The nonpolar polyethylene is transformed by oxidation into a superficially polar polyethylene with a known surface density of carbonyl groups. The dispersion and polar contributions to the free energy of adhesion for the systems oxidized and unoxidized polyethylene with n-octane, water, and methylene iodide are calculated. The variation of γ_s^d, γ_s^p, and γ_sl with temperature is found to verify the geometric mean equation for the interfacial free energy $\text{γ}{}_{\mathrm{<mtext>sl</mtext>}}\text{= γ}{}_{\mathrm{<mtext>s</mtext>}}\text{+ γ}{}_{\mathrm{l}}\text{− 2 (γ}{}_{\mathrm{<mtext>s</mtext>}}{}^{\mathrm{<mtext>d</mtext>}}\text{γ}{}_{\mathrm{l}}{}^{\mathrm{<mtext>d</mtext>}}\text{)}{}^{\mathrm{<mtext>12</mtext>}}\text{− 2(γ}{}_{\mathrm{<mtext>s</mtext>}}{}^{\mathrm{<mtext>p</mtext>}}\text{γ}{}_{\mathrm{l}}{}^{\mathrm{<mtext>p</mtext>}}\text{)}{}^{\mathrm{<mtext>12</mtext>}}$. The results are analyzed and the importance of the dispersion and polar interactions and their dependence on temperature is discussed.

The study of polymer—water interfaces by contact angle methods can be accomplished directly at the polymer—water interface. Using two water-immiscible liquids or a liquid and a vapor, one can deduce the dispersion and polar components of the hydrated solid surface free energy and the solid—water interfacial free energy. The theory is presented and a numerical analysis procedure is developed to solve the equations in the general case. The special case of n-octane and air is also presented. Data and results are given for poly(hydroxyethyl methacrylate-methoxyethyl methacrylate) copolymers of varying composition and equilibrium water contents. The results show that the hydrophilic component dominates the polymer—water interfacial properties, even at relatively low hydrophilic component compositions. The method presented should be useful for the study of polymer—water interfaces, particularly for hydratable or mobile polymers which can reorient to equilibrate differently with a water environment than with the air or vapor environment commonly used in contact angle studies.

Chromic acid solutions were used to oxidatively etch linear (high density) and branched (low density) polyethylene and isotactic polypropylene. Etched surfaces were characterized by surface IR spectroscopy, wettability, electron microscopy and aqueous adhesive bonding (peel test). Polypropylene was found to etch rapidly, but showed little residual chemical or topographical change. The polyethylenes etched more slowly, especially in the case of the linear polymer, but showed large changes in surface chemistry as a result of oxidative attack. Adhesion onto polyolefin films and fabric increased very rapidly during the first few seconds of attack. This increase is interpreted in terms of wettability, topography and cohesive strength of the surface layer. The differences in etch behavior between the polyolefins is interpreted in terms of ease of oxidative attack at branch points, and surface accessibility to the acid.

Surface tension is frequently expressed as the sum of a polar and a nonpolar term. In this paper an empirical approach is proposed for approximating the nonpolar term γ_sd of the surface tension of fluoropolymers. The experimental data were obtained from contact angle measurements employing a series of linear alkanes. These data are plotted by two different methods to evaluate γ_sd. The critical surface tension γ_e obtained from nonpolar contact angle liquids should reasonably approximate the γ_sd of the fluoropolymer surface. This work is based on classical molecular interactions, many concepts of which were established in earlier reports by Fowkes, Good, and Zisman.

It is generally accepted that the surface tension of fluoropolymers is approximately equal to the sum of a polar and nonpolar term. The first paper in this series described an empirical method for approximating the nonpolar term, and this paper proposes a similar approach for determination of the polar term, based upon contact angle measurements of polar liquids. The method is applicable to other solid surfaces provided suitable contact angle liquids are available.

Critical surface tensions γ_e of nine representative polymer surfaces with four series of polar liquids differed considerably from commonly accepted values. The Good-Girafalco-Fowkes-Young equation is used to explain the results, and it is shown that if certain precautions are observed, the equation may be used to predict γ_c of solid polymers for “standardized” series of liquids. The theoretical concepts of Fowkes and Good are shown to be compatible with Zisman's approach to the determination of γ_c. Serious errors may result, however, in the evaluation of contact angle data from misuse of the theoretical concepts of Fowkes or from misinterpretation of critical surface tension values as determined by the Zisman technique. Curve of cos θ vs. γ_L are straight lines only for one particular series of liquids and normal curves are of power form. It is suggested that many of the experimental contact angle data in the literature may be reinterpreted, including those for poly-(styrene), human skin, nylon 11, poly(ethylene), and monolayers of perfluorolauric acid.

A modification of the Good-Girafalco-Fowkes-Young equation is used to calculate nondispersion interactions I_SL^P at the interface for nine polymeric solids and four polar series of liquids. The relationship of I_SL^P to work of adhesion W_A and the spreading coefficient S_e is shown. A linear relationship is found to exist between I_SL^P and γ_L^P, the nondispersion energy component of the liquids, for the series of polar liquids and the solids studied. The slopes of the I_SL^P vs. γ_L^P curves vary depending upon the polymer surface. Intercepts of the curves may be a measure of π_s, the reduction in the surface energy of the solid resulting from adsorption of vapor from the liquid.

By combining four techniques—X-ray photoelectron spectroscopy (ESCA), soft X-ray spectroscopy, contact-angle hysteresis, and electron microscopy—a powerful method to elucidate the nature of solid surfaces is created. ESCA provides semiquantitative elemental analysis of the uppermost 5–100 Å of the sample. Soft X-ray spectroscopy extends the elemental analysis to a depth of about a micron. Contact-angle measurements can be interpreted in terms of the distribution of surface energy and roughness, and a view of the microtopography is obtained with the electron microscope. This method of surface characterization has been applied to several problems in fluoropolymer surface chemistry. For example, certain sodium complex solutions are shown to react with fluoropolymer surfaces, removing most of the fluorine and leaving a sponge-like surface with characteristics of an unsaturated, oxidized hydrocarbon. Also analyzed are surface changes that occur upon exposure of these sodium-etched films to environmental conditions. In another application, films of poly(tetrafluorethylene/hexafluoropropylene) melted and recrystallized against a gold substrate were analyzed. The unusual wettability of such films has been attributed to the presence of a “transcrystalline” surface region, but our analysis indicates the presence at the surface of a very thin layer of materials with the characteristics of an oxidized hydrocarbon. The increased wettability is evidently due to the presence of this layer.

Detailed physical and chemical surface characterization of fractured adhesive joints, guided by qualitative fracture mechanics theory, constitutes a semiempirical method to elucidate adhesive bonding phenomena. Inherent flaws, interfacial separation, viscoelastic and plastic responses, and crazing and crack propagation are the main factors governing overall bond strength. Surface analyses (primarily by SEM/EDAX² and ESCA³) provide an estimate of the nature and extent of each mechanism. Results from various fluoropolymer joints are presented and rationalized in terms of the elastic modulus and fracture work in the failure zone. Bond strength on untreated fluoropolymers is negligible, but ESCA shows a small amount of fluorocarbon transfer to the adhesive. Surface treatments increase surface energy via a hydrocarbon layer ∼20 to >500 Å thick, and useful peel strength results. SEM shows fracture relatively deep in the fluoropolymer with pronounced microdeformation. When the surface treatment is depleted by heat or light, bond strength varies with surface composition. Also, copolymers with perfluoropropylvinyl ether side chains in place of perfluoromethyl groups are superior hot melt adhesives. The combination of SEM and ESCA shows cohesive failure in both instances, but the latter separates closer to the interface and with relatively little deformation.

The thermodynamics of an idealized rough surface is treated, using the geometry of a vertical plate partially immersed in a liquid. Gravity is included explicitly in the theory. The results of this treatment are more general than those of previous studies and are more easily extended to other surface topographies. Some novel results are found, such as a delineation of the conditions under which a macroscopic contact angle of 180° will result from geometric properties of the solid surface. On rough surfaces consisting of material for which, if smooth, the equilibrium contact angle would be different from 90°, the slopes of the asperities will be a very important factor in determining the effective equilibrium contact angles.

A simple but crude theoretical model involves summation of the pair potential function by integration and the use of dispersion, polar, and induction interactions for establishing the pair potential. For single-component systems the cohesive energy density, δ², the surface tension, γ, and the molar volume, V_m, are important. The theoretical model, as related to single-component systems, predicts a proportionality between δ² and $\text{γ/V}{}_{\mathrm{m}}{}^{\mathrm{<mtext>13</mtext>}}$ for molten metals and organic liquids, an increasing trend of γ with δ for polymers, and a maximum ideal tensile strength equal to about one-fourth of δ² for polymers and metals. Most of the experimental results are reasonably consistent with the theoretical predictions. For two-component interactions the model must be further modified. The δ_A or γ_A values are measures of the intensity of interactions within component A. For predicting the A–B interactions, the nature of the interactions within A and B must also be defined in terms of the fractional polarities p_A and p_B. The value of p_A can be determined either from the ionization potential, the polarizability, and the dipole moment of A or by interacting the polar material A with a nonpolar material B. The theory allows the prediction of the heat of mixing and of the ideal butt joint strength from δ_A, δ_B, p_A, and p_B and the prediction of interfacial tension from γ_A, γ_B, p_A, and p_B. While most of the available experimental data are poorly suited for exact quantitative testing of the theory, they are semiquantitatively consistent with it. The theory is useful for interpreting experimental data on polymer solubility, adhesive bond strength, wettability of polymers, and interfacial tension involving organic liquids and water or mercury. In particular, the interfacial tension between mercury and non-hydrogen-bonding organic liquids can be calculated quite accurately with the aid of the fractional polarities.

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 effect of roughness of a solid surface on its wettability by a liquid has been studied theoretically using mechanistic arguments. By calculating the equilibrium shape of a liquid drop resting on a rough surface, we obtain the relation between the true (or microscopic) equilibrium contact angle at the three-phase contact line and the apparent contact angle observed macroscopically at the geometrical contour plane of the solid. By extending a proposal of Shuttleworth and Bailey, we provide a plausible explanation for hysteresis of the drop shape and contact angle which we evaluate for solid surfaces with concentric grooves. To calculate the equilibrium drop shape of a liquid on a solid surface whose roughness is more realistic than concentric grooves, we employ a perturbation method of solving approximately the Young-Laplace equation for the shape. Although the hysteresis in contact angle and drop shape cannot be evaluated by the method, the apparent contact angle and the local contact line positions are approximately predicted when the surface roughness has the form of cross grooves, hexagonal grooves, and radial grooves. Surfaces having random roughness are also considered and a modified form of the well-known Wenzel equation is derived which includes a factor for surface texture in addition to the conventional roughness factor.

Measurements of the interfacial tension of glycerol-dodecane, formamide-dodecane, ethylene glycol-dodecane, and aqueous ethylene glycol solution-dodecane and the surface tension of ethylene glycol-water solutions were carried out. On this basis the surface tension components of these liquids were calculated and they were compared with values from the literature. It was found that they are close to J. Panzer's (J. Colloid Interface Sci.44, 142, 1973) results obtained by using solubility parameters. In order to verify whether the determined components of the surface tension of polar liquids are valid, measurements of equilibrium contact angles for these liquids were made on the surface of paraffin, polytetrafluoroethylene, polyethylene, polyethylene terephthalate, and polymethyl methacrylate. The measured values of contact angles were compared with those calculated, using the well-known components of the surface free energy. Good agreement was achieved among measured and calculated contact angle values and those obtained by other researchers. It was found that the calculated components of the surface tension of polar liquids worked well in the studied systems, and the geometric mean used for dispersion and nondispersion interfacial interactions gives good results despite existing intermolecular forces due to hydrogen bonding.

Employing the values of organic liquid surface tension and interfacial surface tension of water-organic liquid, values of dispersion and nondispersion components of these liquids were calculated and compared with those obtained in another way. For these organic liquids and water, the values of the contact angle on paraffin wax, polytetrafluoroethylene, polyethylene, polyethylene terephthalate, and polymethyl methacrylate were measured. The values of dispersion and nondispersion components of surface free energy of these polymers and paraffin wax were calculated using the measured values of the contact angle for diiodomethane and water and the calculated values of the components of their surface tension. These calculated data were in agreement with the literature data. Taking our values of free energy components of liquids and solids, the values of the contact angle for these solids were calculated and compared with those measured, obtaining good agreement. On the basis of the measurements and calculations it was found that dispersion and nondispersion components of surface free energy of liquids and solids “work well” in the systems studied.

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.

Polymeric solid surfaces were prepared by a radiation-induced graft copolymerization technique using various mixtures of 2-hydroxyethyl methacrylate (HEMA) and ethyl methacrylate (EMA). Low-density polyethylene (PE) films were used as graft substrates. Contact angles on these polymeric surfaces were determined in air and under water. The critical surface tension (γ_c) of each polymeric surface in air was estimated by the Zisman method. Geometric mean and harmonic mean approximation methods were utilized to estimate the dispersion force contribution (γ_s^d) and the polar contribution (γ_s^p) to the total surface free energy (γ_s) from contact angle data in air. The geometric mean approximation was also used to estimate γ_s^d′ and γ_s^p′ from contact angles under water. The calculated values of γ_s^d, γ_s^p are strongly dependent on the pair of liquids chosen for the calculation regardless of the approximation adopted. The values of γ_s, calculated as the sum of γ_s^d and γ_s^p, are close to the γ_c values and are less dependent on the pair of liquids used. A comparison of the ratio γ_s^d/γ_s^p for the same surface in air and under water suggests that major polymer chain conformational changes occur, particularly with respect to the hydroxyl side chain, when such surfaces are immersed in water.

A thermodynamic model is used to predict the adsorption of nonionic surfactants on latexes with different polarity. The model, which is based upon the Flory-Huggins theory of polymer solutions, predicts that the adsorption decreases as the polarity of the latex increases. It is predicted that adsorption should occur even when it is unfavorable to replace a surface-water contact with a surfacesurfactant contact. This is due to a lower number of unfavorable hydrocarbon-water contacts when the surfactant is adsorbed, compared to when it is free in solution. It is also predicted that it is in principle possible to determine the latex polarity or solubility parameter, from adsorption measurements, provided that a similar experiment is carried out on a latex with known polarity, or solubility parameter.

The reorganization of the surface of a polyethylene grafted with 1% acrylic acid during contact with water has been studied using contact-angle measurements, a color test, esterification, inverse gas chromatography and ESCA spectroscopy. The evolution of the surface properties of the polymer in contact with water is explained by movements of the macromolecular chains followed by the orientation at the surface of the acrylic grafts, initially buried in the bulk of the polymer. The concept of “potential” surface energy of a polymer is proposed.

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.

PTFE was treated with oxygen plasma, and the effects of treatment time were evaluated by XPS, SEM, and the contact angles of water and CH₂l₂. Advancing and receding angles were interpreted in the light of current theories on contact angle hysteresis. It was found that at short treatment time wettability reflects chemical modification of the surface, while at longer treatment times surfaces are deeply etched and contact angles are controlled by roughness. With water as the wetting liquid, the typical behavior of composite surfaces was observed.

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.

An equation of state is developed which allows the surface tension of a low-energy solid to be determined from a single contact angle formed by a liquid which is chemically inert with respect to the solid and whose liquid surface tension is known. The equation of state is obtained using two independent methods. In the first one, similar arguments to those in previous papers are used; however, the qualitative argument, based on the general appearance of plots, is replaced by computer curve fitting and statistical analysis. The second method, which has not been employed heretofore, treats the solid surface tension as an adjustable parameter. Molecular arguments in conjunction with the interaction parameter Φ are used to eliminate poor choices of the solid surface tension. The results are in excellent agreement with the first method.

The range of validity of the equation of state and practical points in its application are discussed.

The use of scanning electron microscopy for direct observation of the effects of surface roughness on the spreading of liquids is described, making it possible to view moving liquid drops at distances less than 1 μm from the advancing contact line. Various surfaces were examined including several with simple forms of roughness which can assist in explaining the behavior of more complex surfaces. Spreading is shown to be highly dependent on the orientation and texture of the roughness; in particular, the presence of sharp edges of step height ⩽0.05 μm are shown to influence spreading significantly. These observations reinforce our previously stated doubts of the significance of conventionally measured macroscopic contact angles.

Polystyrene tissue culture vessels are commercially treated by corona discharge or plasma surface oxidation to provide a hydrophilic surface, with 15–20% surface oxygen. ESCA and FTIR showed that oxidation forms hydroxyl, carbonyl, and carboxyl groups. We have discovered that water washing removes about half the oxidized species. It is believed that reaction with the vinyl polymer backbone to form carboxyl groups results in CC bond scission to form soluble fragments; addition and ring reactions would not yield soluble species. This functional group removal could affect the desired properties, such as the use of these groups as anchors in chemical coupling.

Three methods of manipulating wetting data appear satisfactory in providing estimates of the components of solid surface free energy due to dispersion, Keesom, and hydrogen-bonding forces. These include: an extension of the Hansen method with contact angle data, which had been applied to solubility parameters, to surface free energy calculations; Hansen plots of absorption volume data; and Zisman plots using the components of surface tension rather than total surface tension. The Fowkes equation for the dispersion component of surface free energy agrees well with the results from the empirical methods. Extensions and modifications of the Fowkes equation to provide the polar components of solid surface free energy have not worked well when evaluated with a wide range of reference liquids.

The effect of velocity upon the advancing and receding contact angles of water, glycerol, formamide and methylene iodide on glass coated with a dimethyl siloxane layer has been investigated. The cosines of the dynamic contact angles of water and glycerol vary linearly with the interfacial velocity, as predicted by hydrodynamic theory. However, this treatment cannot account for the magnitudes of the velocity effects observed with water and glycerol. Qualitative molecular considerations explain certain features of the velocity dependence of advancing contact angles. Initial rapid relaxation of the contact angles occurs on removal of the drive because of molecular reorientation of liquid molecules at the solid interface. There are further slow changes of water and methylene iodide advancing contact angles with time because of penetration of liquid changing the solid-liquid interfacial tensions. No penetration occurs with glycerol and formamide because of their greater effective molar volumes and with the former liquid the advancing and receding angles relax to a common value of about 90° which is considered to be the equilibrium value.

The relationships of the optical characteristics of monomers (refractive index and specific refraction) and the cohesion parameters of polymers (effective cohesional energy and molar volume of the repeating unit) have been analyzed. New relationships are proposed that allow the calculation of the surface energies of homo- and copolymers using refractometric data of monomers. The relationships were tested with good consistency for a number of polymers of various chemical nature.

In forced spreading systems, three different modes of θ_d-V behavior have been found, each of which predominates in a different velocity range. In the lowest velocity range, with systems involving the low viscosity, low boiling, nonpolar liquid hexane, θ_d was found equal to θ_eq (the Elliott-Riddiford or Hansen-Miotto mode). In the next higher velocity range, which extends to very low velocities for all other systems studied, the behavior described by Eq. 6 (the Blake-Haynes mode) predominates. At still higher velocities, the behavior described by Eq. [9] (the Friz mode) becomes superposed on the Blake-Haynes mode, and eventually predominates up to the range where θ_d approaches 90° and Eq. [9] becomes inapplicable. In the Blake-Haynes mode the major force opposing advance of the liquid front is the solid-liquid interfacial viscosity. In the Friz mode it is the bulk viscosity of the liquid.

The roughness of solid surfaces has no appreciable effect on the θ_d-V relationship, provided the physicochemical character of the surfaces is the same and the roughness is random. If the process of roughening alters the physicochemical character the θ_d-V behavior of the roughened surface may differ from that of the smooth one.

There is no qualitative difference between the θ_d-V behavior of systems in which θ_eq is zero and systems in which θ_eq is positive.

The surface structure and properties of poly(vinyl alcohol)-poly(dimethylsiloxane), PVAL-PDMS, graft copolymers with the controlled PDMS graft chain length as well as chain distribution were studied. The surface of the graft copolymer was examined by XPS technique and was found to be covered with essentially pure PDMS graft component even in only 5 mole% siloxane unit content. The contact angle measurement was carried out with the water-in-air technique for PVAL—PDMS graft copolymers as well as the graft copolymer/PVAL homopolymer blends and the significant surface accumulation of PDMS graft component was confirmed with the graft copolymer/PVAL blend of less than 1 mole% of siloxane unit content. In contact with water, PVAL—PDMS graft copolymer surface was found to transform its surface morphology remarkably, which was noticed by the contact angle measurement with the air-in-water technique, where the contact angle of PVAL—PDMS graft copolymer surface was found to differ from that of pure PDMS—coated surface.