The efficacy of vacuum ultraviolet irradiation for oxidizing the surface of cellulose fibers was compared to that of the conventional wet and dry processes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 357–361, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291099-0518%2819990201%2937%3A3%3C357%3A%3AAID-POLA13%3E3.0.CO%3B2-2

Polyvinylidenefluoride (PVDF) was irradiated by a keV Ar⁺ ion in O₂ environment for improving adhesion between PVDF and Pt, and reaction between PVDF and the ion beam has been investigated by X-ray photoelectron spectroscopy (XPS). The adhesion test between Pt and the modified PVDF was carried out by boiling test, in which the specimens were kept in boiling water for 4 h. Two failure modes (buckling up due to weak adhesion and crack formation due to strong adhesion) of Pt films have been observed in the system. Contact angle of PVDF was reduced to 31 from 75° by the irradiation of 1 × 10¹⁵ Ar⁺ ions/cm² with oxygen flow rate of 8 sccm. The surface of the irradiated PVDF became more rough as ion dose increased. The improved adhesion mechanism and identification of newly formed chemical species have been confirmed by Carbon 1s and Fluorine 1s X-ray photoelectron core-level spectra. The main reaction occurred at the irradiated PVDF surface is an ion-beam-induced oxidation accompanied with preferential sputtering of fluorine. Newly formed chemical species at interface are regarded as ester and carboxyl groups. Adhesion of the Pt–PVDF interface was improved by ion irradiation in O₂ environment. This improvement is originated from the presence of carbon—oxygen bonds on the irradiated PVDF surface. Comparison of failure modes on the irradiated PVDF at various conditions after the boiling test shows that adhesion of Pt film is largely affected by the product of ion-assisted reaction. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 41–47, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1097-4628(19990404)72:1%3C41::AID-APP4%3E3.0.CO;2-J

The deposition of plasma polymers on poly(acrylonitrile) (PAN) fibers has been investigated by X-ray photoelectron spectroscopy and dynamic contact angle measurements. Four polymerizable monomers were examined: tetrafluoromethane (TFM), perfluoropropene (PFP), tetramethyldisiloxane (TMS), and hexamethyldisiloxane (HMS). The deposition rate of TFM was undetectable and the treated fibers exhibited some fluorination and an increase of hydrophilicity, due to posttreatment oxidation after exposure to air. The deposition rate of PFP was quite slow and the formation of an incomplete fluorinated layer was observed, with a remarkable increase of the water advancing contact angles. TMS and more so HMS quickly formed continuous and reproducible polysiloxane layers having pronounced hydrophobic properties. The influence of the position of the fibers in the plasma reactor chamber was also investigated. A good uniformity of deposition was found when the fibers were placed at different points between the electrodes.

To analyze various approaches for the determination of surface free energies of solids from liquid-solid contact angles, comb-like polymers with controlled grafting rates and macromolecular structures have been synthesized. The surface free energy parameters were calculated from the contact angles of standard liquids on the solid surfaces. A mathematical approach of the so-called acid-base theory of adhesion was used to characterize the nucleophilic and/or electrophilic behavior of the polymeric solid surfaces. Thus, correlations were established between the macromolecular structures and the dispersive component of the surface free energy, on the one hand, and the acid and base components, on the other. The main conclusion is that the surface free energy components are relevant for the characterization of functional comb-like polymeric materials: the dispersive and base components increase with the number of grafted electron-donating groups, whereas the acid component decreases.

The modifications induced by excimer laser irradiation of poly(ether ether ketone) (PEEK) surfaces have been investigated as a function of the laser process parameters for laser fluences below the material ablation threshold. In the case of 193 nm laser treatment, a significant increase in the adhesion properties of PEEK was obtained due to the formation of new polar and reactive groups on the surface. The extent of these reactive groups has to be controlled since their presence in high concentration may also have a negative effect on the mechanical properties of the treated surface. Laser treatments using 248 nm radiation did not result in a significant increase in the adhesion properties of PEEK. This probably results from thermal degradation of the surface at this laser wavelength.

The effects of intense pulsed high power ion beam (HPIB) treatment of ultra-high strength polyethylene (UHSPE) fibers on the fiber/epoxy resin interface strength were studied. For this study, argon ions were used to treat Spectra^™ 1000 (UHSPE) fibers in vacuum. Chemical and topographical changes of the fiber surfaces were characterized using Fourier transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR), X-ray photoelectron spectroscopy (XPS), dynamic wettability measurements, and scanning electron microscopy (SEM). The fiber/epoxy resin interfacial shear strength (IFSS) was evaluated by the single fiber pull-out test. The FTIR-ATR and XPS data indicate that oxygen was incorporated onto the fiber surface as a result of the HPIB treatment. The wettability data indicate that the fibers became more polar after HPIB treatment and also more wettable. Although the total surface energy increased only slightly after treatment, the dispersive component decreased significantly while the acid-base component increased by a similar amount. SEM photomicrographs revealed that the surface roughness of the fibers increased following the HPIB treatment. The single fiber pull-out test results indicate that HPIB treatment significantly improved the IFSS of UHSPE fibers with epoxy resin. This enhancement in IFSS is attributed to increased roughness of the fiber surface resulting in mechanical bonding and in increased interface area, increased polar nature and wettability, and an improvement in the acid-base component of the surface energy after the HPIB treatment.

Polytetrafluorethylene (PTFE) was treated with N⁺ , O⁺ and C⁺ ion beams with energies of 20 and 30 keV at 5 mA/cm² current density in the pulse regime. Structural changes were studied by IR ATR, XPS, IR diffuse reflectance spectra and wetting methods. After treatment the PTFE surface became chemically active to isocyanate, acrylamide and epoxy reagents, which caused a change of interface interaction with active adhesives. The durability of the PTFE adhesion joint to an epoxy adhesive increases by more than 100 times. The ion beam treatment can be used to increase adhesion joint durability of PTFE.

The gas-phase reactions of ozone with CH bonds in methane, ethane, propane (secondary CH bond), and isobutane (tertiary CH bond) have been studied by semiempirical AM1 method. Reactions proceed through biradical transition state and lead to alkyl and hydrotrioxyl HOOO radicals. The latter immediately decomposes into molecular oxygen and hydroxyl radical HO. The formation of hydrotrioxides ROOOH in gas-phase reactions between ozone and hydrocarbons is shown to be highly improbable.

Ar plasma etched and Al metallised bisphenol A carbonate was analysed by mass spectroscopy, photoelectron spectroscopy (XPS), and scanning force microscopy (SFM). We mainly used a technical polymer (Makrolon 2808, Bayer) made by injection-moulding, as well as spin coated bisphenol A carbonate (n=1) and polycarbonate (PC) (n=115). The mass spectroscopy during the etching process shows the degradation of the PC in the form of carbon monoxide, carbon dioxide and methyl groups. The photoelectron spectroscopy shows in detail the surface modification after Ar plasma treatment and metallisation. The plasma induces a reduction of the carboxylic carbon (C 1s), a strong reduction of singly bonded oxygen (O 1s) and also a slight reduction of doubly bonded oxygen. After Al metallisation, a reaction of Al with the oxygen groups and an interaction with the aromatic system is documented. Ar plasma etching increases the chemical interaction of Al mainly with the aromatic carbon. The X-ray photoelectron spectroscopy of metallised PC under different initial conditions shows a strong influence of incorporated water in the PC bulk that cannot be seen by XPS on uncoated PC. The O 1s signal increases during metallisation and results in an oxidation of Al probably caused by the fact that the hydrophobic surfaces becomes hydrophillic. Temperature-dependent XPS was done on technical PC samples and on spin coated samples (n=1, n=115) and supports the influence of the bulk state for the Al–PC interface. For n=1 carbonate, a diffusion of Al into the PC volume was observed. The SFM measurements showed a roughening effect on the nanometer scale even after short treatment times. Al can be seen as a weakly bound cluster on the virgin PC, and if a pre-etching is done, Al seems to grow as a good wetting film. The adhesion force of Al films on PC without any influence of the volume can be explained by the chemical bonding of Al to the carboxylic and aromatic systems. The adhesion can be increased by plasma pre-treatment. A breakdown of the adhesion on technical PC is probably induced by a reaction of Al with mobile intercalated gas, that is enriched near the surface after Al coating.

The Lifshitz–van der Waals/acid-base approach proposed by van Oss et al. is found to yield inconsistent solid surface tensions and components from contact angles, for fluorocarbon, polystyrene, and poly(methyl methacrylate) solid surfaces. It is also shown that the approach cannot predict the correct interfacial tensions of all liquid–liquid pairs in question: the predicted interfacial tensions range from 34% lower to 112% higher than the experimental values. Thus, the usefulness of the approach for surface tension components and interfacial tensions is open to question. The liquid surface tension components postulated since 1986 are also summarized.

Using the molecular-kinetic theory of wetting, we analyze the dynamic contact angle of a sessile drop of squalane spreading spontaneously on Langmuir−Blodgett multilayer substrates (behenic acid on glass). This allows the effect of microscale roughness on the parameters appearing in the theory to be determined. In particular, it is shown that the jump frequency of liquid molecules at the wetting line decreases with microroughness, supporting the idea that surface defects induce additional pining potentials. The increase in pinning potential can be explained in terms of a linear increase in the activation free energy of wetting with increasing RMS microroughness.

A droplet of liquid placed on a flat high-energy solid surface spreads to give a thin film so that no macroscopic droplet shape exists. On a chemically identical solid surface with only the geometry changed to a cylinder, the same droplet can have an equilibrium conformation. When the equilibrium conformation is of a barrel type, the profile of the droplet changes rapidly in curvature as the three-phase contact line is approached and the direct measurement of the contact angle is difficult. This work considers the theoretical profile for barrel-type droplets on cylinders and discusses how the inflection angle in the profile depends on droplet parameters. Experimental results are reported for poly(dimethylsiloxane) oils on a range of fiber surfaces and these are used to estimate the equilibrium contact angle from the inflection angle. The drop radius and volume dependence of the inflection angle is confirmed.

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.

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/ethyl methacrylate, 30/70) P(MMA/EMA, 30/70) copolymer were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It was found that five liquids yield nonconstant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining seven 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_l|Kv cos θ depends only on γ_l|Kv for a given solid surface or solid surface tension). This contact angle pattern is in harmony with those from other methacrylate polymer surfaces previously studied.^45,50 The solid–vapor surface tension calculated from the equation-of-state approach for solid–liquid interfacial tensions¹⁴ is found to be 35.1 mJ/m², with a 95% confidence limit of ± 0.3 mJ/m², from the experimental contact angles of the seven liquids. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2039–2051, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1099-0488(19990815)37:16%3C2039::AID-POLB8%3E3.0.CO;2-O

Recent progress in the correlation of contact angles with solid surface tensions are summarized. The measurements of meaningful contact angles in terms of surface energetics are also discussed. It is shown that the controversy with respect to measurement and interpretation of contact angles are due to the fact that some (or all) of the assumptions made in all energetic approaches are violated when contact angles are measured and processed. For a large number of polar and non-polar liquids on different solid surfaces, the liquid–vapor surface tension times cosine of the contact angle, γ_lvcosθ, is shown to depend only on the liquid–vapor surface tension γ_lv, and the solid–vapor surface tension γ_sv when the appropriate experimental techniques and procedures are used. Equations which follow these experimental patterns and which allow the determination of solid surface tensions from contact angles are discussed. Universality of these experimental contact angle patterns is illustrated; other reasons which may cause data to deviate from the patterns slightly are discussed. It is found that surface tension component approaches do not reflect physical reality. Assuming the fact that solid surface tension is constant for one and the same solid surface, experimental contact angle patterns are employed to deduce a functional relationship to be used in conjunction with Young's equation for determining solid surface tensions. The explicit form of such a relation is obtained by modifying Berthelot's rule together with experimental data; essentially constant solid surface tension values are obtained, independent of liquid surface tension and molecular structure. A new combining rule is also derived based on an expression similar to one used in molecular theory; such a combining rule should allow a better understanding of the molecular interactions between unlike solid–liquid pairs from like pairs. Existing static contact angles for 34 different types of solid surfaces from Zisman et al. are evaluated in terms of their solid surface tensions using experimental contact angle patterns. A FORTRAN computer program has been implemented to automate these procedures. It is found that literature contact angles do not have to be discarded completely; they can be used to determine solid surface tensions, with caution. The surface tensions for the 34 solid surfaces from Zisman et al. are also reported.

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 general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects. A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges. It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation. Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results. The link between the stochastic process and observable quantities is derived.

A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns. Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured. Thus each discharge event is described by the two quantities, charge and phase angle. The modelling of the observation process is explicitly derived for this case. However, the used method can easily be generalized to other types of PD measurements.

The proposed approach yields new possibilities for the interpretation and analysis of PD patterns. Features of PD patterns can be derived analytically from the process parameters. Conversely, quantitative information about the discharge physics can be gained from measured patterns. Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly.

Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air).

The influence of the surface modification of pressure-sensitive adhesive tapes on their adhesion behavior has been investigated. PBA [poly(butyl acrylate)] and PIB [poly(isobutylene)] adhesives were chosen as pressure-sensitive adhesives and nitrogen plasma was used for the surface modification of the adhesives. The peel force of PBA or PIB adhesive/stainless steel joints was evaluated. The nitrogen plasma treatment showed large effects on the adhesion behavior of both the PBA and the PIB adhesives. The peel force for the PBA adhesive/stainless steel joint decreased by 57 times as a result of the nitrogen plasma treatment and that for the PIB adhesive/stainless steel joint increased by 2.2 times. There are essential differences in the modification reactions caused by the nitrogen plasma between the PBA and PIB adhesives. For the PBA adhesive, cross-linking reactions occurred among the PBA polymer chains and the surface was hardened. For the PIB adhesive, degradation reactions occurred and products with a low molecular weight were formed on the surface. These differences are due to the different responses of the PBA and PIB adhesives towards the nitrogen plasma. The mechanism of the changes in adhesion behavior caused by the nitrogen plasma is discussed.

Acid–base interactions across interfaces are shown to have predictable influences on adhesion. The history of this development, and methods to assay the acid–base character of solvents, polymers and a variety of powders and fibers are reviewed briefly. Recent studies are described that demonstrate directly how acid–base interactions influence both ‘fundamental’ and ‘practical’ adhesion.

Lifshitz-van der Waals (LW) and Lewis acid-base (AB), together with electrostatic (EL) forces are the non-covalent forces acting in adhesion in condensed phase media, such that the work of adhesion, VEadh= WLW+ WAB+ WEL. In the case of serum albumin (SA) and glass surfaces or silica particles, on a macroscopic scale, WEW> 0, WAB< 0 and WEL< 0, so that W'ddh is negative, ie repulsive. Nonetheless, in aqueous media, at neutral pH, SA adheres to glass surfaces, as well as to silica particles. It may be hypothesized that on a microscopic level, negatively charged, electron-donating SA moieties, located on prominent sites with a small radius of curvature, can penetrate the macroscopic repulsion field and bind to electron-accepting cations imbedded in the glass surfaces (Ca ions) or in silica particles (Si ions). The correctness of the hypothesis is supported by the fact that all adhering SA can be desorbed from, say, silica particles with Na2-EDTA. Furthermore, energy vs. distance diagrams demonstrate that the more prominently located SA sites with a small radius of curvature should indeed be able to overcome the macroscopic repulsion field and to adhere locally to microscopic cationic sites in the glass or silica. Thus, energy vs. distance balances of the extended DLVO type (including AB as well as LW and EL forces), combining macroscopic and microscopic interactions, can be used to predict adhesion in complex systems.

The surface of isotactic poly(propylene) foils was oxidized by corona discharge plasma in order to improve the adhesive characteristics. The dependence of the degree of surface oxidation on either the current density or the time of exposure was determined. Rapid increase of the free surface energy was observed at current densities ranging from 0.4 to 0.6 mA. A reduction of the exposure time of discharge at the foil surface has an effect similar to the reduction of current density. The change of free surface energy of extruded poly(propylene) was rapid, especially during the first 24 h, while for modified biaxially oriented poly(propylene) the decrease of free surface energy was substantially slower.