Study on 172‐nm vacuum ultraviolet light surface modifications of polydimethylsiloxane for micro/nanofluidic applications

In this study, we developed a technique for modifying the surface of the silicone elastomer Poly(dimethylsiloxane) (PDMS) by 172‐nm wavelength vacuum ultraviolet (VUV) light exposure. Such materials have high potential for application to micro/nanofluidic devices if their surface properties can be adequately controlled. The hydrophilicity, zeta potential and bonding strength of the VUV‐exposed surfaces were investigated and compared to surfaces exposed to conventional vacuum oxygen plasma. It was found that the proposed technique was effective at modifying the surface conditions from hydrophobic to hydrophilic, increasing the zeta potential, and allowing good bonding to glass. The time required to produce the maximum bonding strength was found to be similar to that for vacuum oxygen plasma exposure. However, since VUV exposure does not require the creation of a vacuum, it offers a faster turnaround, making it suitable for mass production. Copyright © 2010 John Wiley & Sons, Ltd.     

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     

Photochemical fabrication of a well-defined diblock copolymer nanotemplate using 172-nm vacuum ultraviolet light

Well-ordered nanopore arrays were successfully prepared from polystyrene (PS) and poly(methyl methacrylate) (PMMA) diblock copolymer (DBC) film based on a photochemical approach using 172-nm vacuum ultraviolet (VUV) light. Since the etching selectivity between the PS and PMMA domains against activated oxygen species generated by the VUV irradiation of atmospheric oxygen molecules was markedly different, PMMA was preferentially decomposed, resulting in the formation of PS nanopore arrays. Both the photoetching rate and final morphology depended greatly on the atmospheric pressure during VUV irradiation. Since at 10 Pa the PS domains degraded less due to the shortage of oxygen molecules in the atmosphere, the residual matrix kept its fine nanostructures up to 40 min of irradiation. The matrix could be eliminated completely when irradiation was extended to 60 min at this pressure. On the other hand, at 10(3) Pa the DBC film was removed completely from the substrate within 10 min of irradiation. However, at 10(3) Pa, not only the decomposition of the PMMA domains, but also the photoetching rate of the PS domains accelerated significantly resulting in marked distortion of the generated nanostructures. By selecting an appropriate atmospheric pressure and time for VUV irradiation, we were able to control both nanoarray formation and elimination without the use of any physical and/or chemical treatment.     

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.     

The hydrophilization of polystyrene substrates by 172-nm vacuum ultraviolet light

This paper describes the photochemical surface modification of polystyrene (PS) substrates using vacuum ultraviolet (VUV) light 172 nm in wavelength. We have particularly focused on the effects of atmospheric pressure during VUV irradiation on the obtained surface's wettability and the stability of the wettability, in addition to its chemical structure, morphology, and photooxidation rate. Samples were photoirradiated with VUV light under pressures of 10, 10(3), or 10(5) Pa. Although, in each case, the originally hydrophobic PS surface became highly hydrophilic, the final water-contact angle and photooxidation rate depended on the atmospheric pressure. The samples treated at 10 Pa were less wettable than those prepared at 10(3) and 10(5) Pa due to the shortage of oxygen molecules in the atmosphere. The minimum water-contact angles of the samples treated at 10, 10(3), and 10(5) Pa were about 8 degrees, 0 degrees, and 0 degrees, respectively. With the samples prepared at 10 and 10(3) Pa, photooxidation reactions proceeded in the topmost region closest to the surface, while at 10(5) Pa photooxidation was found to be greatly enhanced in the deeper regions, as evidenced by angle-resolved X-ray photoelectron spectroscopy. Photoetching rates were determined through atomic force microscope observation of microstructured PS samples prepared by a simple mesh-contact method. As estimated from AFM images of the latticed microstructures obtained, the rates of samples prepared at 10(3) and 10(5) Pa were about 1.5 and 1.3 nm/min, respectively. However, no photoetched features were observable on the sample surface prepared at 10 Pa. Hydrophilic stability also varied greatly depending on atmospheric pressure. The hydrophilicity of samples treated at 10 and 10(3) Pa gradually decreased as they were exposed to air. On the other hand, the sample surface prepared at 10(5) Pa showed excellent hydrophilicity even after being left in air for 30 days.     

Investigation of glow discharge plasma for surface modification of polypropylene

Material surface properties of polymers, plastics, ceramics and textiles can be modified by atmospheric or low‐pressure glow discharge plasma. The aim of the present work is to study the surface modification of biaxially oriented polypropylene (BOPP) film in order to improve its hydrophilic and wetting properties. In this article we used low‐pressure, low‐temperature oxygen plasma for the surface treatment of BOPP. Scanning electron microscopy indicates that plasma treatment causes mainly physical changes by creating microcraters and roughness on the surface and increasing surface friction. Attenuated total reflectance infrared spectra show oxygen‐containing groups such as ? OH at 3513 cm^?1 and C?O at 1695 cm^?1. Microscopic investigations of water droplets on BOPP (treated, untreated) show that the interfacial adhesion of treated surfaces is increased. Copyright © 2003 John Wiley & Sons, Ltd.     

Enhancement of polymer hydrophobicity by SF6 plasma treatment and argon plasma immersion ion implantation

In this study sulphur hexafluoride (SF₆) plasmas and argon plasma immersion ion implantation (ArPIII) techniques have been applied to improve the hydrophobicity of poly(tetrafluoroethylene) (PTFE), polyurethane and silicone surfaces. As evaluated by water contact angle measurements, all the treatments resulted in a significant enhancement in the hydrophobicity of the polymers. However, exposure of the treated samples to air induced a strong variation in their hydrophobicity as a consequence of post‐plasma reactions between atmospheric species and remnant surface free radicals. X‐ray photoelectron spectroscopy results strongly suggest that for polyurethane and silicone the surface fluorination by SF₆ plasmas and the creation of new carbon bonds and radicals are the main agents for hydrophobicity enhancement. The PTFE exposed to ArPIII revealed increases in the contact angles after exposure to air. A significant incorporation of oxygen and the formation of new carbon bonds were revealed by XPS measurements. Copyright © 2003 John Wiley & Sons, Ltd.     

Regulation of pattern dimension as a function of vacuum pressure: alkyl monolayer lithography

Photopatterning of a hexadecyl (HD) monolayer has been demonstrated using vacuum ultraviolet (VUV; lambda = 172 nm) light under controlled vacuum pressure with the objective of minimizing the pattern dimension. X-ray photoelectron spectroscopy (XPS) and lateral force microscopy (LFM) studies reveal that photodegradation of the HD monolayer not only is limited to the regions exposed to VUV but also spreads under the masked regions. The strong oxidants generated by VUV irradiation to atmospheric oxygen and water vapor diffuse toward the masked regions through the nanoscopic channels and photodissociate the monolayer under the masked area, near the photomask apertures, resulting in broadening of the photopattern. Such broadening decreases with decreased vacuum pressure inside the VUV chamber, associated with a decrease of oxidant concentration and reduction of their diffusion. Gold nanoparticles (AuNPs) were immobilized on the VUV patterned features to probe the dimension of the chemically active pattern. Field emission electron microscopy reveals the construction of 565 nm wide pattern features at a vacuum pressure of 10 Pa. This pattern widens to 1,030 nm at 10 (4) Pa using the same size apertures (500 nm) as printed on the photomask. This study provides insight for fabricating submicron patterns with high reproducibility and its exploitation for different applications, which includes the patterning of nanoparticles, biopolymers, and other nano-objects at submicron dimensions.     

Vacuum Ultraviolet Irradiation of Polymers

The interest in incoherent sources for wavelength-selective photochemistry has increased lately, but little is still known about the behavior of polymers when exposed to far UV and vacuum UV (VUV) radiation. The same dearth of information exists regarding UV (VUV) radiation emitted by low-pressure plasmas during polymer treatment. In order to study VUV-UV effects on several polymers (polyethylene - PE, polystyrene - PS, hexatriacontane - HTC, and poly(methyl methacrylate) - PMMA), we have used the well-characterized emissions from hydrogen (broad-band emission) and hydrogen/argon mixture (near-monochromatic radiation) plasmas as light sources. During irradiation, samples were kept under vacuum or in a flow of pure oxygen at low pressure; in both cases the radiation fluxes at the sample position have been precisely determined by careful spectroscopic calibration experiments. We have employed a quartz crystal microbalance (QCM) to measure in-situ any possible mass change of the various polymers. Following irradiation, samples were analysed by ellipsometry (for thickness and refractive index), X-ray photoelectron spectroscopy (XPS, to evaluate the near-surface composition and content of various functional groups), and atomic force microscopy (AFM, for surface topography and roughness measurements).     

Stability and ageing of plasma treated poly(tetrafluoroethylene) surfaces

In this study CO₂, H₂/H₂O and H₂O low pressure plasma treatment of poly(tetrafluoroethylene) (PTFE) foils and of thin plasma deposited fluorocarbon polymer (PDFP) films with a structure close to PTFE was investigated. The properties of the plasma were analyzed by mass spectroscopy (MS) and optical emission spectroscopy (OES). The modified fluorocarbon surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), fourier transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry, electrokinetic measurements and dynamic contact angle measurements in order to find optimized treatment conditions. The results of the surface modification were compared with respect to the efficiency of the plasma treatment and the stability of the modification effect at different ambient conditions. It was shown that the H₂O plasma treatment is the most effective process for the intended modification. The hydrophobic PTFE surface was converted into a more hydrophilic one. The introduced radicals after the H₂O plasma treatment can be utilized subsequently for post plasma reactions such as grafting processes.     

Chemical‐surface modification of polymers using atmospheric pressure nonequilibrium plasmas and comparisons with vacuum plasmas

We demonstrate that stable microwave‐coupled atmospheric pressure nonequilibrium plasmas (APNEPs) can be formed under a wide variety of gas and flow‐rate conditions. Furthermore, these plasmas can be effectively used to remove surface contamination and chemically modify polymer surfaces. These chemical changes, generally oxidation and crosslinking, enhance the surface properties of the materials such as surface energy. Comparisons between vacuum plasma and atmospheric plasma treatment strongly indicate that much of the vacuum‐plasma literature is pertinent to APNEP, thereby providing assistance with understanding the nature of APNEP‐induced reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 95–109, 2002     

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.     

Vacuum-ultraviolet induced oxidation of the polymers polyethylene and polypropylene

The emission from low-pressure microwave plasmas in the vacuum-ultraviolet (VUV) region (λ < 200 nm) was investigated in order to use these plasmas as light sources for the study of the VUV photochemistry of polyethylene (PE) and polypropylene (PP) as part of the study of plasma-polymer interaction. These polymers, immersed in low-presure oxygen, were exposed to radiation with wavelengths down to 112 nm, the cut off of magnesium fluoride used as a window to separate the polymer specimen from the plasma light source. Total oxygen incorporation in the surface [O], and the formation of hydroxyl, carbonyl, and carboxyl groups were measured using XPS in combination with chemical derivatizations, particularly their dependence upon the radiation spectrum and the oxygen pressure around the sample. In most experiments the surface oxygen concentration [O] attained a constant value that appears to be related to the initial oxidation rate; this suggests a competition between oxygen incorporation and chain scission reactions, followed by the removal of volatile oxidation products. PE is usually oxidized to a higher level than PP, the latter appearing to be more susceptible to reaction with atomic oxygen than PE. A general initiation mechanism for the VUV experiments is proposed that allows us to explain the observed differences in behavior between PE and PP, and the results obtained under different irradiation conditions. The nature of oxidation products is in both cases very similar to what is observed after direct plasma treatment of the polymers. We conclude that short wavelength radiation contributes very appreciably to the observed surface modification effects during plasma treatment of PE and PP. © 1995 John Wiley & Sons, Inc.     

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     

Regulation of the biological properties of medical polymer materials with the use of a gas-discharge plasma and vacuum ultraviolet radiation

The mechanisms of the interaction of a gas-discharge plasma and vacuum ultraviolet (VUV) radiation with medical polymers were considered. Various techniques were proposed for the use of plasma-chemical and photochemical processes for enhancing the biocompatibility of medical polymers. The plasma-chemical and photochemical processes for the surface functionalization and regulation of the biological characteristics of medical polymers by the immobilization of proteins, biologically active compounds, and liquid-crystal biosensors were described. The relationships of the surface processes that occur in the VUV photolysis of polymers with the kinetics of protein adsorption and the adhesion of blood cell components were analyzed.     

Effect of cold atmospheric plasma treatment on hydrophilic properties of fluorosilicone rubber

Vulcanized fluorosilicone rubber for aviation was treated by argon and oxygen cold atmospheric plasma (CAP) in order to modify its hydrophilic properties. The sample's chemical composition was analyzed by X‐ray photoelectron spectroscopy (XPS). The static contact angle, surface free energy, and adhesion strength were used to indicate the hydrophilic properties. Additionally, the surface morphologies of the specimens were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that the contact angle declined from 101.5° to 22°, and the surface energy rose from 21.3 to 71.2 mJ/m² after they were treated by argon plasma. Alternatively, the water contact angle decreased to 25.5°, and the surface energy increased to 70.6 mJ/m² after they were treated by oxygen plasma. In addition, the SEM and AFM images of the samples illustrate that the treated surface of fluorosilicone rubber becomes rougher than the non‐treated surface. The concentrations of carbon (C) and fluorine (F) elements of the material' surface decreased and the contents of O element greatly enriched after plasma treatment. Additionally, chemical group C―O and C―OH appeared after the treatment. However, the hydrophilic effect of the plasma treatment is aged after the specimens were stored for 8–12 h. Copyright © 2016 John Wiley & Sons, Ltd.     

Stability of polycarbonate and polystyrene surfaces after hydrophilization with high intensity oxygen RF plasma

A general drawback observed with plasma treatment is the limited stability of the hydrophilic-treated surfaces toward washing, storage, or heating. It has recently been found that oxygen, air, or argon radiofrequency plasmas with higher intensities than normally used can give hydrophilic surfaces having good wash stability. High intensity oxygen plasma treatment of polystyrene and polycarbonate surfaces was therefore carried out using two different capacitively coupled RF reactors with internal shelf electrodes. The obtained surface characteristics and stability were evaluated using contact angle measurements, XPS, AFM, and nanoindentation. For both materials, low water contact angles were found to correlate with high surface oxygen content. Only the surfaces exposed to relatively intense treatments, with self-bias voltages above 140 V (polystyrene) or 240 V (polycarbonate), could withstand washing in ethanol and remain highly hydrophilic. Substantial amounts of nonsoluble material were observed on the plastic substrates after treatment. Furthermore, for polycarbonate Young's modulus of the surface was found to increase with increasing intensity of the plasma. These observations were taken as an indication that extensive cross-linking of the surface layer took place. After more than 6 months of storage, the samples treated with the most intense plasmas (self-bias voltages in the range of 480-600 V) still had water contact angles around 20 degrees .     

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     

Surface characteristic of poly(p‐phenylene terephthalamide) fibers with oxygen plasma treatment

Oxygen plasma is widely employed for modification of polymer surfaces. Plasma treatment process is a convenient procedure that is also environmentally friendly. This study reports the effects of oxygen plasma treatment on the surface properties of poly(p‐phenylene terephthalamide) (PPTA) fibers. The surface characteristics before and after oxygen plasma treatment were analyzed by XPS, atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA). It was found that oxygen plasma treatment introduced some new polar groups (O? C?O) on the fiber surface, increased the fiber surface roughness and changed the surface morphologies obviously by plasma etching and oxidative reactions. It is also shown that the fiber surface wettability was improved significantly by oxygen plasma treatment. Copyright © 2008 John Wiley & Sons, Ltd.     

Influence of plasma treatment on the molar mass of poly(ethylene terephthalate) investigated by different chromatographic and spectroscopic methods

The influence of different types of low and atmospheric pressure plasma on poly(ethylene terephthalate) (PET) has been studied in terms of changes in molar mass and molar mass distribution. Apart from a variation of plasma gases (oxygen, helium) different types of plasma (microwave, radio frequency, corona discharge) were used for the plasma surface modification. The changes in molar mass and types of functional end groups of lower molar mass products were investigated by means of matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOFMS), whereas the high-molar mass fraction was analyzed by means of size-exclusion chromatography (SEC). The formation of crosslinked products during exposure to a helium plasma, which emits preponderately energy-rich and intense ultraviolet radiation, was proved by means of thermal field-flow fractionation (ThFFF). This method combined with a multiangle laser light scattering (MALLS) detector allows detection of weakly crosslinked polymers and microgels. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1639–1648, 1998