Polymerizable species

Table 10.1 presents typical specifications for a polymerization-grade product, as well as some physical properties. Prohibited impurities refer to inhibitors (croton-aldehyde, vinyl acetylene), chain-transfer agents (acetic acid, acetaldehyde, acetone) and polymerizable species (vinyl crotonate), while methyl and ethyl acetate impurities are tolerated. 

The copolymerization of macromonomer with comonomer is governed by the general rules of copolymerization, the ability of any of the two polymerizable species present to participate in the process being determined by the radical reactivity ratios r. Let us denote the macromonomer as M and the comonomer as A. The well-known instantaneous composition law applies to the copolymer formed ... 

Figure 4.5 The time dependence of emission intensity of various photo-emitting species detected by OES in TMS DC glow discharge in a closed reactor (a) polymerizable species in dissociation glow, (b) Hoc emission line at 656 nm 50 mtorr TMS, DC power 5 W.

Photon-emitting species create polymerizable species (which does not emit photon) via energy transfer reaction. 

Polymerizable species that do not emit photons are created by the molecular dissociation reaction occurring in the dissociation glow, i.e. A and/or B in Eq. (4.2) should be replaced by A and B, in this case. 

The remaining Ar emits photon according to Eq. (4.10). The photon emission by A and B is described by Eqs. (4.3) and (4.4), respectively. A fraction (p) of fragmented photon-emitting species reacts with the monomer or its fragmented moiety (represented by X) to yield polymerizable species (represented by AX ). 

In the third case described above, the formation of polymerizable species can be represented by a modified Eq. (4.11) as... 

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. 

In any case, dissociation of organic molecules is the main route to create chemically reactive or polymerizable species in LCVD processes. The dissociation of monomer by the luminous gases occurs based on the principle of the energy... 

Irradiation of monomer vapor does not yield substantial polymerization that is observed in plasma polymerization. In radiation polymerization, the bombardment of electrons creates the initiator for polymerization of the monomer, whereas in plasma polymerization, the bombardment of electrons produces the polymerizable species out of a nonpolymerizable organic molecule as well as of a polymerizable monomer. The polymerizable species could act as an initiator if polymerizable monomers exist, which is not affected by the glow discharge. This situation occurs only in the pulsed discharge of a polymerizable organic molecule in short duty cycle (long resting period). 

The deposition of polymerizable species (the quinoid form and singlet-state monomer) can occur without polymerization. (The deposition of polymerizable species is followed by the polymerization.)... 

Since the dissociation glow can be considered to be the major medium in which polymerizable species are created, the location of the dissociation glow, i.e., whether on the electrode surface or in the gas phase, has the most significant influence on where most of the LCVD occurs. The deposition of plasma polymer could be divided into the following major categories (1) the deposition that occurs to the substrate placed in the luminous gas phase (deposition G) and (2) the deposition onto the electrode surface (deposition E). The partition between deposition G and deposition E is an important factor in practical use of LCVD that depends on the mode of operation. 

In low-pressure cascade arc torch (LPCAT), the electrical power is applied in the cascade arc generator, in which only carrier gas, generally Ar, is activated to create luminous gas. The luminous gas created in the cascade arc generator is blown into the second expansion chamber, in which the monomer is introduced. Thus, the luminous gas of Ar neutrals primarily creates polymerizable species, and following these two steps should treat the deposition kinetics. Principles described in this chapter apply to each of the two steps. Details of deposition kinetics in LPCAT are described in Chapter 16. 

Considering the fact that the refractive index continues to increase after most of the polymerizable species are exhausted in the gas phase, DC LCVD of TMS in a closed system contains the aspect of LCVT of once-deposited plasma polymer coating by hydrogen luminous gas phase. In the later stage of closed-system LCVD, oligomeric moieties loosely attached to a three-dimensional network are converted to a more stable form, and significantly improved corrosion protection characteristics (compared to the counterpart in flow system polymerization of TMS) were found, details of which are presented in Part IV. Thus, the merit of closed-system cathodic polymerization is well established. 

In comparison with conventional electrical discharge processes, LPCAT is a very different process in that its activation of carrier gas and the creation of polymerizable species by the activated carrier gas are temporally and spatially separated. When discharge power is applied to the cascade arc generator, the plasma of carrier gas (usually argon) is produced in the cascade arc column and the luminous gas phase is blown into a vacuum chamber where monomers are introduced. The deactivation of the reactive species, some of which lead to the creation of polymerizable species in the luminous gas phase, occurs within the relatively narrow beam of an argon luminous gas jet. The higher the flow rate of Ar, the narrower is the beam and the longer the luminous gas flame. 

The kinetic pathlength is short due to the high one-directional transport velocity of polymerizable species. 

In such a straight tube reactor, it is clear that the activation of monomer, or the creation of polymerizable species, occurs at the tip of glow where the incoming monomer molecules interact with the luminous gas phase. Thus, the opening of double bond and detachment of F occur at this point, and the further reaction of the free radicals, the species created by the detachment of F, and the detached F s with gas phase species occurs while all gaseous species are moving toward the downstream side of the reactor. Under such a one directional flow conditions, particularly with monomer that contain -CF3, the analysis based on the formation of -CF3 from monomers that do not contain -CF3 might become the focal point of discrepancy. 

---