Stress in Glow Discharge Polymers

The practical importance of the internal stress is that if too thick a layer is deposited by plasma polymerization, the overall force reaches the point where the internal stress exceeds the cohesive force of the plasma polymer or the internal stress at the interface exceeds the adhesive force of the plasma polymer to the substrate. 

If the adhesion of plasma polymer to the substrate is good, cracking of the plasma polymer results, as evidenced by many studies of transport characteristics of plasma polymers as a function of the thickness of plasma polymer (see Chapter 34). If the adhesion is poor, buckling of the plasma polymer deposited onto a rigid substrate such as a glass plate occurs. 

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. 

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. 

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