Many approaches have been used to infer the surface tension of solids from liquid contact angles. In most cases the different methods have not been verified by independent means because of the inherent difficulty in directly measuring a solid surface tension. This paper examines a range of diverse experiments which, together with appropriate thermodynamic models, permit such an independent verification to be made.

As part of an ongoing study, the focus has been on two methods of interpreting contact angles which often yield conflicting results; namely, the equation of state approach and the theory of surface tension components. The previous work has led to the conclusion that the latter approach is incorrect. In this paper the accuracy of these two methods is examined in a strictly empirical way through the interpretation of a wide body of experimental results. It is seen that the predictions of the equation of state approach are in much closer agreement with the various experiments than are those derived from the Fowkes equation.

The effect of gamma-radiation on the surface chemical properties of polystyrene was studied by ESCA and FT-IR. Gamma-radiation produces surface >CO and CO containing functional groups only, and also causes oxidation to depths >10 nm as detected by ESCA. FT-IR spectra showed that below the top few molecular layers ester, acid, and carbonyl groups of various types were present. The α,β-unsaturated carbonyl/acid groups form a higher proportion of the total carbonyls with increasing depth. Surface oxidation follows pseudo-first-order kinetics; the extent of interior oxidation is linear with dose.

Adhesion of polyimides to ceramic substrates such as SiO₂, Al₂O_3' and MgO, and interfacial interactions were studied using XPS, SEM, and the peel test. The peel strength of polyimides on SiO₂ and Al₂O₃ is almost identical and higher than that on MgO at the same polyimide thickness. Contrary to the failure within the polyimides on SiO₂ and Al₂O_3' Mg was found on the peeled PMDA-ODA acid-derived polyimide surface, implying weakening of MgO by interfacial reactions with polyamic acid. With the neutral polyamic ethyl ester, the locus of failure on MgO was changed to the apparent weak boundary layer of the ester-derived polyimide. On SiO₂ and Al₂O₃ the peel crack propagated with a discontinuous stick-slip process. The constant interspacing between transverse stick-slip striations on the peeled polyimide surfaces has confirmed that plastic bending is the major energy dissipation process with a minimal contribution from tensile loading.

Polyolefin films were surface-modified by different methods to improve the wetting and adhesion of water-borne printing inks. Polyethylene (PE) films were treated with corona at various energy levels. Surface-modified PE films were characterized by contact angle measurements and electron spectroscopy for chemical analysis (ESCA). Good wetting was already achieved with treatment at a lower energy level. Various degrees of adhesion were obtained at various degrees of treatment. A hydrophilic monomer, 2-hydroxyethylmethacrylate (HEMA), was polymerized onto the surfaces of polypropylene (PP) with radiation-induced grafting, which was carried out at two different radiation doses. In both cases, a thick, visible layer of polyHEMA was formed on the surface of PP, and satisfactory wetting was already achieved at lower radiation doses. Scanning electron microscopy (SEM) showed that different degrees of roughness were achieved at various radiation doses. Like the case of corona-treated PE, different degrees of adhesion were obtained at different degrees of surface treatment. This study shows that improved wetting alone is not satisfactory for good practical adhesion', regardless of the surface modification method used.

This article will describe some of the recent progress in the area of plasma polymerization and plasma treatment. It is not intended to be an exhaustive overview of the field, but instead a summary of the highlights of research studies in this field selected by the authors according to the importance.

Comprehensive reviews on basic phenomena, theory, and reaction mechanisms of plasma-assisted processing and plasma polymerization will be covered in this review. Formation of diamond and amorphous carbon which has attracted considerable attention in the last few years will also be described.

Recent advances in plasma assisted deposition of composite metal/organic (polymeric or carbon) films will be discussed including deposition configurations and processes. Suggested applications particularly in optics and microelectronics will be emphasized.

Possible trends in future research and development of plasma deposition of organic films will also be outlined.

The apolar and the polar (electron-acceptor and electron-donor, or Lewis acid-base) surface tension components and parameters of solid surfaces can be determined by contact angle measurements using at least three different liquids, of which two must be polar. With swelling clay minerals (e.g. smectite clay minerals), smooth contiguous membranes can be fabricated, upon which contact angles can be measured directly. With non-swelling clay minerals (e.g. talc), contact angles can be determined by wicking, i.e. by the measurement of the rate of capillary rise of the liquids in question through thin layers of clay powder adhering to glass plates. The apolar and polar (acid-base) surface tension components and parameters thus found for various untreated and quaternary ammonium base-treated clays allowed the determination of the net interfacial free energy of adhesion of human serum albumin onto the various clay particle surfaces immersed in water. The free energies of adhesion, thus found, correlate well with the experimentally observed degree of adsorption of human serum albumin.

It is now generally recognized that the principal forces contributing to the work of adhesion between two phases, W_A, are the Lifshitz-van der Waals forces (which include a small contribution from permanent and induced dipoles) and acid-base interactions, taken in the most general 'Lewis' sense. One may thus write W_A = W^LW_A + w^ab_A = 2√σ^LWSσ^LWL + fN(-ΔH^ab), where W^LW_A and W^ab_A are the Lifshitz-van der Waals and acid-base contributions to the work of adhesion, and σ^LW_S and σ^LW_L are the Lifshitz-van der Waals contributions to the surface free energies of the solid and the liquid, respectively; ΔH^ab is the enthalpy (per mol) of the acid-base adduct formation between the acid or base functional groups on the adherend and in the adhesive; N is the number (moles) of accessible functional groups per unit area of the adherend; and f is an enthalpy-to-free energy correction factor (which has normally been assumed to be ~1). The present work seeks to evaluate W^ab_A for several systems using wetting measurements and, for at least one system, to obtain a quantitative check of the above equation using independently measured values of f, N, and (ΔH^ab). The total work of adhesion is determined from the measured surface tension of the liquid, σ_L' and its contact angle, 0, against the solid: W_A= σ_L(1 + cos ). σ^LW_S and σ^LW_L are determined using probe liquids, N is determined from conductometric titrations of the solid in finely divided form, and ΔH^ab is determined by flow microcalorimetry. f is determined from a Gibbs-Helmholtz analysis of surface tension and contact angle data obtained over a range of temperatures. Conclusions reached are that the f factor is significantly below unity in most cases and that even including this effect, the above equation is still not verified quantitatively when the terms are measured independently.

The growing realization of the importance of intermolecular acid-base interactions in promoting the solubility, adsorption, and adhesion of polymers to other materials has caused a demand for the quantitative characterization of the acid-base properties of the commonly used solvents, polymers, and inorganic fillers and substrates. There have been several recent advances in the measurement techniques for such determinations, especially in the fields of inverse gas chromatography, microcalorimetry, ellipsometry, FTIR, NMR, and XPS spectroscopy, all leading to the capability of determining the Drago E and C constants or the Gutmann acceptor numbers (AN) or donor numbers (DN) for the acidic or basic sites of solvents, polymers, or inorganic surfaces. In the last year, new studies have also allowed the characterization of the specific acid-base cohesive interactions in solvents and polymers, and the determination, from contact angle measurements on polymers, of the surface concentration and strength of acidic and basic surface sites. All of these techniques are discussed in this paper and it is expected that they will soon become standard laboratory practices.

Photomicrographs were taken of the organic drops formed on surfaces of vertical cylindrical fibers in water. The organic liquids used were 96% paraffin oil + 4% tetrabromoethane, paraffin oil, and 80% paraffin oil + 20% heptane. The fibers studied consised of plyester, a fluoroethylene-propylene copolymer (FEP), and nylon. The upper and lower contact angles, θ_t and θ_b, formed by the drops on the fiber surface were measured as a function of the dimensionless maximum drop radius, N, and length, $\text{L}$, from the projected images. As N increased so did θ_t, whereas θ_b only initially increased and then became more or less constant. Furthermore, θ_b decreased slightly close to the highest investigated values of N for systems involving FEP fiber. For a given system, the difference between the values of θ_t and θ_b increased as N increased, confirming that gravity forces affect the drop shape and contact angles. Good agreement is found between the measured values of θ_t and θ_b, and those obtained by using the theory for the shape of a drop on a vertical fiber [A. Kumar and S. Hartland, J. Colloid Interface Sci. 124, 67 (1988)].

The drop size dependence of the advancing contact angle of dodecane and ethylene glycol on carefully prepared FC-721, Zonyl FSC and DDOA surfaces has been studied by means of axisymmetric drop shape analysis. The contact angles were measured in air and were found to decrease by 3 to 5 degrees as the radius of the three-phase contact line increased from approximately 1 to 5 mm. This phenomenon is interpreted in terms of line tension by the modified Young equation. Our experimental results show that the line tensions are positive and of the order of 1 μJ m⁻¹ for all the three solid-liquid systems in our study; these results are consistent with previous work in our laboratory. The occasionally observed phenomenon that contact angles increase as the radius of the three-phase contact line increased on less carefully prepared surfaces is ascribed to the corrugation of the three phase contact line.

The water vapor permeability of glass-bead-filled phenoxy films was shown to depend strongly on the degree of interfacial interaction between the filler and matrix; the greater the adhesion, the lower the permeability. Scanning electron microscopy (SEM) was used to characterize the glass surface and the corresponding degree of adhesion between the filler and polymer matrix. Maximum interaction between the acidic phenoxy and glass filler was obtained when the glass had been treated with an aminopropyltriethoxysilane, which yielded a basic surface overall. Retention of the cellosolve acetate solvent was also reduced by the glass filler, especially by the more basic glass. The dynamic mechanical properties were affected primarily by the presence of residual solvent.

This work demonstrates the usefulness of flow microcalorimetry for surface characterization of metal foils (aluminum) and polymer [poly (ethylene-co-acrylic acid)] fibers. It shows that the polymer to aluminum adhesion is dominated by Lewis acid/Lewis base type interactions. These interactions are predictable from the measured heats of surface adsorption and desorption of probe molecules from dilute solution. The heats of interaction are a measure of the strengths of these sites. Adhesion between basic aluminum foil and acidic polymer resin increases with increasing numbers of either acidic sites on the polymer or basic sites on the foil. The calorimetry and adhesion results are in good agreement. This study supports recent observations vide infra that wettability of the aluminum is much less important for polymer/aluminum adhesion than chemical bonding.

The role of Lewis acid-base interactions at the fiber-matrix interface in composites is studied with both glass and Teflon fibers. In the glass fiber case, surface chemistry is modified with amino-, methacryloxy- and glycidoxy-silane coupling agents (A-1100, A-174 and A-187, respectively). Silane adsorption mechanisms as well as the properties of filament-wound, unidirectional epoxy and polyester composites are explained by a combination of X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and flow microcalorimetry. The heats of adsorption of pyridine and phenol prove that the coupling agents add acidic sites to the glass fiber surface as well as stronger basic sites. The subsequent adhesion of the matrix polymers and the short beam shear strengths of composites are explained on this basis. The Teflon fibers are first etched with sodium naphthalene solutions, and then sequentially hydroborated and acetylated, producing approximately monofunctional hydroxyl (acidic) and ester (basic) groups on the surfaces, as determined by XPS, FTIR, and electrophoretic mobility analyses. Composites prepared with the acetylated fibers and a chlorinated polyvinyl chloride (acidic) matrix are superior in tensile properties, and SEM fractography shows PTFE fibrillation, indicative of good fiber-matrix adhesion and stress transfer, in this case only.

A method has been developed to measure the surface acidity of solids using the contact angles of seven probe liquids. The geometric mean model was used to calculate the surface free energy. Then the value of the solid polarity, from the geometric mean, was compared with the polarity values calculated using the geometric mean dispersive component and the contact angles of two Lewis acids (liquefied phenol and glycerol) and two Lewis bases (formamide and aniline.) The deviation between these values was used to determine a value for the acidity, referred to as D. D was measured for a series of polymeric and metallic materials. Lap shear joints were fabricated and tested using these substrates and two adhesives, a urethane and an epoxy. The acidity of the substrate surfaces was found to affect the lap shear joint strength.

Many coatings materials are based on polymeric materials and sometimes difficulties arise when trying to marry them to polymer substrates of low surface energy and relatively inert molecular structure. Through the application of tailored coating formulation, substrate surface pretreatment and suitable coating process these problems may be eliminated to produce coated polymers with high bond strength properties.

Etching and modification of polymers by plasmas is discussed in terms of the roles played by atomic and molecular oxygen, atomic fluorine, CF_x radicals, ions, high energy metastable species, and photons. Addition of fluorine-containing gases to oxygen can increase both 0 atom densities in the plasma and polymer etching rates. The etching rate be- havior generally exhibits a maximum at a specific concentration of this additive. Process parameters which alter the concentrations of 0 and F atoms in the plasma or affect the rate of delivery of these species to the polymer surface shift the position of this maximum with respect to feed gas composition. However, the gas composition which yields maximum rates exhibits a strong dependence on polymer structure, specifically, its degree of unsaturation. This is explained on the basis of molecular orbital (MO) arguments which predict that the surfaces of unsaturated polymers have a higher affinity than saturated polymer surfaces for atomic fluorine. Favored reaction pathways leading to volatile etching products are pro-posed based on MO calculations of relative bond strengths for various oxygenated and fluorinated organic model compounds. Although fluorine abstraction of hydrogen plays a major role in generating radical sites on saturated polymer surfaces, it is likely that etching of unsaturated moieties proceeds through a saturated radical intermediate resulting from addition reactions of fluorine atoms. Excessive amounts of fluorine in the plasma result in reduced etching rates and incorporation of fluorine and/or CF_x radicals into the polymer. Polymer film surfaces are also modified by high energy metastables and ultraviolet radiation generated from noble gas plasmas, The effect of vacuum ultraviolet radiation from helium microwave plasmas on films of polytetrafluoroethylene and polyethylene is addressed.

The surface modification of polyethylene surfaces by NO-plasma irradiation was investigated from the point of view of the hydrophilicity and chemical composition. The hydrophilicity was evaluated from the advancing contact angle of water and the surface energy. The chemical composition of the modified surfaces was determined by diffuse reflectance Fourier transform infrared spectroscopy and XPS. NO-plasma irradiation for 5 min made the polyethylene surfaces hydrophilic. The advancing contact angle of water on the modified polyethylene surfaces reached 28 deg, and the surface energy was 57.6 mJ/m². The incorporation of oxygen and nitrogen moieties on the polyethylene surfaces occurred during the NO-plasma irradiation. The main oxygen moieties were carbonyl groups, hydroxyl groups, and ether linkages; the nitrogen moieties were amino groups. NO-plasma irradiation was more effective in improving the hydrophilicity than the O₂ plasma, N₂ plasma, or corona discharge treatment.

Thin plasma polymerized layers of acrylic acid (PPAA) were deposited onto polyethylene and muscovite mica surfaces. Structure and surface properties of the deposited layer depend on the polymerization conditions. The content of carboxylic groups in the layer decreases, whereas the degree of crosslinking or branching increases, with increasing discharge power. A soft, sticky layer with a low contact angle against water is obtained when a low discharge power (5 W) is used. In contrast, a hard film with a rather high water contact angle is obtained when the discharge power is high (50 W). A surface force apparatus was employed to study some film properties including adhesion force, crack formation, and capillary condensation. The adhesion force between plasma polymerized acrylic acid layers prepared at a low discharge power is high in dry air. It decreases remarkably in humid air and no adhesion is observed in water. In dry air, the adhesion force between PPAA layers decreases as the discharge power increases.

The wetting of cylindrical monofilaments by liquid polymers is a problem of much scientific and technological importance. In particular, the characterization of the physicochemical nature of interfaces is a key problem in the field of advanced fibrous composites. The macroscopic regime contact angle, which reflects the energetics of wetting at the solid-liquid interface, is difficult to measure by usual methods in the case of very thin cylindrical fibers.

In the present article a numerical method is proposed for the calculation of macroscopic regime contact angles from the shape of a liquid droplet spread onto a cylindrical monofilament. This method, which builds on earlier theoretical treatments by Yamaki and Katayama [1], and Carroll [2], very much improve the accuracy of the contact angle obtained. Experimental results with high-strength carbon, para-aramid, and glass fibers, are presented to demonstrate the high degree of accuracy of the method proposed.

The surface modification of isotactic polypropylene (PP) in a microwave plasma of CO2 is described. The modified PP is characterized in bulk and also at its surface. The mechanism of plasma modification is discussed in terms of degradation and oxidation. The degradation leads to volatile products and to the formation of a layer of oxidized oligomers of PP. The oxidation leads to ketone, acid or ester groups. The degradation and oxidation rates depend on plasma parameters (duration, discharge power, gas flow, pressure, discharge or post-discharge treatment). The oxidation rates vs the various plasma parameters show a maximum. The crosslinking of PP (Crosslinking Activated Species of INert Gases) seems to be negligible.

Despite the recognition of several sources of variation of contact angle, both between and within sessile drops on plane substrates, no comment has ever been made on the statistical treatment of observed angles, especially those around single drops. Circumperipheral observations are suggested as essential, and analysis using cos theta in an autocorrelation model is proposed as a general means of handling such data.

Polyethylene (PE) and polypropylene (PP) were oxygen plasma treated and aged in carefully reproducible conditions. The effect of aging on the surface chemistry, wettability and adhesion were studied using a combination of techniques: contact angle measurements, XPS, SSIMS, adhesion tests (shear and pull).

PE was found to be relatively insensitive to aging both in terms of wettability and adhesion, due to crosslinking during plasma treatment, which is likely to reduce macromolecular mobility within the surface layer.

In the case of PP, dramatic decreases of wettability occur with time, due to macromolecular motions leading to minimization of oxygen-containing functions at the surface. This behavior was shown to affect the adhesion performance of treated PP.

PTFE was treated with oxygen and argon plasmas and the effects of treatment were evaluated by actinometry, SEM, XPS, static SIMS and contact angle measurements. At short treatment times for both plasmas and at long treatment times for argon plasmas, chemical modification of the surface was dominant, while at longer oxygen plasma treatment times, surfaces are deeply etched but chemically equivalent to untreated PTFE. Interestingly, the change in surface chemistry is paralleled by a simultaneous variation in plasma chemistry, suggesting that the two vary accordingly. The wetting behaviour of treated surfaces is interopreted on the basis of current theories on surface dynamics and contact angle hysteresis.

Oxygen plasma-treated polydimethylsiloxane surfaces were aged in a low-energy (air) and in a high-energy (water) medium. Treated samples were characterized using a combination of surface-sensitive techniques: X-ray photoelectron spectroscopy, static secondary ion mass spectroscopy, and contact angle measurements. Plasma treatments cause large increases in surface tension of treated samples. When aged in air (low-surface-energy medium) the samples returned to a low-surface-tension situation. The mechanism was a combination of diffusive burial of polar groups in the bulk and condensation of silanol groups formed by plasma treatment and consequent crosslinking. When aging was performed in water, a high surface tension was maintained.

Corona discharge (CD) treated polyethylene films were examined using X-ray photoelectron spectroscopy (XPS) and a variety of chemical derivatization techniques. The composition of the CD-treated surfaces were found to be relatively unaffected by aging at temperatures between 70 and 80°F. Ink adhesion testing of films treated under progressively more serve conditions indicated the efficiency of adhesion varied directly with the severity of treatment. Derivatization of CDtreated polyethylene films with pentaflurophenylhydrazine (PFPH) resulted in the formation of a stable hydrazone complex. The PFPH complex extends the detection limit for enolizable carbonyl groups ca. eight-fold and provides relative quantitation of the number of these groups on variously treated polyethylenes. Formation of the hydrazone complex destroyed ink adhesion, indicating that the complex had blocked the site responsible for chemical bonding to the ink. Adhesion of water-soluble printing inks to CD-treated polyethylene is a direct consequence of hydrogen bonding between enolic hydroxyls on the polymer surface and carbonyl groups of the ink.

Macroscopic wetting behavior is investigated theoretically from a thermodynamic viewpoint. The axisymmetric liquid meniscus formed under a conical solid surface is chosen as the subject of the theoretical analysis. Using the meniscus configuration obtained by the Laplace equation, the total free energy of the system is calculated. In the case of the half vertical angle of the cone φ = 90 deg (horizontal plate), the system shows thermodynamic instability when the meniscus attaches to the solid surface at the contact angle. This result, unlike the conventional view, agrees well with the practical wetting behavior observed in this study. On the other hand, when 0 deg < φ < 90 deg, the system shows thermodynamic stability at the contact angle. However, when the solid cone is held at a position higher than the critical height from a stationary liquid surface, the system becomes unstable. It is possible to measure the contact angle easily using this unstable phenomenon.

Using the literature data of the refractive index, the structural unit molar volume of polymers and their dipole moment, as well as the literature data of the polarizability, ionization potential, and dipole moment of many liquids, values of the Φ parameter for paraffin—liquid and polymer—liquid interfaces were calculated. Next, introducing these values of Φ and the earlier measured values of the contact angle for many liquids to the Young equation, values of the surface free energy (γ_S) of paraffin, polytetrafluoroethylene (PTFE), polyethylene (PE), polyethylene terephthalate (PET), and polymethacrylate (PMMA), were determined. It was found that the average values of γ_S for these solids were in agreement with those calculated on the basis of geometric, harmonic, or harmonic—geometric mean approaches. The values of the surface free energy of paraffin, PTFE, PE, PET, and PMMA were also calculated from the Young equation modified by Neumann et al. and, using the earlier measured values of the contact angle for many liquids, they were compared with the values obtained by other methods. Next, employing the mean value of the surface free energy, values of the contact angles for many liquids were calculated and compared with those measured earlier for the same liquids. It was found that for paraffin, PTFE, and PE there were big differences among the values of their surface free energies calculated from the contact angles for some liquids; however, the average values were in agreement with those obtained by other methods. The average values of the surface free energies of PET and PMMA were also in the range of the results obtained by other authors. It was also found that the average deviations of the contact angles calculated from the Young equation modified by Neumann et al. from the measured ones were slightly larger than those of the contact angles calculated from equations employing the geometric and harmonic means of the surface free energy components; the method of Neumann et al. may also be used to predict the wettability in some systems.

To draw a relationship between surface concentration of the siloxane segment and adhesion performance, surface properties of the polydimethylsiloxane—poly(methyl methacrylate) block copolymers(PDMS-b-PMMA) prepared via poly(azo-containing siloxaneamide)s and their PMMA blends have been studied by measurements of FT-IR spectra, water contact angle, ESCA spectra and 180° peel strength toward pressure-sensitive adhesive tape. The water contact angles of the chloroform-cast blend films increased abruptly with siloxane bulk concentrations, or siloxane contents, particularly, on the air-side surfaces to reach almost 100° in low siloxane content. A marked increase of the contact angle was observed in the blends containing siloxane chain length (SCL) of longer than 2000. ESCA data evidently confirmed for these blend systems that the siloxane segments with low surface energy were accumulated or enriched mainly on the air-side surface, and that, on the other hand, polar PMMA segments with high surface energy were oriented to the glass-side surface and the inside of the films. This surface accumulation behavior of the siloxane segments reflected the 180° peel strength, as a measure of adhesion performance. The water contact angle and 180° peel strength were unequivocally correlated to the siloxane surface concentration estimated from ESCA data. Conversely, in the compression-molded blend films made by a hydraulic press between a Teflon and a stainless steel plate, the extent of surface accumulation of the PDMS segment was lower than that of the chloroform-cast films, suggesting lower degree of segment migration in hot-press films, probably due to substrate surface energy and lower relaxation in the blend melts.