The effect of corona discharge on low-density polyethylene (LDPE) film was studied in terms of surface functionality and surface energetics of the film surfaces, improving the dyeability. The introduction of a polar group (OCO, CO, and CO) to a corona-treated LDPE film with acrylic acid could be confirmed by ESCA. The Owens–Wendt and Wu models using geometric means were studied to analyze the surface free energy of corona-treated film. It was found that the corona-treated LDPE film did lead to an increase in surface free energy, mainly due to the increase of its specific (or polar) component as the corona discharge power increased. Also, the K/S values were increased as the concentrations of dye increased. From the acid–base interaction point of view, it was found that the graft polymerization of acrylic acid onto the corona-treated LDPE film plays an important role in growing the acidic character which is one of the specific components of surface free energy, resulting in improving the dyeability with basic dyeing agent. A direct linear relationship is shown between the O_1s/C_1s ratio and the resulting K/S value or the specific component for this work.

Contact angles of 17 liquids on 3 hydrophobic solid surfaces, FC721, fluorinated ethylene propylene, and polyethylene terephthalate, were measured by using the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) technique. Details of the surface preparation and the experiments are presented. The accuracy of these contact angle data is better than 0.2° in most cases. These data were used to calibrate an equation of state for interfacial tensions of solid—liquid systems. The end results of the analysis is an equation of state for interfacial tensions with a single parameter β = 0.0001247 (m²/mJ)², cf., Eqs. [22]–[24]. Within the experimental limitations, there is no evidence for the notion that β might change from system to system.

Absence of drop size dependence of contact angles of sessile drop systems is sometimes observed in experiments. The contact angle data sometimes fluctuate periodically about a horizontal line. Moreover, in cases where a drop size dependence of contact angles exists, the contact angle data often scatter significantly. These fluctuations may be caused by surface roughness. In this paper, two idealized rough surface models are developed. The mean contact angle of a sessile drop in each rough solid surface model is calculated. The fluctuations of the drop size dependence of contact angles produced by these models resemble those obtained experimentally and the fluctuations may therefore be a consequence of the roughness on solid surfaces. It is also concluded that the apparent absence of drop size dependence of contact angles does not necessarily imply zero or extremely low line tension.

The kinetics of modification of a fluoropolymer coating (FC 722, 3M Company) during its contact with octane, dodecane, and hexadecane is studied via measurement of quasi-static (velocity independent) advancing and receding dynamic contact angles. A decrease in both angles with the time of contact between solid and liquid is observed and it is interpreted as the result of swelling of the polymer. By means of a theoretical extrapolation of the θ_R(t) data tot= 0, based on an equation relating θ_R(t) to swelling kinetics, the experimentally inaccessible receding contact angle on dry coating, θ⁰_R, is determined. The contact angle hysteresis on such a surface, θ⁰_A− θ⁰_R, is found to be less than the hysteresis, θ^∞_A− θ^∞_R, obtained on samples that were soaked in the alkanes long enough to reach saturation. This increase is thought to be due to loosening of the polymer chains during the swelling, leading to an exposure of higher-energy segments to the nonpolar liquid and to an enlargement of the solid surface pores filled with liquid. The contact angle data are also interpreted in terms of interfacial free energies.

Low-rate dynamic contact angles of nine liquids on a poly(methyl methacrylate) (PMMA) polymer are measured by an automated axisymmetric drop shape analysis—profile (ADSA-P). It is found that two liquids dissolved the polymer on contact. From the experimental contact angles of the other seven polar and nonpolar liquids, it is found that the liquid–vapor surface tension times cosine of the contact angle changes smoothly with the liquid–vapor surface tension (i.e., γ_lνcos θ depends only on γ_lνfor a given solid surface). The dependence of γ_lνcos θ on γ_sνis explicitly illustrated by replacing the solid surface from the PMMA to other methacrylate polymers: such a procedure shifts the curves in a very regular manner. Thus, because of Young's equation, γ_sldepends only on γ_lνand γ_sν. This contact angle pattern is in harmony with those from other inert and noninert (polar and nonpolar) surfaces. The solid–vapor surface tension of PMMA calculated from the equation of state approach for solid–liquid interfacial tensions is found to be 38.5 mJ/m², with a 95% confidence limit of ±0.5 mJ/m²from the experimental contact angles of the seven liquids.

By employing a new strategy, we show that axisymmetric drop shape analysis (ADSA) can be used to determine simultaneously the surface tension and the density of polymer melts from sessile drops at elevated temperatures. To achieve this, two developments were necessary. First, the ADSA algorithm had to be modified to replace the density by the mass of the drop as an input parameter. Since ADSA also yields the volume, the density became output rather than input. Second, a closed high-temperature chamber whose temperature could be precisely controlled and a sample holder that allowed the formation of highly axisymmetric sessile drops at elevated temperatures had to be developed. For a typical polymeric material (polystyrene), it is demonstrated that measurements with sessile drops yield essentially the same surface tension values and temperature coefficients as measurements with pendant drops. The densities determined with ADSA are comparable to independent PVT results.

A theoretical treatment of the effect of surface heterogeneity on contact angles is given, by means of a model employing the capillary rise of a liquid in contact with a stripwise heterogeneous surface. Local contortions of the liquid-vapor surface are postulated to conform to an assumed periodic shape of the three-phase line. A minimum of free energy is found in a configuration in which Young's equation is obeyed locally. When the width of the strips is below some value of the order of 0.1 μ, the amplitude of the periodic contortion of the three-phase line is less than about 10 A, which is operationally indistinguishable from a straight line. Extension of this model is made to a patchwise heterogeneous surface, and a mechanism for hysteresis is developed. For patches smaller than about 0.1 μ, it is shown that heterogeneity should make a negligible contribution to hysteresis.

Acid/base theory has, over the last decade or so, been developed to describe interfacial free energies, or tensions, in wetting theory. An approach put forward by van Oss and co-workers, involving van der Waals/Lifshitz and Lewis electron acceptor/donor contributions to surface/interfacial free energies, has often been employed. The present study considers use of this theory for evaluating surface data for various polymeric surfaces employing known, characterized liquid probes for obtaining contact angle data. Results are analyzed using extended matrix analyses, originally proposed for treating the dispersive/polar interpretation of wetting results, and good agreement with literature values is obtained. By “inverting” the system, i.e., by treating the known solids as probes and rederiving surface data for liquids, inconsistencies are found to arise. Results for wetting of the same polymers and mica, using a two-liquid system (n-octane/water), are exploited to attempt to rederive the surface characteristics of water. Again, serious incoherence is manifest. Despite the conceptual interest of acid/base theory, clearly the mathematical formulation is presently inadequate. Copyright 1999 Academic Press.

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

The current work incorporates a microscopic study of the effect of fiber orientation on the fiber wetting process and flow of liquid droplets along filter fibers when subjected to airflow and gravity forces. Glass filter fibers in various combinations were oriented at various angles within a plane defined by the airflow direction and were supplied with distilled water in aerosol form. The behavior and flow of the liquid collected by the fibers were observed and measured using a specially developed microscope cell, detailed in the paper. The experimental results were compared to a theoretical model developed to describe the behavior. The theory and experimental results showed good agreement. The developed theory allows an optimum angle to be determined for the internal filter fiber structure in the design of wet filters. A sensitivity analysis of the model was conducted to determine the most important parameters. This will aid design of wet filtration systems such that maximal self-cleaning can be accomplished with minimal water use.

A new method to measure the contact angle θ of sessile drops deposited onto solid substrates is presented. The crux of the method is to use the drop as a convex mirror for large, collimated, incident laser beams. The reflected light is intercepted on a screen in the far field. It appears as a bright circular spot, the diameter of which is simply related to the beam angular divergence 4θ. Overall measuring accuracies can be made better than 0.1° for θ angles less than 45θ. Reproducibility is also excellent because of the natural averaging over the entire three-phase line boundary of the drop. It is estimated to be ±0.25°. An extension of the method, valid for all angles less than 90°, is to use the drop as a plano-convex lens. This is of course applicable only if the solid substrate is optically clear. The refracted light is then intercepted on a screen and analyzed to yield the contact angle through simple geometrical optics considerations. These two methods are demonstrated on a series of short-chain alkanes (hexane to hexadecane) in contact with hydrophobic glass slides and/or polytetrafluoroethylene plates.

We report contact angle measurements of five n-alkanes, dodecane through hexadecane, on Teflon (FEP) as a function of drop size. In all cases the contact angles decreased by approximately 5° when the drop size was increased from approximately 1 to 4 mm contact radius. A complete solution to the problem of mechanical equilibrium of a sessile drop on a solid surface indicates that the dependence of the contact angle on drop size may be explained by including the effect of line tension in the Young equation. The observed drop size dependence of the contact angle yields a line tension of (2.5 ± 0.5) × 10⁻⁶ J/m. Over the range of n-alkanes studied it was not possible to discern any dependence of the line tension on liquid surface tension.

We have established experimentally that, for certain liquid-solid systems, the contact angle formed by a drop of liquid varies with drop size, below a critical diameter which is probably a function of the nonuniformity of the solid surface. This effect has been observed with water on Teflon FEP and on polymethylmethacrylate (PMMA), in regard to both advancing and receding contact angles. The decrease was about 8° in θ_a and 16° in θ_r for water on Teflon FEP, between diameters of 4 mm and about 1 mm; and for other systems, a comparable or even greater effect was observed. For water on PMMA, θ_r decreased from 51° to 26°, between diameters of 8 and 3 mm. With ethylene glycol on Teflon FEP, the decrease in θ with drop size was observed in the retreating angle only. With n-decane on Teflon FEP, the contact angle was independent of drop size, between diameters of 1 and 12 mm. For all liquid-solid systems studied, the limiting contact angles for large drops were in good to excellent agreement with the values obtained by the vertical plate method. Qualitatively, this effect could be explained by hypothesizing the existence of a large, negative line tension. It was found that there was quantitative disagreement between observed results and predictions based on this hypothesis. A theory is proposed to explain the experimental results, based on the known heterogeneity of many polymer surfaces and a previously discussed theory of hysteresis. A negative pseudo-line tension can be used in a phenomenological description of the motion and contortion of the three-phase line.

We report on the surface modification of Sylgard-184 poly(dimethyl siloxane) (PDMS) networks by ultraviolet (UV) radiation and ultraviolet/ozone (UVO) treatment. The effects of the UV light wavelength and ambient conditions on the surface properties of Sylgard-184 are probed using a battery of experimental probes, including static contact angle measurements, Fourier transform infrared spectroscopy, near-edge X-ray absorption fine structure, and X-ray reflectivity. Our results reveal that when exposed to UV, the PDMS macromolecules in the surface region of Sylgard-184 undergo chain scission, involving both the main chain backbone and the side groups. The radicals formed during this process recombine and form a network whose wetting properties are similar to those of a UV-modified model PDMS. In contrast to the UV radiation, the UVO treatment causes very significant changes in the surface and near-surface structure of Sylgard-184. Specifically, the molecular oxygen and ozone created during the UVO process interact with the UV-modified specimen. As a result of these interactions, the surface of the sample contains a large number of hydrophilic (mainly –OH) groups. In addition, the material density within the first ≈5 nm reaches about 50% of that of pure silica. A major conclusion that can be drawn from the results and analysis described in this work is that the presence of the silica fillers in Sylgard-184 does not alter the surface properties of the UVO- and UV-modified Sylgard-184.

The characterization of polymer-water interfaces by contact angle measurement is performed using water-immiscible liquids. It gives the dispersive and the nondispersive components of surface free energy as a function of functional group of copolymer hydrogels. Using the method of Rusanov, the contactangle-induced deformation of the three-phase region in our systems was examined. The systems used were poly(2-hydroxyethyl methacrylate) (HEMA), poly(2-hydroxyethyl methacrylate-methyl methacrylate) (HEMA-MMA), poly(2-hydroxyethyl methacrylate-methoxyethoxyethyl methacrylate) (HEMA-MEEMA), poly(2-hydroxyethyl methacrylate-aminoethyl methacrylate) (HEMA-AEMA), and poly(2-hydroxyethyl methacrylate-diethylaminocthyl methacrylate) (HEMA-DEAMA). The deviation of contact angle due to the surface deformation was found to be appreciable in case of poly(HEMA-AEMA) and poly(HEMA-DEAMA).

Contact angle measurements on polymer hydrogels were performed at various temperatures, and we obtained the dispersive (γ_s^d) and nondispersive (γ_s^p) components of the surface tension of polymer hydrogel at each temperature. Utilizing the temperature dependence values of γ_s^d and γ_s^p, we obtained the surface entropies of polymer hydrogels. The polymer hydrogels used were isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (HEMA), poly(2-hydroxyethyl methacrylate + aminoethyl methacrylate) (HEMA + AEMA), poly(2-hydroxyethyl methacrylate + N-vinyl pyrrolidone) (HEMA + VP), poly(2-hydroxyethyl methacrylate + methyl methacrylate) (HEMA + MMA), poly(2-hydroxyethyl methacrylate + methoxyethyl methacrylate) (HEMA + MEMA), and poly(2-hydroxyethyl methacrylate + methoxyethoxyethyl methacrylate) (HEMA + MEEMA), respectively. The contact angles were also measured by using droplets of water-immiscible liquids under conditions in which the polymer hydrogel was fully hydrated.

The surface tensions of polyethylene, polyisobutylene, and polyvinyl acetate, and the interfacial tensions of polyethylene/polyvinyl acetate and polyisobutylene/polyvinyl acetate systems have been measured by the pendent drop method in the temperature range up to 200°C. The results are analyzed in terms of the equations of Fowkes and of Girifalco and Good, and suggest that the conformational restriction of polymer molecules imparts a limitation on the extent of interfacial contacts and sharp phase boundaries in these systems. Several quantities of interest in adhesion, such as contact angle, spreading coefficient, and work of adhesion are also discussed.

Contact angles of polar liquids on various nonpolar solids immersed in a nonpolar liquid have been measured. They agreed satisfactorily with values calculated by combining Young's equation either with measured contact angle data of the liquids on the substrates in air, or with the Fowkes relation valid at each of the interfaces. The latter method also applies when one of the liquids spreads on the substrate in air. Polyethene, polystyrene, polytetrafluorethene and glyceryltristearate were used as nonpolar solids; paraffin oil and heptane as nonpolar liquids; water and glycerol as polar liquids.

Digital image processing techniques are applied toward the determination of polymer surface tension by pendant drop measurements. Experimental values for poly(dimethylsiloxane) as a function of molecular weight and temperature correspond well with previous measurements of poly(dimethylsiloxane) surface tension, testifying to the applicability of the new technique. Current thermodynamic treatments are found to provide excellent predictions of poly(dimethylsiloxane) surface tension for molecular weights of 3900 and 75,000 at temperatures ranging from 20 to 120°C. Theories developed by K. M. Hong and J. Noolandi (Macromolecules, 14, 1223, 1981) and Y. Rabin (J. Polym. Sci. Polym. Lett. Ed. 22, 335, 1984) yield predictions within 5% of t he experimental results for the materials and conditions studied.

The purpose of this research was to classify fluoropolymer surfaces with regard to their wettability by various liquids. Thin solid blades, coated with various fluoropolymers, were suspended from a force transducer balance. The blades penetrated various hydrocarbon/air and hydrocarbon/water interfaces, and the curves of force acting on the blade versus displacement were recorded. The contact angles hydrocarbon/solid/air and hydrocarbon/solid/water were calculated using: $\text{COS}\text{θ =}\text{t}\text{γL}$ and $\text{COS}{}_{\mathrm{i}}\text{θ =}\text{T}{}_{\mathrm{i}}\text{γi}$ where θ, θ_i: contact and interfacial contact angle; γL, γi: surface and interfacial tension; and τ, τ_i: adhesion and interfacial adhesion tension. Cos θ versus γ_L for various γ_L were plotted and γ_c (for cos θ = 1) was determined. It was found that there are two critical surface (γ_c) and critical interfacial (γ_cⁱ) tensions due to contact angle hysteresis. Three different fluoropolymer surfaces were investigated. When the surface can be used in the form of a well-defined blade and when enough liquid to be tested is available, the method was found to be useful, rapid, and reproducible. Under these circumstances it can be used in place of Zisman's well-known sessile drop method.

A modified Wilhelmy plate technique has been developed for the measurement of surface tensions of viscous polymers. The method requires no knowledge of liquid density and provides a means of assuring a zero contact angle for the polymer on the plate. The surface tensions of several silicone polymers with viscosities as high 106 centipoises have been measured. The method has also been used to determine the surface tensions of several molten polyethylenes as a function of temperature over the range 115° to 215°C.

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.

Contact angle measurements have been carried out on various solid substrates using water-propanol mixtures and α-bromonaphthalene as wetting liquids. These substrates were: polytetrafluorethylene, Parafilm, polyethylene, polyurethane, polystyrene, polymethylmethacrylate, fluorapatite, and hydroxyapatite. The dispersion and the polar components of the surface free energy, γ_s^d and γ_s^p have been calculated from the geometric mean equation. Two approaches have been considered: (1) neglecting the spreading pressure π_e and (2) taking π_e into account (Dann's method). The results show that both approaches actually yield the same results for the surface free energy, γ_s, if a proper interpretation of the approaches is considered. All data indicate, that approach (1) gives γ_s values determined on the adsorbed liquid layer, whereas in approach (2) the free energies of the bare solid surfaces are found.

The wettability of a number of low energy solid surfaces, including hoof keratin and human skin, has been examined using two liquids, water and methylene iodide, and employing Wu's empirical approach to obtain γ_s^d and γ_s^P, the dispersion and polar components of the solid “surface tension.” The sum of these parameters, (γ_s^d + γ_s^p) was found to be in good agreement with reported values of γ_c, the critical surface tension, based on Zisman plots. Using the latter method, γ_c values of solids selected from the above group were determined using aqueous ethanol solutions. The values were lower than those obtained using nonpolar liquids, thus confirming earlier findings. A compilation of our own data and data from the literature reveals that the derived values of γ_c show little or no dependence on the type of solid surface, the type of alcohol or its chain length. The results can be explained in terms of adsorption of alcohol at the surface of the solid.

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 finding that the dispersion force contributions to the surface free energies of octane and water are equal enabled a simple method to be developed to characterize the hydrophilic nature of solid surfaces. This technique involves measuring octane contact angles on solid surfaces under water. Nonhydrophilic solids unable to interact by polar forces exhibit a predicted 50° contact angle, whereas those able to interact by polar forces give values greater than 50°. The greater the contact angle, the stronger are the polar interactions. The deviation of the contact angle from 50° can be used to evaluate, Isw, defined as the interfacial stabilization energy from the nondispersion (polar) forces.

The surface tensions of a series of poly(isobutylenes) in the molecular weight range 400–3000 have been determined at 24°C. These results, together with surface tension values from the literature for poly(dimethyl siloxanes) and three series of different pure chain-molecule homologues, are found to exhibit a linear dependence on (molecular weight)^{$\text{−2}\text{2}$}. A simple free-volume argument seems to be consistent with this empirical observation.

Six solid surfaces were compared with respect to their surface quality, by measuring advancing contact angles along the solid surfaces (in the vertical and horizontal directions) at constant immersion rate. It was found that surfaces of mica, dip coated in FC-721, Teflon (FEP) heat pressed against mica, and siliconized glass yield essentially constant advancing contact angles at different locations of the solid surfaces and, thus, are well suited to dynamic contact angle measurements. Static and low rate dynamic contact angles of a number of pure liquids were therefore measured on these solid surfaces. Low rate dynamic contact angles were found to be identical with the static contact angles and independent of the velocity of the three-phase contact line (up to 0.5 mm/min).

Contact angles of water, glycerol, formamide, ethylene glycol, 1-bromonaphthalene, and bromobenzene were measured in the temperature range 5–160° on surfaces of polyethylene, polystyrene, polyacetal, polycarbonate, poly(ethylene terephthalate), and poly(tetrafluoroethylene-co-hexafluoropropylene). Stable advancing and receding angles were found and these varied linearly with temperature except in the range where solubility or swelling was evidence. Superheated water wet all the polymers to a greater degree than predicted. For the fluoropolymer all the liquids showed a negative temperature coefficient of the contact angle, both advancing and receding, ranging from 0.03 to 0.1 deg/°C. For the other polymers coefficients for advancing angles were nearly all negative and ranged from 0.03 to 0.18 but most receding angle values were positive; several liquid-polymer pairs showed a negligible coefficient. Temperature coefficients of the critical surface tension and of the dispersion surface tension of each solid were evaluated. Correlations of these derived quantities are discussed.

The critical velocity of engulfing V_c of acetal, nylon-6,6, and nylon-12 particles when encountered by the solidification front of salol is reported as a function of particle size. Using the dimensional analysis derived previously, the free energy of adhesion ΔF^adh for the attachment of these particles to the salol solid/melt interface is determined. These values of ΔF^adh, together with the known surface tension values γ_PV of the particles, are used to determine the salol solid/melt interfacial tension γ_SL to be γ_SL = 0.0053 ± 0.0025 erg/cm². Similarly, the free energies of adhesion ΔF^adh for PMMA particles to the solid/melt interfaces of naphthalene, biphenyl, and salol are determined. As all the γ_SL values for these systems are known—in the case of naphthalene and biphenyl from the temperature dependence of contact angles—γ_PV for the PMMA particles is determined. Using the equation of state for interfaces, the contact angle for the system PMMA/water is predicted. This value is in excellent agreement with the contact angle of water on a film of PMMA obtained by solvent casting. It is concluded that measurement of the critical velocity of engulfing represents a unique method for contact angle determinations on small particles.

The surface free-energies of the polyethers, polyethylene glycol, polypropylene glycol, polyepichlorohydrin, and polybutylene glycol, their mixtures and their random and block copolymers were determined by means of the pendant drop method. In all cases, except that of random copolymers, surface excesses of the low surface-energy component have been found. In the mixtures of homopolymers the behavior of surface excess isotherms depends on the molecular weight of the two components, while in block copolymers it depends on the degree of polymerization of the base unit. The Belton and Evans Equation for perfect solutions and the Prigogine equation for r-mer solutions have been applied to the experimental data.

The surface tension and surface entropies of different molecular weight polyethylene glycols and polypropylene glycols have been measured. The surface entropy of a mixture of polyethylene glycol and polypropylene glycol and that of block copolymers have also been determined. In the case of homopolymers, there is no effect of molecular weight on surface free energy and the increase in free energy on passing from the interior to the surface is due mainly to the heat content with the entropic contribution being very small.

In the case of a mixture of homopolymers and block copolymers, a minimum is observed when surface entropy is plotted against composition. At any particular composition, the surface entropy of a mixture is higher than that of a block copolymer of the same composition.

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.

Ellipsometrically determined adsorption isotherms are reported for water on two types of pyrolytic carbon, on polyethylene, and on stearic acid-coated copper, for relative pressures up to close to the saturation pressure, P⁰, and for various temperatures. Contact angle data for bulk water on the same solids are included; advancing angles of 60°–90° were found. The adsorbed film thickness reaches 40–80 Å in the first two systems, but only a few angstroms in the second two; correspondingly, the surface pressures of P⁰, π⁰, are large in the first two cases and small in the second two. Large contact angle thus does not necessarily imply low π⁰. The data are fitted to a previously published potential-distortion model, which allows adsorption and contact angle behavior to be related.

The spreading of surfactant solutions over hydrophobic surfaces is considered from both theoretical and experimental points of view. Water droplets do not wet a virgin solid hydrophobic substrate. It is shown that the transfer of surfactant molecules from the water droplet onto the hydrophobic surface changes the wetting characteristics in front of the drop on the three-phase contact line. The surfactant molecules increase the solid–vapor interfacial tension and hydrophilize the initially hydrophobic solid substrate just in front of the spreading drop. This process causes water drops to spread over time. The time of evolution of the spreading of a water droplet is predicted and compared with experimental observations. The assumption that surfactant transfer from the drop surface onto the solid hydrophobic substrate controls the rate of spreading is confirmed by our experimental observations.

A technique was developed to rapidly measure surface tensions (γ) of viscous molten polymers and polymer solutions. The usual problems of slow meniscus equilibration and low signal-to-noise levels due to thermal convection currents at elevated temperatures have been overcome. Small-diameter fibers were used as vertical probes in the Wilhelmy technique to facilitate rapid equilibration of the wetting meniscus, and a “baffle tube” surrounding the electrobalance wire was implemented to suppress noise from thermal convection currents from the oven. Even with the baffle tube, it was found that computer averaging of the measured wetting force was necessary to obtain the desired precision precision atT 250°C. Measurements of γ up to ∼400°C were routinely made with η> 50 P polymers. Data are given for a molten fluoropolymer, a thermoplastic, and a liquid crystalline polymer. Room-temperature polymer fluids with viscosities extending to η = 500,000 P were studied; at η ⩽ 5000 P the precision was better than 0.04mNm. The dynamic contact angle versus time was measured as a function of fiber diameter, giving a relationship between the rate of meniscus equilibration and fiber diameter. Contact angles of polymer fibers immersed in water and methylene iodiode were used to calculate the surface free energies of the polymer in the solid state. These values are consistent with the extrapolated molten surface tension data and help to characterize the trend in γ over a wide range ofT.