Finally, cell micro-patterning on the FEP surface was carried out based on difference in the affinity between modified and unmodified surfaces patterned by a masking method.

Amidoximation of the Acrylonitrile Polymer Grafted on Poly(Tetrafluoroethylene-co-hexafluoropropylene) Films and Its Relevance to the Electroless Plating of Copper

Argon plasma-pretreated poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films were subjected to UV-induced surface graft copolymerization with acrylonitrile (AN). The AN graft-copolymerized FEP (AN-g-FEP) surface was further reacted with hydroxylamine to introduce amidoxime groups on the surface (the amidoximated AN-g-FEP surface). Electroless plating of copper could be carried out effectively on the AN-g-FEP and amidoximated AN-g-FEP surfaces after sensitization in SnCl2 solution and activation in PdCl2 solution. The plasma pretreatment time of the FEP substrate and the UV graft copolymerization time affected the graft concentration of the AN polymer on the FEP surface. The surface compositions and the degree of amidoximation (DA) of the surface-modified FEP films were determined by X-ray photoelectron spectroscopy (XPS). The amidoximated FEP surface formed a complex with palladium ions and exhibited a strong ability to extract copper ions from the plating solution. The interaction of the AN-g-FEP...

Thermal imidization of poly(pyromellitic dianhydride-4,4′-oxydianiline) precursors on fluoropolymers modified by surface graft-copolymerization with glycidyl methacrylate

Composite films of polyimide (PI) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) or of PI and poly(tetrafluoroethylene) (PTFE) were prepared by thermal imidization of the poly(amic acid) (PAA) precursors of poly(pyromellitic dianhydride-4,4′-oxydianiline) (PMPA-ODA) on glycidyl methacrylate (GMA) graft-copolymerized FEP and PTFE films. The resulting PI/GMA-g-FEP and PI/GMA-g-PTFE composites exhibited T-peel adhesion strength of approximately 7.0 and 6.5 N/cm, respectively, compared to negligible adhesion strength for the laminates prepared from thermal imidization of the PAA on the pristine and the Ar plasma-treated FEP and PTFE films. X-ray photoelectron spectroscopy (XPS) results revealed that both the PI/GMA-g-FEP and PI/FEP-g-PTFE composite films delaminated by cohesive failure inside the FEP and PTFE films, respectively. The so-delaminated PI films with a covalently tethered FEP or PTFE surface layer were highly hydrophobic, having static water contact angles above 140°. The highly hydrophobic property depends on both the composition and roughness of the delaminated surface.

Thermal and electroless deposition of copper on poly(tetrafluoroethylene-co-hexafluoropropylene) films modified by surface graft copolymerization

Surface modification of Ar plasma-pretreated poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) film via UV-induced graft copolymerization, under atmospheric conditions, with 4-vinylpyridine (4VP), 2-vinylpyridine (2VP) and 1-vinylimidazole (VIDz) was carried out to improve the adhesion with the thermally evaporated and the electrolessly deposited copper. The surface composition and structure of the graft-copolymerized FEP films were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM), respectively. The electroless plating of copper on the graft-copolymerized FEP film could be carried out in the absence of sensitization by SnCl/sub 2/ (the Sn-free activation process). The adhesion strength of the thermally evaporated and the electrolessly deposited copper on the FEP surfaces was affected by the type of monomers used for graft copolymerization and the surface graft concentration. The T-peel adhesion strengths of the electrolessly deposited copper on the 4VP, 2VP and VIDz graft-copolymerized FEP films with 90-s of Ar plasma pretreatment were about 9.8 N/cm, 7.3 N/cm and 4.9 N/cm, respectively.

Surface Modifications on Teflon FEP and Mylar C Induced by a Low Energy Electron Beam: A Raman and FTIR Spectroscopic Study

The surface modifications induced on Teflon FEP and Mylar C polymer films by a low energy electron beam are probed using Raman and FTIR spectroscopy. The electron beam, which does not affect the Mylar C surface, may break the copolymer chain into its monomers degrading the Teflon FEP surface. For Mylar C the electron beam decreases the roughness of the polymer surface. This difference in behavior may explain recent results in which the surface modifications investigated by measuring the second crossover energy shift in the electronic emission curve differed for the two polymers (Chinaglia et al [1]). In addition, the Raman data showed no evidence of carbon formation for either polymer samples, which is explained by the fact that only a low energy electron beam is used.

Surface modification of tetrafluoroethylene–hexafluoropropylene (FEP) copolymer by remote H2, N2, O2, and Ar plasmas

AbstractTetrafluoroethylene–hexafluoropropylene (FEP) copolymer sheets were modified by remote H2, N2, O2, and Ar plasmas, and the effects of the modification on adhesion between FEP sheets and copper metal were investigated. The four plasmas were able to modify the FEP surfaces' hydrophilicity. Defluorination and oxidation reactions on the FEP surfaces occurred with exposure to the plasma. The hydrophilic modification by H2 plasma was best, followed by modification by O2, Ar, and N2 plasmas. The surface modification of FEP by all four remote plasmas was effective in improving adhesion with copper metal. The peel strength order of the FEP/Cu adhesive joints was H2 plasma &gt; Ar plasma &gt; N2 plasma &gt; O2 plasma. Mild surface modification is important for the adhesion improvement of FEP with Cu metal. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1258–1267, 2002

Enhancing the neuronal interaction on fluoropolymer surfaces with mixed peptides or spacer group linkers

Embryonic hippocampal neurons cultured on surface modified fluoropolymers showed enhanced interaction and neurite extension. Poly(tetrafluoroethylene- co-hexafluoropropylene) (FEP) film surfaces were aminated by reaction with a UV-activated mercury ammonia system yielding FEP-[N/O]. Laminin-derived cell-adhesive peptides (YIGSR and IKVAV) were coupled to FEP surface functional groups using tresyl chloride activation. Embryonic (E18) hippocampal neurons were cultured in serum-free medium for up to 1 week on FEP film surfaces that were modified with either one or both of GYIGSR and SIKVAV or GGGGGGYIGSR and compared to control surfaces of FEP-[N/O] and poly( l-lysine)/laminin-coated tissue culture polystyrene. Neuron-surface interactions were analyzed over time in terms of neurite outgrowth (number and length of neurites), cell adhesion and viability. Neurite outgrowth and adhesion were significantly better on peptide-modified surfaces than on either FEP or FEP-[N/O]. Cells on the mixed peptide (GYIGSR/SIKVAV) and the spacer group peptide (GGGGGGYIGSR) surfaces demonstrated similar behavior to those on the positive PLL/laminin control. The specificity of the cell–peptide interaction was demonstrated with a competitive assay where dissociated neurons were incubated in media containing peptides prior to plating. Cell adhesion and neurite outgrowth diminished on all surfaces when hippocampal neurons were pre-incubated with dissolved peptides prior to plating.

Polyelectrolyte adsorption on NH3-Plasma-Treated poly(tetrafluoroethylene-co-hexafluoropropylene)

A two-step procedure for a permanently hydrophilic surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP, fluorinated ethylene propylene) was studied. In the first step a cationic polymer surface was created by low-pressure ammonia plasma treatment introducing nitrogen-containing functional groups. Afterwards, the anionic poly(sodium 4-styrenesulfonate) was adsorbed onto the plasma-treated FEP surface. The adsorption was assumed to be controlled by ionic interactions. The modification effects and their long-term behavior were evaluated by means of water contact angle goniometry. Furthermore, electrokinetic measurements and X-ray photoelectron spectroscopy were used for surface characterization.

Effects of the surface modification by remote hydrogen plasma on adhesion in the electroless copper / tetrafluoroethylene - hexafluoropropylene copolymer (FEP) system

FEP sheets were modified with a remote hydrogen plasma and the effects of the modification on the adhesion between copper metal and FEP sheets were investigated. The remote hydrogen plasma treatment is able to make FEP surfaces hydrophilic. In the remote hydrogen plasma treatment process, both defluorination and oxidation occur on the FEP surface. The oxidation reactions on the FEP surface form oxygen functional groups such as C-O and C=O groups. Modification of the FEP surface by the remote hydrogen plasma is effective in improving the adhesion of copper metal. The peel strength of the Cu/FEP system increased form 0 to 195 mN/5 mm, and the failure mode changed from the Cu metal/FEP polymer interface to within the FEP polymer layer. Remote hydrogen plasma treatment may be a preferable pretreatment of the FEP surface for adhesion with copper metal.