Extended chain polyethylene fibres have been treated in ammonia and oxygen lo-pressure gas discharges (plasmas) in order to enhance adhesion to epoxy and unsaturated polyester resins, respectively, and thus significantly improve fibre/resin interfacial properties in fibre-reinforced polymer composites. Ammonia plasma treatment results in the incorporation of amine functional groups onto the fibre suface. The treated fibre surface has been analysed using XPS and spectrophotometric techniques. Extended chain polyethylene/epoxy composites made from ammonia, plasma-treated fibres show a marked increase in interlaminar shear strength over composites made from untreated, corona-treated or oxygen plasma-treated fibres. The increase in fibre/resin adhesion after ammonia plasma treatment is confirmed by SEM observations of fracture surfaces, which show clean interfacial fracture surfaces in composites made from treated fibres. Fibres modified by oxygen plasma treatment contain a significant concentration of carbon-oxygen functionalities, which contribute to the polarity of the surface and hence increase wet-out by unsaturated polyester resins. The concentration and nature of carbon-oxygen species on the fibre surface have been determined by XPS. Pull-out tests on multifilament yarns embedded in a polyester resin confirm the high fibre/matrix adhesion achieved with the oxygen plasma-treated fibres compared to corona-treated or untreated fibres. Tensile properties of the fibres are reduced significantly after prolonged treatment in an oxygen plasma, while in an ammonia plasma the fibre strength is unaffected.

New approaches for electroless plating of nonconductive polymers or polymer-based materials are described. In this work, polyimide substrates were surface-functionalized (i) in nitrogenated (ammonia at reduced pressure) and oxygenated (air at atmospheric pressure) atmospheres under assistance of vacuum-ultraviolet (VUV) irradiation (use of a xenon silent discharge excimer source) or (ii) directly in air at atmospheric pressure using a dielectric-barrier discharge (DBD) device. After functionalization, the substrates were “activated” by dipping in a dilute acidic PdCl₂ solution or by spin-coating of a thin metal-organic film (from a solution of palladium acetate (PdAc) in chloroform). The catalytic activity of the so-deposited palladium species toward the electroless deposition of nickel was studied before and after a VUV post-irradiation (in air at atmospheric or reduced pressure) with a view to understanding better the role of the reducer (sodium hypophosphite) within the electroless bath.

This work confirms the specific interest of grafting nitrogenated functionalities onto polymer surfaces for attaching covalently the palladium-based catalyst (in particular in the case of the PdCl₂ route), forming thus strong Pd - N - C bonds at the metal/polymer interface. This results from the strong chemical affinity of palladium toward nitrogen. On the other hand, when oxygenated functionalities are surface-grafted, the conventional two-step procedure using SnCl₂ and PdCl₂ solutions can be proposed due to the strong chemical affinity of tin toward oxygen. The Ni deposits obtained under these different conditions pass the standard Scotch^®-tape test and, therefore, exhibit a good practical adhesion. For this same purpose, it is interesting to note that the DBD treatment operating in air at atmospheric pressure causes an increase of the surface roughness and, therefore, an improvement in adhesion of metallic films when their initiation is catalyzed through the PdAc route. In addition, this work demonstrates that extensive research still has to be performed to understand and improve the Ni/polymer adhesion when the PdAc route associated with a VUV irradiation is considered.

As shown from the literature data, plasma treatments of polymer materials create chemical and morphological surface modifications which are strongly dependent on the experimental conditions and on the methods and apparatus configurations employed. In the present work, poly(tetrafluoroethylene) (PTFE) substrates were treated by RF plasma (RIE mode) in various gaseous atmospheres (H2, He, Ar, O2, N2, NH3). The main objective was to compare, under similar experimental conditions, the capabilities of these different gases to modify the morphology of the PTFE surface and to graft specific chemical functionalities for a subsequent metallization through an electroless process. The relevant chemical modifications were characterized by XPS and surface energy measurements while the morphology changes were observed by SEM. Under similar experimental conditions (treatment time, working pressure) and with the reactor operating in the RIE mode, defluorination capabilities of the plasma treatments vary as a function of the gaseous atmosphere according to the following sequence H2 > Ar, NH3 > N2 > He ≫ O2. In addition, O2 and He plasmas are shown to strongly etch the PTFE surface. The very low ability of O2 plasmas to graft oxygenated functionalities is largely due to their strong etching power. A discussion is also focused on the complex interactions between nitrogenated plasmas and PTFE surfaces. Especially, it is shown that nitrogen is not grafted in the same chemical form after an NH3 plasma as after a N2 plasma. Furthermore, after treatment in non-nitrogenated and non-oxygenated plasmas (He, Ar, H2) both nitrogen and oxygen species are grafted on the polymer surface, probably in the molecular form. Such a grafting occurs on venting the reactor with dry nitrogen and on exposing plasma-treated samples to ambient atmosphere.

This paper deals with the electroless metallization (nickel plating) of poly(tetrafluoroethylene) substrates which were previously submitted to RF glow-discharge plasmas in various gaseous atmospheres (H2, He, Ar, O2, N2, NH3) and subsequently to sensitization/activation or direct activation processes in order to chemisorb palladium which is the catalyst of the plating reaction. As shown in previous works and confirmed in this one, the use of the conventional sensitization/activation treatment (immersion of the plasma-treated samples successively in acidic tin chloride and palladium chloride solutions) is made possible due to the strong chemical affinity of tin to oxygen. On the other hand, when nitrogenated species are grafted on the PTFE surface, the chemisorption of the catalyst can be directly accomplished using only a simple acidic palladium chloride solution. It is shown, in more detail, in this paper that O2 and H2 plasmas cannot be used to deposit electroless Ni films through the conventional sensitization/activation process. This is due to the negligible oxygen content grafted onto the PTFE surface (case of O2 plasma), and to the strong crosslinking of this same surface (case of H2 plasma) even though the amount of oxygen grafted during the post reactions in air is relatively high. On the other hand, bright and adherent Ni deposits are obtained by using either He or N2 plasmas via the conventional two-step process again due to the oxygen species grafted during the post-reactions in air, or by N2 and NH3 plasmas via the direct one-step process due to the nitrogen species grafted during the plasmas themselves.

Metallization techniques based on electroless plating are widely used to coat polymer materials in a large variety of technological applications. Traditionally, dilute tin chloride (SnCl₂) and palladium chloride (PdCl₂) solutions in HCl are used to render the surface of such non-conductive substrates catalytically active towards metal deposition in the electroless plating solution. In the present work, it is shown how X-ray photoelectron spectroscopy has allowed to monitor the chemical and compositional surface modifications of polymer substrates (polypropylene, polycarbonate) subjected to plasma and UV or VUV irradiation (use of ArF^* excimer laser and Xe₂^* incoherent excimer lamp, respectively) in oxygenated (O₂, air) or nitrogenated (N₂, NH₃) atmospheres, as well as to understand the mechanisms of the catalyst (palladium species) chemisorption on the so-grafted surfaces through the use of a simple dilute palladium chloride solution in HCl. In addition, this work has allowed to bring into light the precise role that the reducer plays (sodium hypophosphite) present in the electroless nickel bath. In short, this research has been successful in allowing the development of new approaches for the electroless metallization of polymer surfaces.

The electron beam-initiated surface modification of films prepared from various blends of low-density polyethylene (LDPE), ethylene vinyl acetate (EVA), and ditrimethylol propane tetraacrylate (DTMPTA) was carried out over a range of radiation doses (20-500 kGy) and concentrations of DTMPTA. The films were characterized by Fourier transform infrared-attenuated total reflectance (FT-ATR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), contact angle measurements, and peel adhesion. The printability of the films was also measured. FT-ATR and XPS revealed that the surface polarity of the films made from a 50 : 50 blend of LDPE and EVA increased up to a radiation dose of 100 kGy, compared with the unirradiated sample. The polarity decreased after 100 kGy radiation. Surface pitting and roughness were observed in the SEM photomicrographs of the same films, irradiated at higher radiation doses. Higher values of the surface energy were obtained at 100 kGy for the samples without DTMPTA and for the samples containing 3 wt% DTMPTA. Excellent printability was observed for all the films irradiated above an irradiation dose of 20 kGy. The data on the printability and peel adhesion of the irradiated films could be explained by surface energy, XPS, and SEM results.

Self-assembled monolayers (SAMs) of alkylsiloxanes on elastomeric PDMS (polydimethylsiloxane) were used as model systems to study interactions between surfaces. Surface free energies (γsv) of these chemically modified surfaces were estimated by measuring the deformations that resulted from the contact between small semispherical lenses and flat sheets of the elastomer under controlled loads. The measured surface free energies correlated with the surface chemical compositions of the SAMs and were commensurate with the values estimated from the measurements of contact angles. This study provides direct experimental evidence for the validity of estimates of the surface free energies of low-energy solids obtained from contact angles.

The reversible work of adhesion (W) is the free energy change per unit area in creating an interface between two bodies (Fig. 1). The work W is related to the intermolecular forces that operate at the interface between two materials, eg, an adhesive and an adherend. However, in practice, the reversible work of adhesion may be obscured by other factors (eg, mechanical interlocking, interdiffusion) because it is always a few orders of magnitude lower than the measured adhesive joint strength [1, 2]. One important contribution to practical joint strength is the energy loss due to irreversible deformation processes within the adhesive. Nevertheless, Gent and Schultz [3] showed using peel strength measurements that viscoelastic losses were proportional to the reversible work of adhesion. For this reason, one of the important tasks is to determine the nature of interfacial chemical and physical forces and to understand how they control the reversible work of adhesion.

This paper reviews the background to the theory of Lewis acid-base (AB) interactions in adhesion, adsorption, wetting and mixing of polymers and other materials (pigments, fillers, fibres, etc.). These specific materials interactions require the revision of old concepts («polar» interactions) and the development of new analytical techniques and methodologies. Four of the most currently used techniques to characterize AB interactions are described: contact angle measurements, inverse gas chromatography. X-ray photoelectron spectroscopy, and atomic force microscopy.

Properties of polymer surfaces are very sensitive to minute quantities of impurity and different preparation procedures. Baier and Zisman reported that wettabilities of polypeptides and polyamides such as Nylon 2,6, and 11 are different when the polymers are cast from different solvents. They attributed this difference to the existence or absence of the surface hydrogen bonding site. They also proposed a Zisman plot split criterion for the recognition of hydrogen-bonding functionality in a polymer specimen's surface. However, we found that this criterion does not always work. We then apply our new model for acid-base interactions to interpret their data. The model fits data well. Moreover, it is noticed that cases with exposed surfacehydrogen bonding sites are all bipolar and surfaces without hydrogen bonding sites are all monobasic.

This paper describes two methods of modifying the surface energy of a plasma-polymerized film. One method is to use diphenylamine (DPA) to stabilize the surface energy increase of the polymer caused on exposure to air or a polar liquid. Another method is to use heptafluorobutyric anhydride (HFBA) to reduce the surface energy of aged (oxidized) film. The HFBA-treated film displays the same surface energy (20 mJ/m²) as the freshly prepared film. It is, however, much more stable than the as-polymerized film in propylene glycol. Other silylation and fluorinated esterification reagents were found to be much less effective. The changes in surface energy were caused by changes in the chemical structure. The chemical changes were analyzed by infrared (IR) spectroscopy and electron spectroscopy for chemical analysis (ESCA). These changes were either caused by oxidation of the film in air, water, and propylene glycol or were induced by fluorination of the oxidized film. The polymer used in this study is a copolymer of perfluoropropane (PFP) and 3,3,3-trifluoropropylmethyldimethoxysilane (TFPS). Other physical properties, such as solubility, thickness, coefficient of friction, adhesion, and thermal transitions of the polymer, have also been studied.

Electrons produced in atmospheric pressure corona discharges are used for a variety of beneficial purposes including the destruction of gaseous contaminants, and surface treatment. In other applications, such as electrostatic precipitators and photocopiers, unintended reactions such as ozone production and deposition of silicon dioxide are detrimental. In both situations, a kinetic description of the electron distribution in the corona plasma is required to quantify the chemical processes. In this paper, the electron density and energy distributions are numerically determined for a positive dc corona discharge along a wire. The electron density distribution is obtained from the 1-D charge carrier continuity equations and Maxwell's equation. The non-Maxwellian electron kinetic energy distribution is determined from the Boltzmann equation. The effects of wire size (10-1000 μm) and current density (0.1–100 μA/cm of wire) on number density and energy distribution of electrons are presented. With increasing current, the electron density increases, but the thickness of the plasma and the electron energy distribution are not affected. Smaller electrodes produce thinner plasmas and fewer, but more energetic electrons, than larger wires. The effect of electrode size on the electron-impact chemical reaction rate is illustrated by the rates of dissociation and ionization of oxygen and nitrogen.

The rate of production and the spatial distribution of ozone in the negative DC corona discharge are predicted with a numerical model. The results are compared to prior experimental data and to results previously presented by the authors for the positive corona discharge. In agreement with experimental data, ozone production rate in the negative corona is an order of magnitude higher than in the positive corona. The model reveals that this significant difference is due to the effect of discharge polarity on the number of energetic electrons in the corona plasma. The number of electrons is one order of magnitude greater and the chemically reactive plasma region extends beyond the ionization region in the negative corona. The paper also extends our prior modeling effort to lower velocities where the Joule heating reduces ozone production. The magnitude of the reduction is characterized by a new dimensionless parameter referred to as the electric Damkohler's third number(Da_III–e).

The surface of poly(ethylene terephthalate) (PET) film was modified by low-temperature plasma with O₂, N₂, He, Ar, H₂, and CH₄ gases, respectively. After being treated by low-temperature plasma, their surface wettability and chemical composition were investigated by means of electron spectroscopy for chemical analysis (ESCA) and contact angle measurement. The result shows that the surface wettability of PET can be improved by low-temperature plasma, and the effect of the modification is due mainly to the kind of the gases. Mainly because of the contribution of hydrogen bonding force γ[STACK]^c_S[ENDSTACK], the surface wettability of PET treated with O₂, N₂, He, and Ar plasma for a short time (3 min) increase sharply, and the surface wettability is also improved by H₂ plasma treatment; but the CH₄ plasma treatment does not improve the wettability of PET. ESCA shows that the effect of wettability of PET is tightly related to the presence of polar functional groups that reside in the outermost surface layer of PET. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1327–1333, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2819990606%2972%3A10%3C1327%3A%3AAID-APP13%3E3.0.CO%3B2-0

The surface free energy and surface structure of poly(tetrafluoroethylene) (PTFE) film treated with low temperature plasma in O₂, Ar, He, H₂, NH₃, and CH₄ gases are studied. The contact angles of the samples were measured, and the critical surface tension γ_c (Zisman) and γ_c (max) were determined on the basis of the Zisman's plots. Furthermore, the values of nonpolar dispersion force γ^a_s, dipole force γ^b_s, and hydrogen bonding force γ^c_s to the surface tensions for the plasma-treated samples were evaluated by the extended Fowkes equation. Mainly because of the contribution of polar force, the surface free energy and surface wettability of PTFE film which was treated with H₂, He, NH₃, Ar, and CH₄ for a short time increased greatly. Electron spectroscopy for chemical analysis (ESCA) shows that the reason was the decrease of fluorine and the increase of oxygen or nitrogen polar functional group on the surface of PTFE. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1733–1739, 1997
https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1097-4628(19970328)63:13%3C1733::AID-APP4%3E3.0.CO;2-H

The effect of ion implantation including ion species (N₂ and C₃H₈⁺) and the fluences (1x10¹⁴-5x10¹⁵ ions/cm²) on the surface energy of ultrahigh molecular weight polyethylene (UHMWPE) were investigated. The total surface energy increases significantly after implanting with the fluence of 1x10¹⁴ ions/cm² regardless of ion species, then, the total surface energy slightly increases for N₂⁺ implanted UHMWPE and decreases slightly for C₃H₈⁺ implanted UHMWPE with a further increase of fluence. The structural changes of UHMWPE with different fluence for different ion species are very similar. The linear chains of UHMWPE are damaged and cross linking is generated after implantation. As the fluence increases, the polymer surface becomes more disordered, and the surface becomes hydrogenated amorphous carbon when the fluence exceeds 1x10¹⁵ ions/cm². The surface roughness increases with the increase of the fluence regardless of ion implantation species.

A polyester film surface was graft-polymerized with various water-soluble monomers by a combination of plasma pretreatment and photoirradiation. The grafted surfaces showed strong adhesion to another non-grafted or grafted film when brought into direct contact in the presence of water and subsequently dried. The adhesion force depended on the hydrophilicity of the adhering surfaces and the graft density. The film having a larger graft density generally showed stronger adhesion in the final stage of drying, but it took longer to achieve high adhesion because of the larger amount of water existing in the interfacial region between the two surfaces. On the other hand, substantial adhesion was obtained almost instantaneously upon contact when one was grafted with an anionic polymer and the other grafted with a cationic polymer. Adhesion between similarly charged surfaces was very weak at the beginning of drying, probably because of the electrostatic repulsion between the charged groups.

This chapter discusses axisymmetric drop shape analysis (ADSA) and its application. It provides an account of these ADSA methodologies. It contains a description of the numerical algorithms and their implementation. The applicability of ADSA is illustrated extensively for the investigation of surface tension measurements with pendant and sessile drops and contact angle experiments with sessile drops using both axisymmetric drop shape analysis - profile (ADSA-P) and axisymmetric drop shape analysis - diameter (ADSA-D). The advantages of pendant and sessile drop methods are numerous. In comparison with a method such as the Wilhelmy plate technique, only small amounts of the liquid are required. Drop shape methods easily facilitate the study of both liquid-vapor and liquid-liquid interfacial tensions. Also, the methods have been applied to materials ranging from organic liquids to molten metals and from pure solvents to concentrated solutions. There is no limitation to the magnitude of surface or interracial tension that can be measured: The methodology presented in this chapter works as well at 10³ mJ/m ² as at 10 ^-3 mJ/m ².

In this paper, the corona charge stability in electret polytetrafluoroethylene Teflon film is investigated after the film is treated by radio-frequency plasma. It is found that the charge stability depends strongly on the plasma composition and the film exposure to plasma, especially for negative charge. When a non-metalized film is held horizontally on the ground holder, i.e. with one side facing the plasma, oxygen plasma treatment achieves a superior negative charge retention on the front side, while its rear side retention decreases significantly. Under the same conditions in oxygen/helium and helium plasmas, the charge stability also increases but the potentials are lower compared with pure oxygen plasma after annealing. In a hydrogen plasma, the stability only slightly enhances. If the film is held vertically on the holder, so that both sides contact the plasma, the surface potential on both sides decreases dramatically and arrives at a few volts within 2 min, after annealing at 170°C. By Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), we conclude that oxidation formed on the front side is responsible for increasing the stability of negative charge. The positive carriers, generated in the film during plasma treatment, recombine with charge from corona charging and causes the surface potential in the rear side of the horizontal non-metalized film, or in both sides of the vertical non-metalized film, to discharge after heating.

In recent work, we found that the stability of the negative corona charge in radio frequency plasma treated polytetrafluoroethylene (PTFE) films (18 µm thickness) strongly depends on the plasma sources, the exposure time and the condition of the film in the plasma, i.e. the film orientation on the holder and whether the film is one-sided metallized or non-metallized, as well as the film side for corona charged. Using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, we conclude that two factors affect the negative charge stability: oxide formed on the surface and positive charges trapped in the film. The oxides serve to retain the negative corona charges and the plasma-generated positive charges recombine with the negative corona charges and cause the corona charge discharge after heating.

An 18 μm nonmetallized polytetrafluoroethylene (PTFE) film is treated in radio frequency (RF) plasma before a point-to-grid corona charged. The isothermal (170°C) surface potential measurement shows that the surface charge stability is significantly dependent on the plasma sources and treatment conditions. Oxygen (O₂), oxygen/helium (O₂/He) mixture gases and helium (He) plasma treatment enhance the film negative charge stability significantly but not hydrogen (H₂) plasma. Electron spectroscopy chemical analysis confirms that this superior negative charge retention for O₂ plasma treatment is a result of the high concentration of oxide groups on the subsurface during the plasma treatment.

Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections, such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solid-solid or solid-liquid interfaces after they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy-gamma by the Johnson-Kendall-Roberts theory, for example, F = 3-pi-R-gamma for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.

Ozone application to an extrudate web has been used for over a decade to enhance adhesion of polymer to the substrate in the extrusion coating process. However, to date, ozone’s effectiveness has not been quantified by published statistical data. A two level fractional factorial design consisting of 64 experimental runs was utilized to study the effects of ozonation and other variables (nine total variables) thought to affect adhesion and heat seal strength in the extrusion coating process. The 64 experimental runs were performed by coating LDPE (0.923 g/cc, 10 g/10 min) onto a 40-pound Natural Kraft paper. Logistic regression was utilized to study the factors affecting adhesion in extrusion coating and ordinary linear regression techniques were used to quantify the affects of the variables on heat seal strength. The coating line variables found to have a statistically significant effect on adhesion and heat seal strength were corona treatment of the substrate, melt temperature, air gap, line speed, coating weight and ozone treatment of the extrudate.

A “blister test” technique has been used to determine the fracture surface energy of a range of adhesive joints formed using a polyurethane adhesive and a range of solid substrates. For each adhesive pair examined the work of adhesion was calculated from the contact angles formed by liquids for which the polar and dispersion force components of the surface tension are known. For each adhesive pair, the solubility parameter of adhesive and substrate were determined by swelling measurements in a range of liquids. Although cohesive failure of the joints was observed for some of the pairs for which the solubility parameters were matched, this was not true for all such pairs and an explanation of this behaviour has been sought in a new calculation of the volume interaction component of the molecular interaction parameters.