Adhesion-Promoting Plasma Polymer Layers

Surface functionalization of PP films in the O2 plasma was performed in the cw mode. PP films which were coated with plasma polymer layers of functional-group carrying monomers had been used without any additional plasma pretreatment. PTFE films were exposed first to H2 radio-frequency (RF) plasma (cw mode) for 1-1800 s at pressure p=6Pa and power P=300W, followed by the deposition of adhesion-promoting plasma polymer layers. 

Deposition of Adhesion-Promoting Plasma Polymer Layers 

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A few variants for producing well-adhering Al-PTFE systems were tested (a) deposition of plasma polymers bearing a single type of functional group as adhesion promoters on virgin PTFE (b) H2 plasma pretreatment of PTEE followed by deposition of an adhesion-promoting plasma polymer layer and (c) H2 plasma pretreatment of PTFE alone. 

After peeling of Al-polymer systems with adhesion-promoting plasma copolymer layers, the peeled surfaces were inspected again with XPS to determine the locus of failure, i.e., whether the peel front propagated along the interface (interfacial failure) or within the material (cohesive failure). 

By means of OH- and COOH-containing plasma polymer layers the quahfica-tion of these layers as models of single-type functionalized adhesion promoters with variable concentrations of functional groups should be proved. The plasma-initiated copolymerization of acrylic acid with ethylene or 1,3-butadiene is shown in terms of measured COOH concentration as a function of the composition of the comonomer mixture in Fig. 18.3. Depending on the co-monomer reactivity, a more linear correlation (butadiene), or a parabolic behavior (ethylene), between precursor composition and COOH groups produced was observed. For each type and concentration of functional group, its concentration was determined by chemical derivatization followed by XPS analysis as described in Section 18.2.5. 

The introduction of oxygen is most likely due to post-plasma reactions of plasma-produced C-radical sites at the PTFE surface by reaction with molecular oxygen when the samples were transferred from the plasma reactor to the XPS spectrometer with transient contact with air [73, 80]. This post-plasma reaction is unavoidable, but in this case it may help to promote the adhesion between the deposited plasma polymer layer and the pretreated PTFE. 

The introduction of an adhesion-promoting pulsed plasma polymer interlayer onto the H2-plasma pretreated (each for 10 s) PTFE substrate improved the peel strength further to a range of 350—400 N m limited by the adhesion of the adhesive (supporting) tape to the evaporated A1 layer. Because of this limitation of the peel test, the measured peel strength is no measure for the difference between OH and COOH groups at the PTFE interface in this case (Fig. 18.9). 

Very little of the research that has been done on these proteins has involved the use of electrochemical techniques. Instead, ellipsometry, FTIR/ATR spectroscopy, radioactive labeling, and photon correlation spectroscopy have been used. Many of the studies have been directed toward the development of biocompatible polymer surfaces. The first event that takes place after contact of blood or plasma with an artificial surface is the rapid adsorption of proteins from the blood onto the material surface. It is generally assumed that all subsequent events, such as platelet adhesion and surface activation of blood coagulation, are determined by the composition and structure of the initially adsorbed protein layer. It is known from in vitro experiments that the adhesion of platelets is promoted when fibrinogen has been adsorbed on a material surface and that platelet adhesion is reduced when preadsorbed albumin is present on the surface. In a study of the adsorption behavior of three of the more abundant... 

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