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.

The effect of nonpolymer-forming plasma (e.g., plasma of hydrogen, helium, argon, nitrogen) can be viewed as the following two reactions: 1) reaction of active species with polymer, and 2) formation of free radicals in polymer which is mainly due to the UV emitted by the plasma. The incorporation of nitrogen into the polymer surface by N₂ plasma and the surface oxidation by O₂ plasma are typical examples of the first effect. The latter effect generally leads to incorporation of oxygen in the form of carbonyl and hydroxyl and to some degree of cross-linking depending on the type of substrate; however, the degradation of polymer at the surface manifested by weight loss occurs in nearly all cases when polymers are exposed to plasma for a prolonged period of time. The effects of polymer-forming plasma is predominated by the deposition of polymer (plasma polymer); however, with some plasma-susceptible polymer substrates the effect of UV emission from polymer-forming plasma cannot be neglected. The mechanism of polymer formation can be explained by the stepwise reaction of active species and/or of an active specie with a molecule, and the chain addition polymerization of some organic compounds (e.g., vinyl monomers) is not the main route of polymer formation.

Plasma polymers contain appreciable amount of trapped free radicals; however, the concentration is highly dependent on the chemical structure of the monomer. In plasma polymerization, 1) triple bond and/or aromatic structure, 2) double bond and/or cyclic structure, and 3) saturated structure are three major functions which determine the rate of polymer formation and the properties of plasma polymers. The changes of some properties of plasma polymers with time are directly related to the concentration of trapped free radicals in plasma polymers. The amount of trapped free radicals in a plasma polymer is also influenced by the conditions of discharge; however, the UV irradiation from the polymer-forming plasma is not the main cause of these free radicals. Excess amount of free radicals are trapped during the process of polymer formation (rather than forming free radicals in the deposited polymer by UV irradiation). The properties of a plasma polymer is generally different from what one might expect from the chemical structure of the monomer, due to the fragmentation of atoms and/or functions during the polymerization process. This is another important factor to be considered for the modification of polymer surfaces by plasma polymerization.

The chemical species created in a low-pressure electrical discharge in oxygen attack the polymer at the surface, converting it to gaseous products. This process is interesting because: 1) the chemical changes on the resulting surface facilitate the formation of strong adhesive bonds and provide sites for the chemical attachment of other molecules, 2) significant morphological features lying below the surface may be revealed, 3) polymer can be cleanly removed from surfaces which are resistant to oxidation, and 4) dielectric breakdown frequently is preceded by the attack on the polymer of chemical species created in a corona discharge. Atomic oxygen is an important chemical species created in such a discharge. It reacts with organic substances rapidly at room temperature, but lives long enough in the low-pressure gas that it can be separated from many other reactive species created in the discharge. “Titration” with NO₂ provides a straightforward chemiluminescent means for determining the concentration of atomic oxygen to which the sample is exposed. This paper characterizes the attack of atomic oxygen, perhaps in the presence of long lived but less reactive species such as excited O₂molecules, on polymer surfaces, using electron microscopic observations of known morphological features of polyethylene to observe the changes produced by atomic oxygen. Lamellar polyethylene crystals were attacked both at the edges and the fold surfaces. Layers many microns thick were removed from spherulitic samples and replicas obtained from the surfaces thus exposed. Thick samples were thinned to the point at which they were transparent to an electron beam and interior morphological features were directly observed.

Electrical characterization is based on a display of voltage and charge which appears as a simple parallelogram. The area is a measure of energy input per cycle and is independent of voltage waveform but very dependent on the maximum voltage. A useful model for such corona discharges employs a Zener diode to simulate the corona drop. The buffer dielectric plays a major roll in controlling the corona power, and the air gap importance depends on the electrode system employed. Proper interpretation of the voltage-charge traces provides insight as to the corona performance and serves as a diagnostic procedure for obtaining optimum performance.

The autohesion of polyethylene sheets was markedly improved by corona discharge treatments in oxygen, nitrogen, argon and helium. Equal bond strength was produced by an equal number of discharge cycles regardless of the time or frequency of application. At a given operating voltage the power consumed in a discharge rather than the chemical nature of a gas proved to be a factor controlling the enhancement of autohesion. The detrimental effect of the oxidation upon autohesion was noted after a prolonged treatment in an oxygen corona.

The ability of corona treatment to render polyethylene film self-adherent has been previously reported and the mechanism explained. A similar effect has now been found with corona-treated poly(ethylene terephthalate) film which adheres strongly to itself when joined under conditions of heat and pressure that give no adhesion with untreated film. Poly(ethylene terephthalate) films irradiated with short-wave UV light also become self-adherent. The behavior of the adhesive joints in both cases is the same as that reported for corona-treated polythylene film in that the joint strength is zero in the presence of hydrogen-bonding liquids, but recovers completely if the joint is allowed to dry undisturbed. Chemical and physical tests have shown that the adhesive bond is a hydrogen bond between the hydrogens of phenol groups created by corona or UV irradiation in one surface with carboxyl carbonyl groups in the other surface. Thin-layer chromatography of surface extracts from corona- and UV-treated films has shown the products of treatment to be practically identical for both types of treatment, supporting the conclusion that the mechanism of corona treatment resembles that of greatly accelerated photo-oxidation.

Corona-treated polyethylene films have been reported to exhibit strong self-adhesion when joined together under conditions of heat and pressure that give no adhesion with untreated films. The present study of this effect has shown that the adhesion is completely destroyed by the application of any hydrogen-bonding liquid to the adhesive joint and that the effects of liquids is completely reversible. Joints allowed to dry recover full strength. These facts together with the results of chemical reactions conducted on the treated film surface have established that the adhesive bond is a hydrogen bond. Corona treatment forms keto groups on the polyethylene chain; these groups enolize and the enolic hydrogens bond with carbonyl groups in the adjacent sheet of film when two sheets are heated together under pressure.

Polypropylene films were treated with chromic acid mixture. The change in the treated films was investigated by comparing change in amount of 2,4-dinitrophenylhydrazones formed in the treated films with their change in wettability with water. Oxidation of the film surface zone, partial breakdown of polymer in the film surface zone, and oxidation of surface zone bared from the film inner zone seemed to occur with increase in treatment time or with increase in treatment temperature.

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.