X-ray photoelectron spectroscopy (XPS) and contact angle methods were used to examine the surfaces of an homologous series of poly(ether urethane) (PEU) samples before and after cleaning treatments. Four PEU films with Shore hardnesses varying from 45 to 75 D were studied as well as two commercially available intravenous catheters of related PEUs. The four as received PEU films have similar surface compositions (~79% C, ~17% O, ~2% N, and ~ 2% Si) although they differ in bulk composition. Critical surface tension (γ_c) values are all similar and high (45-46 dynes/cm). The similarity in the surfaces of the four PEUs, despite the differences in their mechanical properties, demonstrates that surface properties do not necessarily reflect bulk properties. Soap washing and methanol-acetone extraction of the PEU films resulted in surfaces more representative of the bulk compositions of the PEUs. Analysis of the intravenous catheters confirmed that they are lubricated with PDMS, a common practice in the medical device industry. This study documents the value of detailed surface analysis for an enhanced understanding of the surface zone of PEUs. It also illustrates how cleaning protocols can remove labile surface species.

The effects of the modification of polystyrene (PS), polyethylene (PE), poly(vinyl chloride) (PVC), silicone rubber (SR), and fluorinated ethylene propylene (FEP) copolymer by radio frequency glow discharge in a helium environment were presented in part I. The hydrated polymer surfaces were characterized by XPS, SEM, visual microscopy, and by contact angle measurements. In general, exposure of the polymers to RFGD produced an oxidized hydrophilic surface, yet the roughness of the surface was unaltered by the relatively mild plasma conditions used. In this article, the frictional behavior of oxidized and unoxidized SR, PE, and FEP in distilled water, isotonic saline, and blood plasma environments is examined experimentally. The results are discussed in relation to the properties generally believed to affect frictional phenomena and to the surface properties as determined in part I. Results indicate that RFGD-treated SR generates less friction than untreated SR when dragged across all untreated and treated polymer surfaces, whether the medium is distilled water or an isotonic saline solution. Friction is consistently lower in a blood plasma medium between all surfaces investigated, most probably because of the presence of adsorbed proteins at the polymer interfaces.

Circular tubings are used extensively in biomedical implants and devices. It is desirable to determine contact angles on the inner or outer surfaces of such tubing in its final fabricated form. In this study, a technique for the measurement of contact angles on tubing surfaces in an aqueous environment is reported. This has particular applications to biomaterials research, where polymer tubings contact the biologic environment. In this technique, air or octane captive bubble dimensions can be measured, and an underwater contact angle calculated from these dimensions. The validity of the technique was experimentally confirmed using Solution Grade Biomer and NIH standard polyethylene surfaces.

The effect and interrelationship between primary (segmental backbone) and secondary (side chain) molecular motions on thrombogenesis, independent of morphological order/disorder, crystallinity, and/or associated water is elucidated using an amorphous hydrophobic polymer of poly-[(trifluoroethoxy) (fluoroalkoxy)phosphazene], PNF. The results indicate that thrombogenesis for an amorphous hydrophobic polymer is sensitive and dependent on the degrees and types of primary and secondary molecular motions at the polymer interface.

Osteointegration is dependent on a variety of biomechanical and biochemical factors. One factor is the wettability of an implant surface that is directly influenced by its surface energy. This investigation used the Zisman plot to determine critical surface energy. The effects of surface treatment, bulk grain size, and surface roughness on the critical surface tension of unalloyed titanium (Ti) were examined. Radio frequency glow discharge-treated Ti had the highest critical surface tension, followed by the passivated and heat-sterlized conditions. Titanium with no surface treatment had the lowest critical surface tension. The surface energy of Ti with an average grain size of 23 μm was not significantly different from that with a grain size of 70 μm. Surface roughness was shown to cause significant difference in measurements and definitely should be considered in studies of this kind. © 1994 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jbm.820281206

How to convert an analytical balance into an accurate ring tensiometer or densimeter.

The paper describes a physical chemistry experiment for measurement of surface tension of solids. It includes description of the Zisman-plot method for obtaining the surface tensions of solids as well as a detailed experimental procedure for surface tension measurements using contact angle goniometry. Four surfaces were analyzed and the critical surface tension (gc) for each of them was obtained. Excellent reproducibility was achieved despite measurement of less accurate Young's contact angles. The experiment includes an extensive data analysis section combined with graphical interpretation of the results. It can be included in an undergraduate physical chemistry laboratory sequence on its own or combined with existing experiments pertaining to surface phenomena.

A basic and affordable experimental apparatus is described that measures the static contact angle of a liquid drop in contact with a solid. The image of the drop is made with a simple digital camera by taking a picture that is magnified by an optical lens. The profile of the drop is then processed with ImageJ free software. The ImageJ contact angle plugin detects the edge of the drop and fits its profile to a circle or an ellipse. The tangent to the triple line contact is calculated and drawn by the ImageJ software, thus, returning the value of the contact angle with acute precision on the measurement.

It is shown that in any two‐phase mixture of fluids near their critical point, contact angles against any third phase become zero in that one of the critical phases completely wets the third phase and excludes contact with the other critical phase. A surface layer of the wetting phase continues to exist under a range of conditions when this phase is no longer stable as a bulk. At some temperature below the critical, this perfect wetting terminates in what is described as a first‐order transition of the surface. This surface first‐order transition may exhibit its own critical point. The theory is qualitatively in agreement with observations.

FUNDAMENTAL quantities associated with the wetting of solids are the free energies of the solid/vapor boundary ('Y SV) and the solid/liquid boundary ('Y SL). These have been obtained for the special case of a glassy solid fluorocarbon on which the con tact angles (8) have been measured for liquids of widely different surface tension ('Y LV). The excellent agreement between the observed angles and those calculated from the theoretical relation,

Molecular dynamics simulations have been carried out for liquid water between flat hydrophobic surfaces. The surfaces produce density oscillations that extend at least 10 Å into the liquid, and significant molecular orientational preferences that extend at least 7 Å into the liquid. The liquid structure nearest the surface is characterized by “dangling” hydrogen bonds; i.e., a typical water molecule at the surface has one potentially hydrogen‐bonding group oriented toward the hydrophobic surface. This surface arrangement represents a balance between the tendencies of the liquid to maximize the number of hydrogen bonds on the one hand, and to maximize the packing density of the molecules on the other. A detailed analysis shows that the structural properties of the liquid farther from the surface can be understood as effects imposed by this surface structure. These results show that the hydration structure of large hydrophobic surfaces can be very different from that of small hydrophobic molecules.

The rigid‐sphere theory of liquids is shown to yield an expression for the surface tension of a one‐component liquid which is essentially identical to an expression previously derived by the author employing a qualitative approach. Such a correlation suggests that the rigid‐sphere theory of liquids approach is applicable to a wide variety of nonpolar polymeric liquids.

We discuss the behavior of a liquid partially wetting a solid surface, when the contact angle at equilibrium θ₀ is small, but finite. The solid is assumed to be either flat, but chemically heterogeneous (this in turn modulating the interfacial tensions), or rough. For weak heterogeneities, we expect no hysteresis, but the contact line becomes wiggly. For stronger heterogeneities, we first discuss the behavior of the contact line in the presence of a single, localized defect, and show that there may exist two stable positions for the line, obtained by a simple graphic construction. Hysteresis shows up when the strength of the defect is above a certain threshold. Extending this to a dilute system of defects, we obtain formulas for the “advancing” and “receding” contact angles θ_a, θ_r, in terms of the distribution of defect strength and defect sharpness. These formulas might be tested by controlled contamination of a solid surface.

We have explored the possibility of acquiring information on the molecular nature of some novel polymer films by following Zisman's approach of determining the critical surface tension γ_C which is considered to reflect the molecular composition of the solid surface. This led us to obtain equilibrium contact angle data, using both a series of pure liquids and various alcohol solutions, not only with the polymer films but also with solid surfaces of polystyrene and polymethylmethacrylate.

Lower values of γ_C were obtained with the solutions than with the pure liquids; these lower values are attributed to the preferential adsorption of the alcohol molecules at both the polymer-liquid and the polymer-vapour interfaces. The value of γ_C depends on the alcohol used, and is relatively independent of the solid : it is inferred that the alcohol is adsorbed with the hydroxyl group towards the polymer surface.

It is concluded that in certain cases, the value of γ_C obtained using solutions cannot be used as being characteristic of the solid (as has been suggested by Zisman), and that changes at the solid-vapour interface cannot be neglected when interpreting contact angle data. Several sets of data reported in the literature are discussed from this viewpoint. The Gibbs adsorption isotherms is applied to the contact angle data and the results add further weight to the conclusions regarding the occurrence of adsorption at both interfaces.

Low-pressure plasma treatment has been introduced for the practical pretreatment of automobile bumpers. The differences between the conventional solvent vapor pretreatment and low-pressure plasma or corona treatment, are shown schematically.

Low dynamic surface tension is an important factor in achieving superior film formation in water-borne coatings. Dynamic coating application methods require surfactants with low dynamic surface tensions in order to prevent defects such as retraction, crawling and cratering. Comparative basic and empirical data are presented that will demonstrate the ability of acetylenic diols to lower the dynamic surface tension of water-borne coatings and hence improve the quality of the cured film.

Progress of research on wetting, spreading, and liquid-to-solid adhesion is reviewed with emphasis on advances since 1963. Carefully controlled experiments with pure materials, clean solids, special atmospheres, and thermodynamically acceptable physical conditions have resulted in new knowledge of surface science, polymer properties, and the role of interfering properties and chemicals. New uses and relationships have already been produced in many fields of technology and science. Recent advances shed much light on molecular conformational differences, polymer tacticity, and hydrogen bonding. Several current research problems are identified.

The influence of pigment shapes and pigment blends on the surface energy was investigated and compared with the surface chemistry of pigmented latex coatings. The coatings were made of different volume ratios of two pigments: plate-like kaolin clay pigment and prismatic precipitated calcium carbonate (PCC) pigment. These were mixed together with carboxylated styrene–butadiene–acrylonitrile latex (SBA), and applied over nonabsorbent substrates as well as absorbent substrates. The composition of the surface of the coatings was investigated by X-ray photoelectron spectroscopy (XPS). Two approaches were used to estimate the total surface energy and the components of the coatings: a conventional approach—“the Kaelble approach”—and a more modern approach—“the van Oss approach.” Pigment blends with different shapes and increments caused a change in the surface chemistry and the surface energy of the latex coatings. As the prismatic PCC pigment particles increased in the kaolin/SBA coating system, the SBA latex content at the coating surface increased and the total surface energy of the coating decreased. This is valid for both nonabsorbent as well as absorbent substrates. It was found that there was a strong correlation between the surface energy and the surface composition. The surface energy of the coatings estimated by the Van Oss approach was always lower than that estimated by the Kaelble approach. Colloidal interactions between pigment–pigment and/or pigment–binder were thought to play an essential role in determining the final coating surface energy and its components. Changes in the surface latex content and the surface energy due to the different pigment blends investigated were found to fit straight-line equations.

1. Methods of preparing surfaces of polytetrafluoroethylene (TFE) are described which permit obtaining reproducible and reliable measurements of contact angles with liquids. Polytetrafluoroethylene is found to be an ideal low energy surface for the study of the wetting relations of a solid with a wide variety of organic and inorganic liquids.

2. It is shown that all but the lowest-boiling liquids studied do not adsorb on TFE sufficiently from the vapor to produce a significant spreading pressure. Hence the π_E term in the expression for the work of adhesion (W_A) is negligible and W_A = γ_LV(1 + cos θ). The calculated values of W_A are found to be consistent and reasonable.

3. It is found that for each homologous series of liquids cos θ is a linear function of γ_LV. The constants of the resulting relations are given. From this W_A − W_c can be expressed as a function of γ_LV alone.

4. The observations show that there is a critical surface tension (17.5–20.5 dynes/cm.) below which liquids wet TFE.

5. It is concluded that the corrected Young-Dupré relation coupled with the spreading coefficient permits a rational description of the results. The Doss and Rao relation for the surface condensation leads to such high values for high-boiling liquids as to make it unlikely that the theory is correct.

1. Data are presented on the equilibrium contact angles of a wide variety of liquids on specially prepared surfaces of halogenated derivatives of polyethylene. The free energy decrease of immersion (f_SL), the work of adhesion (W_A), and the spreading coefficients have been calculated for the liquids which do not spread. The free energy decrease on immersion of these solids in the saturated vapor of most of the liquids of this study is shown to be negligible.

2. For each solid surface the contact angle and spreading coefficient follow the relation to liquid surface tension reported previously for polytetrafluoroethylene, i.e., as the liquid surface tension increases, the contact angle increases and the spreading coefficient decreases.

3. Substitution of chlorine or hydrogen for fluorine in these polymers increases W_A, f_SL, and the spreading coefficient with respect to a given liquid, the increase depending on the proportion of nonfluorine substituent. Substitution of hydrogen for fluorine has a smaller effect in this respect than substitution of chlorine for fluorine.

4. It is shown that when f_SL for a given solid in a series of liquids is constant, the plots of W_A υs. surface tension and spreading coefficient υs. surface tension should be straight lines with positive and negative 45° slopes, respectively. The data presented for the fluorinated polymers and the liquid n-alkanes and di-n-alkyl ethers fall on such lines. The values of f_SL for the fluorinated polymers plot as a straight line against mole per cent fluorine substitution.

5. There appears to be a maximum value of W_A characteristic of the polymer, which is identical with the work of adhesion given by the liquid having a contact angle of 90° on the given solid.

6. It is shown experimentally that there are many exceptions to the “rule” that nonpolar liquids wet nonpolar solids.

7. From the data it is shown that recent attempts to measure the surface tension of solids by contact angle measurements alone involve unjustifiable assumptions.

1. Contact angle measurements have been made of a wide variety of liquids on clean, smooth surfaces of polyethylene, paraffin, and surfaces of single crystals of n-hexatriacontane (C₃₆). The calculated value of the final spreading coefficient (S_{LV^∘/SV^∘}) is given and from the data there can be calculated the values of the free energy of immersion (f_SL), and the work of adhesion (W_A). The free energy of immersion of the solid in the liquid vapor can be neglected in these calculations since it is believed to be quite small for surfaces of low adsorptivity and low free surface energy. It is shown that the methyl-rich surfaces of C₃₆ and paraffin are not wetted by a wide variety of organic liquids, including the n-alkanes, so that the rule that nonpolar solids are wetted by nonpolar liquids is again found to be erroneous.

2. Wettability is found to decrease in the order polyethylene, paraffin, C₃₆. This is attributed to the increase in the proportion of methyl to methylene groups in the surface. The C₃₆ surface, like all higher n-alkane crystals, is shown to be the least wettable of all hydrocarbon surfaces since its surface comprises only methyl groups arranged in the closest possible packing. It is shown that it should be possible to estimate the packing of adsorbed monolayers of straight-chain hydrocarbon derivatives by comparing the hydrophobic contact angle to the angle on C₃₆. Many of the variations of the hydrophobic contact angle on paraffin found in the literature are shown to be attributable to variations in the methylmethylene ratio in the surface.

3. In contrast to the fluorinated polymers, paraffin and C₃₆ are shown to have multiple curves of cos θ vs. γ. The multiplicity of these curves is attributed to differing dependencies of γ_SL on γ_LV^∘ due to variation in the constitution of the liquid. Increase in adhesion of hydrocarbon surfaces to liquids is found to be in the order: liquids containing oxygen or fluorine; aliphatic hydrocarbons, aromatic hydrocarbons.

4. It is shown that variation of the free energy of immersion of the n-alkane series of liquids on a given hydrocarbon surface is the resultant of two competing tendencies: i.e., increased adhesion due to increase in methylene to methyl ratio in the liquid, and decrease in adhesion due to increase in surface tension of the liquid. For the C₃₆ surface, the latter tendency predominates; for the paraffin surface, the former tendency predominates, f_SL is shown to be the upper bound of the solid surface tension for systems where γ_SL ⩾ 0.

5. It is shown that in general there is more than one value of the critical surface tension (below which liquids spread on a given surface) for hydrocarbon surfaces, depending on the value of γ_SL given by a liquid of given surface tension. The fluorinated polymers are shown to be a special case where the surfaces are independent of the nature of the liquid and therefore have essentially a single value of γ_C.

A study has been made of the wettability of adsorbed monolayers of branched-chain and cyclic molecules. It is shown that such monolayers are nonwetted by organic liquids with surface tensions higher than a critical value characteristic of the monolayer. Oleophobicity is therefore not restricted to monolayers whose surfaces comprise close-packed methyl groups, the latter merely representing an extreme case. It is shown, further, that adsorbed layers of nonpolar liquids behave, as regards wettability, like low-energy solids with the same atomic groups exposed in the surface. It is concluded that the free surface energy of adsorbed liquid or solid films depends only on the atomic groups in the surface and their packing.

A simple apparatus and technique are described for measuring contact angles formed by liquid drops on solid surfaces. The procedure is a modification of one described 25 years ago by Langmuir and Schaeffer. It is based on observation of the angle at which light from a point source is reflected from a liquid drop surface at its contact point with a solid. The technique gives results the same, within experimental error, as those obtained by other investigators who used the usual “drop profile” method. The apparatus has the advantages of ease of construction and operation, ruggedness, and low cost coupled with high precision and accuracy.

In order to obtain information on the wettability of some polymers, the sliding of water drops down tilted polymer slides has been examined. The angle of sliding is defined as the limiting angle between the slide and the horizontal plane at which the drop moves at a uniform rate, and can be taken as a measure of the wettability of the surface.

Previous studies have shown that small amounts of amphipathic (i.e., polar non-polar) molecules, in particular oleamide, cause a marked lowering of the coefficient of friction between thin films of polyethylene. In this paper the surface chemical properties of the aging film and the nascent film (i.e., during the flat film extrusion process) have been studied. The wettability (contact angle) and friction of the aging film at room temperature show that the friction is reduced only when sufficient additive is present to form a weakly held monomolecular layer. This monolayer is formed by almost complete exudation of the additive from the bulk. Contact angle measurements show that the molecules become oriented on the surface such that the polar groups are in contact with the polyethylene and the hydrocarbon chains project into the air. The stability of the monolayer to water condensation is much improved by flame treatment of the film immediately on extrusion. On preparing film by extrusion through a water-quenching bath it has been found that water adheres more easily to films containing oleamide. By suitable adjustments, the water bath was modified to form a “dynamic” Langmuir trough. Contact angle measurements on the emerging film and studies on the effect of sweeping the water surface around the emerging film show that the surface tension of the water is lowered by the amphipathic molecule being quantitatively stripped off the film as it emerges through the water/air interface. From surface pressure/area relationships, the surface concentrations of the spreading molecules are calculated. It is found that there is a surface concentration about one hundred times greater than would be expected from a uniform distribution of the additive throughout the polymer. This large surface excess is approximately proportional to the bulk concentration and implies a pronounced adsorption at the polymer melt/metal or polymer melt/air interface.

The equilibrium contact angle (θ_E) has been measured for some sixty diverse liquids with respect to a smooth platinum surface coated with an adsorbed oriented monolayer of n-octadecylamine. Linear relations were found between cosine θ_E and the liquid surface tension (gg_LV^∘) for every homologous series. When homology was disregarded, the cos θ_E-υs.-γ_LV^∘ data for all the liquids collected on three straight lines, two of which were approximately parallel. Simple curvilinear relations obtained between the work of adhesion (W_A) and γ_LV^∘ and between the final spreading coefficient (S_{LV^∘/SV^∘}) and γ_LV^∘, the constituents of each set of three curves being the same as before. The grouping onto multiple lines corresponds to differences in the solid/liquid interfacial tension, γ_SL, and to the relative solvent power of the liquids for the adsorbed octadecylamine. The same correlation obtained for the critical surface tension (γ_C), which was shown to be specific both to the homologous series and to the solid surface.

Constant values of the free energy decrease on immersion (f_SL) were observed for the homologous series of n-alkanes and n-alkyl ethers, while the alkylbenzene series showed a linear decrease with increasing γ_LV^∘. From the small range and low experimental values of f_SL observed for many unrelated liquids, it is concluded that the free surface energy of the monolayer-coated solid is probably not much more than 28.5 erg/cm.².

The striking similarity observed for the wetting properties of the monolayer-coated surfaces compared with those reported previously for surfaces of single crystals of n-hexatriacontane and bulk paraffin (5) demonstrates that the wetting behavior of a surface is essentially controlled by the nature and packing of the outermost group of atoms in the molecules. Intercomparison of wetting data for the monolayer with reference data obtained for a surface of methyl groups in closest packing (i.e., n-hexatriacontane single crystals) is proposed as an approach for determining, from contact angle measurements, the packing of adsorbed films at the solid/air interface.

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.