Plasma polymer dangling bonds

Some monomer show a more or less anticipated decrease in polymer deposition rates based on the concept that a pulsed discharge decreases the initiation rate, but some monomers show dramatically increased deposition rates. The most significant effect of pulsed discharge, however, can be seen in the concentration of free radicals trapped in plasma polymers (dangling bonds), which reflects the unique mechanisms of polymer formation in plasmas. 

XPS data, on the other hand, showed that the ETC AT treatment of Ar + CF4 and Ar + C2F4 yielded just as good, if not better, fluorination of PET fibers than radio frequency plasma treatment with these gases [14,15]. These examples clearly demonstrate that polymerizable species in plasma polymerization are not photon-emitting species in most cases. This is in accordance with the growth and deposition mechanism based on free radicals, which account for the presence of large amount of dangling bonds in most plasma polymers. 

TMS deposition on PE showed only substrate signals with no detectable TMS signal (Fig. 6.12b). The absence of the TMS signal in this system could be due to the fast reaction of TMS radicals with the surface radicals generated from PE. The more likely explanation is that the number of free radicals in the plasma polymer layer is too small in comparison with the free radicals created in the bulk of the substrate, PE. What we see in Figure 6.13 is the decay of PE polymer free radicals, which were created by the luminous gas of TMS. With substantial decay of the PE free radicals, TMS dangling bonds, which decay much slower, became discernible. 

It is tempting to use such relatively simple wet chemical methods to determine the amount of free radical on plasma polymers and on polymers treated with glow discharge. However, these methods have serious limitations when applied to the dangling bonds in plasma polymers or polymer free radicals in polymers treated with glow discharge. The most serious limitation is the accessibility of the chemical to the free radicals to be analyzed. Another serious limitation is the specificity of chemical reactions. 

Figure 7.14 depicts the influence of copolymerization of N2 and H2O on the dangling bonds in plasma polymers. 

In summary, the addition of H2O, N2, CO, or various combinations of these unusual comonomers to a plasma polymerization of acetylene produces chemically distinct polymers. The copolymerization of H2O reduces the quantity of dangling bond in a remarkable manner, as shown in Figure 7.14, and enhances the stability of the polymers. 

Because H2O acts as an efficient modifier of the growth mechanism, which shifts the major growth path (if it is applicable) from cycle 2 to cycle 1, the addition of H2O decreases the concentration of dangling bonds in the plasma polymers. 

The long-term stability of a plasma polymer seems to be related to the concentration of dangling bonds and the tightness of the network which dangling... 

Table 7.18 Dangling Bonds in Plasma Polymers of Acetylene with H2O, N2, and CO...

LPCAT polymerization or coating could be considered more or less the same as the plasma polymerization or coating by other conventional plasma processes, except that the kinetic pathlength of growth is short. 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... 

The role of the second plasma treatment by HFE or Ar seems to be the removal of type B plasma polymer of TMS from the top surface region or possibly converting the type B plasma polymer to type A plasma polymer. Electron spin resonance (ESR) data (described in Chapter 6) indicate that the number of Si-based dangling bonds decreases by these second plasma treatments. The weight loss observed with some plasma polymers and the ESR data for TMS film suggest that type B plasma polymer in the top surface region of an LCVD film could be up to nearly 30% of the... 

In both the polymerizations, free radicals are the species that are responsible for the formation of bonds in the depositing materials. The growth mechanism, however, is not by the conventional chain-growth free-radical polymerization. In a conventional free-radical chain-growth polymerization, two free radicals and 10,000 monomer molecules yield a polymer with degree of polymerization 10,000, which does not contain free radicals. In contrast to this situation, in plasma polymerization and Parylene polymerization, 10,000 species with free radical(s) recombine to yield a polymer matrix that has an equivalent degree of polymerization, and contains numbers of unreacted free radicals (dangling bonds). 

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