Recently receding contact angles have increasingly attracted attention in studies of wetting phenomena. The difference between the advancing and receding contact angles of the same liqнid on the same solid surface is termed 'contact angle hysteresis'. The hysteresis is usually ascribed to the solid surface roughness and/or its chemical heterogeneity. These possible mechanisms of the hysteresis appearance do not exclude another interpretation of the receding contact angle origin (E. Chibowski et al., in: Surfactants in Solution, A.K. Chattopadhyay and K.L. Mittal (Eds.), pp. 31-53, Marcel Dekker, 1996). In this approach, the presence of liquid film behind the drop is considered to be the cause for the ovbserved hysteresis, except, of course, for cases of rough and/or macro-chemically heterogeneous solid surfaces. In this paper, a new approach is presented and then verified using experimental advancing and receding contact angles taken from the literature. This approach allows an evaluation of the total surface free energy of a solid if the advancing and receding contact angles for a probe liquid are known. It does not require values of the solid surface free energy components for estimation of the value of total surface free energy.

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.

Two different experimental approaches based on ellipsometry and zeta potential measurements have been employed to determine the dispersion and polar surface free energy components of glass. From ellipsometry the adsorption isotherms of n-octane and water have been determined, yielding values for the film pressures of n-octane and water and the dispersion and polar surface free energy components of glass. Similarly, zeta potentials in water of glass covered with various amounts of n-octane and n-hexanol have been determined. Next, the film pressures of these liquids and surface free energy components of glass were also calculated. Thus determined values are 32 and 80 mJ/m² (from ellipsometry) and 25 and 80 mJ/m² (from zeta potentials) for the dispersion and polar components, respectively. The correspondence between the surface free energies obtained by two completely independent methods gives confidence to the approaches employed.

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.

Four different approaches to determination of solid surface free energy (van Osset al.’s (LWAB), Owens and Wendt’s (O–W), Chibowski’s contact angle hysteresis (CAH) and Neumann’s equation of state (EQS)) were examined on glass, silicon, mica and poly (methyl methacrylate)(PMMA) surfaces via measurements of advancing and receding contact angles. Sessile drop and tilted plate methods were employed to measure the contact angles of probe liquids water, formamide and diiodomethane. The results obtained show that on a given solid the advancing contact angle is slightly larger and the receding one smaller if measured by tilted plate method. Hence, the resulting hysteresis is larger than that from the contact angles measured by sessile drop. The calculated (apparent) surface free energy is the greatest if determined from O–W equation. Unexpectedly, EQS fails for weakly polar polymer PMMA surface, giving significantly lower value of the calculated energy. In rest of the tested systems LWAB, CAH and EQS approaches give comparable results for the apparent surface free energy of the tested solids. A hypothesis is put forward that using a probe liquid only apparent surface free energy of a solid can be determined because the strength of interactions originating from the solid surface depends on the strength of interactions coming from the probe liquid surface.

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry. Both XPS and IR-RAS results indicated the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surface. Oxygen and argon plasmas were highly ablative, whereas ammonia plasma was only moderately so. Oxygen and argon plasmas appear to react with the LaRC-TPI via a fragmentation/oxidation mechanism; the effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups.

The effects of aging temperature and time on the adhesion properties of oxygen plasmatreated low-density polyethylene (LDPE) were investigated. As the aging temperature and time increased, surface rearrangement and the migration of molecules containing polar functional groups into the bulk were accelerated to the surface to form a hydrophobic surface. The adhesion strength of oxygen plasma-treated LDPE/aluminum joints was measured using a 90° peel test by varying the plasma treatment time and aging temperature. The adhesion strength was constant, regardless of the plasma treatment time. As the aging temperature increased, the adhesion strength of the LDPE/aluminum joints decreased and the locus of failure changed from cohesive to interfacial failure. It was also found that the polar functional groups buried in the bulk could be reoriented to the surface in a polar environment. This study also investigated whether repeated oxygen plasma treatment would increase the concentration of polar functional groups at the surface and reduce the surface rearrangement and the migration of molecules containing polar functional groups during aging. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) showed that repeated oxygen plasma treatments increased the concentration of polar functional groups at the surface. However, the aging time between plasma treatments had a negligible effect on the concentration of polar functional groups at the surface.

Improvement of the paint adhesion to a polypropylene (PP) bumper has been investigated without using a primer by treating the bumper surface with O₂, H₂O, and acetylene plasmas. All the plasma treatments resulted in an increase of the adhesion strength in dry conditions. The adhesion strength could be increased up to a value comparable to that obtained by applying a primer. The treated surfaces were quite stable for 7 days in air. After exposure to wet conditions, however, the adhesion strengths for both O₂ and H₂O plasma-treated samples decreased significantly, while the adhesion strength for the acetylene plasma-treated sample did not change much.

Thin plasma polymerized layers of acrylic acid (PPAA) were deposited onto polyethylene and muscovite mica surfaces. Structure and surface properties of the deposited layer depend on the polymerization conditions. The content of carboxylic groups in the layer decreases, whereas the degree of crosslinking or branching increases, with increasing discharge power. A soft, sticky layer with a low contact angle against water is obtained when a low discharge power (5 W) is used. In contrast, a hard film with a rather high water contact angle is obtained when the discharge power is high (50 W). A surface force apparatus was employed to study some film properties including adhesion force, crack formation, and capillary condensation. The adhesion force between plasma polymerized acrylic acid layers prepared at a low discharge power is high in dry air. It decreases remarkably in humid air and no adhesion is observed in water. In dry air, the adhesion force between PPAA layers decreases as the discharge power increases.

Ar⁺1 ion irradiation on a polycarbonate (PC) surface was carried out in an oxygen environment in order to investigate the effects of surface chemical reaction, surface morphology, and surface energy on wettability of PC. Doses of Ar⁺ ion were changed from 5 × 10¹⁴ to 5 × 10¹⁶ at 1 keV ion beam energy by a broad ion beam source. Contact angle of PC was not reduced much by Ar⁺ ion irradiation without flowing oxygen gas, but decreased significantly as Ar⁺ ion was irradiated with flowing 4 sccm (ml/min) oxygen gas and showed a minimum of 12° to water and 5° to formamide. A newly formed polar group was observed on the modified PC surface by Ar⁺ ion irradiation with flowing oxygen gas, and it increased the PC surface energy. On the basis of x-ray photoelectron spectroscopy analysis, the formed polar group was identified as a hydrophilic CO bond (carbonyl group). In atomic force microscopy (AFM) study, the root mean square of surface roughness was changed from 14 Å to 22–27 Å by Ar⁺ ion irradiation without flowing oxygen gas and 26–30 Å by Ar⁺ ion irradiation with flowing 4 sccm oxygen gas. It was found that wettability of the modified PC surface was not greatly dependent on the surface morphology, but on an amount of hydrophilic group formed on the surface in the ion beam process.

High surface energy (60–70 mJ/m2) polymers, i.e., totally wettable by water, from polypropylene to fluoropolymers have been obtained by ion-assisted reaction (IAR), in which the polymer surface was irradiated by energetic ions in a reactive gas environment. The ion energy was 1000 eV and the ion dose was varied in the range of 5 × 1014 – 1 × 1017 ions/cm2. Oxygen gas was introduced near the polymer surfaces during ion irradiation. The change in wettability was critically dependent on the ion dose and on the flux of oxygen gas. The surface energy was mainly increased due to the polar component related to the hydrophilic groups generated such as carbonyl and carboxyl, etc. The reaction generating the hydrophilic groups on the polymer surface modified by ion assisted reaction (IAR) was explained according to a two-step mechanism. The improvement in adhesion between the IAR-modified polymers and other materials was also explained in terms of the increased surface energy as well as surface roughness of the polymers modified by IAR.

Atmospheric low-temperature plasma was produced using dielectric barrier discharge (DBD) plate-type plasma reactor and high frequency of 13.56 Hz. The surfaces of polyimide films for insulating and packaging materials were treated by the atmospheric low-temperature plasma. The contact angle of 67 was observed before the plasma treatment. The contact angle was decreased with deceasing the velocity of plasma treatment. In case of oxygen content of 0.2 %, electrode gap of 2 mm, the velocity of plasma treatment of 20 mm/sec, and input power of 400 W, the minimum contact angle of 13 was observed. The chemical characteristics of polyimide film after the plama treatment were investigated using X-ray photoelectron spectroscopy (XPS), and new carboxyl group bond was observed. The surfaces of polyimide films were changed into hydrophilic by the atmospheric low-temperature plasma. The polyimide films having hydrophilic surface will be very useful as a packaging and insulating materials in electronic devices.

Wettable surface of polymers (advanced wetting angle ∼10° and surface energy ∼ 60 ∼ 70 erg/cm²) have been accomplished by the ion assisted reaction, in which energetic ions are irradiated on polymer with blowing oxygen gas. The energies of ions are varied from 0.5 to 1.5 keV, doses 10¹⁴ to 10¹⁷ ions/cm², and blowing rate of oxygen 0 ∼ 8 ml/min. The wetting angles are increased when the wettable polymers were exposed in air, but are remained in pure water. Improvement of surface energy is mainly due to the polar force. Surface analysis shows hydrophilic functional groups such as CO, (CO)O, CO, etc., are formed without surface damage after the ion assisted reaction treatment. Comparisons between the conventional surface treatments and the ion assisted reaction are described in term of physical bombardment, surface damage, functional group, and chain mobility in polymer.

Measurements have been made of the energy required to break through unit area of polystyrene (PS), polymethylmethacrylate (PMMA), and joints prepared by molding the two polymers in contact. The results were: 1.23 ± 0.5 kJ/m² (PS), 0.46 ± 0.10 kJ/m² (PMMA), and 0.22 ± 0.04 kJ/m² for the bonded joint. Thus, the interface was significantly weaker than either adherend, but surprisingly strong for two incompatible materials. Microscopy and selective dyeing revealed that fracture took place at the interface itself, with no appreciable transfer of material from one side to the other. It is concluded that Van der Waals interactions are sufficient to create relatively strong bonds.

Low density polyethylene (LDPE) and high density polyethylene (HDPE) were plasma-treated with N₂ and O₂ plasma. The wettability and polar component of surface free energy of plasma-treated polyethylene were investigated by contact angle measurement. The concentration of functional groups formed by plasma treatment such as hydroxyl and carbonyl groups was measured using attenuated total reflection Fourier transform infrared spectroscopy (ATR FTi.r.). The concentration of polar functional group increased rapidly with 5–10s of plasma treating time and then very slowly after that. The adhesion strength of epoxy resin/plasma-treated polyethylene joints was examined by a 90° peel test. The increase of the adhesion strength was similar to that of concentration of polar functional groups. The higher adhesion strength of epoxy resin/plasma-treated HDPE joints was observed than that of epoxy resin/plasma-treated LDPE joints since HDPE deformed more during the peel tests and had more polar functional groups on the surface.

Atmospheric pressure N₂ cold plasmas are generated with a torch-type generator using 60-Hz AC power. High flow rate N₂ gas is injected into the plasma generator and high voltage of about 2 kV is introduced into the power electrode through transformer. Discharge characteristics of N₂ cold plasma, such as current–voltage profile, gas temperature and radial species in plasma, are measured. As one possible application, the N₂ cold plasma is used to modify the surface of polymer, especially polypropylene, for adhesion improvement. Power dissipation in discharge has the maximum value at optimal power electrode position, z=3 mm, which lead to the generation of more energetic electrons capable of creating N₂* and N₂⁺ excited states in plasmas effectively. These excited species can induce high population of oxygen and nitrogen atoms on polymer surface through creation of polymer excited states. Maximum bonding strength about 10.5 MPa is obtained at optimal power electrode position.

New helical coupling plasma system for continuous surface treatment and modification (surface processing) of fiber bundles has been developed and tested for glass fibers. The system enables surface processing of single filaments and flat substrates as well. Surface processed glass fibers and their bundles were examined as reinforcements for glass fiber/polyester composite systems. Processing of fibers comprised a surface treatment using argon gas and a surface modification using hexamethyldisiloxane and vinyltriethoxysilane monomers. Interfacial and interlaminar shear strengths of plasma processed glass fiber/polyester systems were compared with those of untreated and commercially sized fibers.

Commercial polymers in thin film form were used for modification by atmospheric RF plasma. The influence of the plasma treatments using Ar and Ar+O₂ on surface energy, morphology and chemical structure of the films was investigated. It was revealed that both modifications caused surface activation of the polymer film, but they obeyed different mechanisms enhancing polymer wettability. First, surface graphitization due to argon sputtering caused hydrogen to free the surface and then reacts with oxygen in the air. Second, surface oxidation is connected with the functional group formation. The reactions of Ti with the polymer led to the simultaneous formation of TiCl₂, TiC, Ti-oxide and they contributed to film adhesion. In comparison with Ar, the mixed Ar+O₂ RF plasma treatment was a more timesaving process and had more influences on surface activation and film adhesion.

The crosslinking of an ethylene–;tetrafluoroethylene copolymer by exposure to an argon plasma, excited by an inductively coupled RF field, is studied over a wide range of pressures and power loadings. The results are interpreted in terms of a two-component, direct and radiative energy-transfer model showing that the outermost monolayer crosslinks rapidly via direct energy transfer from argon ions and metastables.

The crosslinking of an ethylene–tetrafluoroethylene copolymer by exposure to a variety of inert gas plasmas, excited by an inductively coupled radiofrequency (RF) field, has been studied. The rates for direct and radiative energy-transfer processes are determined within the framework of a kinetic model of the system and are shown to have a strong dependence on the sustaining gas, as do the average depths of penetration of the ions and metastable species. Helium is found to be the most efficient gas for the crosslinking of the outermost few monolayers whereas the crosslinking of the subsurface and bulk polymer is best effected by neon. Madelung charge potential calculations have been performed to simulate the experimentally determined x-ray photoelectron spectroscopy (ESCA) spectra to elucidate several features of the mechanisms involved.

The oxidation of polyethylene, polypropylene, and polystyrene by exposure to plasmas excited in pure oxygen and helium–oxygen mixtures at low power levels has been studied. A detailed curve resolution procedure is outlined, and the rate of oxidation is shown to be a strong function of the polymer structure for pure oxygen plasmas, as is the composition of the oxidized layer; this is not the case, however, for oxidation effected by helium–oxygen mixtures. It seems likely, from a consideration of the available data, that the oxidation is confined to the outermost monolayer and is initiated by a crosslinking mechanism that involves oxygen-containing functionalities.

This chapter summarizes our present understanding of surface preparations for structural adhesive bonding of aluminum, titanium, and steel adherends. Both the initial bond strength and the subsequent bond durability depend critically on the interaction of the adhesive (and/or primer) with a pretreated adherend. There are two mechanisms of adhesion that are prominent in structural adhesive bonding: mechanical interlocking of the polymer adhesive with a microscopically rough adherend surface, and chemical bonding (with either covalent bonds or weaker van der Waals bonds) of adhesive molecules to the (intentional) adherend oxide. The magnitude and relative importance of both of these interactions depend greatly on the nature of the adherend surface prior to bonding and on the rheology and chemistry of the adhesive.

Contact angle data for sets of probe liquids allow the determination of components of solid surface energies which in turn can be used to calculate the work of adhesion of other materials to the solid surface. There is much debate currently about the correct choice of the acid–base components for the probe liquids. For many systems, the strength of adhesion measured independently correlates well with the calculated work of adhesion. Recent trends in this area include adhesion under water and the adhesion of bacterial and other cells to immersed solids.

As adaptable as polymeric materials are in their many applications to our daily lives, the need exists to tailor the polymer surfaces to provide even more flexibility in regard to their uses. Plasma treatments offer an unprecedented spectrum of possible surface modifications to enhance polymers, ranging from simple topographical changes to creation of surface chemistries and coatings that are radically different from the bulk polymer. Furthermore plasma treatments are environmentally friendly and economical in regard to their use of materials.

Plasma processing can be classified into at least four categories that often overlap. These are the following: (1) surface preparation by breakdown of surface oils and loose contaminates, (2) etching of new topographies, (3) surface activation by creation or grafting of new functional groups or chemically reactive, excited metastable species on the surface, and (4) deposition of monolithic, adherent surface coatings by polymerization of monomeric species on the surface. Key features of these processes will be briefly discussed, with a rudimentary introduction to the chemistries involved, as well as examples. Focus is placed on capacitively coupled radio-frequency (rf) plasmas (see Figure 1 in the article by Lieberman et al. in this issue of MRS Bulletin) since they are most commonly used in polymer treatment.