Nitrogen plasmas at atmospheric pressure produced by 2.45 GHz microwaves at a power density of approximately 10 MW m^-3 have a degree of ionization less than about 10^-7. Nevertheless they have interesting and potentially important effects on polymer and metal surfaces exposed to them. An experimental programme is underway to identify the active species in the plasma and its afterglow. This paper describes a simplified model of the chemical kinetics in the plasma that allows species concentrations to be estimated in a range of conditions, for comparison with experimental data. It predicts a high degree of dissociation combined with low gas temperature in microwave-generated plasmas.

As shown from the literature data, plasma treatments of polymer materials create chemical and morphological surface modifications which are strongly dependent on the experimental conditions and on the methods and apparatus configurations employed. In the present work, poly(tetrafluoroethylene) (PTFE) substrates were treated by RF plasma (RIE mode) in various gaseous atmospheres (H2, He, Ar, O2, N2, NH3). The main objective was to compare, under similar experimental conditions, the capabilities of these different gases to modify the morphology of the PTFE surface and to graft specific chemical functionalities for a subsequent metallization through an electroless process. The relevant chemical modifications were characterized by XPS and surface energy measurements while the morphology changes were observed by SEM. Under similar experimental conditions (treatment time, working pressure) and with the reactor operating in the RIE mode, defluorination capabilities of the plasma treatments vary as a function of the gaseous atmosphere according to the following sequence H2 > Ar, NH3 > N2 > He ≫ O2. In addition, O2 and He plasmas are shown to strongly etch the PTFE surface. The very low ability of O2 plasmas to graft oxygenated functionalities is largely due to their strong etching power. A discussion is also focused on the complex interactions between nitrogenated plasmas and PTFE surfaces. Especially, it is shown that nitrogen is not grafted in the same chemical form after an NH3 plasma as after a N2 plasma. Furthermore, after treatment in non-nitrogenated and non-oxygenated plasmas (He, Ar, H2) both nitrogen and oxygen species are grafted on the polymer surface, probably in the molecular form. Such a grafting occurs on venting the reactor with dry nitrogen and on exposing plasma-treated samples to ambient atmosphere.

This paper deals with the electroless metallization (nickel plating) of poly(tetrafluoroethylene) substrates which were previously submitted to RF glow-discharge plasmas in various gaseous atmospheres (H2, He, Ar, O2, N2, NH3) and subsequently to sensitization/activation or direct activation processes in order to chemisorb palladium which is the catalyst of the plating reaction. As shown in previous works and confirmed in this one, the use of the conventional sensitization/activation treatment (immersion of the plasma-treated samples successively in acidic tin chloride and palladium chloride solutions) is made possible due to the strong chemical affinity of tin to oxygen. On the other hand, when nitrogenated species are grafted on the PTFE surface, the chemisorption of the catalyst can be directly accomplished using only a simple acidic palladium chloride solution. It is shown, in more detail, in this paper that O2 and H2 plasmas cannot be used to deposit electroless Ni films through the conventional sensitization/activation process. This is due to the negligible oxygen content grafted onto the PTFE surface (case of O2 plasma), and to the strong crosslinking of this same surface (case of H2 plasma) even though the amount of oxygen grafted during the post reactions in air is relatively high. On the other hand, bright and adherent Ni deposits are obtained by using either He or N2 plasmas via the conventional two-step process again due to the oxygen species grafted during the post-reactions in air, or by N2 and NH3 plasmas via the direct one-step process due to the nitrogen species grafted during the plasmas themselves.

Polymer surfaces can be covered with functional groups by exposure to a plasma. The species of the plasma gas are attached to surface carbon atoms forming functional groups of different compositions. To produce a modified polymer surface with a high density and homogeneity of functional groups several possibilities such as plasma grafting of intact monomers, selective plasma bromination, plasma oxidation followed by conversion to OH groups as well as introduction of spacers with functional groups were tested. Thus, to produce exclusively OH groups at the polymer surface, the O functional groups formed by an oxygen plasma were chemically reduced by diborane, Vitride™ (Na complex) or LiAlH4. Typical yields were 9 to 14 OH groups per 100 carbon atoms as detected by XPS. The segment lengths of the spacers were varied between 1 to 22 ethylene or ethylene oxide units. At the end of the different spacers OH, NH2, COOH, Br or C=C groups are bound. These specifically functionalized polymer surfaces are used in pharmacy and medicine. Especially C=C or OH group terminated spacers have been found to “preserve” the plasma activation of the polymer surface by converting the unstable radical sites into stable functional groups. On further processing these groups can react with polymer coatings by classic radical mechanisms (C=C) or by polyaddition (OH) with polyurethanes or other polymers forming pure covalent bonds.

Glow discharge plasma is an effective method for treating surfaces, sputtering, etching, plasma-assisted deposition, ashing, and is used in a range of other processes. Sigma Technologies Int’l, Inc., has developed a novel plasma system which can be operated at atmospheric pressure, thereby eliminating the need for vacuum chambers and pumps. This atmospheric plasma system can be effectively used for surface treatment and for plasma-assisted deposition. This plasma system has been tested successfully for the functionalization of various polymer films. The surface energies of the films treated by the newly developed atmospheric plasma system have been shown to increase substantially, thereby enhancing the wettability and adhesion properties of these films. Details of the atmospheric pressure plasma system, and the results from treatment tests are presented.

Ultrahydrophobic polymeric surfaces were prepared using two plasma chemistry approaches: (1) fluorocarbon plasma polymerization, and (2) simultaneous argon plasma etching of polypropylene (PP) surfaces and sputtering of poly(tetrafluoroethylene) (PTFE) onto these rough surfaces. In the first case, several fluorinated monomers were selected to study the effect of the monomer structure on the plasma polymer morphology and wettability. Ultrahydrophobic surfaces were generated for those monomer gases that were capable of forming powders. For fluoromonomers that did not form powders, the wetting characteristics were similar to that of PTFE. Plasma polymerization of perfluorohexane does not lead to powder deposition and the highest advancing water contact angle measured was 118° (the receding contact angle was 74°). Fluorinated acrylates and ethyl heptafluorobutyrate were tested as well and in all cases, the powder formation of the polymer led to highly hydrophobic surfaces (advancing and receding contact angles between 164°–174° and 8°–173°, respectively). In the second technique, argon plasma was used to etch PP surfaces, creating a rough surface (the roughness is controlled by the reaction time). Simultaneously, PTFE was sputtered onto the roughened PP surface to create fluorinated surfaces. The most hydrophobic surface exhibited an advancing contact angle of 172° and a receding contact angle of 169°. AFM and SEM analyses of these samples show that the powder deposition of the polymers and the etching of PP concurrent with the sputtering of PTFE lead to rough surfaces resulting in a highly nonwettable surface.

Low-pressure gas plasma treatments were proposed as an alternative to the chemical surface treatments (e.g. halogenation, cyclization) of vulcanized styrene-butadiene (SBR) rubber. The effectiveness of low-pressure oxygen plasmas has been already shown. In this study the influence of the oxygen/nitrogen ratio on the adhesion performance of rubber/polyurethane adhesive joints was considered. Different mixtures of oxygen (20–40 vol%) and nitrogen (80–60 vol%) were used for the plasma treatment of an SBR rubber between 1 and 15 minutes, using a power of 50 watts and a residual pressure of 1 Torr. The modifications produced on the rubber surface by the plasma treatment were assessed using advancing and receding contact angle measurements, ATR-IR spectroscopy and scanning electron microscopy. Adhesion was determined from T-peel tests on plasma treated rubber/polyurethane adhesive joints. The treatment of rubber with oxygen-nitrogen mixture plasmas decreased the advancing and receding contact angle values and increased the T-peel strength (a cohesive failure in the rubber was produced). This increase was due to the partial removal of hydrocarbon moieties from the rubber surface and to the creation of oxygen containing species. The increase in the time of treatment decreased the peel strength and made the locus of failure mainly cohesive in the rubber. The higher the percentage of oxygen in the gas mixture, the greater the degree of oxidation on the rubber surface, the higher the degree of roughness and the more effective the treatment. A minimum percentage of 20 % oxygen in the gas composition was required to achieve good adhesion. Nitrogen plasma produced a different effect than the oxygen-nitrogen mixture plasma due to crosslinking reactions on the treated rubber surface which directed the failure to be cohesive in the adhesive.

Polycarbonate surfaces have been treated with radiofrequency plasmas of oxygen, air and argon to hydrophilise the surfaces and to provide good cell culture properties. Surfaces treated at high RF power/gas flow ratios were highly hydrophilic and stable towards washing in 70% ethanol, while those treated at lower ratios were not wash-stable. Cell growth properties as good as on commercial tissue-culture polystyrene could be obtained down to 20° water contact angle (measured after ethanol washing) on the treated surfaces for three different human cell lines (HeLa cervix carcinoma cells, MRC-5 lung fibroblasts and Chang hepatoma cells). The HeLa cells were most sensitive to the treatment conditions, while the Chang cells showed the most robust behaviour. Cells grown on surfaces with around 20° water contact angle were assessed by immunofluorescence staining methods and phase contrast microscopy. The cells showed normal behaviour with respect to morphology, spreading, cytoskeleton structure, cell-surface contacts and DNA synthesis.

The chemical and morphological influences of SF6 and Ar plasmas on bisphenol-A-polycarbonate (PC) and the influence of plasma treatments on Al metallisation were investigated. The treatment of the sample, X-ray photoelectron spectroscopic (XPS) and scanning force microscopic (SFM) analyses were made in an ultrahigh vacuum (UHV) chamber without breaking the vacuum. Using SF6 for the etching process, a significant inclusion of fluorine (C-F, C-F2) takes place. After argon plasma treatment of the PC surface a reduction in the carboxylic carbon was observed in the C1s spectrum. Both kinds of plasma treatments reduce the double and single bonded oxygen. During the metallisation process on an Ar-plasma treated PC surface aluminum couples via oxygen to the aromatic carbon. Al-metallisation on the SF6 pre-etched surface leads to the formation of an Al-F interlayer. With the SFM, the roughening effects on the nm scale after the two plasma treatments is observable. On the virgin PC, Al layers can be seen as slightly bound clusters. On both plasma pre-treated PC surfaces the Al grows as a film.

Poly(vinylidene fluoride) (PVDF) samples were treated in plasma atmospheres of ammonia, pure N2 and N2/H2 mixtures in order to enhance their adhesion to evaporated copper. The chemical and physical modifications occurring on the plasma treated PVDF films were studied by XPS measurements. The main effects resulting from these treatments were a substantial defluorination and the grafting of oxygen- and nitrogen-containing groups. The adhesion of 20 nm thick copper layers was evaluated by peel test measurements. XPS depth profiles of the samples with Cu overlayers were used to identify chemical bonds at the Cu-PVDF interface.

Aramid cords and fibers and polyester tire cords were treated in a continuous or pulsed DC plasma containing organic monomers such as pyrrole or acetylene in a custom-built reactor. For the treated cords the rubber adhesion was measured in a standard pull-out test. It was found that the plasma polymer coating significantly increased the pull-out forces. The effect of the power-to-pressure ratio and the pulsing of DC power on the performance of the treated cords or fibers were investigated. It was found that, in general, low power / high pressure conditions gave better results than high power / low pressure conditions. Coatings obtained under these conditions were thoroughly characterized by a range of analytical tools. Based on these data and on failure analysis, models were developed to explain the experimental findings.

Ultra-high strength polyethylene fibers were treated with excimer laser and high power ion beams (HPIB) to enhance their adhesion to epoxy resins. Laser treatments were carried out in air, argon, and helium environments and HPIB treatments were carried in vacuum. The effects of these treatments on the surface topography and chemistry were characterized using several techniques. It can be seen from the results that both laser and HPIB treatments increased the fiber surface roughness as well as the surface polarity. HPIB treated fibers had a characteristic bumpy surface while the laser treated fibers had deeper striations or a rougher surface. Although the total surface energy did not change after the treatments, the acid-base component increased significantly and the dispersive component decreased by almost the same amount. After the treatments the fiber-epoxy interfacial shear strength (IFSS) increased between 200 to 300%. This enhancement is attributed to the increased roughness of the fiber surface and increased specific interface area, increased polar nature and wettability, as well as improvement in the acid-base component of the surface energy.

The exposure of a polymer to ozone in the presence of ultraviolet light (UV/ozone) is a simple and effective way to improve the wettability of the surface. Using atomic force microscopy (AFM) we were able to examine the changes in morphology and the increase in the adhesion force at the surface of biaxially oriented polypropylene (PP) films after treatment with UV/ozone. It is clearly shown by atomic force microscopy (AFM) that UV/ozone treatment modified the original, fine, fiber-like structure to one displaying the formation of mounds or droplets. These droplets are most likely comprised of short chains of oxidized polymer or low-molecular weight oxidized materials (LMWOM). The size of the mounds increased with increasing treatment time. More interestingly, lateral force imaging AFM were capable of distinguishing these mounds from the surrounding surface, indicating that the mounds were formed on aggregation of the loose LMWOM during the UV/ozone treatment, while the surrounding surface was covered by bound moderately oxidized materials. The adhesion force was estimated from measurements made on the amount of force required to retract the tip from the surface after the two had made contact. A clear increase in adhesion force was observed on the modified PP film surface, which indicates an increase in the surface energy. We have demonstrated that mechanical scratching can alter the surface morphology and increase the surface energy of a polymer on a micrometer scale. The mechanically-scratched areas are more susceptible to modification than the surrounding unscratched surface when exposed to UV/ozone.

High surface energy (60–70 mJ/m2) polymers, i.e., totally wettable by water, from polypropylene to fluoropolymers have been obtained by ion-assisted reaction (IAR), in which the polymer surface was irradiated by energetic ions in a reactive gas environment. The ion energy was 1000 eV and the ion dose was varied in the range of 5 × 1014 – 1 × 1017 ions/cm2. Oxygen gas was introduced near the polymer surfaces during ion irradiation. The change in wettability was critically dependent on the ion dose and on the flux of oxygen gas. The surface energy was mainly increased due to the polar component related to the hydrophilic groups generated such as carbonyl and carboxyl, etc. The reaction generating the hydrophilic groups on the polymer surface modified by ion assisted reaction (IAR) was explained according to a two-step mechanism. The improvement in adhesion between the IAR-modified polymers and other materials was also explained in terms of the increased surface energy as well as surface roughness of the polymers modified by IAR.

Wettability of polyimide (PI) films modified by atomic oxygen (AO) was investigated by contact angle measurements. The PI films with/without being covered by a metal mesh were exposed to the AO beam with fluences from 1.4x 1016 to 9x 1018 atoms/cm2. The atomic force microscopy (AFM) and the X-ray photoelectron spectroscopy (XPS) were used to characterize the PI film surfaces. Both the roughness and the oxygen concentration at the PI surface increased by the AO exposure. The advancing and the receding contact angles of water on the PI films were measured both by the sessile drop method and by the Wilhelmy method. The contact angles measured by these two methods were identical for the PI samples both with/without AO exposures. In the case of the AO-exposed PI films being covered by the metal mesh, the contact angles were evaluated by the Wilhelmy method. A difference in contact angles on the exposed and the covered areas was clearly observed. It was also found that the wettability of the AO-exposed PI films was related to the amount of oxygen detected by the XPS.

The idea of the treatment of textiles with plasma is a few decennia old. There is no consensus about who was “the first,” but it is clear that the treatment of textiles is historically linked to the plasma treatment of polymers in general. As one of the most promising alternatives in many fields, the importance of plasma treatments results from the exceptional advantages it offers. It does have specific action only at the surface, keeping the bulk properties unaffected. The future of plasma is closely linked to the fact that this technique gives the treated surface some properties that cannot be obtained by conventional techniques, and this is without the need to use water as a reaction medium. At the level of textiles, this means changing an almost inert surface into a reactive one, and in this way, it becomes a surface engineering tool. The transfer of research results into the technological field would lead to nonpolluting and very promising operating conditions. In the prospect of chemical finishing using plasma, two main methods can be considered: grafting of a compound on the fiber or surface modification by means of discharges.

Polyester monofilaments were treated by a pulsed surface electrical discharge in nitrogen at atmospheric pressure, to increase their adhesion to an epoxy resin matrix. The treatment resulted in an eight-fold increase in adhesive strength, without any change in mechanical properties of the monofilaments. It is concluded that polar group interactions, rather than increased surface area, are responsible for the improved adhesive strength.

The use of plasma treatment for the modification of polystyrene microtiter wells has been evaluated by contact angle measurements, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The contact angle data suggests that the effect of plasma treatment is first to clean the surface of volatile contamination, increasing the hydrophobicity, and then to introduce oxygen functionality into the surface, decreasing the hydrophobicity. The cleaning effect appears to occur in the first few seconds of treatment while the oxygenation effect increases with increased exposure to the plasma. The XPS and ToF-SIMS measurements show increasing surface oxygen concentration with plasma treatment time, with a concomitant reduction in aromaticity. Scanning tunnelling microscopy (STM) imaging reveals that plasma treatment significantly affects the adsorption of bovine serum albumin (BSA). Untreated surfaces exhibited areas where no BSA adsorption occurred. These regions ranged in size between 20 and 60 nm in diameter. Plasma treated surfaces, however, exhibited no such areas, with BSA adsorption appearing to be more uniform across the surface. The regions on the untreated surfaces where no BSA adsorption occurred are thought to be hydrocarbon (volatile) in nature possibly from the moulding process, which is removed in the first few seconds of plasma treatment.

The hydrophobic recovery of polydimethylsiloxane elastomers was studied after exposure to partial electrical discharge. Silicone elastomers that were thoroughly extracted of free oligomeric impurities as well as those deliberately contaminated with low molecular weight (LMW) silicone fluids were used for these studies. Contact angle and X-ray photoelectron spectroscopy revealed that the recovery rates of the oxidized extracted samples are strongly influenced by the applied voltage, humidity, and aging condition. The recovery rates increase considerably as the applied voltage and the humidity during discharge increase. Remarkably, the oxidized samples stored under high vacuum (10⁻⁷ Torr) exhibit lower recovery rates than those aged in air. Free silicone fluid, when added to the elastomer, affects the recovery rate as well; however, significant recovery is seen even without any added fluid. These results imply that the LMW species that are formed in situ during electrical discharge are sufficient to cause the hydrophobic recovery of oxidized PDMS elastomers.

Improvement of the paint adhesion to a polypropylene (PP) bumper has been investigated without using a primer by treating the bumper surface with O₂, H₂O, and acetylene plasmas. All the plasma treatments resulted in an increase of the adhesion strength in dry conditions. The adhesion strength could be increased up to a value comparable to that obtained by applying a primer. The treated surfaces were quite stable for 7 days in air. After exposure to wet conditions, however, the adhesion strengths for both O₂ and H₂O plasma-treated samples decreased significantly, while the adhesion strength for the acetylene plasma-treated sample did not change much.