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.

Adhesion properties between branched polyethylene (PE) and isotactic polypropylene (PP) were studied by a peel test and scanning electron microscopy. In this study, two types of branched PEs were used; one is a linear low density polyethylene (LLDPE) and the other is a high pressure low density polyethylene (LDPE). The adhesive strength of the LLDPE/PP is much higher than that of LDPE/PP. Furthermore, the formation of PE influxes between PP spherulites has a small effect on the adhesion. The dynamic viscoelastic measurements for the binary blends composed of branched PE and PP were also carried out to estimate the interfacial tension by using a rheological emulsion model proposed by Palierne. The interfacial tension is 1.0 mN for LLDPE/PP and 2.1 mN for LDPE/PP, suggesting that the interfacial thickness of LLDPE/PP is about twice that of LDPE/PP. The adhesive strength between branched PE and PP will be determined by the interfacial thickness, which represents the entanglements between two polymers. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 457–463, 1998
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2819981017%2970%3A3%3C457%3A%3AAID-APP5%3E3.0.CO%3B2-M

Polystyrene and polyethyleneterephthalate surfaces were exposed to helium, oxygen/helium and nitrogen/hydrogen plasmas singly and in combination. The treated surfaces were evaluated by water contact-angle measurements and by x-ray photoelectron spectroscopy. It was found that the oxygen and nitrogen tend to graft to common carbon atoms, to form amide groups. The water wettability was found to correlate with the fraction of electronegative atoms incorporated into the surface. © 1998 John Wiley & Sons, Ltd.
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291096-9918%28199808%2926%3A9%3C674%3A%3AAID-SIA414%3E3.0.CO%3B2-5

The interfacial adhesion of ultrahigh-modulus polyethylene (UHMPE) fibre–vinylester composites was improved by the oxygen plasma treatment of the UHMPE fibre. The chemical functional group formations on the UHMPE fibre surface by oxygen plasma treatment were analysed using diffuse reflectance Fourier transform infrared spectroscopy and the morphological changes of the UHMPE fibre surface by plasma etching were observed by scanning electron microscopy. The wettability enhancement by the chemical functional group formation and the mechanical interlocking due to the micropits were important factors in improving the interfacial adhesion of the UHMPE fibre–vinylester composites by oxygen plasma treatment. In order to investigate the relative importance of the two factors, wettability enhancement and mechanical interlocking, in the improved interfacial adhesion of the UHMPE fibre–vinylester composites, nitrogen plasma treatment was also performed. Nitrogen plasma treatment of the UHMPE fibre was proved to be effective in the formation of the micropittings and ineffective in the chemical functional group formation in comparison with the oxygen plasma treatment. The interlaminar shear strengths of the nitrogen-plasma-treated UHMPE fibre–vinylester composites showed almost the same value as those of the oxygen-plasma-treated UHMPE fibre–vinylester composites. The wettability enhancement and mechanical interlocking are important in the improvement of interfacial adhesion of UHMPE fibre–vinylester composites by plasma treatment and mechanical interlocking seems to be more important.

Surfaces of poly(ethylene terephthalate); PET, films were irradiated with Ar⁺ at 1 keV using various ion doses (ID) from 10¹⁴ to 10¹⁷ ions/cm² (isc) with and without an O₂ environment. The wettability of the modified surfaces of PET was determined by measuring the contact angle between water droplets and the modified surfaces. The modified surfaces were also characterized by AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy) for changes in the surface morphology, and the chemical composition and molecular structure, respectively. The contact angle decreased from 70° for unmodified surfaces to 45° for modified surface with ID = 10¹⁴ isc without O₂ and remained relatively constant with higher ID. The contact angle, however, reached a minimum value of 8° for modified surfaces with ID = 10¹⁶ isc with O₂. The improved wettability may be due to a combination of the formation of hydrophilic groups, chemical and molecular structural changes, physical structural or morphological changes, and increased roughness of the surface. The wettability of the modified surfaces also depended on the time of exposure to air. The wettability worsened with exposure time to air, but was revived by immersing the films into water. Possible mechanisms for the change of the wettability of the modified surfaces are given.

We have used Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) in combination with X-ray photoelectron spectroscopy (XPS) to study chemical changes taking place at the surface of pure cellulose paper samples treated in N2 plasma for periods of time up to 60 seconds. High resolution TOF-SIMS spectra permit the detection of various functionalities containing nitrogen, even following very brief (∼2s) plasma exposure. Correlations between chemistry and surface properties, such as water wettability, are presented and discussed.