XPS has been used to elucidate the mechanisms of surface modification of low density polyethylene by electrical (“corona”) discharge treatment and by chromic acid treatment. The use of derivatisation techniques for improving the precision of functional group analysis is described. These techniques also allow the role of specific interactions in adhesion to discharge treated surfaces to be investigated. The role of residual Cr on the adhesion of deposited metal to polymer surfaces is discussed.

Poly(ethylene terephthalate) (PET) film has been discharge-treated under controlled conditions and the resulting surface modifications analysed via X.p.s. (ESCA), contact angle and surface energy measurements. Changes in surface properties have been followed as a function of ageing time. These measurements have been correlated with the adhesive properties of the treated surfaces using autoadhesion (treated-treated seals) as the probe. Discharge treatment introduces phenolic -OH and carboxylic acid -COOH groups into the surface resulting in increased wetting and much enhanced autoadhesion via hydrogen bonding of phenol groups to carbonyl groups. Much chain scission also occurs; the low molecular weight material is easily removed by washing and migrates into the film on ageing. The new functionalities in relatively immobile chains slowly reorientate and internally H-bond. The former process is largely responsible for the wettability change on aging, the latter for the loss of adhesive properties.

A series of low-density polyethylene (LDPE) surfaces, chemically modified using a number of oxidative techniques employed for adhesion enhancement (pretreatments), have been studied by time-of-flight (ToF) SIMS and XPS. The methods consisted of corona discharge, flame, electrochemical, chromic acid, acid dichromate and acid permanganate treatment. All except flame treatment were performed under mild and fairly severe conditions to yield a range of surface chemistries. The XPS analysis, using high energy resolution and a refined approach to C 1s curve-fitting, provided some new insights into the quantitative assessment of the type and concentration of functional groups. Both positive and negative ion ToF-SIMS spectra were obtained at high mass resolution. The oxygen-containing fragments were identified by accurate mass analysis and subjected to a detailed comparison with the XPS results. No convincing relative intensity correlations could be identified that would allow particular secondary ion fragments to be associated strongly with particular functional groups (in this multi-functional surface situation). Inorganic residues resulting from wet chemical treatments were also investigated and here the two techniques were found to be more complementary. Copyright © 2003 John Wiley & Sons, Ltd.
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/sia.1515

X-ray photoelectron spectroscopy showed that a normal flame treatment caused a high level of oxidation in low-density polyethylene. 0.02% of the antioxidant 2,6-ditertbuty-p-cresol did not reduce the degree of oxidation or the level of adhesion in contrast to the extrusion of low-density polyethylene. It is estimated that the depth of oxidation is between 40 and 90 Å which is much less than for a moderate chromic acid treatment or with extrusion. There were no significant changes in the XP-spectra or adhesion levels of flame treated samples after 12 months.

The previously observed, but unexplained, deleterious effect of high relative humidity on the efficiency of electrical (‘corona’) discharge treatment for rendering polypropylene film printable has been re-examined. The effect of film temperature during treatment has also been studied. A consistent explanation of both effects based on the degree of surface coverage by physically adsorbed water is put forward, supported by X-ray photoelectron spectroscopy analysis of treated film surfaces.

Films of poly(methyl methacrylate) (PMMA) of both medium and high molecular weight have been prepared by casting onto clean glass. The difference in water contact angle of the surface originally in contact with glass. and air and the variation over time of this parameter have been studied. By use of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and, particularly, static secondary ion mass. spectroscopy (SSIMS), it has been shown that migration of low molecular weight impurities from the bulk of the film to the film/air interface is responsible for the contact angle behavior.

Specific reactions for the derivatization of oxygen-containing functional groups in polymer surfaces have been developed in order to improve the precision of analysis by X-ray photoelectron spectroscopy (xps). These have been used to probe the chemical composition of low density polyethylene (ldpe) surface-modified by electrical discharge treatment. Simultaneously the effect of derivatizing particular groups on the auto-adhesive behaviour of these surfaces has been examined. Two independent specific interaction mechanisms have been identified.

A method is described for measuring the level of pretreatment of polyethylene films in terms of critical surface tensions. Drops of water-dioxan mixtures of various surface tensions are placed upon the pretreated films and their critical surface tensions assessed from the spreading behaviour of the liquids.

A suggestion is made for using this method as a process control test.

A review is presented of the adhesion between polymers with particular emphasis on the processes that occur during failure at the level of polymer chains and how these processes relate to the macroscopic interface toughness. The same processes at the chain level, pull-out and scission, occur in both glassy polymers and elastomers, but the two classes of material are considered separately because their deformation processes around a crack tip are so different. Emphasis is placed on the work in which the author has participated and so the review makes no attempt to be an unbiased survey of the field.

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus. Scanning electron and optical microscopic observations have been used to examine the microscopic basis for these results, which are compared with results previously obtained for aramid/epoxy and extended-chain polyethylene/epoxy composites. It is concluded that the increased interlaminar shear and flexural properties of vinylester matrix composites are due to improved wetting of the surface-treated fibres by the vinylester resin, rather than covalent chemical bonding.

Joining of thermoplastic composite primary structures is an area of great importance in the aerospace industry. In order to achieve a strong and durable joint, an effective surface treatment is needed. Preadhesion surface treatment of thermoplastic composites is limited due to their chemical inertness to aggressive chemicals or mechanical treatments. The feasibility of using a new technique of ArF UV (193 nm) excimer laser irradiation as a preadhesion treatment of PEEK (polyaryletheretherketone) reinforced with carbon fibers is demonstrated. This method presents an alternative to other limited and polluting conventional surface treatment methods, such as sand blasting, etching or welding. Experimental results indicated that laser preadhesion treatment significantly improved the shear and tensile adhesion strength of various structural epoxy adhesives FM 300 2K and AF 163-2 bonded to PEEK composite adherends compared with untreated and SiC blasted substrates. Best results were obtained with laser pulse fluences of 0.18 or 1 J/Pcm2. Shear strength of laser treated PEEK composite joints improved by 450% compared with that of untreated PEEK composite and by 200% compared to SiC blasted adherends, at ambient and at—30 C and 120 C temperatures. An order of magnitude improvement was found in the tensile strength of laser treated PEEK composite in a sandwich structure compared to non-treated or abraded sandwich joints. The mode of failure changed from interfacial to cohesive as the number of pulses or laser energy increased during treatment. A similar improvement was achieved in fracture toughness (Mode I and II) performance of laser treated compared to abraded or non-treated PEEK composite adherends. Surface analysis of laser treated adherends and of the fractured joints revealed surface cleaning (XPS), morphology changes (Scanning Electron Microscopy), chemical modification (FTIR spectroscopy and XPS), changes in crystallinity (X-ray) and in wetting property (contact angle), all correlated with the laser irradiation improving the joint’s performance. The bulk properties of the PEEK composite adherend did not deteriorate by the laser irradiation during treatment, as indicated by the identical flexural strength before and after laser treatment. Durability tests showed no change in performance of joints produced from laser treated adherends compared to untreated and abraded ones, even after exposure of 60 days at 60 C/95% RH. It can be concluded that the excimer laser has a potential as a precise, clean and simple preadhesion surface treatment for PEEK composite.

Contact angle measurements with three different liquids were performed on: (i) butyl rubber PB 101-3 (Polysar Ltd.) and (ii) Dow Corning 236 dispersion. Contact angles were measured at different temperatures within the range from 23°C (room temperature) to 120°C. The surface tensions, γsv, of the polymeric coatings at each temperature were calculated from the contact angles. The temperature coefficients of the surface tensions, d_γsv/dT, i.e., the surface entropies, were established for the temperature range covered.

Polar and dispersion surface free energy (SFE) can be determined with the Owens-Wendt method. Thereby, contact angles (CAs) of at least two liquids with known surface tension (ST) components are measured. The ST components can either be determined through experiment or drawn from literature. However, it is important to know how big the difference is between SFE component values that have been calculated with experimentally-determined ST values or values derived from literature. In this study, STs of different test liquids were analyzed by Pendant Drop method and the components by CA measurement on a non-polar surface. CAs on different polymer surfaces were measured to calculate SFE components with the Owens-Wendt method. The calculations conducted were either based on experimentally-determined ST parts or different sets of values found in the literature. The findings of the survey show that, depending on the set of literature values used, the SFE results deviate significantly from the values obtained from experiment. Expressing this deviation in figures, in extreme cases the polar part differs for some polymers by -100% to +100%, with the dispersion component spanning -50% to +43%. In comparison, the expected relative uncertainties exhibited by the experimentally-determined ST values are about 15% for the polar and approximately 5% for the dispersion SFE part. Hence, the results show that the SFE uncertainty can be reduced significantly by means of analyzing the ST parts experimentally.

The interfaces formed by evaporating copper, nickel, and chromium layers on polystyrene, polyvinyl alcohol, polyethylene oxide, polyvinyl methyl ether, polyvinyl acetate, and polymethyl methacrylate have been studied with x‐ray photoemission spectroscopy (XPS). The adhesion strengths of the metal films to the polymers were measured by a tensile‐pull test. At submonolayer coverages of the metals, the peak positions and widths of the metallic electron core levels measured with XPS vary significantly from one polymer substrate to another. Most of these variations can be accounted for in terms of changes in the atomic and extra‐atomic relaxation energies during the photoemission process. Much of this change is brought about when the metal atom deposited on an oxygen‐containing polymer interacts with the substrate oxygen and forms a metal‐oxygen‐polymer complex. The presence of this complex is verified by changes in the photoemission lineshapes of the substrate carbon and oxygen atoms. The XPS signatures of these various complexes are quite similar and suggest that they are chelate‐like complexes. The adhesion strength of any metal on an oxygen‐containing polymer is greater than on the oxygen‐free polystyrene. In general, the increased adhesion strength correlates with the presence of the metal‐oxygen chelate complexes.

X-ray photoelectron spectroscopy is used to measure the surface composition of polycarbonate/ poly(butylene terephthalate) blends. The blend surface is enriched in PC compared to the bulk, with the surface PC/PBT ratio equal to about 1.6 times to bulk formation. For blends containing an impact modifier as a third component, the XPS spectra of the molded surface indicates that no impact modifier is present within the XPS sampling depth. A spectral simulation scheme improves the accuracy of the computed PC/PBT ratio over conventional data reduction schemes involving curve fitting.

Contact angle measurements have been carried out on various solid substrates using water-propanol mixtures and α-bromonaphthalene as wetting liquids. These substrates were: polytetrafluorethylene, Parafilm, polyethylene, polyurethane, polystyrene, polymethylmethacrylate, fluorapatite, and hydroxyapatite. The dispersion and the polar components of the surface free energy, γ_s^d and γ_s^p have been calculated from the geometric mean equation. Two approaches have been considered: (1) neglecting the spreading pressure π_e and (2) taking π_e into account (Dann's method). The results show that both approaches actually yield the same results for the surface free energy, γ_s, if a proper interpretation of the approaches is considered. All data indicate, that approach (1) gives γ_s values determined on the adsorbed liquid layer, whereas in approach (2) the free energies of the bare solid surfaces are found.

Equilibrium, advancing and receding contact angles for five different liquids have been determined on twelve commercial polymers after various surface roughening procedures. It was found that influences of surface roughening on contact angles disappear if the stylus surface roughness R_A is < 0.1 μm. Surface roughening tends to increase observed contact angles, if the contact angle on the smooth surface is above 86°, whereas contact angles decrease if the contact angle on the smooth surface is below 60°. For contact angles on the smooth surface between 60° and 86°, surface roughening was found not to influence measured contact angles. These results show a broad similarity in the trend, predicted by the early Wenzel equation, describing the influence of surface roughness on contact angles, although of course the stylus surface roughness R_A is not identical to the theoretical r-parameter in the Wenzel equation.

It is shown that in any two‐phase mixture of fluids near their critical point, contact angles against any third phase become zero in that one of the critical phases completely wets the third phase and excludes contact with the other critical phase. A surface layer of the wetting phase continues to exist under a range of conditions when this phase is no longer stable as a bulk. At some temperature below the critical, this perfect wetting terminates in what is described as a first‐order transition of the surface. This surface first‐order transition may exhibit its own critical point. The theory is qualitatively in agreement with observations.