A wide variety of plasma treatments was performed on polyethylene surfaces, resulting in a wide range of total surface energies. The linear correlation of with cos θ was discussed in light of the Young–Dupré equation. Hundred percent of the surface energy variation was accounted for by the polar component of surface energy; the dispersive component was not affected by surface treatment. These data show that for this polymer the contact angle of a single polar liquid can be used as a robust quantitative indicator of treatment level, and because of its excellent linear correlation with total surface energy for this system, can be used as a quantitative measure of total surface energy.

The use of plasma deposition to introduce sulfonate groups to the surface of a polyurethane was attempted. In previous work, the bulk incorporation of sulfonate groups was found to improve the blood contacting properties of the base polyurethane but physical properties in the hydrated state were adversely affected. Plasma deposition schemes involving ammonia and sulfur dioxide were utilized in an attempt to incorporate sulfonate groups. Surface characterization by X-ray photoelectron spectroscopy (XPS) and contact angle measurements was used to follow polymer surface rearrangement dynamics and to address the issue of plasma chemistry specificity. Concerns of reaction specificity were alleviated by using the plasma as a pretreatment which is followed by a chemical surface derivatization.

World-class, fully automated manufacturing processes rely more and more on advanced, environmentally friendly surface treatment technologies. An innovative atmospheric pressure plasma technique allows inline rubber and plastic manufacturing processes to become fully automated with total process control. A thorough pretreatment must produce surfaces with reliable and repeatable characteristics to achieve optimal adhesive bonding, coating and printing results. In addition, pretreatment must be delivered in a cost-effective and safe manner. The new process uses the high effectiveness of plasma for microfine cleaning, high-surface activation and nanocoating. In most cases the plasma application takes the place of environmentally unfriendly and costly solvent cleaning or chemical adhesion promoters and primers.

The effects of surface modification of polyethylene (PE) and aluminum on the adhesion strength have been investigated. PE was modified by KMnO₄/H₂SO₄ treatment followed by adsorption of different cations, Ca²⁺, Ba²⁺, and Zn²⁺. The aluminum surface was treated with alkali and was also modified by adsorption of titanates. The surfaces were characterized by means of multiple internal reflection (MIR) IR and ESCA. The adhesion strength was measured by the T-peel test. Both ESCA and MIR analyses show the presence of hydroxyl, carbonyl, ester, and carboxyl groups on the KMnO₄/H₂SO₄-treated PE surface. In addition, sulfate and sulfonate groups are present. The sulfonate groups are apparently localized in crevices extending beneath the ESCA sampling depth of 50 A. Vinylidene groups are also present on the surface. Cation adsorption on the oxidized PE surface seems to be determined by the solubility constant of the corresponding sulfate salts and is in the order Ba²⁺ > Ca²⁺ > Zn²⁺. Adsorption of Ca²⁺ and Ba²⁺ increases the relative concentration of oxygen-containing groups on the KMnO₄/H₂SO₄-treated surface. A further increase is seen after annealing. KMnO₄/H₂SO₄ treatment almost doubled the adhesion strength of PE to aluminum. Ca²⁺ adsorption on the surface prior to lamination increased the adhesion strength nearly three times and caused cohesive failure in the T-peel test. However, when Zn²⁺ was adsorbed, the adhesion strength decreased drastically. Alkaline treatment of the aluminum surface had only a minor effect on adhesion. The chemical structure of the adsorbed titanates has a great influence on the adhesion strength.

To enhance polyimide-to-polyimide adhesion, we have investigated the effect of surface modification in water vapor plasma. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface before subsequent layers of polyimide are applied results in dramatically enhanced interfacial adhesion. The polyimide-to-polyimide interfacial adhesion strength attained following water vapor plasma treatment exceeds the cohesive strength of the applied polyimide layer. The effect of surface modification in water vapor plasma on metal-to-polyimide adhesion has also been investigated. The use of a water vapor plasma to treat a fully cured polyimide (PMDA–ODA) surface prior to metallization results in increased metal-to-polymer interfacial adhesion. A study of both electroless and vacuum-deposited metal was conducted. The use of contact-angle measurements, peel tests, Fourier transform infrared spectroscopy, optical emission spectroscopy, nuclear forward scattering, and X-ray photoelectron spectroscopy has led us to a preliminary understanding of the resulting surface modification and the subsequent effect of adhesion promotion. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1992.070461216

The main properties of corona discharges are reviewed, with emphasis on the features which make them unique for use as non-equilibrium chemical reactors : Their stability andease of operation over a wide range of gasesand pressures, including atmospheric : their sharply confined ionization regions where hot electrons interact with cold gas, inducing reactions without back reactions ; and their extended low field drift regions which act as gaseous electrolytes, inducing electrochemical reactions on surfaces. Present and future applications are discussed : Synthesis of ozone and ammonia, promotion of flames and combustion, surface treatment, and electrical insulation improvement.

The process of modifying the surface of polytetrafluoroethylene in a glow discharge plasma in vapors of organic compounds of various classes was investigated. It was established that the greatest increase of wettability is seen when modification is done in acrylic acid vapor. Multiple attenuated total internal reflection infrared spectroscopy was used to study the spectra of the coatings that formed and to demonstrate their difference in the case of acrylic acid.

The ESCA spectra of Kapton polyimide film exposed to atomic oxygen O(3P), either in low earth orbit (LEO) on the STS-8 Space Shuttle or downstream from a radio-frequency oxygen plasma, were compared. The major difference in surface chemistry induced by the two types of exposure to O(3P), both of which caused surface recession (etching), was a much larger uptake of oxygen by Kapton etched in the O2 plasma than in LEO. This difference is attributed to the presence of molecular oxygen in the plasma reactor and its absence in LEO: in the former case, O2 can react with radicals generated in the Kapton molecule as it etches, become incorporated in the etched polymer, and thereby yield a higher steady-state ‘surface oxidation’ level than in LEO.

The ESCA (electron spectroscopy for chemical analysis) spectra of films of poly(vinyl fluoride) (Tedlar), tetrafluoroethylene-hexafluoropropylene copolymer (in the form of a Teflon FEP coating on Kapton H, i.e. Kapton F) and polytetrafluoroethylene (Teflon or Teflon TFE) exposed to atomic oxygen (O(³P)) - either in low Earth orbit (LEO) on the STS-8 Space Shuttle or within or downstream from a radio-frequency oxygen plasma - were compared. The major difference in surface chemistry of Tedlar induced by the various exposures to O(³P) was a much larger uptake of oxygen when etched either in or out of the glow of an O₂ plasma than when etched in LEO. In contrast, Kapton F exhibited very little surface oxidation during any of the three different exposures to O(³P), while Teflon was scarcely oxidized.

The ESCA (electron spectroscopy for chemical analysis) spectra of films of poly(vinylidene fluoride) (PVDF), tetrafluoroethylene-ethylene copolymer (TFE/ET) and polyethylene (PE) exposed to atomic oxygen (O(³P)), in or out of the glow of a radio-frequency O₂ plasma, were compared. ESCA spectra of PE films exposed to O(³P) in low Earth orbit (LEO) on the STS-8 Space Shuttle were also examined. Apart from O(³P)-induced surface recession (etching), the various polymer films exhibited surface oxidation, which proceeded towards equilibrium saturation oxygen levels. The maximum surface oxygen uptakes for in-glow or out-of-glow exposures were in the order: $\text{PE}\text{>}\text{TFE}\text{ET}\text{>}\text{PVDF}$; for PE itself, the oxygen uptakes were in the order: in glow > out of glow > LEO. Given prior ESCA data on poly(vinyl fluoride) and polytetrafluoroethylene films exposed to O(³P), the extent of surface oxidation is seen to decrease regularly with increase in fluorine substitution in a family of ethylene-type polymers.

Low pressure plasma treatment using radiofrequency (rf) discharge of argon gas was employed to improve the hydrophilicity of polypropylene. The effects of argon plasma on the wettability, surface chemistry and surface morphology of polypropylene were studied using static contact angle measurements, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Increase in surface energy of polypropylene was observed as a result of argon plasma treatment. SEM and AFM images revealed the increased surface roughness. A set of identified process variables (rf power, pressure, argon flow rate and time) were used in this study and were optimized using central composite design (CCD) of response surface methodology (RSM). A statistical model was developed to represent the surface energy in terms of the process variables mentioned above. Accuracy of the model was verified and found to be high.

An atmospheric pressure oxygen and helium plasma was used to activate the surface of poly(methyl methacrylate) (PMMA). The plasma physics and chemistry was investigated by numerical modeling. It was shown that as the electron density of the plasma increased from 3 × 10¹⁰ to 1 × 10¹² cm⁻³, the concentration of O atoms and metastable oxygen molecules (¹Δ_g) in the afterglow increased from 6 × 10¹⁵ to 1 × 10¹⁷ cm⁻³. Exposing PMMA to the afterglow for times between 0 and 30 s led to a 35° ± 3° decrease in water contact angle, and a ten-fold increase in bond strength to several adhesives. X-ray photoelectron spectroscopy of the polymer revealed that after treatment, the surface carbon attributable to the methyl pendant groups decreased 5%, while that due to carboxyl acid groups increased 7%. The numerical modeling of the afterglow and experimental results indicate that oxygen atoms generated in the plasma oxidize the polymer chains.

A low-temperature, atmospheric pressure oxygen and helium plasma was used to treat the surfaces of polyetheretherketone, polyphenylsulfone, polyethersulfone, and polysulfone. These aromatic polymers were exposed to the afterglow of the plasma, which contained oxygen atoms, and to a lesser extent metastable oxygen (^1δ_g O₂) and ozone. After less than 2.5 seconds treatment, the polymers were converted from a hydrophobic state with a water contact angle of 85±5 to a hydrophilic state with a water contact angle of 13±5 . It was found that plasma activation increased the bond strength to adhesives by as much as 4 times. X-ray photoelectron spectroscopy revealed that between 7% and 27% of the aromatic carbon atoms on the polymer surfaces was oxidized and converted into aldehyde and carboxylic acid groups. Analysis of polyethersulfone by internal reflection infrared spectroscopy showed that a fraction of the aromatic carbon atoms were transformed into C=C double bonds, ketones, and carboxylic acids after plasma exposure. It was concluded that the oxygen atoms generated by the atmospheric pressure plasma insert into the double bonds on the aromatic rings, forming a 3-member epoxy ring, which subsequently undergoes ring opening and oxidation to yield an aldehyde and a carboxylic acid group.

The surfaces of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were treated with an atmospheric-pressure oxygen and helium plasma. Changes in the energy, adhesion, and chemical composition of the surfaces were determined by contact angle measurements, mechanical pull tests, and X-ray photoelectron spectroscopy (XPS). Surface-energy calculations revealed that after plasma treatment the polarity of PET and PEN increased 6 and 10 times, respectively. In addition, adhesive bond strengths were enhanced by up to 7 times. For PET and PEN, XPS revealed an 18-29% decrease in the area of the C 1s peak at 285 eV, which is attributable to the aromatic carbon atoms. The C 1s peak area due to ester carbon atoms increased by 11 and 24% for PET and PEN, respectively, while the C 1s peak area resulting from C-O species increased by about 5% for both polymers. These results indicate that oxygen atoms generated in the plasma rapidly oxidize the aromatic rings on the polymer chains. The Langmuir adsorption rate constants for oxidizing the polymer surfaces were 15.6 and 4.6 s(-1) for PET and PEN, respectively.

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.

A RECENT communication by Gray¹ illustrates a possible pitfall in the use of the theories of Fowkes^2–5 and Good and Girifalco^6,7 to estimate surface energies, and the various components of surface energy, from contact angles. This source of error is the incorrect identification of the surface tension terms, and the equating of the contact angle in a contaminated, experimental system to that in a system composed of properly pure components. Thus, Gray wrote Fowkes's equation in the form

This chapter, and the following one by Neumann and Good, deal with the measurement of contact angles in three-phase systems. The contact angle is, intrinsically, a macroscopic property, and one that should be amenable to a phenomenological treatment, e.g., to measurement without regard to its thermodynamic or microscopic interpretation. But inevitably, the desire for a thermodynamic and molecular interpretation arises. In addition, some fundamental questions about solid surfaces come up, and the problem of hysteresis must be given explicit consideration. So there is a need to go beyond phenomenology.

It has been found that if a film of a polymer such as polypropylene or Teflon FEP is oriented by stretching, the contact angle of liquid becomes anisotropic, being higher for liquid front advancing or retreating perpendicular to the direction of stretch than for advance or retreat in the parallel direction. Electroscanning microscopy has revealed a small degree of anisotropy of the surface roughness. Comparison with samples that have been abraded with a single stroke of abrasive paper of various grit sizes showed that a far greater degree of anisotropic roughness would be required, to produce the observed contact angle anisotropy and hysteresis, than is actually observed on the stretched samples. It is concluded that the observed contact angle anisotropy is probably due to the anisotropic force field of the oriented polymer molecules.

The dependence of cell or bacteria adhesion and growth on the polarity of a polymer substrate, as controlled by the composition of a HEMA-EMA copolymer, has been studied by contact angle measurements. These have been analyzed by the acid/base hydrogen bonding methodology of van Oss, et al. It was found that adhesion and growth of mouse 3T3 cells occurred on surfaces for which the acidic parameter, γ^⊕_S, was negligibly small. This was the case above 50% EMA, for which γ^p was zero, and both attachment and growth occurred. The γ^⊕ parameter was appreciable, but approximately constant, independent of composition of the copolymer. The acid/base theory thus supplants the simple polar-nonpolar (γ^pandγ^d) hypothesisin regard to cell adhesion. A new 3-dimensional representation of hydrophilic/hydrophobic behavior is suggested, to implement the acid/base description.

Contact angle hysteresis on rough surfaces is caused by the contortion of the liquid surface that must occur as the liquid front passes from one metastable configuration to another. We have combined the Wenzel equation for the effect of roughness on the contact angle, 0, with the well-known equation relating contact angles to the surface free energy of the solid and of the liquid, and with Good’s hypothesis of a free energy barrier to liquid front motion. The method that is developed calls for measuring 0 for a series of liquids and plotting cos0a vs./yi and extrapolating to the limit of (\/yi) 0. On a perfectly smooth, homogeneous surface, the intercept is—1 and the Wenzel ratio for a rough surface is given, approximately, by the negative of the value of the intercept. A shift of the yc value for the solid, due to roughness, is also predicted. Experimental data are presented for measurements with Teflon FEP.

The theory of the apolar components of interfacial forces was examined in the previous chapter of this volume.(1) It has been possible to develop that theory of apolar components at this time owing to the existence of quantitative, mathematically formulated theories of forces between molecules (e.g., the London theory) together with the Lifshitz electromagnetic theory of the interaction of macroscopic bodies. (See the previous chapter for references.)

The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γ^LW, γ^⊕ and γ^⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed.

The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ^⊕ and γ^⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ^⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m². The acidic component, γ^⊕, rose from zero to as high as 1.9 mJ/m². Its decay with time was only qualitatively the same as that of γ^⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data.

The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.

A method has been devised to determine the acid/base parameters of reference liquids as absolute numbers, and not as values relative to a conventional set of parameters for water. Contact angle measurements are employed, using three liquids on three solids. The theory calls for the solution of nine simultaneous, nonlinear equations in nine variables–and unreasonably formidable task.

A preliminary set of solutions has been computed, for one set of polar liquids on five solids. These results must be rejected on grounds of physical reasonableness. They also fail the test of predicting liquid-liquid interfacial tension, which for miscible liquids must be negative or zero.

We owe a great debt to W. A. Zisman and his colleagues at the Naval Research Laboratory for their extensive, pioneering work that opened up the field of contact angles and made possible the development of the modern theory of wetting and adhesion. Their data on the wetting of solids by apolar liquids and by hydrogen bonding liquids pointed the way to the recent introduction of a theory of hydrogen bond interactions across interfaces. We will devote this chapter to a review of this new theory.