Internal Stress in Plasma Polymer

The internal stress in plasma polymer films is generally expansive, i.e., the force to expand the film is strained by external compressive stress. According to the concept presented by Yasuda et al. [1], the internal stress in a plasma polymer stems on the fundamental growth mechanisms of plasma polymer formation. A plasma polymer is formed by consecutive insertion of reactive species, which can be viewed as a wedging process. The internal stress is related to how frequently the insertion occurs as well as on the size of inserting species. The both factors are dependent on the operational factors of plasma polymerization. 

This curling can be attributed to an internal stress arising in the plasma polymer during polymer deposition. It is important to recognize that the internal stress in the plasma polymer (in the as-polymerized state) is an expansive stress and that this is in marked contrast to what would be expected if the adsorbed monomer were polymerized at the surface of the substrate, which would create, with very few exceptions, contractive stress due to the contraction of volume on the polymerization of a monomer. 

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The internal stress of plasma polymers is dependent not only on the chemical nature of monomer but also on the conditions of plasma polymerization. In the plasma polymerizations of acetylene and acrylonitrile, apparent correlations are found between and the rate at which the plasma polymer is deposited on the substrate [2], as depicted in Figure 11.3. The effect of copolymerization of N2 and water with acetylene on the internal stress is shown in Figures 11.4 and 11.5. The copolymerization with a non-polymer-forming gas decreases the deposition rate. These figures merely indicate that the internal stress in plasma polymers prepared by radio frequency discharge varies with many factors. The apparent correlation to the parameter plotted could be misleading because these parameters do not necessarily represent the key operational parameter. 

INTERNAL STRESS IN PLASMA POLYMERS PREPARED BY LPCAT... 

Figure 11.7 Monomer feed rate dependence of internal stress in plasma polymers 750 seem Ar, 4.0 A arc current.

Figure 11.4 Changes in the internal stress of plasma polymers by copolymerization of acetylene with nitrogen ( ) or water (Q) ...

Figure 11.6c shows the monomer feed rate dependence of internal stress in VpMDSO plasma polymer films at different argon flow rates. The overall values of internal stress in plasma films obtained with argon flow rate at 1500 seem are much higher than those obtained at 750 seem. 

From Figure 11.6c it can also be noted that the internal stress in CAT polymers deceased with increasing VpMDSO monomer feed rate. In plasma deposition process, when the other plasma parameters are kept the same, the increase of monomer feed rate indicates that the same amount of energy input is consumed by a larger number of monomer molecules. In other words, when the other plasma parameters are kept constant, the increase of monomer feed rate will actually reduce the relative energy input in plasma polymerization process. 

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. 

Figure 11.7b shows the internal stress in LPCAT films of cyclic siloxanes 1,3,5,7-tetramethylcyclotetrasiloxane (TMTSO) and 2,4,6,8-tetravinyl-2,4,6,8-tetra-methylcyclotetrasiloxane (TVTMTSO). The large siloxane ring structure in these two monomers did not provide any decrease of internal stress in resultant plasma polymer films, compared with simple siloxane monomers, i.e., TMTSO, HMDSO, and VpMDO. 

If a comparison is made between Figures 11.11 and 11.13, a correlation of higher internal stress to larger refractive index in plasma polymer films was observed. In the plasma polymer films with larger refractive index, the internal stress that developed during the deposition process is more difficult to release afterward due to the tighter structure. 

Low-pressure cascade arc torch polymerization or coating could be considered more or less the same as the plasma polymerization or coating by other conventional plasma processes. The ultrathin layers prepared by LPCAT polymerization have the general characteristics of plasma polymers, i.e., amorphous (noncrystalline), high concentration of the dangling bonds (free radicals trapped in immobile solid phase), and the high degree of the internal stress in the layer. 

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. 

Type A plasma polymers have the characteristic internal stress built in the film, and the plasma polymerization coating acts as the tempered ultrathin layer on the substrate. The internal stress is caused by the wedging effect of the deposition process, and the total stress increases linearly with the thickness. As the thickness increases, the internal stress reaches the critical point beyond which the internal stress becomes greater than the cohesive force or the adhesive force of the plasma polymerized coating. Above the critical thickness, therefore, the coating cracks (not necessarily in macroscopic sense) or delaminates (buckles) from the substrate. Consequently, there exists a thickness limit of plasma polymerization coating. The tighter the structure, the smaller is the thickness limit. 

With a cross linked structure, it is often characteristic of plasma polymers, that internal stress builds up as the film thickness increases. The internal stress can in fact be measured from the curling force produced when plasma polymer films of known thickness are deposited on a substrate (e.g. LIffE) for which the bulk modulus is known. Typical data are shown in Figure 13. 

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