Applications of Plasma Polymers

Both electronic and medical applications of plasma polymers have been reported [54-61]. Most of these investigations are on the interface between polymers and inorganic materials, for instance, metal/polymer interfaces in structural adhesive joints, and cation diffusion along polymer/metal interfaces under an applied electric potential. In another reference, more specific aspects for electrical and electronic applications [59] were treated, wherein protective films for microcircuitry, and for wettability were explained. The use of such film for surface treatment has also been examined. 

Thin films of high thermal resistivity and electrical insulation were prepared by plasma polymerisation of silazane and subsequent pyrolysis in air. The film had strong adhesion, high thermal resistivity, and very good insulating properties in a broad range of temperatures. The author described how the chemical constitution of the films transformed into a silicon oxide type and a polycrystalline structure after the pyrolysis process [63]. 

The hybrid films for the pinhole-free electrical insulators were prepared using hexamethyldisiloxane (HMDSO) and silicon monoxide (SiO) [64]. The HMDSO hybrid films were prepared on the substrates by evaporating SiO during HMDSO plasma polymerisation in RF discharge. SiO was evaporated by heating in RF plasma consisting of HMDSO and oxygen at a pressure of 10 Torn 

The films for insulators can be fabricated not only by inorganic compounds as mentioned above, but also by organic compounds. Plasma-polymerised films with uniform thin 

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The expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

The reactive species in Parylene C deposition that interacts with the substrate surface is para-xylylene, in which two free radicals exist in the para position of a benzene ring. Para-xylylene is relatively stable and reacts only with other free radicals or with other para-xylylene units. In order to create a good adhesion of Parylene C film to a smooth-surface substrate, it is necessary to create free radicals on the substrate surface. With the aid of plasma interface modification, it is possible to achieve strong adhesion of Parylene coatings to such smooth surfaces. Strong adhesion of Parylene C coating to bare 7075-T6 (an aluminum alloy) panels was achieved with the application of plasma polymers [16]. 

Because of significantly lower levels of solubility, permeation through a plasma polymer is often not governed by a truly solution-diffusion principle. Many plasma polymers show characteristics in between solution-diffusion type polymers and molecular sieves. This difference in permeation mechanisms might be utilized favorably in applications of plasma polymers. 

In view of fact that most time-dependent failures, such as the fatigue of polymers, initiate at the surface, more precisely at the interface of the polymer and the surrounding medium, it is to be expected that the application of plasma polymers will contribute to the improvement of the wear characteristics of polymers at least in certain cases. 

Zhang Z. (2003) Surface modification by plasma polymerization and application of plasma polymers as biomaterials, PhD Thesis, Johannes Gutenberg-Universitat Mainz. 

Applications of Plasma Polymers as Corrosion-Resistant Layers on Reactive Metals... 

The potential applications of plasma polymers as materials for electrochemistry are... 

On the whole, the applications of plasma-source emission detection to GC in the field of polymer/additive analysis are limited. The same holds for GC-atomic absorption spectrometry [370]. 

The hydrodynamic factors that influence the plasma polymerization process pose a complicated problem and are of importance in the application of plasma for thin film coatings. When two reaction chambers with different shapes or sizes are used and when plasma polymerization of the same monomer is operated under the same operational conditions of RF power, monomer flow rate, pressure in the reaction chamber etc., the two plasma polymers formed in the two reaction chambers are never identical because of the differences in the hydrodynamic factors. In this sense, plasma polymerization is a reactor-dependent process. Yasuda and Hirotsu [22] systematically investigated the effects of hydrodynamic factors on the plasma polymerization process. They studied the effect of the monomer flow pattern on the polymer deposition rate in a tubular reactor. The polymer deposition rate is a function of the location in the chamber. The distribution of the polymer deposition rate is mainly determined by the distance from the plasma zone and the... 

The rapid development of solid state physics and technology during the last fifteen years has resulted in intensive studies of the application of plasma to thin film preparation and crystal growth The subjects included the use of the well known sputtering technique, chemical vapour deposition ( CVD ) of the solid in the plasma, as well as the direct oxidation and nitridation of solid surfaces by the plasma. The latter process, called plasma anodization 10, has found application in the preparation of thin oxide films of metals and semiconductors. One interesting use of this technique is the fabrication of complementary MOS devices11. Thin films of oxides, nitrides and organic polymers can also be prepared by plasma CVD. 

A great deal of research has been focused on the evaluation of plasma polymers and plasma treated materials for blood and soft tissue contacting-applications (2,3). A number of studies have involved the physical adsorption or covalent attachment of a variety of biomolecules to various gas plasma-treated polymer surfaces (4,5). In such studies, however, the covalent immobilization is often assumed to take place through precursor groups formed at the biomaterial surface from ill-defined oxygen and nitrogen functionalities obtained directly from the plasma. 

Clark, D.T., Dilks, A. and Shuttleworth, D., "The Application of Plasmas to the Synthesis and Surface Modification of Polymers" in Polymer Surfaces, Ed. D.T. Clark and W.J. Feast, J. Wiley, London 1978. 

A thorough understanding and careful control of plasma parameters can enable the process to be tailored to the desired application. It must be acknowledged however, that in spite of an impressive number of studies reported, an accurate prediction of the molecular structure of plasma polymers is not available. However, attempts have been made to examine the effects of various deposition parameters on the growth of plasma derived polymer thin films, and some of the results are summarized below. 

In addition to microelectronic and optical applications, polymers deposited using thermal and plasma assisted CVD are increasingly being used in several biomedical applications as well. For instance, drug particles microencapsulated with parylenes provide effective control release activity. Plasma polymerized tetrafiuoroethylene, parylenes and ethylene/nitrogen mixtures can be used as blood compatible materials. An excellent review of plasma polymers used in biomedical applications can be found in reference 131. 

In the middle of a spectrum, the gain in one feature is attained on sacrifice of another feature. Therefore, one must choose a plasma polymerization process, including type of reactor, reaction conditions, and type of monomer (starting gas or vapor), aimed at a specific type of plasma polymer i.e., type A or type B, suitable for an application. 

The inclusion of particles in a film of plasma polymer was once considered by some investigators to be a characteristic problem due to the plasma polymerization mechanism, which hampers the practical use of plasma polymers in some applications. In contrast to this view, the formation of powder or the inclusion of particles in a film is related to the polymer deposition part of polymerization-deposition mechanisms. The inclusion or elimination of particles, therefore, could be accomplished by selection of the proper operational parameters and reactor design. The data of Tiepins and Sakaoku [7] are a typical demonstration that powders can be formed nearly exclusively if all conditions are selected to favor powder formation. An important point is that the monomers used in their study were those commonly used by other investigators for the study of film formation by plasma polymerization in other words, no special monomer is needed to form powders exclusively. 

Thus, recognition of the characteristic internal stress buildup in a plasma polymer is important for estimating the upper limit of thickness of a plasma polymer for a practical application. Poor results with respect to such parameters as adhesion and barrier characteristics are often due to the application of too thick a plasma polymer layer. The tighter the network of plasma polymer, the higher is the internal stress. Consequently, the tighter the structure, the thinner is the maximal thickness... 

Plasma polymers of certain kinds of monomers have very little, if any, internal stress, and thickness is not a limiting factor of application. However, because of this very feature such polymers may not provide certain coating functions that are sought for the application of plasma polymerization. In other words, the internal stress is not a drawback of plasma polymer but an important characteristic of the materials formed by LCVD. 

In certain applications of plasma polymerization, the incorporation of electrode material, particularly in a controlled and designed manner, is extremely useful and becomes a great asset in LCVD. For instance, a thin layer of plasma polymer of methane with a tailored gradient of copper has been shown to improve the adhesion of the thin layer to a copper substrate as well as the adhesion of metal to a polymer film [3,4]. In general applications of LCVD, in which the metal contamination should be avoided, it is important to select the electrode material that has low sputtering yield. Titanium has been used successfully in such cases. 

It has been generally observed that polymer deposition occurs mainly on a surface exposed to glow. More precisely, the deposition rate of polymer onto a surface that does not make contact with glow is several orders of magnitude smaller than that onto a surface that contacts glow. The results outlined here clearly demonstrate that the rate of deposition of a polymer onto a surface that contacts glow is dependent on the S/pV of the glow. This factor seems to have important implications in the application of plasma polymerization, which may involve substrates of various sizes and shapes. 

When an LDPE film is immersed in a salt solution (0.9% NaCl), the AC resistivity decreases as a function of the immersion time, as shown in Figure 24.11. These figures include the effect of a nanofilm of plasma polymer deposited on the surface of LDPE. With hydrophobic plasma polymer (HFE + H2), the decrease of AC resistivity was not observed. These figures indicate that the surface state breakdown occurs when the salt intrusion takes place. The salt intrusion can be prevented by the application of a plasma polymer, which is an amorphous network (one phase and no weak boundary). The extent of protection seems to be dependent on the hydrophobicity of the network. 

Parylene N to smooth surface materials has been reported with the application of plasma depositions [13,14]. It was reported that excellent adhesion of Parylene C coating to a cold-rolled steel surface was achieved using plasma polymer coatings, in turn giving rise to corrosion protection of the metal [15]. Another major deficiency of Parylene C is its poor painting properties when paint is applied on a Parylene C film, due to its extremely hydrophobic surface. Because of this, surface modification of Parylene films is necessary to enhance their adhesion performance with spray primers. 

Figure 33.7 depicts the influence of plasma pretreatment of CRS surface as well as the hydrophilicity of the plasma polymers on corrosion test results. The left half of the bar graph represents hydrophilic interface and some of top surface are also hydrophilic. The right half of the bar graph represents the water-insensitive interface and nonhydrophilic top surfaces except the plasma polymer of CH4, which was intentionally kept in air for 10 min before application of E-coat. The figure indicates two important factors, i.e., the removal of oxides from CRS/plasma polymer interface, and nonhydrophilic top surface of plasma coatings, for corrosion protection of CRS by plasma interface engineering, which involves application of cathodic E-coat. While the air exposure of plasma polymer of CH4 severely deteriorated the corrosion protection of E-coated sample, the same exposure of TMS surface showed no effect. This difference seems to reflect the reactivity of double bonds described in Chapter 7. 

The author has chosen to draw attention to some possible applications for plasma polymers in the Life Sciences. Whilst considerable attention is given (quite rightly) to recent significant advances in our understanding of biological... 

Many other applications for plasma polymers in the Life Sciences have been dted, often in relation to implantable medical devices or materials, with the goal of concealing the device from the bodies defence mechanisms, or improving cell colonisation of the material, e.g. endothelial cell growth into vascular grafts. A number of excellent studies from the group of Hans Griesser (CSIRO, Australia) describe the use of plasma polymers as substrates to which biomolecules can be immobilised. These immobilisations have been demonstrated to enhance the medium-term acceptability of contact lens materials and may prove relevant to implantable devices. 

The purpose of this paper is to discuss some of the recent advances in our understanding of the kinetics and mechanism of plasma polymerization, the structure and properties of plasma polymers and some of their potential applications. It is not intended to be exhaustive, as earlier reviews (5-] ) are already available. Interested readers are referred to the literature cited for further details. 

Millard,M.M.,Polymer Deposition in Glow Discharge,Chapter V, in "Application of Plasma Chemistry",ed. by Bell,A.T.,Hollahan,J.,... 

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