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.

Etching and modification of polymers by plasmas is discussed in terms of the roles played by atomic and molecular oxygen, atomic fluorine, CF_x radicals, ions, high energy metastable species, and photons. Addition of fluorine-containing gases to oxygen can increase both 0 atom densities in the plasma and polymer etching rates. The etching rate be- havior generally exhibits a maximum at a specific concentration of this additive. Process parameters which alter the concentrations of 0 and F atoms in the plasma or affect the rate of delivery of these species to the polymer surface shift the position of this maximum with respect to feed gas composition. However, the gas composition which yields maximum rates exhibits a strong dependence on polymer structure, specifically, its degree of unsaturation. This is explained on the basis of molecular orbital (MO) arguments which predict that the surfaces of unsaturated polymers have a higher affinity than saturated polymer surfaces for atomic fluorine. Favored reaction pathways leading to volatile etching products are pro-posed based on MO calculations of relative bond strengths for various oxygenated and fluorinated organic model compounds. Although fluorine abstraction of hydrogen plays a major role in generating radical sites on saturated polymer surfaces, it is likely that etching of unsaturated moieties proceeds through a saturated radical intermediate resulting from addition reactions of fluorine atoms. Excessive amounts of fluorine in the plasma result in reduced etching rates and incorporation of fluorine and/or CF_x radicals into the polymer. Polymer film surfaces are also modified by high energy metastables and ultraviolet radiation generated from noble gas plasmas, The effect of vacuum ultraviolet radiation from helium microwave plasmas on films of polytetrafluoroethylene and polyethylene is addressed.

Webs of Kapton 200-H and Upilex-S polyimide films were treated using oxygen plasma prior to sequential sputter deposition of chromium and copper in a roll metallization system. Two plasma system configurations were employed for treatment. In one configuration, the sample traveled downstream from a microwave plasma; in the other, the web moved through a DC-generated glow discharge. For the DC-glow treatment, the potential difference between the plasma and the web, Φ_f, and relative ion densities, n+, were measured at various values of chamber pressure and DC power using a Langmuir probe. Although samples treated downstream from the microwave plasma were not subjected to bombardment by energetic ions, Φ_f for the DC-glow operating conditions was between 5 and 13 eV. For both films, advancing DI water contact angles of less than 20° were achieved using both modes of treatment. Contact angles for untreated films were greater than 60°. However, 90° peel tests yielded values of 15 to 20 g/mm for microwave plasma treatments and 40 to 60 g/mm for DC-glow treatment. Peel values for untreated Kapton and Upilex films were about 25 g/mm. High-resolution X-ray photoelectron spectroscopy in the C1s region for Kapton film surfaces treated downstream from the microwave plasma showed increases in carbonyl groups, with concentrations inversely proportional to web speed. In contrast, DC-glow modification was due mainly to formation of carboxylates with a small increase in carbonyl component. It is proposed that treatment downstream from the microwave plasma results in formation of a weak boundary layer at the polyimide surface. Ion bombardment occurring in the DC-plasma configuration results in relatively more crosslinking at the polymer surface. Furthermore, adhesion between the sputter-deposited chromium and the DC-glow modified polyimide improved with increasing values of Φ_fn+.

Polymers have wide-ranging applications in food packaging and decorative products, and as insulation for electronic devices. For these applications, the adhesion of materials deposited onto polymer substrates is of primary importance. Not all polymer surfaces possess the required physical and/or chemical properties for good adhesion. Plasma treatment is one means of modifying polymer surfaces to improve adhesion while maintaining the desirable properties of the bulk material. This paper addresses the interaction of organic surfaces with the various components of a plasma, with examples taken from a review of the pertinent literature.

A viscous-liquid drop spreads on a smooth horizontal surface, which is uniformly heated or cooled. Lubrication theory is used to study thin drops subject to capillary, thermocapillary and gravity forces, and a variety of contact-angle-versus-speed conditions. It is found for isothermal drops that gravity is very important at large times and determines the power law for unlimited spreading. Predictions compare well with the experimental data on isothermal spreading for both two-dimensional and axisymmetric configurations. It is found that heating (cooling) retards (augments) the spreading process by creating flows that counteract (reinforce) those associated with isothermal spreading. For zero advancing contact angle, heating will prevent the drop from spreading to infinity. Thus, the heat transfer serves as a sensitive control on the spreading.

In this investigation the fracture surface between bovine dentine and bovine enamel and a dental cement was observed using the scanning electron microscope at magnifications up to 10,000 ×. The results indicated that the topography of the adherend plays an important role in the formation of an adhesive bond and in the fracture pattern of an adhesive joint, even when cohesive failure is involved.

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.

Gas discharge plasma treatment can be used to modify the surface properties of biomaterials for a variety of biomedical applications. An established application is the use of oxygen and nitrogen plasmas to improve the hydrophilicity of surfaces, encouraging cell attachment and subsequent growth. The physical properties and surface chemistry of the biomaterial influences cell attachment and subsequent culture. In-situ cells are surrounded by a complex extracellular matrix (ECM) containing fibronectin, laminin, collagen types I-V, and proteoglycans. In this study, ammonia and sulphur dioxide gases have been chosen with the objective of incorporating carboxylic acid, sulphur and nitrogen containing groups on the surface.

This paper describes theoretical and experimental investigations of the effect of an electrode coating on the onset voltage of a corona on negatively stressed electrodes. Dielectric-coated hemispherically-capped rod-to-plane gaps positioned in air are investigated. The onset voltage is calculated based on the self-recurring single electron avalanche developed in the investigated gap. Accurate calculation of the electric field in the vicinity of a coated rod and its correlation to the field values near a bare rod of the same radius are obtained using the charge simulation method. The calculated field values are utilized in evaluating the onset voltage of the corona. Also, laboratory measurements of the onset voltage on bare and coated electrodes are carried out. The effects of varying the field nonuniformity, the coating thickness and its permittivity on the onset voltage values are investigated. The results show that coating the electrodes with a dielectric material is effective in increasing the onset voltage of the corona on its surface. The calculated onset voltage values for coated and bare electrodes agree satisfactorily with those measured experimentally.

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.

(1) A study hasbeen made of the wettability of adsorbed monolayers of monohydroperfluoroundecanoic acid. The results obtained on this surface (comprising CF₂H groups) are compared with previous results on adsorbed monolayers of perfluorodecanoic acid (comprising CF₃ groups) and solid polytetrafluoroethylene (comprising CF₂ groups).(2) It is shown that for “normal” liquids [ie, those for which only van der Waals forces of adhesion are operative) the contact angles on CF₂H surfaces are larger than on CF₂ surfaces and nearly as large as on CF₃ surfaces.(3) Alcohols, acids and amines are found to give abnormally low contact angles on CF₂H and CF₃ surfaces, but only amines give low angles on polytetrafluoroethylene. This can be accounted for by the ability of these polar liquids to form hydrogen bonds with the fluorinecontaining surfaces. Esters, lacking a suitable hydrogen atom, are shown to bond only to the CF₂H surface (which can supply the necessary hydrogen atom); alcohols and acids bond to CF₂H and CF₃ surfaces; primary amines bond to all three fluorinated surfaces because of their ability to form “double” hydrogen bonds. These phenomena are believed to be new examples of the “unsymmetrical hydrogen bond” described by Pauling.

The wettability by organic liquids containing Cl, Br or I is less affected by the amide or ester groups as might be expected from the inability of halogenated liquids to form hydrogen bonds. Reasons are given for believing that hydrogen-bonding takes placein the wetting of nylon by water, glycerol, formamide and thiodiglycol and does not take place in the wetting of polyethylene terephthalate by these liquids. The postulated mechanism of wetting led to an experiment which showed that perfluorolauric acid could be adsorbed on nylon from n-decane solution rendering thenylon surface oleophobic.

Wettability of solid surfaces containing covalent chlorine increases greatly with the chlorine content. There is no indication of hydrogen-bonding at the solid/liquid interface for surfaces containing carbon, hydrogen and chlorine. A close packed monolayer of perchloro-2, 4-pentadienoic acid adsorbed on a polished metal is shown to behave with respect to wetting like an organic surface comprising Í00 atom per cent, of chlorine substitution. Increased wettability of fluorine-containing surfaces by hydrogen-bonding liquids is reported for a number of new, partially fluorinated plastic surfaces. The wettability of fluorinated surfaces varies with the type of spreading liquid. For non-polar liquids the wettability decreases with increasing fluorine substitution. For hydrogen-bonding liquids, the wettability increases in the order: polytetrafluoroethylene, polytrifluoroethylene, polyethylene, polvvinylidene fluoride and polyvinyl fluoride. The corresponding order for the haligenated liquids is polytetrufluoroet. hylene, polytrifluoroethylene, polyvinylidene fluoride, polyethylene and polyvinyl fluoride. Explanations are offered for the relation between wettability and the atom per cent, fluorine substitution in the surface based on the electronegativity of the fluorine atoms in the surface and the molecular structure of the spreading liquid.

Natural rubber thermoplastic elastomers (NRTPEs) made by dynamic vulcanization of natural rubber during its mixing with polypropylene were subjected to various halogenation surface treatments. Marked reduction in the coefficient of friction is possible depending on the chemical treatment employed, TPE composition and the presence of a lubricant. As a result of halogenation there is an increase in the microroughness and hardness of the NRTPE surface. These effects in part explain the large decrease in the friction coefficients since the contact area is decreased. Thus NRTPE can be employed in applications requiring low friction, such as certain types of seals. Another consequence of halogenation of NRTPE is the increase in its surface energy which in turn promotes adhesion to various polar substrates. Indeed it was determined that halogenation of NETPE is an effective way of priming the surface of these materials for adhesion to acrylic and other substrates. Ethylene Propylene Diene Monomer rubber-Polypropylene thermoplastic elastomers (EPTPEs) were used as a control in this study to assess how a low unsaturation EPDM-based TPE compares with the high unsaturation NRTPEs in different halogenation surface treatments.

The article describes applications of a novel windowless argon excimer source for surface modification. Experimental results on etching of polymeric surfaces, degradation of organic surface residues, surface activation and modification of gas permeability and selectivity of polymeric membranes are presented. Moreover, radical formation from the excimer source and surface curing of liquid acrylates are examined. Typical treatment times are in the range of minutes for photolytic decomposition effects and seconds for UV curing effects. The surface modification effects induced by the argon excimer source were analysed by XPS, ESR, IR-spectroscopy, white light reflection spectroscopy, scanning electron microscopy, micro-hardness and permeation measurements.

In the present work three common polyolefins: high density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) have been treated with an atmospheric pressure air plasma torch (APPT) in order to improve their wettability properties. The variations in surface energy (γ_s), as well as the durability of the treatment are determined by means of contact angle measurements for different aging times after plasma exposure (up to 270 days) using five test liquids which cover a wide range of polarities. The introduction of new polar moieties (carbonyl, amine or hydroxyl) is confirmed by Fourier transform infrared spectroscopy in attenuated total multiple reflection mode (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, scanning electron microscopy (SEM) provides information on the morphological changes and variation on surface roughness, revealing that smoother, lamellar and semispheric micrometric structures are created on the LDPE, HDPE and PP surfaces, respectively. Results show that APPT treatment enhances both the total and polar components of the γ_s under study, with an unprecedent stability (> 8 months) in time.

Most polymeric materials, particularly polyolefins and their derivatives, present a low surface energy which is the cause of their poor wettability and limits processes such as adhesive bonding, painting, or metalizing. Many methods have been developed and used to modify polymer surfaces for improved wetting, including mechanical treatments, wet-chemical treatments with strong acids or bases, and exposure to flames or corona discharge. In this paper the improvement of wetting properties of several polymeric materials widely used in the automotive industry, such as high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and silicone, is studied by means of surface mechanical abrasion using sandpapers of different grain sizes (1000, 180 and 80). Measurements of the surface roughness are performed using a Hommel Tester T8000 device equipped with a diamond stylus, which provides data on the arithmetic average roughness R_a parameter and Abbott–Firestone curve. Variations in the polymers surface energy (SE) are estimated through contact angle measurements using five test liquids of different polarities. Both components of the SE, dispersion (σ^D) and polar (σ^P), as well as total (σ^T) at different conditions of treatment are analyzed using the Owens–Wendt–Rabel–Kaelble (OWRK) method. Morphological changes induced in the surface are analyzed by Scanning Electron Microscopy (SEM). Additionally, measurements of the static friction coefficient (μ_s) are carried out by the standard method ASTM D 1894-08. A slight enhancement in surface wettability is found with the mechanical abrasion pre-treatment from the SE increase. Finally, a higher value of μ_s is achieved for the abraded specimens as the normal force acting onto the system is increased.

In 1878, Gibbs [1] published his celebrated “Theory of Capillarity,” the standard reference of surface thermodynamics ever since. In a rather compact-yet exhaustive and profound-manner, Gibbs treated fluid-fluid, as well as solid-fluid, interfaces and their equilibrium properties while representing the interfacial region in an Euclidean manner by a single dividing interface, preferably the so-called surface of tension. For this particular dividing surface, the standard Laplace (or Young-Laplace) equation [2]: ∆P=2Hγ (1) holds exactly for a majority of cases. Here H=(c1+c2)/2 denotes the mean curvature, γ is the interfacial tension, and ∆P is the pressure jump at the interface, and c1 and c2 are the principal curvatures of the surface of tension. Moreover, for any given interface, the interfacial tension γ attains a minimum value when the surface of tension is chosen to be the dividing surface, as may readily be verified.

In this study, atmospheric plasma treatment has been used to modify the wetting properties of ethylene-methacrylic acid sodium ionomer. The effects of the plasma treatment on surface properties of this ionomer have been followed by contact angle measurements, Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). With the use of these techniques, the overall effects on activation–functionalization and surface topography changes have been determined in terms of the processing parameters of the atmospheric plasma treatment (rate and distance). The obtained results show a remarkable increase of the wetting properties and optimum balanced behavior is obtained for atmospheric plasma treatment with a rate of 100 mm/s and a distance of 6 mm; in this case, surface free energy is increased from 33 mJ/m² (untreated ionomer) up to 62 mJ/m², maintaining good transparency. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Electroless plating of non-conducting materials needs, prior to the metal deposition itself, to make the sample surface catalytically active. The route involving the chemical reduction of a thin solid metal–organic coating has, for this purpose, a significant potential in reducing the number of steps which are required today in conventional wet chemical metallization processes. In this work, a novel activation process using a dielectric barrier discharge (DBD) is described for the first time. This process is based on the plasma-induced chemical reduction at atmospheric pressure in air of palladium acetate (PdAc) layers resulting in the formation of palladium (Pd) on non-active surfaces. Fast surface activation of polymers like polyimide (PI) was found to occur in only a few seconds using a simple DBD device instead of expensive excimer UV lamps or complicated laser systems. The DBD-induced Pd layers on PI exhibit high activity with regard to initiation of the electroless copper plating. Indeed, copper deposition starts immediately after dipping the activated PI samples in the electroless solution without any inhibition time. Homogeneous copper coatings on PI were achieved under optimal plasma treatment conditions. The results are compared to those achieved with excimer UV lamps and excimer UV lasers.

Knowledge of the surface free energy of solids is important to understanding a number of processes involving wetting and adhesion to solid surfaces. The measurement of surface free energy has been a subject of active interest for at least 50 years. Despite the importance of the problem to a variety of industries a universally accepted method or set of methods for determination of solid surface free energies has not been agreed upon. In this review article various methods that have been used for the calculation of surface free energies are discussed. The limitations and concerns for employment of each of these methods are furthermore highlighted. Of principal concern is the use of contact angles that meet the requirements to be Young’s contact angles and the mixing of quantities obtained by contact angle measurements with those obtained by IGC, as surface free energies obtained by IGC tend to be larger than those obtained from contact angle measurements. Calculated values from IGC data are presumably larger than those from contact angle data as IGC data are often collected at very low surface coverages.

The surface chemistry and surface energetics of materials are important to the performance of many products and processes—sometimes in as yet unrecognized ways. This review is written for the researcher interested in exploring the nature of surfaces and their relation to processes involving spreading, wetting, liquid penetration and adhesion. Researchers concerned with many types of products including pharmaceuticals, printing and the making of composite materials should have interest in this topic. More specifically, this work is a review of the literature concerning the surface free energy of solids. Both theoretical approaches for understanding the surface free energy of solids are explored and contrasted, as are experimental methods for measuring surface free energy of solids. Experimental methods that offer insight into the chemical nature of surfaces but do not measure surface free energy are also discussed as these two subjects are intertwined.

An understanding of the surface free energy and surface chemistry of solids is needed for investigation into the nature of processes involving adhesion, wetting and liquid penetration. Frequently the contact angles of several probe liquids on a given solid are used for calculation of solid surface free energy. Models by Fowkes, Kwok and Neumann, van Oss, Chaudhury and Good, as well as by Chang and Chen have been used for such calculations. Each of the above models has been championed in the literature. It has been noted by the present author and others that the use of different models may lead to different qualitative interpretations of the nature of a solid surface. A disinterested comparison of the various available models has not been made. In the present paper, a comparison of the calculations is undertaken in order to better understand the limitations of each model. Particular attention to the assumptions required for contact-angle data to be used for surface free energy calculations is given. The effect of the degree to which the experimental contact-angle data meet the required assumptions have on the calculated surface free energy is addressed in this work. When data meeting the theoretical assumptions common to the various published models are used, all of the published models fit the data, to a good approximation, equally well. A poor fit of the experimental data is an indicator that at least one liquid does not fully meet the assumptions re-quired by the chosen model. Differences in the acid—base character of the solid surface appear to re-sult from the acid—base scale used by the model. The paper is intended to raise the awareness of the difficulties in assigning surface free energy and predicting wetting behavior.

In order to determine the surface free energy of a solid, it is necessary to measure contact angles of a variety of liquids on a given solid. The models investigated, here, include those proposed by Zisman, Kwok and Neumann; Owens and Wendt; van Oss, Chaudhury and Good, as well as Chen and Chang. In this chapter, the relative merits of these models are explored. The use of an overdetermined data set allows one to assess the statistical quality of the model and the estimated parameters. Liquids that show unusual behaviors (eg stick-slip) are unsuitable for determination of surface free energy. In this work, it will not be possible to examine the quality of each contact angle measurement. Rather, a relative assessment of various models is made. The results reported here indicate that no more than two adjustable parameters can be statistically justified. The Zisman, Kwok-Neumann models and a version of the van Oss, Chaudhury and Good model where the value of γ+ for the solid surface equals zero appear to be statistically viable. γ+ is the parameter that assesses the acidic character of the surface. These models yield similar values for the total surface free energy of the polymer surfaces.