The dispersion force component of surface tension γ_S^d and the nondispersive interaction energy at the water/solid interface (or the nondispersive work of adhesion) I_SWⁿ were evaluated for poly-(tetrafluoroethylene) (PTFE), poly(vinylchloride) (PVC) and poly(methylmethacrylate) (PMMA) by the analysis of the contact angles of water drops in hydrocarbon (the two-liquid-contact-angle method). The results were compared with those obtained by the one-liquid-contact-angle method with α-bromonaphthalene and methylene iodide as probe liquids, which is the method usually adopted. The values of γ_S^d from the two-liquid method were considerably larger than those from the one-liquid method, whereas their high sensitivity to error in the measurement of contact angles was taken into account. This discrepancy may be attributed to the neglect of the surface pressure π in the one-liquid method and the π values of the liquids used on the sample solids were calculated.

A new method has been developed for calculating surface free energies of polymeric materials using a simplified solution to the Fowler equation and polarizabilities and diamagnetic susceptibilities for polymer constituent groups. Comparison of this new method with estimates from group parachors, contact angle measurements, or extrapolation of data for melts indicates generally good agreement among the different methods. Discussions are also included on the effect of limited rotation on dipolar interactions and on the proper application of Good and Girifalco's method for estimating surface free energies of solids.

Ellipsometrically determined adsorption isotherms are reported for water, bromobenzene, nitro-methane, benzene, amyl, butyl, propyl, and ethyl alcohols, carbon tetrachloride, n-octane, and n-hexane on a polished polytetrafluoroethylene surface. These are nonwetting systems, and contact angles were also measured. In addition, isotherms were determined for two wetting systems, carbon tetrachloride on oxide-coated stainless steel and n-hexane on oxide-coated chromium-plated glass. For most of the nonwetting cases, the film pressure of the adsorbed film was not negligible, and should not not be omitted in semiempirical treatments of contact angle. The isotherms may be fitted by a previously proposed potential-distortion model, the choice of parameters also giving the observed contact angle. Alternatively, the isotherms are found to be segments of a single characteristic isotherm of the Polanyi type and thus obey a corresponding state principle. This characteristic isotherm for nonwetting systems does not fit the data for the two wetting cases, and the possibility is discussed that in the nonwetting cases the adsorbed state consists of bulk-like liquid in the form of micropatches or lenses rather than as a film of uniform thickness.

The contact angle may be measured by magnifying the projected image of a sessile drop, assuming the profile is elliptical, and finding the Cartesian coordinates of selected points on the profile. By selecting groups of three such points the mean equation to the outline can be determined and thus the tangent at the observed point of contact. The method was tested using water drops on various steel surfaces and using mercury on glass. Results showed generally satisfactory agreement with those obtained using the tilting plate method and also with those obtained by other investigators who have employed analytical methods to define the drop shape. If the sessile drop shape is assumed to be a part-oblate spheroid, a minimum free-energy analysis illuminates several experimentally observed features of its shape.

The dispersion force component of surface free energy, γ, and the nondispersive interaction free energy between solid and water, I, were determined by the two-liquid contact-angle method, i.e., by the measurement of contact angles of water drops on plain solids in hydrocarbon, for commercialy available organic polymers such as nylons, halogenated vinyl polymers, polyesters, etc. A method to estimate the I values from the knowledge of the polymer composition is also proposed, on the basis of the assumption of the spherical monomer unit and the sum of interactions between functional groups and water molecules at the surface.

Plasma chemistry of polymers may be categorized into two major types of reactions as (1) surface reaction of polymers and (2) polymerization of monomers by plasma. So far as these two types of reactions are concerned, plasma is very similar to other ionizing radiation, such as ..gamma.. radiation, x radiation, UV radiation, and high-energy electron beams, which can (1) initiate polymerization of certain monomers, and create free radicals on polymer exposed, which lead to (2) crosslinking of the polymer and/or (3) degradation of the polymer, or can be further utilized as the initiation sites of (4) graft copolymerization. The characteristic features of plasma are (1) the radiation effect is limited to the surface, and the depth of the layer affected by the plasma is much smaller than that by other more penetrating radiation, and (2) the intensity at the surface is generally stronger than that by the more penetrating radiations. Therefore, plasma treatment provides an ideal means of modifying surface properties of polymers. Examples are presented and discussed.

The, nature of polymer surfaces has received increasing attention as the use of these materials, in a variety of forms, increases yearly. Modifications of polymer surfaces for adhesion, friction, and diffusion oriented appiications have necessitated a careful analysis of the surfade region morphology (surface physics) and chemical properties of the surface layer (surface chemistry). The behavior of composite structures has involved the discipline of classical fracture mechanics. The orientation of polymeric species or additives which migrate to the interface may modify the wetting characteristics and, most certainly, the frictional properties in addition to the diffusion of penetrant species beyond the boundary layer. The above topics are discussed within the framework of recent analytical and theoretical developments in surface science. The findings of these recent studies have facilitated many exciting technological advances.

A method of determining the polar term of the adhesion energy of several liquids to a high-energy solid, I_SL^P, has been developed, based on the measurement of the contact angle of water on a solid in a liquid medium. The I_SL^P values for mica are found to be a linear function of the square root of the polar term of the surface free energy of liquids. This finding agrees with the suggestion that the polar term of the energy of adhesion may be represented by the geometric mean of the polar term of the surface free energy of a solid and a liquid. The slope of the straight line provides the value of γ_S^P = 90 ergs/cm² for the polar term of the surface free energy of mica. The results were compared with those obtained by a cleavage method and also discussed in terms of each component of the surface free energy of mica. The present method is useful for the determination of the polar part of the energy of adhesion of a high-energy solid to liquids, and its surface free energy.

A method for measuring the dispersive part of the surface free energy γs^D of a high-energy solid, and its interaction energy with water and n-alkanes, W_SL, has been developed. It is based on the measurement of the contact angle of water on the solid under n-alkanes. Muscovite mica was chosen as a model high surface energy solid. The results obtained for γs^D and W_SL of mica are in good agreement with the results obtained by other techniques. The present method can be considered to be applicable for other solids.

A sphere in continuum model, with an internal surface, is used to relate surface tension and internal pressure. The results support the previous use of this model for polar interactions. The agreement of theory and experiment is close to that obtained with a recent lattice model.

The problem of the deformation of a quiescent air/liquid interface by a plunging solid surface is considered in the context of a differential force balance of the type used in withdrawal theory. Interfacial deformation and air entrainment which eventually arises at high speeds are discussed in terms of three separate regions: where the dynamic contact angle, θ, is >90°, 90° > θ > 180°, and θ → 180°. This latter condition leads to the development of a dimensionless correlation between Weber and Bond numbers correlating air entrainment data which were found to be in substantial agreement with the experimental results. The theoretical and experimentally measured profiles also showed good agreement, particularly for viscosities up to 6.77 P and dynamic contact angles less than 180°, for surface tensions in the range 34 < π < 65 dyn·cm⁻¹.

The crosslinking of an ethylene–;tetrafluoroethylene copolymer by exposure to an argon plasma, excited by an inductively coupled RF field, is studied over a wide range of pressures and power loadings. The results are interpreted in terms of a two-component, direct and radiative energy-transfer model showing that the outermost monolayer crosslinks rapidly via direct energy transfer from argon ions and metastables.

Surfaces of polymers [polyethylene, polystyrene, poly(ethylene terephthalate), poly(oxymethylene), cellulose acetate, polyacrylonitrile, nylon 6, and polytetrafluoroethylene] treated with argon (inert) and nitrogen (reactive) plasma were examined by ESCA (electron spectroscopy for chemical analysis). Argon plasma treatment generally introduces oxygen functionalities into the polymer surface. Nitrogen treatment generally incorporates nitrogen and oxygen functionalities into the treated surface. The extent of oxygen incorporation is typically less than that produced by argon plasma. When nitrogen and oxygen functional groups are already in a polymer structure, the extent of additional incorporation of these two elements as a result of plasma treatment is very much less than with other polymers. Polymers which contain only one of the elements tend to incorporate the other element to much the same degree as polymers without either element initially present.

Low density polyethylene sheet was subjected to treatment by corona discharge in oxygen, nitrogen, helium and argon; in addition some sheets were treated with ozone gas. The bond strength between two similarly treated sheets was then measured using a commercial flexographic ink as an adhesive. The results showed that although surface oxidation improved both the ink adhesion and the wetting properties of polyethylene it is not a necessary prerequisite for good bonding. When the sheet was subjected to electrical discharge in nitrogen, argon or helium, considerable enhancement of ink adhesion was obtained without any detectable change in the surface chemistry of the polymer. The results indicate that ink adhesion after treatment in various gases follows closely the trends established previously in corona-induced autohesion of polyethylene. This suggests that the mechanism of bonding is similar in the two cases.

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.

Corona discharge treatment of poly(ethylene terephthalate) (PET) films produces chemical and physical modification of the surface leading to the formation of cavities and bumps. The roughness of the surface increases with the time of treatment and may be detected by scanning electron microscopy for the samples treated above 10 cycles, which corresponds to the duration of the exposure of the film under the electrodes. The degree of chemical modification, producing OH groups, is observed by adsorption of radioactive calcium ions and contact angle measurements. The results of these measurements are discussed and evidence presented shows that increase of the surface density of functional groups up to the value of 0.2 × 10¹⁴ sites/cm² leads to a rapid increase in wettability of PET films.

This paper discusses a recently developed surface energetics criterion for adhesive bonding and fracture and its applications in such diverse areas as structural adhesive bonding, fiber reinforced composites, biomaterials development, and lithographic printing. The theoretical relations describe systematic methods for the surface energy analysis of solid adhesive and adherend surfaces. The surface tension properties for the adhesive and adherend can then be introduced into a modified Griffith fracture mechanics relation to obtain predictions of bond strength under varied conditions of liquid or gas phase immersion such as water and dry air.

This report applies recently developed surface energy and fracture mechanics relations to the analysis of bioadhesion and biocompatibility. The dispersion α and polar β components of 190 biological and implant surfaces are analysed. The surface energetics relations between bioadhesion and biocompatibility point out that a strongly adsorbed plasma protein film on the implant surface provides the best blood compatibility and low thrombogenic effects. The surface energy relations provide means of selecting optimum implant surface properties and mapping on surface energy diagrams the three phase interactions which define bioadhesion.

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.

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.

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.

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.

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.

Analytical expressions have been derived relating the length, surface area, volume, and Laplace excess pressure of a liquid drop adhering to a cylindrical fiber to linear drop dimensions and the contact angle. Extensive tables of dimensionless forms of these quantities have been computed. The calculations form the basis of a precise and accurate method for measuring contact angle in such systems. A description of experimental technique for contact angle measurement is given, together with results for some well-defined systems.

ESCA and contact angle measurements have been combined in a detailed study of the effect of surface-to-volume ratio or thickness on the surface composition of coatings of a mixture of two fluoroalkyl methacrylate polymers which differ in the length of their fluoroalkyl side chains. These measurements show that the polymer component with the longer side chain is surface active in the mixture. The surface concentration of this component was found to decrease with increasing surfaceto-volume ratio of the coating.

The temperature effect on the wettability of oxidized polyethylene films with known surface densities shows a decrease in the free energy of adhesion at about 85°C for different liquids employed with varying numbers of OH groups. The thermograms obtained by differential thermal analysis show that the beginning of the melting transition is at about 85°C. The close agreement between the temperature at the beginning of the melting transition and the decrease of the wettability of oxidized polyethylene films is interpreted by the increase of the chain mobility leading to the redistribution of external polar groups initially located at the solid—air interface. We express the observed phenomenon as a degree of the overturn of macromolecular chains. The results obtained are discussed in relation to the number of OH groups present in the liquids and their ability to form hydrogen bonds.

One technique for the experimental determination of the dispersion and polar contributions to solid tension, γ_s ^d and γ_s ^p , is to measure the contact angle θ of a set of m liquids of known dispersion and polar contributions to surface tension on the solid and then to calculate γ_s ^d and γ_s ^p . There are two common techniques for this calculation, graphically¹ or analytically.^2,3 The graphical technique is limited in that it only considers dispersion forces (i.e., nonpolar systems) and so only isolates γ_s ^d . For this reason the analytical procedures which isolate both γ_s ^d and γ_s ^p are more commonly used, and they can be expressed in matrix notation as:

>where A is a 2 × 2 matrix containing information about the characterizing liquids and their contact angles, and the vector ◯ is related to γ_s ^d and γ_s ^p . Equation (1) is solved for all ^mC₂ different liquid pairs to give a set of values for γ_s ^d and γ_s ^p which can then be subjected to statistical analysis.

A critical review of some fundamentals of surface chemistry revealed several areas in which current interpretations of data or interrelationships are erroneous or misleading.

Correct forms of fundamental equations interrelating surface energies, equilibrium contact angles and adhesion are given and plotted in a convenient, illuminating, dimension-less form. These curves provide a basis for comparing some recently published empirical equations with the fundamental ones showing that discrepancies result from changing values of the interaction parameter φ.

With increase in sample density, corona treatment was found to be decreasingly effective in enhancing the autohesion of polyethylene sheets. The effect of higher density could be offset in part by an increase in temperature of lamination. This parallel behaviour suggests that similar molecular mechanisms govern the phenomena of thermally-induced and corona-induced autohesion.

By fluorinating the surface of a polymer, the hydrogen bonding energy of a polar surface has been defined. The contact angles for three solvent classes; nonpolar, polar and hydrogen bonding, on a polar surface results in the separation of dispersion, polar, and hydrogen bonding energies. Both critical surface tension plots and theoretical calculations were used to define the surface energy for fluorinated polyethylene.