Low-pressure plasma processing of materials can be divided into three categories, namely (A) etching (removal of material),(B) deposition (addition of new material to a surface), and (C) modification (morphological, structural, and physicochemical change of the surface or near-surface region). In industries which make extensive use of low-pressure plasmas (for example, in the manufacture of integrated circuits-IC, the treatment of polymers for improved adhesion, etc), the above-named changes are deliberate and highly beneficial. However, there exist many instances where treatment can turn into a liability, or where plasma-chemical changes occur involuntarily and are a priori detrimental. The main objective of this chapter is to sensitize the reader to the existence of circumstances where plasma effects can be deleterious, for example:(1) Corona discharges, also known as silent discharges or dielectric barrier discharges, are a form of plasma which occurs when insulating materials are exposed to an alternating source of high voltage (~ 10 kV). Corona is comprised of multitudes of ultra-rapid (~ 100 ns), narrow (~ 100 μm) filamentary micro-discharges, which impinge upon the dielectric surface. Since the 1950s corona is being used commercially for treating polymeric webs up to 8 m in width, so as to render them printable (process category (C) above). However, corona treatment (like its low-pressure counterpart) can be detrimental if" overtreatment" occurs: If the reagent gas, like ambient air, contains oxygen, low-molecular-weight oxidized materials (LMWOM) form on the surface, and these can give rise to a weak boundary layer. This laboratory has compared corona and glow discharge treatment of LDPE and PET, using peel strength and XPS measurements, and has found similar" optimum" treatment criteria for both types of processes: High treatment (oxidation) levels could be correlated with elevated concentrations of acidic (O= C-O) reaction products and low peel strength.(2)

A low pressure plasma comprises a complex mixture of electrons, charged and neutral molecules and fragments in the ground state and excited states, and a broad spectrum of radiation ranging from the infrared to the far ultraviolet. The specific role of each of these components in a plasma treatment of polymers is still not understood completely. The experimental data reported in the literature seem to be contradictory.

Many useful processes for treating solid surfaces can be carried out by plasma methods. However, most previous work was done at low pressure, usually less than a few torr. For such low pressure processes, the vacuum apparatus requires great cost and is not suitable for the treatments of large scale substrates such as long film rolls. We previously reported that surface fluorination and thin film deposition could be carried out with the atmospheric pressure glow plasma (APG) process [1]. This approach can reduce apparatus costs and can also be applied to high vapor pressure substances such as gum, textiles and biomaterials. In this article, we will discuss the mechanism of stabilization of glow plasma at atmospheric pressure and report examples of applications of this technology.

According to the concept described by Langmuir in 1938 [1], the surface properties of a solid are determined by the surface-configuration (spatial arrangement of atoms at the interface) rather than the configuration of molecules which occupy the top surface region. In other words, whether a polymer surface is hydrophilic or hyrophobic cannot be predicted by the presence or absence of hydrophilic moieties in the molecules, but is determined by whether or not the hydrophilic moieties are located at the interface. In recent years, it has been recognized that the surface of a solid, particularly polymeric solid, is very different from what can be anticipated from the bulk characteristics of the same material. This discrepancy has been a focal point of the general phenomena recognized by the terms surface dynamics, surface reconstruction, etc., which deal with the change of chemical and morphological properties of polymer surface due to the change of the surrounding medium [2-14]. The surface dynamic change depends on the reference state from which the change takes place, and if one cannot define the reference state, the surface dynamics cannot be dealt in a generic sense. This problem was indeed found with moderately hydrophilic copolymer of ethylene/vinyl alcohol. The reference state depends on the history of a sample, and the change cannot be reproduced without precise knowledge of the history of a sample [15]. According to the view that a polymeric surface is an ever-changing entity depending on the surrounding medium [16], the restructured surface is not necessarily the final one to stay, ie, restructuring of once restructured surface or multiple repeated restructuring occur with highly perturbable polymeric surfaces. Therefore, the term" surface reconstruction" is intensionally avoided in the discussion of surface dynamics in this article. The change of surface is expressed by the change of surface-configuration.

In recent years, fluorocarbon plasmas have been extensively used for the treatment of polymer surfaces in an increasing number of applications. A decrease of the surface wettability is observed after exposure of the polymer to the discharge, to a degree depending on the treatment time and discharge parameters. Fluorination of the polymer surface, following exposure of the surface to the discharge, is believed to be the result of functionalization and/or polymerization, depending on the plasma composition. However, due to the complexity of the chemical reactions both in the gas phase and at surfaces, the underlying mechanisms are not yet well understood. The characterization of the reactive species formed in the discharge and the possible correlation of their behaviour to the plasma-induced modification of the surface properties is essential for understanding the role of the species and for the identification of the mechanisms of surface modification. Fluorocarbon radicals can be detected in situ by a number of diagnostic techniques, that include optical emission spectroscopy [1], laser-induced fluorescence (LIF)[2-4], UV absorption [5], infrared diode laser absorption spectroscopy (IRLAS)[6-8], and threshold ionization mass spectrometry [9-11].

The lack of data concerning all the species and the microscopic phenomena involved in low pressure plasmas has always been the major obstacle for the complete understanding of the process mechanisms. As a matter of fact, even today, there is no gas-discharge system for which we can have, by whatever diagnostic tools, a complete picture of the concentration profiles of the species, either charged or neutral, involved in the various gas phase, gasfield, and gas-surface interactions (figure 1). This is more pronounced as we go from simpler noble or molecular gas plasmas to the more and more complex" chemical" plasmas used for the deposition of thin films. The main difference between classical chemical reactors and these plasma reactors comes from the presence of the electromagnetic field in interaction with various charged particles and surfaces. This makes the different plasma processes not easily predictable, controllable and comparable with each other. The rf power used in all these processes implies special reactor design and operation regimes which are different from the idealized plug-flow (PFR) or continuous stirred tank (CSTR) chemical reactors. Therefore, what one measures outside the reactor has no straightforward relation with what is happening inside, and there is no universal way of translating this information since, the" reacting gas volume" is not known, isotropic or homogeneous, while all the microscopic plasma quantities are also functions of space, in the specific reactor. On the other hand, a variation of one of the process parameters, like power or pressure, is not only associated with a change in the value of other macroscopic and microscopic quantities, but also with the way they interact with each other, with the electric field, and the surrounding surfaces. Two basic requirements arise from the discussion above: the need for more efficient, yet simple, non-intrusive diagnostics, and the need for more accurate process control. In fact, both requirements end up to the need for more accurate measurements. This is essential if the characterization of the discharge in a universal way is to be pursued, and it is also the main prerequisite for understanding the basic mechanisms governing the process, for building realistic mathematical models and in general for the development of the discharge theory.

In order to achieve a complete understanding and control of plasma processes an appropriate knowledge of the structure of the particular glow discharge utilized is necessary. This is extremely important because the electrical potential distribution inside a plasma reactor is not uniform and therefore, as a function of the reactor geometry and sample position, charged particles are accelerated from the plasma bulk to the substrate to be treated by different potential drops, ie they impinge on different surfaces with different energy.

Ion bombardment of polymer surfaces is a method used in promoting metal/polymer adhesion. The adhesion of these multicomponent interfaces can be attributed to chemical bonding, physical bonding, or a combination of both. By evaluating the resistivity of thin films of Cr, TaSiz, Pd and Au deposited on polymer, the interfacial reactivity was determined, and the contribution due to chemical bonding identified. The adhesion strength of these interfaces, determined by peel strength measurements, increases with interfacial reactivity. Interfacial reactivity increases with the total energy of all the ions bombarding the polymer surface (dosage). Cr and TaSi₂ show extensive interfacial reactivity than noble Au and Pd.

Knowing that the oriented positive ion bombardment plays an important role in the plasma treatments of polymers, some investigations using a positive ion beam-plasma system were carried out. Preliminary results concerning the surface modifications of poly (ethylene terephthalate) films induced by the action of oxygen ion beam are presented. Ion energies (50-500 eV) and doses (3.0 x1015 1.5 x1016 ions/cm²) are those used in a reactive ion etching device. Techniques such as: determination of the surface free energy components by the contact angle method, thermal methods (DTA, DSC, etc.), IR spectroscopy, SEM, XPS, were used to characterize the surface modifications. The relation between chemical and physical modifications is discussed.

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.

Static contact angle and dynamic (advancing and receding) contact angles of water on polymeric surfaces were investigated using microscope cover glasses coated with various plasma polymers of trimethylsilane and oxygen. By variation of the mole fraction of the TMS/oxygen mixture, glass surfaces having varying degrees of wettability were prepared. The advancing contact angle of a sessile droplet, which is independent of the droplet volume, is considered as the static contact angle of water on a polymeric surface, θ_S, which is a parameter characteristic to a polymeric surface. The dynamic contact angle of water refers to the contact angle of which three-phase contact line is in motion with respect to the surface. The dynamic advancing (immersing) contact angle, θ_D,a, and receding (emerging) contact angle, θ_D,r, were measured by the Wilhelmy balance. The difference between θ_D,a and θ_D,r is mainly due to the direction of dynamic force acting on the three-phase contact line. The discrepancy between the immersion and the emersion buoyancy lines and the corresponding values of contact angles can be used to indicate the hysteresis due to the dynamic factor (the dynamic hysteresis). The dynamic hysteresis is largely determined by the critical immersion depth in which the three-phase contact line remains at the same place on the surface while the shape of meniscus changes when the motion of the sample is reversed. The dynamic hysteresis may contain the contribution of the change of static contact angle due to the surface-configuration change caused by the wetting of the surface (the intrinsic hysteresis). The dynamic hysteresis varies according to the value of cos θ_S, with the maximum at the threshold value around 0.6 and linearly decreases above this value, as the emersion line approaches the limiting buoyancy line determined by the surface tension of the liquid. The intrinsic hysteresis follows the same trend with the maximum at around 0.8. The three contact angles are related by cos θ_S = (cos θ_D,a + cos θ_D,r)/2.

The main available theories for the dynamics of wetting are brieflysummarized and discussed in reference to experiments. In partial wetting,hydrodynamic and molecular theories are equivalently efficient, even if thephysical meaning of parameters is not so clear in the former ones. Incomplete wetting, hydrodynamic theories are the only ones valid at lowangles, but some care has to be taken in the interpretation of the “slip length” introduced to remove the divergence of thedissipation at the contact line. The situation is less favourable at themolecular scale, where the theoretical description is still at itsbeginning, due to the multiplicity of behaviours.

The chemical and physical effects incurred at the surface of biaxially oriented polypropylene film during silent discharge plasma treatment have been investigated using XPS, NMR, TOF-SIMS, and AFM techniques. It is found that chain scission accompanied by oxidative attack leads to the formation of low molecular weight oxidized material which agglomerates into globules at the surface due to a large difference in interfacial free energy between the underlying hydrophobic substrate and the oxygenated overlayer.

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.

We have been able to generate the wide and stable plasma in open air (discharge distance, 35 cm; discharge-electrode length, 16 m at maximum) using a pulse with a high voltage and narrow wave form. This was applied to treat the surface of rather non-polar plastics intended for the improvement of adhesion of over-coated layers such as coatings, adhesives and printing inks. The treating system (APPS) consists of the apparatus for generating the plasma and the treating process. Polypropylene (PP) and tetrafluoroethylene perfluorovinyl ether copolymer (PFA) have been examined as typical examples of the plastics. The adhesion strength of urethane paint on PP molding and of a PFA film on steel was significantly improved by the APPS treatment. The characteristics of the surface layer were evaluated by means of scanning electron microscopy, electron spectroscopy for chemical analysis, atomic force microscopy, and contact angle measurement, and it was found that hydrophilic functional groups were introduced into the surface layer of the plastics. The level of the improvement changed with time after treatment; this is discussed from the viewpoint of functional group movement from the surface to the interior. Application of paints on PP bumpers by the electrostatic spray method was also accomplished. The use of a small amount of nitrogen-containing compound following APPS treatment decreased the electrical resistance of the PP surface from 10¹⁶ to 10¹¹ Ω, and highly effective electrostatic coatings of PP bumpers could be realized.

A method has been devised to determine the acid/base parameters of reference liquids as absolute numbers, and not as values relative to a conventional set of parameters for water. Contact angle measurements are employed, using three liquids on three solids. The theory calls for the solution of nine simultaneous, nonlinear equations in nine variables–and unreasonably formidable task.

A preliminary set of solutions has been computed, for one set of polar liquids on five solids. These results must be rejected on grounds of physical reasonableness. They also fail the test of predicting liquid-liquid interfacial tension, which for miscible liquids must be negative or zero.

Wetting on ambient, heterogeneous surfaces is characterized by contact angle hysteresis. Quantitative models of contact angle hysteresis are essential in order to design surfaces with specific wetting behavior or to interpret experiments seeking to characterize a surface through its wetting properties. We focus on the successes and failures of theoretical models as well as experiments on model surfaces in describing contact angle hysteresis on ambient surfaces. We describe experimental observations of contact line structure and dynamics as well as contact angle hysteresis on laboratory surfaces. We discuss three general classes of models treating one-dimensional periodic heterogeneity, two-dimensional periodic heterogeneity, and random heterogeneity. We show where these models succeed and where they fail to agree quantitatively and qualitatively with experimental observations. New models treating strong, dense heterogeneity as well as temporal relaxation of contact angles in experimental environments need to be developed to provide quantitative descriptions of contact angle hysteresis on ambient surfaces.

Ion Irradiation on polytetrafluoroethylene(PTFE) has been carried out to improve adhesion to metal and to adhesive cement. Argon ion was irradiated on the polymer, and amount of Ar⁺ was changed from 10¹⁴ ions/cm² to l×10¹⁷ ions/cm² at 1 keV, and 4 ml/min of oxygen gas was flowed near the polymer surface during the ion irradiation. Wetting angle was changed from 100 degree to 70 - 150 degree depending on the ion beam condition. The changes of wetting angle and effects of Ar⁺ irradiation in oxygen environment were explained in a view of surface morphology due to the ion beam irradiation onto PTFE and formation of hydrophilic group due to a reaction between irradiated polymer chain and the blown oxygen. Strongly enhanced adhesions were explained by interlock mechanism, formation of electron acceptor groups on the modified PTFE, and interfacial chemical reaction between the irradiated surface and the deposited materials.

Modified surfaces of polyethylene terephthalate)(Mylar® or PET film) have been studied by surface energetics, ESCA, atomic force microscopy (AFM), and optical profilometry. For the surface energetics studies, receding contact angle titrations were used to evaluate the surface functional groups in the outer few angstroms of the surface. This sensitive method of determining the contact angle with buffer solutions of different pHs allows one to investigate the nature of the chemical species introduced by the various energetic treatments. The data are consistent with a surface that is covered by a low density of carboxylic acid moieties in the case of corona and flame treatments, applied in a high-speed commercial type of a process at low doses. The high contact angle hysteresis indicates that the coverage is moderately heterogeneous but on a very small length scale, less than a few micrometers. ESCA qualitatively supported this, although this technique is not optimum for the low degrees of surface modification. A comparison is made of the two surface treatments in terms of depth of penetration, roughness, and surface density of chemical moieties introduced. UV laser-treated surfaces showed no indication of surface chemical modification.

Hydrophilic polymer surfaces are desirable for many applications, such as adhesion and wettability. In this study, we have developed a tenaciously bound hydrophilic surface coating which can be applied to a hydrophobic polymer using a plasma treatment process. In this process, porous polyethylene (PE) was used and pretreated with the plasma discharge of an oxidizing gas, eg carbon dioxide. The treated surface, containing mainly anionic groups, was then soaked in a polycation solution, eg polyethyleneimine (PEI). A tenaciously bound hydrophilic coating was formed due to multiple anchors (ionic interactions) between PEI and the plasma-treated surface. The coated surface was characterized using water contact angle goniometry and X-ray photoelectron spectroscopy (XPS). Both the stability and the durability of the coating have been evaluated using various storage conditions and repeated washing in water. The coating process developed in this study is useful in many applications which require a permanent and lasting wettable polymer surface.

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.

Synthetic and analytical procedures for the preparation of surface-functionalized polyolefins are described within the general context of polymer surface functionalization. Chemistry leading to simple functionalization of both polyolefins in general and polyethylene in particular is described. Selected examples of further chemistry leading to graft copolymers attached to these surfaces are described. The effects of such graft chemistry leading to various sorts of chemically modified surfaces with different solvent-polymer interactions specific to the graft microstructure and to temperature are discussed.