The effect of a cold remote N₂ plasma (CRNP) or N₂ + O₂ plasma (CRNOP) on poly(ether ether ketone) (PEEK) is studied. The amount of nitrogen or oxygen uptake and functionalities are determined by x-ray photoelectron spectroscopy (XPS). After CRNP treatment, the N/C and O/C atomic ratios are 0.301 and 0.333, respectively. Nitrogen functional groups are not detected by CRNOP treatment, and the O/C atomic ratio is then 0.785. The ageing process of the treated PEEK surface in the open air is investigated in both cases. For CRNOP treatment the O/C atomic ratio decreases by carbonate function departure, whereas for CRNP treatment the total amount of nitrogen and oxygen graft atoms goes through a maximum after 1 h of air exposure.

An investigation of the surface by XPS photoelectron spectroscopy has shown that the process of production of cast contact lenses based on poly(2-hydroxyethyl methacrylateco-diethyleneglycol methacrylate) is accompanied by mass transfer at the lens-mold boundary. This phenomenon, which impairs the compatibility of the lens during its application, can be considerably suppressed by employing a suitable surface modification of polypropylene molds. The surface treatment consisting in the oxidation of the mold surface by an AC corona discharge in the oxygen atmosphere increased hydrophilicity of the material, thus facilitating separation of the lens from the mold. The results of the XPS study were also confirmed microscopically by employing the SEM method. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jab.770030406

The effects of oxygen plasma treatment on ultra-high-modulus polyethylene monofilaments have been investigated with particular reference to their adhesion to epoxy resins. The adhesion strength was monitored by pull-out tests, and the effects on the monofilaments was also studied by contact angle measurements, determination of gel content and scanning electron microscopy. The results of this investigation suggest that there are three contributions to the improvements in adhesion obtained from the plasma treatment. First, at comparatively short treatment times there is a general oxidation of the surface. Secondly, at intermediate treatment times cross-linking of the surface occurs, which increases the cohesive strength of the fibre surface. Finally, at long exposure times there is a pitting of the surface, which could give rise to a mechanical keying effect but may also reduce the fibre strength.

The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γ_s, is reviewed. The basis for the three components γ^LW_s, γ⊕_s, and γ⊖_s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γ^p') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, π_e. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ^⊖ and γ^⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γ^LW at the hydrophobic corner, and γ^⊕ and γ^⊖ at the two hydrophillic corners, is suggested.

Contact-angle measurements in air and water environments and X-ray photoelectron spectroscopy (XPS) were used to characterize the surface properties of air-corona-treated polypropylene (PP) and poly(ethylene terephthalate) (PET) films. Surface properties were examined as a function of the storage time at various temperatures. Corona treatment forms water-soluble, low-molecular-weight oxidized materials on both polymer films. Corona-treated PP and corona-treated PET films have markedly different responses to aging. For corona-treated PP stored at ambient temperatures, only a slight decrease in wettability was observed. This decrease was attributed to the reorientation of oxidized functionalities within the surface region. At elevated storage temperatures, migration of oxidized species out of the surface region occurred under some conditions. For corona-treated PET, extensive migration and reorientation of oxidized groups occurred even at ambient temperatures, leading to significant decreases in wettability and a loss of surface oxidation. The contrasts in the responses of PP and PET to corona treatment are primarily due to differences in the properties of the base polymer resins.

A polyester film surface was graft-polymerized with various water-soluble monomers by a combination of plasma pretreatment and photoirradiation. The grafted surfaces showed strong adhesion to another non-grafted or grafted film when brought into direct contact in the presence of water and subsequently dried. The adhesion force depended on the hydrophilicity of the adhering surfaces and the graft density. The film having a larger graft density generally showed stronger adhesion in the final stage of drying, but it took longer to achieve high adhesion because of the larger amount of water existing in the interfacial region between the two surfaces. On the other hand, substantial adhesion was obtained almost instantaneously upon contact when one was grafted with an anionic polymer and the other grafted with a cationic polymer. Adhesion between similarly charged surfaces was very weak at the beginning of drying, probably because of the electrostatic repulsion between the charged groups.

Modification of polyethylene and polypropylene film and powder surfaces with oxygen and hydrogen peroxide is promoted by nonporphyrinic, nonfree radical based iron reagents such as Fe₃O(OCOCH₃)₆(C₆H₅N)_3.5 and FeCl₃ • 6H₂O/picolinic acid. These oxidation systems introduced small amounts of carbonyl groups onto the surface of these hydrocarbon polymers. The most visible manifestation of this reaction was increased polyolefin wettability toward water. IR spectroscopy, XPS spectroscopy, and chemical derivatization all were used to verify that the reaction had occurred and that a chemically derivatizable surface had been prepared. The overall process produced a fraction of the density of functional groups introduced by conventional etching chemistry.

The effects of flame treatment on the surfaces of a propylene-ethylene copolymer have been studied using XPS, contact angle measurement, vapour-phase derivatization and an adhesion test. The results obtained were compared to those from the homopolymer. An optimum air-to-gas ratios of ∼11:1 has been found. Close correspondence between water contact angle and oxygen concentration was found, with the exception of high oxygen concentrations. The orientation or migration of functional groups away from the surface has been proposed to cause the non-correspondence between water contact angle and oxygen concentration. Diiodomethane advancing contact angle was found to remain constant, independent of flame conditions. XPS analysis in conjunction with vapourphase derivatization with trifluroacetic anhydride (TFAA) suggests that up to 20% and 30% of the oxygen introduced in the surfaces is present as hydroxyl groups for propylene homopolymer and the copolymer, respectively. High adhesion levels of the flame-treated copolymer with a polyurethane-based paint were found. In most cases, the adhesion failure was complex, but involved the cohesive failure of the copolymer.

Peroxides formed on the surface by corona treatment of low-density polyethylene film can be used to initiate grafting of polar vinyl monomers such as acrylic acid. Different types of peroxides are probably formed on the surface, but at least hydroperoxides could be detected by XPS analysis. The grafting reaction was carried out directly after corona treatment, by placing the corona-treated film above a solution of acrylic acid heated to 100°C. The grafting reaction takes place in a vapor phase of the monomer. After extracting the reacted films with hot methanol and drying, surface analysis by XPS, IR, and contact angle measurements were carried out. Effect of degree of corona treatment and reaction time have been studied. The conclusion from this work is that acrylic acid in vapor phase can successfully be grafted onto corona-treated polyethylene film by this method. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1992.070460916

Hydrophobic recovery of oxygen-plasma-treated polystyrene has been studied with regard to its dependence on temperature, molecular weight and plasma parameters, namely radio frequency power. Plasma chemistry has been studied by actinometry, while surface analysis has been performed by X-ray photoelectron spectroscopy (X.p.s.) and contact angle measurements. Molecular weight changes have been investigated by gel permeation chromatography. Plasma-treated surfaces become richer in oxygen and more crosslinked than untreated ones. Two basic mechanisms have been observed: one is based on short range motions within the plasma-modified layer, burying polar groups away from the surface without modifying its X.p.s. composition; the other involves long range motions, i.e. diffusion of non-modified macromolecules or segments from the bulk to the surface, altering its X.p.s. composition. The latter mechanism becomes important upon increasing ageing temperatures, or lowering molecular weight or extent of crosslinking of the surface layer.

A new method for preparing wettability gradients on polymer surfaces was developed. Wettability gradients were produced on low density polyethylene surfaces by treating the polymer sheets in air with corona from a knife-type electrode whose power gradually increases along the sample length. The wettability gradient surfaces prepared by the corona discharge treatment were characterized by the measurement of water contact angle, Fourier-transform infrared spectroscopy in the attenuated total reflectance mode, electron spectroscopy for chemical analysis, and scanning electron microscopy. The gradient surfaces prepared can be used to systematically investigate the interactions of biological species in terms of the surface hydrophilicity/hydrophobicity of polymeric materials.

Kapton films were treated with seven plasmas: Ar-, N₂-, O₂-, CO-, CO₂-, NO-, and NO₂- plasmas. Surface properties and chemical composition of the plasma-treated Kapton films were investigated from the contact angle measurement, and the IR and XPS spectra. The plasmas, especially NO- and NO₂-plasma, made the Kapton film surface hydrophilic. The XPS and IR spectra showed that the plasma led to the modification of the imide groups in the Kapton film to secondary amide and carboxylate groups.

A roll-coating experimental system is used to study the effect of pre-wetting on dynamic contact angles, the interfacial displacement depth, and the associated phenomenon of air entrainment. The system consists of a roll, which is horizontally rotating in a liquid pool. The dynamic contact angle is recorded by a macrophotography system. The test liquids are glycerol solutions with viscosities in the range 104 < μ < 748 mPa · s. The value of (μV/ρg)^0.5 is taken as the characteristic length to be used in the dimensionless relationships which correlate experimental measurements. The effect of base layer entry angle into the liquid pool on the dynamic contact angles and other flow parameters is studied. Comparison is made with measurements in dry tape-coating and other pre-wet roll–coating systems.

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus. Scanning electron and optical microscopic observations have been used to examine the microscopic basis for these results, which are compared with results previously obtained for aramid/epoxy and extended-chain polyethylene/epoxy composites. It is concluded that the increased interlaminar shear and flexural properties of vinylester matrix composites are due to improved wetting of the surface-treated fibres by the vinylester resin, rather than covalent chemical bonding.

Historically, the technologies most interested in the wetting of fibers have been those involved in the processing of textiles.(1, 2) Much of the early scientific literature on wetting was concerned with liquid penetration into fabrics and other porous solids.(3) More recently, the rapid development of fiber reinforced composites, notably carbon fiber and glass fiber reinforced polymers (CFRP, GFRP), has generated a renewed interest in the wetting of fibers. However, in the interim there has been a change in the scientific attitude toward the use of contact angle measurements as a means of characterizing the surface chemical constitution of solids. In the early literature, the contact angle was viewed as a characteristic of the fiber and a parameter in the capillarity equations for liquid penetration. Due in large measure to the studies by W. A Zisman and co-workers, there has been a change in attitude toward the physical significance of contact angle measurements. It is now recognized that the contact angle can be a highly sensitive tool for surface characterization. Consequently, there is a growing body of literature on the wetting of textile fibers and fibers used in composites aimed at surface chemical characterization as well as the processing of these fibers into composite materials.

Applications of corona discharge induced plasmas and unipolar ions are reviewed. Corona process applications emphasize one of two aspects of the discharge: the ions produced or the energetic electrons producing the plasma. The ion identities depend on the polarity of the discharge and the characteristics of the gas mixture, specifically on the electron attaching species. The electron energies depend on the gas characteristics and on the method of generating the corona. In general, in an application using ions, the corona induced plasma zone will occupy a small fraction of the total process volume, while a process using the electrons will fill most of the volume with the plasma. Current state-of-the knowledge of ionized environments and the function of corona discharge processes are discussed in detail.

Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections, such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solid-solid or solid-liquid interfaces after they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy-gamma by the Johnson-Kendall-Roberts theory, for example, F = 3-pi-R-gamma for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry. Both XPS and IR-RAS results indicated the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surface. Oxygen and argon plasmas were highly ablative, whereas ammonia plasma was only moderately so. Oxygen and argon plasmas appear to react with the LaRC-TPI via a fragmentation/oxidation mechanism; the effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups.

The aim of this work was to improve adhesion to tefzel using plasma surface treatment. The plasmas used were O₂, and NH₃. Joints ,ade from the adherends using several commercially available epoxy adhesives were tested using a double lap shear configuration. Measured bond strenghts for the treated adherends were as much as 30 times greater than those for the untreated materials. Examination of the O₂ plasma-treated Tefzel by electron spectroscopy for chemical analysis indicated a surface oxidation increase of about 7–8% over the untreated material, with the oxide being primarily in the form of an ester.

Properties of polymer surfaces are very sensitive to minute quantities of impurity and different preparation procedures. Baier and Zisman reported that wettabilities of polypeptides and polyamides such as Nylon 2,6, and 11 are different when the polymers are cast from different solvents. They attributed this difference to the existence or absence of the surface hydrogen bonding site. They also proposed a Zisman plot split criterion for the recognition of hydrogen-bonding functionality in a polymer specimen's surface. However, we found that this criterion does not always work. We then apply our new model for acid-base interactions to interpret their data. The model fits data well. Moreover, it is noticed that cases with exposed surfacehydrogen bonding sites are all bipolar and surfaces without hydrogen bonding sites are all monobasic.

A viscous-liquid drop spreads on a smooth horizontal surface, which is uniformly heated or cooled. Lubrication theory is used to study thin drops subject to capillary, thermocapillary and gravity forces, and a variety of contact-angle-versus-speed conditions. It is found for isothermal drops that gravity is very important at large times and determines the power law for unlimited spreading. Predictions compare well with the experimental data on isothermal spreading for both two-dimensional and axisymmetric configurations. It is found that heating (cooling) retards (augments) the spreading process by creating flows that counteract (reinforce) those associated with isothermal spreading. For zero advancing contact angle, heating will prevent the drop from spreading to infinity. Thus, the heat transfer serves as a sensitive control on the spreading.

A technique was developed to rapidly measure surface tensions (γ) of viscous molten polymers and polymer solutions. The usual problems of slow meniscus equilibration and low signal-to-noise levels due to thermal convection currents at elevated temperatures have been overcome. Small-diameter fibers were used as vertical probes in the Wilhelmy technique to facilitate rapid equilibration of the wetting meniscus, and a “baffle tube” surrounding the electrobalance wire was implemented to suppress noise from thermal convection currents from the oven. Even with the baffle tube, it was found that computer averaging of the measured wetting force was necessary to obtain the desired precision precision atT 250°C. Measurements of γ up to ∼400°C were routinely made with η> 50 P polymers. Data are given for a molten fluoropolymer, a thermoplastic, and a liquid crystalline polymer. Room-temperature polymer fluids with viscosities extending to η = 500,000 P were studied; at η ⩽ 5000 P the precision was better than 0.04mNm. The dynamic contact angle versus time was measured as a function of fiber diameter, giving a relationship between the rate of meniscus equilibration and fiber diameter. Contact angles of polymer fibers immersed in water and methylene iodiode were used to calculate the surface free energies of the polymer in the solid state. These values are consistent with the extrapolated molten surface tension data and help to characterize the trend in γ over a wide range ofT.

Filter paper and greaseproof paper have been exposed to hydrogen or oxygen plasma. The paper surface composition was determined by ESCA measurements. The unmodified and modified papers then were laminated with polyethylene and the adhesive strength was measured. The hydrogen plasma treatment reduces the cellulose surface and forms low-molecular-weight degradation products. It is shown that the reduction of the cellulose surface has no influence on the adhesion, but the degradation products strongly decrease the adhesion. Oxygen plasma treatment increases adhesion, probably by removing low-molecular-weight wood resin from the surface and by forming covalent bonds across the interface.