Low-pressure Plasma Treatment Research Articles

Combination of FTIR reflection absorption spectroscopy and work function measurement for in-situ studies of plasma modification of polymer and metal surfaces

An in-situ plasma cell for low and atmospheric pressure plasma treatment of polymer and metal substrates was developed. It enables the investigation of the sample surface with FTIR spectroscopy at grazing incidence as well as the simultaneous measurement of the volta potential difference between the outmost surface layer and a reference electrode in vacuum or a defined atmosphere. To demonstrate the benefits of the device, the plasma modification of a model polymer and an oxide covered metal surface has been investigated. It could be shown that even sub-monolayer surface changes, e.g., the etching of the CH3 and CH2 groups and the generation of carbonyl functions on an octadecylmercaptane (ODM) self-assembly monolayer on gold in a low-pressure oxygen plasma can be detected. Moreover, plasma induced changes of the oxidation states of iron and the thickness and composition of its oxide or oxyhydroxide layer could be monitored. The presented applications of this new in-situ analytical device prove that the measurement of work function in combination with monolayer sensitive vibrational spectroscopy provide new insight in plasma induced surface modifications.

In Situ Study of the Thermoresponsive Behavior of Micropatterned Hydrogel Films by Imaging Ellipsometry

A patterned hydrogel was immobilized on a polymer substrate by low-pressure argon plasma treatment using a masking technique. The polymer sample showed a thermoresponsive aggregation behavior in the region of 35-37 degrees C. The micropatterned, thermoresponsive hydrogel film has been characterized with imaging ellipsometry. The characterization was carried out on the dry film as well as on a swollen sample in water. The thermoresponsive behavior was studied in deionized water by temperature-dependent measurements in a solid-liquid cell. Through imaging ellipsometry, it was possible to distinguish the different regions of interest on a micrometer scale and to follow the swelling of the hydrogel part as a function of the temperature. It was possible to visualize the swelling as 3D profiles of Delta at various temperatures. Long-term changes of the sample could also be detected, which cannot be picked up by conventional ellipsometry.

Effects of different laser and plasma treatments on the interface and adherence between evaporated aluminium and polyethylene terephthalate films: X-ray photoemission, and adhesion studies

Chemical functionalities, topography and adherence between Al and polyethylene terephthalate (PET) films, modified by laser treatment below the polymer ablation threshold fluence in air and in helium at different wavelengths, fluences, pulse numbers and by low pressure plasma treatment in He 95% with O 2 5%, at 0.2 W/cm 3 were investigated by XPS, AFM, water contact angle and adhesion measurements. XPS results revealed (i) formation of polar-oxygenated functional groups (C O, O C O) by laser treatments in air and (ii) incorporation of ester and ether groups by plasma treatment in He 95% with O 2 5%. After deposition of thermally evaporated Al on laser or plasma treated PET films, U-Peel tests indicated that laser treatment in air at 248 nm and 20 mJ/cm 2, and plasma treatment improved Al/PET adhesion. XPS studies revealed the presence of an increasing number of Al O C stable and cohesive interfacial complexes formed by chemical interaction between aluminium and the laser treated in air or plasma treated in He 95% with O 2 5% PET films. These interfacial compounds play an important role in the enhancement of the metal/polymer adhesion.

Thermo-Reversible Swelling of Thin Hydrogel Films Immobilized by Low-Pressure Plasma

Thin films of graft copolymers consisting of poly(N-isopropylacrylamide) (PNiPAAm) or poly(N,N-diethylacrylamide) (PDEAAm) as polymer backbone and poly(ethyleneglycol) as side chains were cross-linked on fluoropolymer substrates by low-pressure plasma treatment. All immobilized polymers exhibit a lower critical solution temperature between 34 and 40 degrees C. The swelling and collapsing of the hydrogels was examined with temperature-dependent spectroscopic ellipsometry. Two time ranges of swelling were observed: a fast 'dynamic' and a slow 'equilibrium' swelling. The dynamic swelling occurs within minutes or less, whereas the equilibrium swelling needs several days to complete. The surface-bound hydrogels show a shift in the transition temperature toward lower temperatures compared with the behavior in solution. Full reversibility of the dynamic swelling/collapsing was found, but the temperature scan exhibits a hysteresis between heating and cooling cycles. The PNiPAAm-containing hydrogels show a sharper transition compared to the PDEAAm-containing hydrogels, which is almost linear over a wide temperature range.

Influence of low-pressure plasma treatment on adhesive bonding between polybutylene terephthalat (PBT) and aluminum

Adhesive joints between polybutylene terephthalat (PBT) and aluminum with epoxy (EP) and polyurethane (PU) resins have been tested after surface treatment by low-pressure plasma. By this means the adhesive bonding strength increased slightly in case of PU adhesive, but 3.5–10 times in case of EP adhesive. In accordance the fracture turns from an adhesive failure to a fracture predominantly in the plastic part. Freshly injected surfaces showed high adhesive strength with EP adhesive as compared with unfreshly injected ones. The increase of joining strength can be attributed to a corresponding increase of the PBT surface tension.

Thermo-Responsive PNiPAAm-g-PEG Films for Controlled Cell Detachment

A series of graft copolymers consisting of either poly(N-isopropylacrylamide) (PNiPAAm) or poly(N,N-diethylacrylamide) (PDEAAm) as a thermo-responsive component in the polymer backbone and poly(ethyleneglycol) (PEG) were immobilized as thin films and cross-linked on a fluoropolymer substrate using low-pressure argon plasma treatment. The surface-immobilized hydrogels exhibit a transition from partially collapsed to completely swollen, which is in the range of 32-35 degrees C and corresponds to the lower critical solution temperature of the soluble polymers. The hydrogels were used as cell carriers in culture experiments with L929 mouse fibroblast cells to probe for cell adhesion, proliferation, and temperature-dependent detachment of cell layers. The fibroblast cells adhere, spread, and proliferate on the hydrogel layers at 37 degrees C and become completely detached after reducing the temperature by 3 K. The cell release characteristics were further correlated to the swelling and collapsing behavior of the hydrogel films and the polymer solutions as measured in PBS solution and RPMI cell cultivation medium. It could be shown that, long before the swelling has completed upon temperature reduction, the cells detach. This can be attributed to the large content of PEG present in the hydrogel, which weaken the cell adhesion strength to the hydrogel layers.

Low pressure plasma treatment for improving the strength and durability of adhesively bonded aluminium joints

The effectiveness of radio frequency plasmas of oxygen, air and argon in removing the organic layer from the deliberately contaminated surface of AA6016T6 alloy has been assessed. Plasma treatment of the alloy surface has been characterised using wettability (water contact angle) and surface chemical analysis (X-ray photoelectron spectroscopy, XPS). The performance of bonded single lap shear joints has been characterised before and after immersion in water for 2 weeks at 60 °C. Oxygen plasma treatment had the most marked effect on joint performance; resulting in a 40% increase in initial joint failure stress and a 38% increase of aged joint failure stress compared with the contaminated alloy. This improvement is correlated with low carbonaceous over layer thickness and carbon concentration, determined by XPS, and increased surface energy, as indicated by low water contact angle.

Multi-jet atmospheric glow plasma cleaning of ablation debris from micro-via drilling process

We devised a multi-jet atmospheric glow plasma system and successfully verified a possibility to ignite and maintain an atmospheric pressure discharge using helium, argon or a mixture of helium and oxygen gas. In this study, as an alternative method to the conventional wet cleaning process, the multi-jet atmospheric glow plasma system was applied to clean the ablation debris inside the blind via holes from micro-via drilling process of printed circuit boards (PCBs) prior to electroplating process. Scanning electron microscope images, after the multi-jet atmospheric glow plasma treatment of the PCBs, showed effective removal of residual organic impurities and massive fibrous debris when compared with low-pressure vacuum plasma treatment. Consequently, we demonstrated that multi-jet atmospheric glow plasma system could not only effectively remove organic impurities and massive fibrous debris generated during micro-via drilling process, but also enhance the electroplating process and the wire bondability.

XPS, AES, and AFM as tools for study of optimized plasma functionalization.

The plasma-based surface modification of polymer materials with desirable bulk properties is a useful way to obtain polymers with tailor-made surface properties. This is necessary because the surface properties of most engineering polymers in use today are less then optimum for many applications. New functionalities such as biocompatibility, adhesion, special functional groups as well as lubricative, friction and wear-and-tear properties are demanded. By optimization of the process parameters during a low pressure plasma treatment, most of these requirements can be fulfilled. A specific functionalization with, e.g., carboxyl, amino, epoxy or hydroxyl groups as well as the generation of ultra thin layers with those functionalities is possible. The most challenging problem is not only to find parameters which do not lead to a fragmentation of the monomeric structure, but moreover the adhesion of the thin films to the substrates must overcome a stability test without delamination. To optimize plasma processes, with their great variety of parameters influencing the obtained surface properties, several surface analytical techniques are indispensable. XPS, AES as well as AFM are helpful tools to characterize the modified sample surfaces and consequently optimize the set of parameters for the glow discharge treatment. With XPS the retention of the monomer structure can be controlled. AES depth profiling clarifies the elemental composition of gradient layers, necessary for a good adhesion of scratch-resistant coatings. AFM visualizes the surface morphology which is important for, e.g., the friction properties of plasma-coated substrates.

Graft-polymerization of a hydrophobic monomer onto PAN textile by low-pressure plasma treatments

In order to confer a good and durable water and oil repellence to polyacrylonitrile (PAN) fabrics, the fluorinated monomer 1,1,2,2, tetrahydroperfluorodecyl acrylate (AC8) was graft-polymerized onto PAN samples using a low-pressure microwave (MW) plasma process. The property was achieved by creating covalent bonding between the PAN fabrics and the fluorinated polymer. The grafting and the polymerization processes were followed by weighing measurements, XPS, IR(ATR) spectroscopies and SEM. The impermeability of the fabrics and the durability of the coating were investigated by Schmerber tests. Moreover, these tests allowed us to appreciate the effect of a reorientation of the fluorinated chains at the surface and also the role played by the addition of a small amount of a cross-linking agent on the surface properties.

THE USE OF PLASMA AND NEURAL MODELLING TO OPTIMISE THE APPLICATION OF A REPELLENT COATING TO DISPOSABLE SURGICAL GARMENTS

Abstract The modification of the surface properties of and the application of coatings to textile materials by means of exposure to plasma has attracted much attention in recent years. The advantages of using excited gases include low process cost and duration, and the avoidance of effluents such as solvents or chlorine. Low-pressure plasma treatment with hexafluoroethane has been shown to create a hydrophobic surface on cotton, which would normally be hydrophilic. Cotton is a popular material for surgical garments and drapes because of its comfort and low cost. The acquisition of hydrophobic behaviour will provide haemo-repellancy and the prevention of bacterial attachment. This paper describes a series of designed experiments to vary three parameters for the plasma process, gas concentration, power and duration, and to measure the resulting degrees of hydrophobic behaviour at the cotton surface by means of observing water droplets. Neural networks can provide rapid development of simulation models of processes by adaptation to observed conditions as inputs and the results as outputs. The data from the plasma trials has been used to develop a neural model to predict surface hydrophobic behaviour. The model is itself optimised for interpolative ability, and allows a search to be made through the data space to find the best possible combination of the process parameters to encourage optimal surface treatment, and thus make the cotton most hydrophobic. The work indicates how the environmentally-friendly approach of plasma treatment can be used to provide garments and drapes for surgery which are comfortable but also protective.

Low pressure plasma treatment of poly(3-hydroxybutyrate): Toward tailored polymer surfaces for tissue engineering scaffolds

Low pressure plasma treatment of poly(3-hydroxybutyrate): Toward tailored polymer surfaces for tissue engineering scaffolds

Durable surface modification of poly(tetrafluoroethylene) by low pressure H 2O plasma treatment followed by acrylic acid graft polymerization

In order to obtain a durable and homogeneous surface functionalization of poly(tetrafluoroethylene) (PTFE), a two step procedure based on low pressure plasma treatment followed by a grafting reaction was used. PTFE foils and plasma deposited fluorocarbon polymer films with a chemical structure close to PTFE were investigated. The fluoropolymer surfaces were treated with microwave water vapor plasma to introduce functional groups and radicals. The generated radicals were utilized as reactive species for the gas phase graft polymerization of acrylic acid. A homogeneous and stable poly(acrylic acid) (pAAc) layer with a thickness of about 70 nm was formed. The influence of the grafting period and the acrylic acid vapor pressure on the amount of grafted pAAc was studied. The properties of the modified surfaces were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, spectroscopic ellipsometry, electrokinetic measurements and dynamic contact angle measurements. The grafted surfaces can be used for a covalent protein immobilization.

Stability and ageing of plasma treated poly(tetrafluoroethylene) surfaces

In this study CO 2, H 2/H 2O and H 2O 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 H 2O 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 H 2O plasma treatment can be utilized subsequently for post plasma reactions such as grafting processes.

Enhanced performance of organic light-emitting devices by atmospheric plasma treatment of indium tin oxide surfaces

Atmospheric plasma treatment of indium tin oxide (ITO) surfaces has been studied and demonstrated to be the most efficient method in improving the performance of vacuum-deposited double-layer organic light-emitting diode devices, among various plasma treatment methods including low-pressure Ar plasma and low-pressure O2 plasma treatment. Although with a current–voltage characteristic close to low-pressure O2 plasma treatment, the atmospheric plasma treatment exhibits a 40% increase of electroluminescence efficiency. X-ray photoelectron spectroscopy results show that the atmospheric plasma treatment increases the work function and reduces the carbon contamination of ITO surfaces. Our results suggest that atmospheric plasma treatment is a cheaper, more convenient, and more efficient method than low-pressure O2 plasma treatment for improving device performance.

Improved adhesion of low-density polyethylene/EVA foams using different surface treatments

The adhesion between a polyurethane (PU) adhesive and four foams containing different low-density polyethylene (LDPE)/ethylene vinyl acetate (EVA) blends was improved by using different surface treatments. UV-ozone, corona discharge, and low-pressure oxygen plasma treatments for different times were used to increase the surface energy of the foams. The low-pressure oxygen plasma was the most successful surface treatment to promote the adhesion of the foams. A reduced length of treatment was needed to improve the adhesion of the foams containing higher EVA content. The surface treatments produced a noticeable decrease in contact angle values due mainly to the creation of different carbon–oxygen moieties and to the formation of cracks/heterogeneities on the foams surfaces. After oxygen plasma, removal of non-polar material from EVA surfaces allowed to expose acetate groups which are likely to be important for increasing the adhesion of the foams.

Low pressure plasma treatment of poly(3-hydroxybutyrate): toward tailored polymer surfaces for tissue engineering scaffolds.

Thin films of poly(3-hydroxybutyrate) were modified by microwave ammonia plasma treatment. The results of the modification were studied by means of contact angle goniometry, ellipsometry, Fourier transform infrared-attenuated total reflection spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. To prove the presence of amino groups on the poly(3-hydroxybutyrate) surface, chemical labeling with 4-trifluoromethyl benzaldehyde was performed before X-ray photoelectron spectroscopy analysis. Under the applied plasma conditions, a hydrophilic surface with a good long-term stability was obtained.

Effects of hydrogen, oxygen, and ammonia low-pressure glow plasma on granular starches

Cassava, corn, high amylose corn, potato, rice Indica, rice Japonica, sweet potato, waxy corn, and wheat starches were exposed to low-pressure ammonia, hydrogen, and oxygen plasma. In every case, depolymerization of the starch polysaccharides was noted. The extent of the depolymerization depended on the nature of the starch as well as the type of plasma applied. Among three fractions of polysaccharides distinguished by their molecular weight average, the fraction of the highest molecular weight suffered the most efficient depolymerization. The relative depolymerization for the middle- and low-molecular fractions of polysaccharides was found to be starch and plasma specific. The chemical character of the plasma had very little influence on the starch polysaccharides. Only subtle oxidation effects were observed in oxygen plasma. Low-pressure glow plasma treatment appeared to be a convenient tool for a waste-less dextrinization of starch. Manipulation of the plasma variety and the time of exposure resulted in a wide spectrum of dextrins of various molecular weights and paste-forming properties.

Recent studies of insulating polymers with high-resolution electron energy loss spectroscopy

The application of high-resolution electron energy loss spectroscopy (HREELS) to the study of insulating polymer surfaces is reviewed. Such studies require the use of a secondary electron beam to prevent charging and to achieve stable surface potentials. Industrial grade polymers, obtained as thin sheets or cut from rod stock, are also rough and disordered compared to the single-crystal or thin film surfaces normally studied by HREELS. Therefore, the practical application of HREELS to insulating polymer samples has been limited by resolution, intensity and stability under secondary electron beam neutralization. This paper reviews recent progress of a method which combines modern instrumentation, low-energy secondary electron beam neutralization and spectral restoration algorithms. It is demonstrated that this method approaches a practical resolution criterion of 5 meV (40 cm −1) at relatively high spectral intensities as shown for polystyrene and polytetrafluoroethylene. The application of HREELS to surfaces modified by low-pressure plasma treatment is also illustrated.

Low pressure plasma treatments inside a closed package

An object is sealed under reduced pressure in a flexible plastic package. If it is put in a vacuum chamber the residual gas in the package inflates it. Adjusting the pressure inside the package and in the chamber, it becomes possible to generate a plasma inside the package by applying an external electrical field. This plasma can be used to alter the surface properties of the object. After the treatment, the package will prevent a contamination of the clean and modified surfaces. Polyethylene and polystyrene incorporate substantial concentrations of oxygen and nitrogen in their surfaces during a plasma treatment in a package, which contains air or nitrogen as a residual gas. The contact angle with water is decreased accordingly to approximately 40°. The XPS C1s spectra show features of amide/imide structures. In the polyethylene surface we also found a component that represents urethanes.