Solid‐state bonding of silicone elastomer to glass by vacuum oxygen plasma,atmospheric plasma,and vacuum ultraviolet light treatment

We experimentally demonstrated that treating a silicone elastomer by a vacuum oxygen plasma, an atmospheric pressure plasma, and vacuum ultraviolet (VUV) radiation resulted in different surface modifications that gave different contact angles, contact angle aging, and bond strengths. The aim of this study was to assess whether high‐throughput surface modification techniques of atmospheric pressure plasma and VUV radiation have the potential to replace conventional oxygen plasma modification. Four silicone elastomers with different hardnesses were used as specimens. The surfaces of all four silicone elastomers were successfully modified from hydrophobic to hydrophilic and they were also bonded to glass surfaces by the three surface modification techniques, although considerable variations were observed in the surface hydrophobicity and the bonding properties. The results clearly reveal that atmospheric pressure plasma and VUV treatment have the potential to replace conventional oxygen plasma treatment. In particular, VUV irradiation produced the most hydrophilic surface that was preserved for a long time. Thus, VUV irradiation is the most promising technique for realizing high‐throughput surface modification and bonding of silicone elastomers. Copyright © 2012 John Wiley & Sons, Ltd.     

Activation of Spruce Wood Surfaces by Plasma Treatment After Long Terms of Natural Surface Inactivation

Surface inactivation is a phenomenon that causes poor adhesion. A wood surface exposed to contaminants such as dust or atmospheric grime can experience surface inactivation. Inactivation mechanisms can reduce the attractive forces on the wood surface and lead to a decrease in wettability. Plasma treatment has been applied to recover inactivated wood surfaces for better adhesion and bonding. Plasma treatment technology is very simple and the cost is rather low. In addition, this treatment produces no environmental pollution. In this study, low pressure plasma treatment was applied to reactivate the surfaces of spruce wood for glue bonding and to increase wettability after a 9-year period of natural surface inactivation. Changes in contact angles, surface energy, surface colour and bonding strength of inactivated and oxygen plasma treated wood surfaces were studied. Wettability, bonding and other mechanical strength properties of plywood panels increased with the oxygen plasma treatment.     

Chemical alteration of poly(tetrafluoroethylene) Teflon induced by exposure to vacuum ultraviolet radiation and comparison with exposure to hyperthermal atomic oxygen

The chemical alteration of poly(tetrafluoroethylene) Teflon by vacuum ultraviolet radiation (VUV) (115–400 nm) has been examined with X‐ray photoelectron spectroscopy (XPS). The initial F/C atom ratio of 1.98 decreases to 1.65 after a 2‐h exposure. The F/C atom ratio is further reduced to a steady‐state value of 1.60 after a 74‐h exposure. The high‐resolution XPS C1s data indicate that new chemical states of carbon form as F is removed and that the relative amounts of these states depend on the F content of the near‐surface region. The states are most likely due to C bonded only to one F atom, C bonded only to other C atoms, and C that has lost a pair of electrons through the emission of F^?. The exposure of the VUV‐damaged surface to research‐grade O₂ results in the chemisorption of a very small amount of O, and this indicates that large quantities of reactive sites are not formed during the chemical erosion by VUV. Further exposure to VUV removes this chemisorbed oxygen. A comparison of the XPS data indicates that the mechanisms of chemical alteration by VUV radiation and hyperthermal (～5 eV) atomic oxygen are different, as expected, because the excitation sources are quite different. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 552–561, 2005     

Evaluation of piezoelectric PVDF polymers for use in space environments. III. Comparison of the effects of vacuum UV and gamma radiation

Films of piezoelectric PVDF and P(VDF‐TrFE) were exposed to vacuum UV (115–300 nm VUV) and γ‐radiation to investigate how these two forms of radiation affect the chemical, morphological, and piezoelectric properties of the polymers. The extent of crosslinking was almost identical in both polymers after γ‐irradiation, but surprisingly, was significantly higher for the TrFE copolymer after VUV‐irradiation. Changes in the melting behavior were also more significant in the TrFE copolymer after VUV‐irradiation due to both surface and bulk crosslinking, compared with only surface crosslinking for the PVDF films. The piezoelectric properties (measured using d₃₃ piezoelectric coefficients and D‐E hysteresis loops) were unchanged in the PVDF homopolymer, while the TrFE copolymer exhibited more narrow D‐E loops after exposure to either γ‐ or VUV‐radiation. The more severe damage to the TrFE copolymer in comparison with the PVDF homopolymer after VUV‐irradiation is explained by different energy deposition characteristics. The short wavelength, highly energetic photons are undoubtedly absorbed in the surface layers of both polymers, and we propose that while the longer wavelength components of the VUV‐radiation are absorbed by the bulk of the TrFE copolymer causing crosslinking, they are transmitted harmlessly in the PVDF homopolymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3253–3264, 2006     

Fick's law of diffusion depth profiles applied to the degradation of oxidized polystyrene during ARXPS analysis

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) measurements were made, using Al Kα and Mg Kα radiation alternately, on a polystyrene sample that had been exposed to a helium plasma. It was observed that oxygen was introduced into the sample surface by the plasma treatment, and that some of it was lost over a period of 5 h under x‐ray irradiation in the vacuum of the spectrometer. Laplace transforms of Fick's law of diffusion profiles were derived and applied to the data. The ARXPS results obtained in this study are consistent with a sample history in which the oxidation of the polymer surface resulting from exposure to plasma is controlled by a diffusion process, whereas the loss of oxygen during exposure to x‐rays is principally controlled by a first‐order reaction such as the liberation of oxygen (presumably as CO₂) from carbon–oxygen groups by the action of radicals created by the ionizing radiation. Copyright © 2005 John Wiley & Sons, Ltd.     

Subsurface investigation of the surface modification of polydimethylsiloxane by 172‐nm vacuum ultraviolet irradiation using ToF‐SIMS and VUV spectrometry

We investigate the mechanism of polydimethylsiloxane (PDMS) surface modification by 172‐nm vacuum ultraviolet (VUV) light. Time‐of‐flight secondary ion mass spectrometry and optical spectrometry are used to measure the chemical composition and VUV transmittance of the PDMS before and after surface modification, respectively. For modified samples of bulk PDMS, the VUV transmittance and the depth of the modified region increased with increasing VUV dose. This can be explained by the following self‐reinforcing cycle of (1) modification of PDMS by VUV light to a more silica‐like composition, (2) improvement of the VUV light transparency, and (3) deeper modification. For thin‐film samples of PDMS formed on sapphire substrates, the transmittance at 172 nm also increased with increasing VUV dose and exceeded that of sapphire in the region from 172 to 300 nm. Finally, thin‐film samples of PDMS formed on silicon substrates, which function as a VUV reflector, were also investigated. For these samples, the secondary ion depth profiles for several chemical species in the PDMS were oscillatory, probably due to the interference of the incident and reflected VUV light. These results strongly suggest that the photon energy of the VUV light plays an important role in modifying PDMS.     

Surface modification of high-performance polymeric fibers by an oxygen plasma. A comparative study of poly(p-phenylene terephthalamide) and poly(p-phenylene benzobisoxazole)

Poly(p-phenylene terephthalamide) (PPTA) and poly(p-phenylene benzobisoxazole) (PBO) fibers were exposed to an oxygen plasma under equivalent conditions. The resulting changes in the surface properties of PPTA and PBO were comparatively investigated using inverse gas chromatography (IGC) and atomic force microscopy (AFM). Both non-polar (n-alkanes) and polar probes of different acid-base characteristics were used in IGC adsorption experiments. Following plasma exposure, size-exclusion phenomena, probably associated to the formation of pores (nanoroughness), were detected with the largest n-alkanes (C(9) and C(10)). From the adsorption of polar probes, an increase in the number or strength of the acidic and basic sites present at the fiber surfaces following plasma treatment was detected. The effects of the oxygen plasma treatments were similar for PPTA and PBO. In both cases, oxygen plasma introduces polar groups onto the surfaces, involving an increase in the degree of surface nanoroughness. AFM measurements evidenced substantial changes in the surface morphology at the nanometer scale, especially after plasma exposure for a long time. For the PBO fibers, the outermost layer - contaminant substances - was removed thanks to the plasma treatment, which indicates that this agent had a surface cleaning effect.     

Wettability interpretation of oxygen plasma modified poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) has been modified via a dc pulsed oxygen plasma for different treatment times. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), optical profilometer, zeta potential, and advancing contact angle measurements. The measured advancing contact angles of water decreased considerably as a function of discharge. Several oxygen-based functionalities (carbonyl, carboxyl, carbonate, etc.) were detected by XPS, while zeta potential measurements confirmed an increase in negative charge for the treated PMMA surface. Evaluating the correlation between the concentration of polar chemical species and zeta potential, we found that increase in surface hydrophilicity results from the coeffect due to incorporation of oxygen functional groups and creation of charge states. The electrical double layer (EDL) effect was also considered in contact angle interpretation by introducing an additional surface tension term into Young's equation. We also found that EDL contribution to the solid-liquid interfacial tension is negligible and can be safely ignored for the systems considered here.     

Effect of nitrogen plasma treatment on the crystallinity and self‐bonding of polyetheretherketone (PEEK) for biomedical applications

Polyetheretherketone (PEEK) is a thermoplastic material with outstanding properties and high potential for biomedical applications, including hermetic encapsulation of active implantable devices. Different biomedical grade PEEK films with initial degree of crystallinity ranging from 8% to 32% (with or without mineral filling) were inspected. PEEK surfaces were treated with nitrogen RF plasma and the effects on materials crystallinity and self‐bonding were evaluated. In particular, the relationship between auto‐adhesive properties and crystalline content of PEEK before and after plasma treatment was examined. PEEK samples showed different bonding strength depending on their degree of crystallinity, with higher self‐bonding performance of mineral‐filled semi‐crystalline films. XRD did not show any modification of the PEEK microstructure as a result of plasma treatment, excluding a significant influence of crystallinity on the self‐bonding mechanisms. Nevertheless, plasma surface treatment successfully improved the self‐bonding strength of all the PEEK films tested, with larger increase in the case of semi‐crystalline unfilled materials. This could be interpreted to the increase in chain mobility that led to interfacial interpenetration of the amorphous phase.     

Plasma surface modification and characterization of POSS-based nanocomposite polymeric thin films

The effect of a remote oxygen plasma on nanocomposite hybrid polymer thin films of poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsequioxane)-co-(methylmethacrylate)] (POSS-MA) has been examined by advancing contact angle, X-ray photoelectron spectroscopy (XPS), and variable-angle spectroscopic ellipsometry (VASE). Exposure to a 25 W remote oxygen-containing plasma was found to convert the surface of POSS-MA films from hydrophobic to hydrophilic within 20 s. The exposure time needed for this conversion to occur decreased as the O2/N2 ratio in the plasma environment increased, indicating a positive correlation between the hydrophilicity and the presence of oxygen in the plasma. Local bonding information inferred from high-resolution XPS data showed that the isobutyl bonding to the POSS moiety is replaced with oxygen as a result of plasma exposure. Finally, VASE data demonstrates that increasing the weight percent of POSS in the copolymer significantly impedes the oxygen plasma degradation of POSS-MA films. On the basis of these results, a model is presented in which the oxygen plasma removes isobutyl groups from the POSS cages and leaves a SiO2-like surface that is correspondingly more hydrophilic than the surface of the untreated samples and is more resistant to oxidation by the plasma. The ability to modify surfaces in this manner may impact the utility of this material for biomedical applications such as microfluidic devices in which the ability to control surface chemistry is critical.     

Effect of imaging techniques on the observed surface morphology of oxygen plasma etched polyethylene fibres

The present work examined the interactions of Activated Oxygen (AO) and Vacuum UV (VUV) radiation generated by oxygen RF plasma with Ultra-Strong Ultra High Molecular Weight Polyethylene (UHMWPE) fibres. The samples were exposed in the afterglow region, beyond the RF reactor exit. Surface morphology changes were characterized by Atomic Force Microscopy (AFM) in contact and tapping modes and Low-Vacuum (LV) and High-Resolution (HR) Scanning Electron Microscopy (SEM).Simultaneous exposure to AO and VUV caused surface erosion characterized by the formation of ordered domains and a morphological reconstruction into row structure. However, the different imaging techniques showed a variety of morphologies of the same surface depending on the imaging contrast mechanism. An explanation for the origin of these observed morphologies is presented.     

A comparison of the strength of autohesion of plasma treated amorphous and semi‐crystalline PEEK films

Polyether ether ketone (PEEK) is a promising material for the encapsulation of electronic components for medical implants but a strong and hermetic joining technology is required. Autohesion is a self‐bonding method that avoids the need for adhesives. The strengths of autohesive joins using amorphous and semi‐crystalline PEEK films after surface activation using RF plasma were compared. Both types of PEEK films showed successful autohesion after activation with the bond strength of the amorphous sample being twice as high as the bond strength of the semi‐crystalline sample. Plasma treatment increased the autohesion strength of PEEK with no observed change in surface roughness (as measured by profilometer). The water contact angle was reduced by the treatment. X‐ray photoelectron spectroscopy (XPS) was carried out to determine surface chemistry. In the case of the semi‐crystalline surface, plasma treatment increased the relative percentage of C? O functional groups compared to the untreated surface. For treated surfaces nitrogen concentration correlated positively with bond strength while oxygen concentration correlated negatively with the semi‐crystalline PEEK samples and positively with the amorphous PEEK samples. The oxygen groups most likely are formed after the treatment by ambient oxidation are not conducive to bond formation, possibly because of the quenching of radicals that would otherwise form links. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of electrochemical and plasma treatments on carbon fibre surfaces

This paper summarizes the chemical changes induced on carbon fibre surfaces (examined by X-ray photoelectron spectroscopy, XPS) by a variety of electrochemical treatment in aqueous electrolytes together with the improvements in fibre/resin bonding in the corresponding composite materials. It was found that there was no correlation between the amount of chemical functionality introduced onto the fibre surface and the fibre/resin bond strength, i.e. chemical bonding does not play a major role in fibre resin adhesion. This does not rule out the possibility of chemical bonding between the fibre and resin—it simply implies that it is not the governing factor. It is suggested that the immediate surface concentration of chemical groups is too low to make a significant contribution. To tailor interfacial properties it would be desirable to promote chemical bonding between fibre and matrix. The use of a specially designed plasma treatment cell has led to an increase in the surface concentration of chemical groups ( C OH, hydroxyl) that have the potential to react chemically with the resin. By exploiting grazing angle data taken from XPS analysis, it is shown that changes in the chemical nature of the fibres only occurs in the outermost layers, whereas the electrochemical reaction proceeds well into the fibre sublayers. Selective introduction of nitrogen-containing functionality (such as amines,  NH₂) has been achieved. The reactivity towards a particular plasma is shown to be largely dependent on the structure of the fibre surface. The number of C/N groups produced on higher modulus fibres was undesirably low. Their concentration was increased by biasing the fibres to a negative potential (10–30 V) during plasma exposure.     

Modification of porous poly(ether sulfone) membranes by low‐temperature CO2‐plasma treatment

A general method of modifying the entire cross section of porous poly(ether sulfone) membranes with a low‐temperature CO₂‐plasma treatment is reported. Both surfaces of the membranes are highly hydrophilic, with a water drop on the surface disappearing in less than 1 s, even 6 months after plasma treatment. This high hydrophilicity of both membrane surfaces results from the incorporation of hydrophilic functionalities, as evidenced by Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy. The incorporation of these hydrophilic functionalities takes place primarily during plasma treatment, with some incorporation of atmospheric oxygen and nitrogen immediately upon exposure to air. Scanning electron microscopy shows that the membrane surface is covered by a thin, white layer that is likely the result of etching and redeposition of sputtered surface fragments. An increase in the water bubble point and glass‐transition temperature is also observed for CO₂‐plasma‐treated membranes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2473–2488, 2002     

Photolysis of a fluorinated polymer film by vacuum ultraviolet radiation

We studied the photolysis of a fluoroethylene–fluoropropylene copolymer (FEP) film by vacuum ultraviolet (VUV) radiation from a resonance Xe lamp at a wavelength of 147 nm and air pressures of 0.05 and 2.5 Torr. The chemical changes in the FEP surface layer were investigated by Fourier-transform infrared spectroscopy with attenuated total reflection attachment and X-ray photoelectron spectroscopy. Double bonds were found to be the main product in the case of VUV treatment at 0.05 Torr, while photo-oxidation of FEP occurred predominantly by VUV treatment at 2.5 Torr under formation of the —CF₂C(O)F group. This oxygen-containing group was more effectively formed in the FEP surface layer by VUV photo-oxidation than by conventional surface oxidation techniques such as treatments by plasma and corona discharge and ozone. Storage of the VUV-treated polymers in air at 50% relative humidity resulted in hydrolysis of —CF₂C(O)F to the —CF₂COOH group. Substantial improvement of the film wettability was noticed after VUV photo-oxidation. These findings suggest that VUV irradiation provides a high potential for surface modification of fluorinated polymers which are known to be particularly resistant against functionalization by conventional surface modification techniques such as plasma treatment. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2215–2222, 1998     

Zeta potential of anoxygenic phototrophic bacteria and Ca adsorption at the cell surface: possible implications for cell protection from CaCO3 precipitation in alkaline solutions

Electrophoretic mobility measurements and surface adsorption of Ca on living, inactivated, and heat-killed haloalkaliphilic Rhodovulum steppense, A-20s, and halophilic Rhodovulum sp., S-17-65 anoxygenic phototrophic bacteria (APB) cell surfaces were performed to determine the degree to which these bacteria metabolically control their surface potential equilibria. Zeta potential of both species was measured as a function of pH and ionic strength, calcium and bicarbonate concentrations. For both live APB in 0.1M NaCl, the zeta potential is close to zero at pH from 2.5 to 3 and decreases to -30 to -40 mV at pH of 5-8. In alkaline solutions, there is an unusual increase of zeta potential with a maximum value of -10 to -20 mV at a pH of 9-10.5. This increase of zeta potential in alkaline solutions is reduced by the presence of NaHCO(3) (up to 10 mM) and only slightly affected by the addition of equivalent amount of Ca. At the same time, for inactivated (exposure to NaN(3), a metabolic inhibitor) and heat-killed bacteria cells, the zeta potential was found to be stable (-30 to -60 mV, depending upon the ionic strength) between pH 5 and 11 without any increase in alkaline solutions. Adsorption of Ca ions on A-20s cells surface was more significant than that on S-17-65 cells and started at more acidic pHs, consistent with zeta potential measurements in the presence of 0.001-0.01 mol/L CaCl(2). Overall, these results indicate that APB can metabolically control their surface potential to electrostatically attract nutrients at alkaline pH, while rejecting/avoiding Ca ions to prevent CaCO(3) precipitation in the vicinity of cell surface and thus, cell incrustation.     

A study of a simulated low Earth environment on the degradation of FEP polymer

Spacecraft flying in a low Earth orbit environment require thermal blankets to provide protection from direct solar heat from the sun. Fluorinated ethylene propylene copolymer is one of the major components of these thermal blankets. In this study, the effect of a simulated low Earth orbit environment on FEP was investigated. UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using ESR and XPS. The ESR study revealed the formation of a terminal polymer chain radical. The stability of this radical has been investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF₂ in the polymer surface decreased with exposure time and the percentage of CF, CF₃ and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen produced a rough surface with regular undulations similar to sand dunes. © 1998 John Wiley & Sons, Ltd.     

Polyelectrolyte adsorption layers studied by streaming potential and particle deposition

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface zeta as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential zeta/zeta0 on the PAH coverage Theta(PAH), calculated using the convective diffusion theory. It was found that for the ionic strength of 10(-2) M, the dependence of zeta/zeta0 on Theta(PAH) can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ThetaPAH > 0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.     

On the correlation of surface charge and energy in non‐thermal plasma‐treated polyethylene

Non‐thermal non‐equilibrium oxidative air 40‐kHz frequency, 13.56‐MHz radiofrequency and 2.46‐GHz microwave discharge plasma treatment were used for modifying low‐density polyethylene foils. The untreated and treated samples were chemically characterised by X‐ray photoelectron spectroscopy. In order to estimate the extent of the plasma sources at distinct treatment times, surface charge and energy were determined by zeta potential (ζ) and surface tension measurements. In addition, the isoelectric points (IEPs) of the studied samples were ascertained, and surface property variations were appraised by ageing time. The overall outcome indicated that ζ‐potential and surface energy progressively changed after each treatment, as well as the influences of ageing on surface features, the IEP shifting to lower pH values and how all of these changes are associated with the new surface chemistry. This contribution seeks to shed light on topics related to polymer science and plasma‐based strategies for surface modification. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of reactive oxygen species generated with ultraviolet lamp and plasma on polyimide surface modification

Understanding the effect of reactive oxygen species (ROS), such as singlet oxygen molecule and atomic oxygen, on polyimide (PI) film properties, such as wettability, morphology, and chemical bonding state, is essential for further development of PI‐based surfaces. We investigated the effect of different ROS generated during ultraviolet (UV) and plasma treatment in oxygen gas on surface modification of Kapton PI. Different surface modification techniques, UV and plasma treatment, are known to generate different ROS. In this work, we demonstrate the effect of different ROS on PI surface modification. From the diagnostics of ROS by means of electron spin resonance and optical emission spectroscopy, we confirmed that during UV treatment, excited singlet oxygen molecules are the main ROS, while plasma treatment mainly generated atomic oxygen. The wettability of PI surface treated by UV and plasma resulted in hydrophilic PI surfaces. XPS results show that the wettability of PI samples is mainly determined by their surface O/C ratio. However, chemical bonding states were different: while UV treatment tended to generate C=O bonds, while plasma treatment tended to generate both C―O and C=O bonds. Singlet oxygen molecules are concluded to be the main oxidant during UV treatment, and their main reaction with PI was concluded to be of the addition type, leading to an increase of C=O groups on the surface of PI film. Meanwhile, atomic oxygen species were the main oxidant during plasma treatment, reacting with the PI surface through both etching and addition reaction, resulting in a wider variety of bonds, including both C―O and C=O groups.