A new method to detect surface oxidation of an otherwise untreated, cross-linked and filled silicone rubber is described. Our method is established on the principle that surface wettability increases during the progress of oxidation. Surface wettability is determined in terms of critical surface tension.

Abhesive polymers, of which silicone rubber is a typical example, are characterized by low surface energy, low friction coefficient and low release value. The problem associated with silicone rubber is its poor adhesion to other polymers. Its adhesional ability, however, can be improved by surface modification, e.g. oxidation, treatment with corona discharge, or ionic bombardment with inert gases.

By our method we found that the oxidation of silicone rubber is comparatively mild below 260°C, but is intensified at 287°C. Excessive oxidation at 316°C results in the formation of low molecular weight siloxanes which lower the wettability of the oxidized surface. Mechanisms of thermal oxidation are discussed.

We present our unified Lewis acid–base approach to adhesion and solvation at the liquid-polymer interface. This approach is to complement the original methodologies proposed by Fowkes and by van Oss, Chaudhury and Good (VCG). Intermolecular interactions are primarily dominated by dispersion, d, hydrogen bonding, h, and secondarily affected by orientation, o, and induction, i. Generally, the polarization component, p, represents both i and o interactions. Fowkes suggested that the acid–base component, γ^ab, of the surface tension should consist of both h and p interactions. However, VCG proposed that the acid–base components, γ^ab, result solely from hydrogen bonding, γ^h, that is equivalent to 2(γ⁺ γ⁻)^1/2, where γ⁺ and γ⁻ are the two hydrogen bonding parameters. VCG defined γ^LW as the Lifshitz-van der Waals component consisting of d, o and i contributions, thus, surface tension, γ, equals γ^ab(VCG)+γ^LW. Both Fowkes and VCG assumed that the polar interactions for a liquid on a low energy surface are negligible.

Now, we assume otherwise, and we treat the specific acid-base interaction to be hydrogen bonding. In addition, we also take into account the nonspecific polarization, p, interaction in terms of the equilibrium spreading pressure, π_e, resulting from the adsorption of a liquid vapor on the polymer surface. Thus, our unified approach uses the dispersion component, γ^d, of Fowkes, the hydrogen bonding, h, of VCG and the polarization, p, in terms of π_e. The difference between the initial (theoretical) and equilibrium (experimental) surface tensions is π_e, and others have observed that π_e on some polymers is substantial. The determination of several initial surface tensions of polymers by considering the effect of polarization is discussed.

In the Appendix, we shall illustrate that this polar component, π_e, is equivalent to the LESR polarity-dipolarity parameter, π^e, (represented by the same symbol but in different context) for the solvatochromic treatment. Furthermore, the surface tension components, π^d, γ⁺, γ⁻ and π_e, are now somewhat comparable with the four parameters in the original Taft-Kamlet relationship, δ, α, B, and π^e. Thus, our proposed unified approach may finally help elucidate the long-debated Lewis acid–base theories pertaining to adhesion and solvation of polymers.

In this paper, we briefly discuss several ways to determine the work of adhesion and the requirements for achieving maximum adhesion and spontaneous spreading. Specifically, we will concentrate on the methodology developed by van Oss. Chaudhury and Good [5–7] for the determination of the work of adhesion and interfacial tension. Recently, Good [4] has redefined the surface interaction components γ⁺ and γ⁻ as hydrogen bond acidic and basic parameters. We have related the surface−hydrogen−bond components γ⁺ and γ⁻ to the Taft and Kamlet's [28, 29] linear solvation energy relationship (LSER) solvatochromic α and β parameters. We [8, 9] have found that, for water at ambient temperature, α [hydrogen-bond-donating (HBD) ability] and β [hydrogen-bond-accepting (HBA) ability] are not equal, and the ratio for the normalized α and β is 1.8. This new ratio is assumed to be equal to that of γ⁺ & γ⁻ for water at 20°C. On the basis of the new ratio, we will present our recalculated surface-hydrogen-bond components for several polymers and biomaterials. This change in the ratio did not affect the total surface tension or the sign of the interfacial tension. The net improvement is in the lowering of the γ⁻ values. These data may be useful for predicting the adhesion between an adhesive and an adherend.

The capability of the PDMS based antiadhesion surface treatment strategy for high resolution unconventional lithography using hard or soft molds as representatives of imprint lithography or soft lithography was investigated. A thin film of PDMs was used as an antiadhesion release layer as PDMS has a fairly low surface energy and allows for the easy release of the mold from the patterned polymer on the substrates. The surface of the Si wafer was coated with a thin film of PDMS and using this PDMS-coated Si wafer as a hard mold line/space patterns were printed on the SU-8-coated PET substrates. Using this photoresist replica mold as a template for a soft mold the same PDMS-based coating strategy was applied. The imprinting of nanostructure-patterned mold onto a polymer composed of the same chemical as the mold led to pattern collapse during the release of the assembly because of the extremely strong adhesion between the mold and the polymer.

The wettabilities of fluorinated polymers were evaluated using a series of contacting probe liquids ranging in nature from nonpolar aprotic to polar aprotic to polar protic. Fully fluorinated polymers were wet less than partially fluorinated polymers, highlighting the weak dispersive interactions of fluorocarbons. For partially fluorinated polymers, the interactions between the distributed dipoles along the polymer backbone and the dipoles of the contacting liquids were evaluated using both polar and nonpolar probe liquids. The results demonstrate that the surface dipoles of the fluoropolymers generated by substituting fluorine atoms with hydrogen or chlorine atoms can strongly interact with polar contacting liquids. The wettabilities of the partially fluorinated polymers were enhanced by increasing the density of dipoles across the surfaces and by introducing differentially distributed dipoles.

Ultra-high molecular weight polyethylene (UHMWPE) fibers were subjected to oxygen plasma treatment in order to improve interfacial adhesion. The treated fibers were characterized by contact angle analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and mercury porosimetry. The surface free energy, O 1s/C 1s ratio, and surface area increased dramatically with 1 min treatment. However, as the treatment time increased further, these parameters either increased slowly at 30, 60, and 100 W, or decreased at 150 W. The increased surface free energy is attributed to the polar component, while the increased O 1s/C 1s ratio is explained by the oxygen-containing moieties introduced by the plasma treatment. The oxygen plasma treatment also roughened the initially smooth surface of the UHMWPE fibers by forming micro-pores and thus increased the surface area. The interfacial shear strength of UHMWPE fibers to vinylester resin was measured by micro-droplet tests and exhibited an increasing trend, believed to result from the increased surface area, the surface free energy, and the oxygen-containing moieties due to the plasma treatment.

The plasma source ion implantation (PSII) was utilized to improve the wettability and the stability of surface layer formed in the modification of polymeric materials. Polystyrene was treated with different kinds of plasma ions to render the surface more hydrophilic or hydrophobic. Hydrophobic recovery of PSII-treated polystyrene was also observed as a function of aging time, aging temperature, and treatment parameters. Treatment parameters involve kinds of gases, pressure, plasma power, pulse frequency, pulse voltage, etc. To study the effect of inert gas on hydrophobic recovery, polystyrene samples were prepared by helium, argon, or gas-mixture treatment. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been used to interpret the PSII-treated polystyrene surface and its hydrophobic recovery, with the assistance of x-ray photoelectron spectroscopy and water contact angle measurements. TOF-SIMS spectra of O218 PSII-treated samples showed the presence of O18-containing peaks from the modified surfaces. PSII modifications provide more stable surfaces of polystyrene as a function of aging time than plasma treatments. The comparison of aging behavior data allowed for examination of the differences in the stability of the functionality introduced by the two different treatment techniques.

Chemical composition, morphology, and crystalline structure of low density polyethylene (LDPE) films surface grafted with acrylic acid (AA) using corona discharge were studied by attenuated total reflection infrared (ATR-IR), electron spectroscopy for chemical analysis (ESCA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide angle X-ray diffraction (WAXD) techniques. The grafted film surface is covered with grafted chains. After grafting for 3.0 h in 20% aqueous solution of AA, the depth of the grafted layer is more than 10 nm. A grain structure was observed on the grafted surfaces which was probably caused by the isolated dispersion of active sites generated by corona discharge, and these active sites initiated the graft copolymerization. However, surfaces of grafted films were smoother than that of ungrafted ones. DSC curves of grafted films show a small peak at about 100°C due to vaporization of adsorbed water. The longer the graft copolymerization time, i.e. the higher the graft degree of AA on LDPE, the higher the amount of adsorbed water. The position of each peak in WAXD patterns, crystal axial length, crystal plane distance and crystal grain size remain almost unchanged during the graft copolymerization time of 2.0 h. However, when the graft copolymerization time reaches 3.0 h, twin peaks at about 21.4° and 22.0° are observed, indicating that a different crystal form is formed at longer copolymerization time, i.e. at a higher graft degree.

The corona discharge technique was explored as a means of forming chemically active sites on a low-density polyethylene (LDPE) film surface. The active species thus prepared at atmospheric pressure in air was exploited to subsequently induce copolymerization of 2-hydroxyethyl methacrylate (HEMA) onto LDPE film in aqueous solution. The results showed that with the corona discharge voltage, reaction temperature, and inhibitor concentration in the reaction solution the grafting degree increased to a maximum and then decreased. As the corona discharge time, reaction time, and HEMA concentration in the reaction solution increased, the grafting degree increased. With reaction conditions of a 5 vol % HEMA concentration, 50°C copolymerization temperature, and a 2.0-h reaction time, the degree of grafting of the LDPE film reached a high value of 158.0 μg/cm² after treatment for 72 s with a 15-kV voltage at 50 Hz. Some characteristic peaks of the grafted LDPE came into view at 1719 cm⁻¹ on attenuated total reflectance IR spectra (CO in ester groups) and at 531 eV on electron spectroscopy for chemical analysis (ESCA) spectra (O_1s). The C_1s core level ESCA spectrum of HEMA-grafted LDPE showed two strong peaks at ∼286.6 eV (CO from hydroxyl groups and ester groups) and ∼289.1 eV (OCO from ester groups), and the C atom ratio in the CO groups and OCO groups was 2:1. The hydrophilicity of the grafted LDPE film was remarkably improved compared to that of the ungrafted LDPE film. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2881–2887, 2001
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1738

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 O₂ and O₂-N₂ ([N₂] < 15%) post-discharge microwave plasma modifications of high-density polyethylene (HDPE) and hexatriacontane (HTC) surfaces have been studied as a function of the distance from the discharge and the gas composition. They are compared in terms of the in-situ XPS O/C ratios and C 1s components, and the ex-situ ToF-SIMS O^-/CH^- ratios and relative intensities of series of peaks. The results on the effect of the distance from the discharge showed a clear correlation between the in-situ XPS results and the O₂ post-discharge modeling, exhibiting the double role of oxygen atoms: functionalization initialization by creating radicals (which react with molecular oxygen) and degradation due to the energy released by the oxygen atom recombination process. Specific in-situ XPS and ex-situ ToF-SIMS signatures of this in-situ degradation related to the oxygen atom recombination process were exhibited. When N₂ was introduced in the plasma gas, the in-situ XPS results and the ex-situ ToF-SIMS results were very different. The in-situ functionalization decreased as a function of the N₂ addition and the ex-situ functionalization exhibited a high maximum for the 5% N₂-95% O₂ post-discharge plasma and then decreased. Despite the absence of a complete O₂-N₂ post-discharge modeling, it can be assumed that there is also a maximum of the oxygen atom content for the 5% N₂-95% O₂ post-discharge. Thanks to the in-situ XPS and ex-situ ToF-SIMS specific signatures, it was also shown that this maximum corresponds to a low in-situ degradation effect. Nitrogen introduction could influence the role of oxygen atoms in such a way that there is a decrease in oxygen atom recombination processes (thus in degradation) for small N₂ addition and even a decrease in oxygen functionalization initialization for further N₂ incorporation in the plasma gas. No nitrogen was observed in the in-situ XPS results, whereas some ex-situ ToF-SIMS nitrogen-containing ions were observed for the O₂ and O₂-N₂ post-discharge. However, their relative intensities followed the variation in oxidation and not the variation in N₂ concentration in the plasma gas. They could be related to a post-treatment functionalization effect. Differences observed between HDPE and HTC are explained in terms of their structural differences (desorption of low molecular weight oxygen-containing fragments for HTC).

The effects of atmospheric air-plasma treatments on woven and non-woven polyester (PET) textile structures were studied by surface analysis methods: wettability and capillarity methods, as well as atomic force microscopy/lateral force microscopy (AFM/LFM). The water contact angle on plasma-treated PET decreased from 80° to 50–40°, indicating an increase in the surface energy of PET fibres due to a change in the fiber surface chemical nature, which was confirmed by a higher fiber friction force measured by the LFM. The extent of water contact angle decrease, as well as the wash fastness of the treatment varied with the structure of the textile. Indeed the more porous the textile structure is (such as a non-woven), the fewer are the chain scissions of the PET at the fiber surface, during the plasma treatment. Thus, the level of surface oxidation and the weak boundary layers formation depend not only on plasma treatment parameters but also on the textile structure.

Dielectric barrier discharge (DBD) technologies have been used to treat a polypropylene film. Various parameters such as treatment speed or electrical power were changed in order to determine the treatment power impact at the polypropylene surface. Indeed, all the treatments were performed using ambient air as gas to oxidize the polypropylene surface. This oxidation level and the surface modifications during the ageing were studied by a wetting method and by X-ray photoelectron spectroscopy (XPS). Moreover polypropylene film surface topography was analyzed by atomic force microscopy (AFM) in order to observe the surface roughness modifications. These topographic modifications were correlated to the surface oxidation by measuring with a lateral force microscope (LFM) the surface heterogeneity. The low ageing effects and the surface reorganization are discussed.

Contact angles of binary liquid mixtures on Teflon FEP were measured. It was found that the equation of state for interfacial tensions, γ_SL = f (γ_LV, γ_sv), cannot be used to determine solid surface tensions from these contact angles of binary liquid mixtures. These findings are explained in terms of the thermodynamic phase rule.

The interfacial energetics and wettability of small particles are of technological interest in many areas of applied science. Areas where such phenomena are important include the preparation of stable suspensions of particles (e.g., colour pigments in paints), the adhesion of particles to solid surfaces in various scenarios (e.g., lubrication), the dispersion of particles into a liquid or melt of a polymer, and the modification of particle surface properties through the adsorption of polymeric macromolecules or surfactants. The successful manipulation of the process being considered is largely determined by the physicochemical surface properties of the interacting surface components, and particularly the wettability and the surface (or interfacial) tension of the particles. The complexities of contact angle phenomena and surface tensions were discussed in Chapter 3.

The effects of a thin liquid film on contact angles are studied using a simplified thermodynamic model. (in this model, the small transition zone between the liquid-vapour interface and the fiat thin liquid film is neglected). A set of mechanical equilibrium conditions have been derived for contact angle systems with a flat thin liquid film. The equilibrium condition at the three-phase intersection explicitly predicts the effects of the film tension, the disjoining pressure and the film thickness, on contact angles.

The number of degrees of freedom for a two-component solid-liquid-vapour surface system with a flat thin liquid film is shown to be three, implying the existence of an equation-of-state-type relationship among the solid-liquid interfacial tension, γ_sl, liquid surface tension, γ_lv, the disjoining pressure, Π, and the film tension, γ_f. An approximate, explicit form of such an equation of state has been derived. The combination of this equation of state with the equilibrium condition of the the three-phase intersection can be used to estimate the film tension, γ_f, and the solid-liquid interfacial tension, γ_sl, from the measured data for the vapour pressure, P_v, the film thickness, h, the curvature of the liquid-vapour meniscus, J, the liquid surface tension, γ_lv, and the contact angle, θ.

The effect of the thin film on the drop-size dependence of contact angles is also investigated and found to be negligible.

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.

The effect of surface heterogeneity on contact angle hysteresis is studied by using the model of Neumann and Good of a vertical plate with horizontal heterogeneous strips. The results of this study explain well known, but not understood patterns of contact angle behaviour: On the one hand, the advancing contact angle on a carefully prepared solid surface is generally reproducible; on the other hand, even a very small amount of surface heterogeneity may cause the receding contact angle to be less reproducible and to depend on several non-thermodynamic factors.

The drop size dependence of the advancing contact angle of dodecane and ethylene glycol on carefully prepared FC-721, Zonyl FSC and DDOA surfaces has been studied by means of axisymmetric drop shape analysis. The contact angles were measured in air and were found to decrease by 3 to 5 degrees as the radius of the three-phase contact line increased from approximately 1 to 5 mm. This phenomenon is interpreted in terms of line tension by the modified Young equation. Our experimental results show that the line tensions are positive and of the order of 1 μJ m⁻¹ for all the three solid-liquid systems in our study; these results are consistent with previous work in our laboratory. The occasionally observed phenomenon that contact angles increase as the radius of the three-phase contact line increased on less carefully prepared surfaces is ascribed to the corrugation of the three phase contact line.

Surface biomedical effects of plasma treatment and plasma polymerization on medical-grade polyetherurethane were studied. N2 and Ar plasma treatments and hexamethyldisiloxane (HMDS) plasma polymerization were performed at a power of 100 W with exposure times ranging from 1 to 15 min. The results showed that the contact angle of water was decreased from 79° to 62° by N2 and Ar plasma treatments, and N2 plasma treatment caused a slight enhancement in anti-coagulability and anti-calcific behavior. HMDS polymerization resulted in a decrease from 79° to 43° in the contact angle and an increase from 30.5 to 37.4 s in the recalcification time. At the same time, the anti-coagulability of polymerized samples for the exposure time of 2-5 min was 2.5 times that of the untreated sample. Results of XPS and ESR analyses showed that HMDS deposited onto the polyetherurethane surface and formed new Si-N bonds, and increased the number of radicals in the sample. XPS analysis also showed that N2 and Ar plasma treatments broke some of the CO and CO bonds at the surface and resulted in oxidation of the surface.

The interfacial adhesive behavior between acrylic pressure sensitive adhesive-like networks (PSA-LNs) and poly(vinyl N-alkyl carbamate) release coatings was studied using a contact mechanical method and peel tests. Surface energy and interfacial energy were directly measured in JKR tests using a novel sample construction. The surface energy of the poly(vinyl N-alkyl carbamates) was found to be around 20 mJ/m². Interfacial energies between PSA-LNs and the release coatings were found to be quite high – between 7 and 24 mJ/m². Changes in adhesion dynamics were governed by acid-base interactions between the carbamate in the release coating and the acid groups in the PSA-LN. The length of the alkyl chain in the release coating moderated this effect. We also found a correlation between fundamental adhesion energy and peel strength. Examination of this phenomenon provides a basis for understanding the poor storage stability of PSA tapes made using alkyl carbamates and acid-containing PSAs.