A study on metal-polymer interface formation following an in situ plasma treatment is presented. The plasma treatment is performed in a dual frequency ECR plasma. This enables to control some of the main plasma parameters. The study is focused on a model system consisting of a polypropylene substrate and a magnesium metal overlayer. Due to large variations in the interface properties depending on the surface treatment, this system allows deeper insight in the interface formation.

Plasma-treated poly-p-xylylene films have been characterized by neutron activation oxygen analysis, internal reflection (IRS) and transmission infrared spectroscopy, transmission electron microscopy (TEM), and surface contact angle measurements. The results indicate that an oxygen plasma roughens the surface and that oxygen is incorporated into the surface. Oxygen is not detected in the bulk of the sample. The infrared transmission spectra exhibited no carbonyl band, but the relative band intensities changed, indicating a change in ring substitution by a loss of chlorine in the chlorinated poly-p-xylylenes. The IRS spectra of the surface of films treated with oxygen plasma did contain carbonyl bands at 1730 and 1640 cm⁻¹. Argon and helium plasmas generally decreased the water contact angle measured on plasma-treated poly-p-xylylene surfaces more than oxygen or nitrogen plasma treatments. Regardless of the plasma utilized, the water contact angles increased with time after the treatment but did not recover to the original level. IRS spectra of the surface of films treated with argon plasma contained carbonyl bands at 1730 and 1695 cm⁻¹. The adhesion of a polyurethane thermosetting material to a poly-p-xylylene surface is greatly improved if a plasma treatment is used prior to the application of the polyurethane. The degree of improvement in adhesion was dependent on the type of plasma and the treatment time.

We describe a procedure to modify the surface of polyethylene (PE) film using a combination of gas discharge and wet chemical techniques. This method generates high densities (10¹⁴-10¹⁶ cm^-2) of a specific functionality, largely unaccompanied by other groups, in a 50-100-Å surface layer. The topography of the polymer surface remains unchanged after treatment and functions as an effective starting material for subsequent derivatization by standard synthetic chemical reactions. A plasma of either oxygen, water, or hydrogen is generated under comparable experimental conditions. In all cases a 1-2-s, 5-W, 0.2-Torr treatment produces about the same degree of surface modification as does longer treatment. High-resolution X-ray photoelectron spectroscopy (XPS) shows that either an oxygen or a water plasma produces a variety of oxidation products ranging from alcohols to carboxylic acids. Chromic acid oxidizes the plasma-oxidized surface further to give high densities of carboxylic acid groups which can be readily converted to acid chlorides and derivatized. Borane/tetrahydrofuran reduces the plasma-oxidized surface to give alcohols which can be esterified readily. Contact-angle measurements show that the water-plasma-treated PE surface has a higher surface free energy (γ_s ∼ 62 dyn/cm) than the oxygen-plasma-treated surface (γ_s ∼ 50 dyn/cm). A 5-s, ambient-temperature, 0.2-Torr, 2-W hydrogen plasma generates a significant number of quenchable radical sites. XPS spectra of this treated surface, exposed to either nitric oxide or nitrosotrifluoromethane, show that both compounds bond to the surface.

The effect of energy of corona discharge treatment (CDT) on the physico-chemistry of the surface of a polyester film was investigated systematically using a number of complementary surface analytical techniques: contact angle analysis, x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and atomic force microscopy. The energy of treatment can be controlled by either varying the speed of the treatment with constant power input or by varying the power of the treatment with constant speed. The changes in surface energy, surface chemistry and surface morphology of poly(ethylene terephthalate) (PET) induced by these two modes of CDT have been investigated.

The surface energy and the polar contribution of the film increased with increasing energy of corona. Phenolic-OH, carbonyl and carboxylic acid (HOC=O) have been identified as the functional groups incorporated onto the surface. Low-molecular-weight oxidized materials were observed in the form of a globular morphology on the surface of the film. By washing the film in methanol–water prior to surface analysis, it was shown that the oxygen content at the surface of the film decreased and the globular morphology was removed.

Differences in the surface energy of corona-treated PET films have been observed under similar corona energy conditions, depending on whether the sample was treated with constant speed or constant power. However, the total surface oxygen content was found to be similar at low-energy treatments. When the high-resolution C 1s XPS spectrum was peak fitted, the relative concentrations of functional groups introduced and other changes to the original polymer structure were shown to be independent of the mode of treatment.

These observations may be explained because although the total energy transferred from the power source to the PET under the conditions of ‘constant power, variable speed’ and ‘variable power, constant speed’ is theoretically the same, these experimental conditions are not interchangeable. At constant power, the concentration of active species, radiation and dielectric breakdowns in the discharge are independent of speed. However, in the case of constant speed, the concentration of active species, radiation and dielectric breakdown increase with increasing power, resulting in a different surface energy between the two sets of samples. However, because the composition of the active species will not change, the functional groups on the surface of the film will be the same across comparable energy levels, independent of mode of treatment. Copyright © 2002 John Wiley & Sons, Ltd.
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/sia.1429

Corona discharge treatment (CDT) is a surface modification technique commonly used to treat plastic films prior to adhesive bonding, printing with inks, lamination to other films and other coating applications. In this study, the treatment conditions are, in energy terms, representative of those used in industrial and laboratory coating applications.

The physicochemistry of the surface of untreated and corona-discharge-treated biaxially oriented polypropylene (BOPP) film was investigated using a number of complementary surface analytical techniques: contact angle analysis; x-ray photoelectron spectroscopy (XPS); atomic force microscopy (AFM). This report describes the surface energetics, chemical functionality and morphology of polypropylene film before and after CDT. Both AFM and XPS were utilized, along with washing experiments, to investigate the presence of a weak boundary layer.

The surface energy was found, as expected, to increase with increasing energy of the corona. The functional groups incorporated onto the surface have been identified as hydroxyl [COH], peroxy [COO], carbonyl [CO], ester [COCO], carboxylic acid [HOCO] and carbonate [OC(O)O]. These groups are present in varying relative concentrations, depending on the energy of the corona utilized.

The morphology of the film changed after CDT. Initially, a fibrillar crystalline structure was observed, whereas after CDT a globular morphology became apparent. These globular features were attributed to low-molecular-weight oxidized material (LMWOM) created by CDT. The roughness of the film was not found to increase under the corona conditions employed.

Formation of LMWOM was found to be independent of treatment energy. However, two mechanisms have been suggested for its formation, dependent on the energy of treatment: below a threshold energy of ∼4 kJ m⁻², oxidation and scission of the inherent low-molecular-weight boundary layer present on polyolefin films is the dominant means for the formation of LMWOM; above 4 kJ m⁻², oxidation and scission of the polymer backbone is the main process.

This work provides a comprehensive reference around CDT of polypropylene film for industrial applications, while also informing how the optimal level of treatment can be determined. In the case of adhesion of silicones, it would be expected that optimal adhesion would be obtained where the maximum amount of oxygen incorporated was in a water-insoluble form. Copyright © 2002 John Wiley & Sons, Ltd.
https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/abs/10.1002/sia.1217

Mild plasma cleaning of metal surfaces was shown to be effective in removing organic contaminants. Auger electron spectroscopy and surface wettability measurements were used to evaluate the plasma cleaning procedure and to provide a comparison with conventional solvent cleaning methods.

SSIMS (static secondary ion mass spectroscopy) has been used to aid in the interpretation of hydrophobic recovery of oxygen plasma treated PS (polystyrene), together with XPS (X-ray photoelectron spectroscopy) and water contact angle measurements.

The heterogeneity in sputter yields of different ions did not allow complete qualitative and quantititative information to be obtained. Yet negative ion spectra of surfaces treated with plasmas of isotopically enriched oxygen allowed us to follow closely the disappearance of polar groups during hydrophobic recovery. Furthermore, using isotopically enriched PS samples, it was possible to obtain unambiguous information about molecular weight and temperature-induced changes in the hydrophobic recovery mechanism, which could not have been provided by XPS.

XPS, SSIMS, water contact angle and each rate measurements have been used to characterize the effect of CF₄/O₂ discharges on bisphenol-A-polycarbonate (PC). 1% O₂ discharges resulted in the grafting of fluorocarbon radicals to the polymer surface. At higher O₂ percentage in the gas feed fluorine atoms reacted with the polymer surface inducing degradation of the aromatic rings and etching. Oxidation of the PC surface was observed after all the treatments. Due to considerations based on reactivity and relative concentration in the discharge, oxygen molecules are considered more effective than oxygen atoms in inducing oxidation. Etching was found to occur as well; fluorine atoms and oxygen molecules are considered most important in promoting it.

Oxygen plasma treatment of polypropylene (PP) surfaces led to introduction of oxygencontaining functionalities, with consequent improvement of surface wettability. A combination of X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectroscopy (SSIMS), and contact angle measurements (water-in-air and air-in-water) allowed us to characterize the behavior of the treated surface in contact with air (low-energy surface) and water (high-energy surface).

The treated surface showed the tendency to rearrange itself to minimize its interfacial energy. When contacted with air (low-energy surface), polar groups were buried away from the polymer/air interface, while in contact with water (high-energy surface) polar groups remained at the polymer/water interface.

When contacted with air, the polymer surface layer rearranged by macromolecular motions within itself, since interdiffusion with the bulk polymer seems forbidden. These motions are thermally activated and it was possible to obtain an apparent activation energy (58.1 kJ/mol) close to those reported for other vinyl polymers.

XPS, SEM, SSIMS, FTIR-ATR, water-in-air, and air-in-water contact angle measurements have been used to unambiguously characterize the locus of failure of PP/epoxy joints. In the case of untreated PP, the fracture has been found adhesive, whereas in oxygen plasmatreated PP, it is cohesive, within bulk PP, but close to the modified PP-bulk PP interface. The smoothness of fracture surfaces allowed us to exclude mechanical interlocking effects. Shear-strength measurements showed that the mechanical strength of the joint was improved by plasma treatment. Preliminary thermal equilibration of the plasma-treated PP sample and changes in the curing cycle of the epoxy resin did not change either the locus of failure or the shear strength of the joint. The reason is probably because the number of polar functions left at the surface after thermal equilibration is sufficient to induce adhesion. The mechanical strength of the PP surface layer may be the determining factor. Fracture energy calculations showed that the observed locus of failure is the same as predicted on the basis of surface energy considerations.

The effect of oxygen plasma treatment on adhesion and surface properties of polypropylene (PP) was assessed. An oxygen rich modified PP layer, immiscible with bulk PP, was formed by the treatment. Contact angle measurements showed that macromolecular motions led with time to rearrangements of the surface layer drastically decreasing its wettability, while its composition, measured by XPS, remained unaffected.The shear strength of PP-epoxy joints increased after plasma treatment. The locus of failure was found to occur at the PP/epoxy interface for untreated PP, within PP in the case of oxygen-plasma-treated samples, close to the modified PP/bulk PP interface. This result suggests that the plasma treament improves the interaction at the PP/epoxy interface, but weakens the mechanical strength of the surface layer thereby creating a weak point at the modified PP/bulk PP interface.

Hydrophobic recovery of oxygen-plasma-treated polystyrene has been studied with regard to its dependence on temperature, molecular weight and plasma parameters, namely radio frequency power. Plasma chemistry has been studied by actinometry, while surface analysis has been performed by X-ray photoelectron spectroscopy (X.p.s.) and contact angle measurements. Molecular weight changes have been investigated by gel permeation chromatography. Plasma-treated surfaces become richer in oxygen and more crosslinked than untreated ones. Two basic mechanisms have been observed: one is based on short range motions within the plasma-modified layer, burying polar groups away from the surface without modifying its X.p.s. composition; the other involves long range motions, i.e. diffusion of non-modified macromolecules or segments from the bulk to the surface, altering its X.p.s. composition. The latter mechanism becomes important upon increasing ageing temperatures, or lowering molecular weight or extent of crosslinking of the surface layer.

Corona discharge treatment was conducted for ultrahigh molecular weight polyethylene (UHMWPE) fiber. The functional groups and surface roughness of the polyethylene fiber surface were determined by an X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The interfacial shear strength of UHMWPE fiber with HDPE film was determined by microbond pullout method. The interfacial shear strength increased by corona treatment. Then, the effect of the chemical and physical factors on the interfacial shear strength was discussed based on the results of multivariate regression analysis. The results indicated that the contribution of functional groups and surface roughness to the interfacial shear strength was expressed as 50 and 50%, respectively. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 243–249, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/%28SICI%291097-4628%2819990110%2971%3A2%3C243%3A%3AAID-APP7%3E3.0.CO%3B2-I

The interfacial shear strength of an ultrahigh molecular weight (UHMW) polyethylene (PE) fiber/epoxy-resin system was greatly improved by the corona-discharge treatment of the fiber. The UHMW PE-fiber/epoxy-resin composite was prepared with corona-discharge-treated UHMW PE fiber. The mechanical properties of the composite sheet were determined by tensile testing. The tensile strength of the composite was also very much improved. However, the tensile strength of the composite was about one-half of the theoretical strength. This result was due to the molecular degradation of the PE-fiber surface caused by surface modification. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1162–1168, 2001
https://onlinelibrary.wiley.com/doi/abs/10.1002/1097-4628%2820010214%2979%3A7%3C1162%3A%3AAID-APP20%3E3.0.CO%3B2-Y

The surface structures of low-density polyethylene (LDPE) film exposed to plasma or γ-ray in air were characterized by ESCA, IR, and EMS. The formation of trans C[dbnd]C bond on the LDPE film surface was observed by the exposure to ac air plasma (2 × 10⁻² torr, 19 W plasma power). Large amounts of O and N atoms as an amide structure were incorporated into the polymer surface by the plasma treatment. These plasma reactions occurred mainly in the amorphous region, and the polymer surface became rough enough to have a microdomain structure upon increasing the plasma treatment time up to 3 h. γ-Irradiation of LDPE in air only brought about O-atom incorporation as ketone and ether linkages. The polymer surface did not undergo etching under γ-irradiation as it did in plasma treatment.

The surface free energy parameters of ethylcellulose (EC) films were determined using the Lifshitz-van der Waals/acid-base approach and the influence of plasticizers on their surface energetics was assessed. Films were prepared by dip-coating glass slides in organic solvents containing EC and the advancing angles of drops of pure liquids on the EC films were measured with a contact angle goniometer using the captive drop technique. EC has lower surface free energy than cellulose. The acid-base (AB) term made only a slight contribution to the total surface free energy and the surfaces exhibited predominantly monopolar electron-donicity. The addition of plasticizer (dibutyl sebacate or dibutyl phthalate) resulted in a small decrease in the total surface free energy. The effects of film forming variables, including solvent system, concentration and post-formation treatment (annealing), on the surface free energy parameters of EC films were also investigated. These data were then used to analyze how the surface energetics affect the interaction of the EC films with other surfaces based on interfacial tension, work of adhesion and spreading coefficient calculations. Lifshitz-van der Waals (LW) interactions provided the major contribution to the work of adhesion for EC with all of the solid substrates analyzed. However, the AB interactions contributed significantly to the work of adhesion for EC with 'bipolar' substrates and to the spreading coefficients of EC over substrates. The consideration of work of adhesion and spreading coefficient based on surface free energy parameters may have potential use in evaluating factors affecting film adhesion and, furthermore, in optimizing pharmaceutical film coating processes.

Adhesion of polyimides to ceramic substrates such as SiO₂, Al₂O_3' and MgO, and interfacial interactions were studied using XPS, SEM, and the peel test. The peel strength of polyimides on SiO₂ and Al₂O₃ is almost identical and higher than that on MgO at the same polyimide thickness. Contrary to the failure within the polyimides on SiO₂ and Al₂O_3' Mg was found on the peeled PMDA-ODA acid-derived polyimide surface, implying weakening of MgO by interfacial reactions with polyamic acid. With the neutral polyamic ethyl ester, the locus of failure on MgO was changed to the apparent weak boundary layer of the ester-derived polyimide. On SiO₂ and Al₂O₃ the peel crack propagated with a discontinuous stick-slip process. The constant interspacing between transverse stick-slip striations on the peeled polyimide surfaces has confirmed that plastic bending is the major energy dissipation process with a minimal contribution from tensile loading.

A new method of quickly and precisely measuring the surface tension of liquids and solutions is described. Utilizing the fact that the size of the bubbles formed from a gas flowing out of a nozzle is dependent on the nozzle diameter and the surface tension of the liquid used, the surface tension of a liquid can be determined by simply counting the number of bubbles formed from a gas flowing out at a constant flow rate or by measuring the period of bubble formation. The expected accuracy of the method is below 0.1% of variance. An evident correlation between the period of bubble formation and the surface tension was shown with several kinds of liquids which differ in surface tension. Changes in surface tension with varied degree of neutralization were determined in an aqueous solution of polyacrylic acid (PAA), 20-30 points of measurement with an accuracy of about 0.1% could be easily obtained within one hour.

Peroxides formed on the surface by corona treatment of low-density polyethylene film can be used to initiate grafting of polar vinyl monomers such as acrylic acid. Different types of peroxides are probably formed on the surface, but at least hydroperoxides could be detected by XPS analysis. The grafting reaction was carried out directly after corona treatment, by placing the corona-treated film above a solution of acrylic acid heated to 100°C. The grafting reaction takes place in a vapor phase of the monomer. After extracting the reacted films with hot methanol and drying, surface analysis by XPS, IR, and contact angle measurements were carried out. Effect of degree of corona treatment and reaction time have been studied. The conclusion from this work is that acrylic acid in vapor phase can successfully be grafted onto corona-treated polyethylene film by this method. © 1992 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1992.070460916

The use of scanning electron microscopy for direct observation of the effects of surface roughness on the spreading of liquids is described, making it possible to view moving liquid drops at distances less than 1 μm from the advancing contact line. Various surfaces were examined including several with simple forms of roughness which can assist in explaining the behavior of more complex surfaces. Spreading is shown to be highly dependent on the orientation and texture of the roughness; in particular, the presence of sharp edges of step height ⩽0.05 μm are shown to influence spreading significantly. These observations reinforce our previously stated doubts of the significance of conventionally measured macroscopic contact angles.

This paper presents and discusses particle behavior at solidification fronts from a thermodynamic point of view. Engulfing or rejection of particles embedded in a melt by solidification fronts depends on whether such quantities as the free energy of adhesion or of engulfing are positive or negative. As the relevant energy balances contain solid‐liquid interfacial tensions which are difficult to determine, these studies may also be viewed as tests for the validity of such data and the underlying theories used to determine them. In this paper, solid‐liquid interfacial tensions are derived from contact angle data and the equation of state approach for interfacial tensions [Neumann et al., J. Colloid Interface Sci. 49, 291 (1974)]. The thermodynamic predictions obtained in this way for approximately 60 systems agree very well with microscopic observations with particles in the range of 10–200 mm in diameter.

The critical velocity of engulfing V_c of acetal, nylon-6,6, and nylon-12 particles when encountered by the solidification front of salol is reported as a function of particle size. Using the dimensional analysis derived previously, the free energy of adhesion ΔF^adh for the attachment of these particles to the salol solid/melt interface is determined. These values of ΔF^adh, together with the known surface tension values γ_PV of the particles, are used to determine the salol solid/melt interfacial tension γ_SL to be γ_SL = 0.0053 ± 0.0025 erg/cm². Similarly, the free energies of adhesion ΔF^adh for PMMA particles to the solid/melt interfaces of naphthalene, biphenyl, and salol are determined. As all the γ_SL values for these systems are known—in the case of naphthalene and biphenyl from the temperature dependence of contact angles—γ_PV for the PMMA particles is determined. Using the equation of state for interfaces, the contact angle for the system PMMA/water is predicted. This value is in excellent agreement with the contact angle of water on a film of PMMA obtained by solvent casting. It is concluded that measurement of the critical velocity of engulfing represents a unique method for contact angle determinations on small particles.

Irradiation of polystyrene by 15 Mrad gamma or exposure to a 254 nm ultraviolet (UV) light source leads to surface oxidation of the polymer to depths greater than 10 nm as opposed to ∼ 3 nm depth offered by either plasma or corona-discharge treatment. Oxidation increases linearly with UV irradiation time. More carboxyl (OCO) acid functionality, which increases with depth, was detected for UV-treated polymer. With 3 Mrad gamma irradiation, only hydroxyl (CC) groups were detected by XPS as the surface-oxidized species. ADXPS, GPC, and static SIMS data suggest that chain scission is the dominant degradation mechanism for polystyrene exposed to high gamma and UV radiation, respectively. © 1993 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1993.070471213

The surface chemistry of polystyrene, used as tissue culture ware, subjected to electron beam irradiation was studied. Core-level and valence-band (VB) X-ray photoelectron spectroscopy (XPS) showed that electron beam (EB) treatment resulted in surface oxidation plus sterilization of the polymer material. The extent of oxidation by EB is linear with the dose and, as such, is analogous to gamma-radiation-induced oxidation. The data indicate that EB-radiation treatment alone provides a polystyrene surface analogous to that obtained by corona discharge or plasma plus low gamma sterilization.

Polystyrene tissue culture vessels are commercially treated by corona discharge or plasma surface oxidation to provide a hydrophilic surface, with 15–20% surface oxygen. ESCA and FTIR showed that oxidation forms hydroxyl, carbonyl, and carboxyl groups. We have discovered that water washing removes about half the oxidized species. It is believed that reaction with the vinyl polymer backbone to form carboxyl groups results in CC bond scission to form soluble fragments; addition and ring reactions would not yield soluble species. This functional group removal could affect the desired properties, such as the use of these groups as anchors in chemical coupling.

The effect of gamma-radiation on the surface chemical properties of polystyrene was studied by ESCA and FT-IR. Gamma-radiation produces surface >CO and CO containing functional groups only, and also causes oxidation to depths >10 nm as detected by ESCA. FT-IR spectra showed that below the top few molecular layers ester, acid, and carbonyl groups of various types were present. The α,β-unsaturated carbonyl/acid groups form a higher proportion of the total carbonyls with increasing depth. Surface oxidation follows pseudo-first-order kinetics; the extent of interior oxidation is linear with dose.