Plasma-initiated polymerization

Although in certain respects plasma polymerization may be thought of as plasma-initiated polymerization this latter description is normally reserved for polymerization reactions that take place after the plasma (normally excited in the vapour of the monomer above the liquid monomer) has been extinguished. Polymers formed in such a way are generally linear and of high molecular weight. Typical monomers are vinyl compounds and the products are often crystalline. 

Acknowledgement.—1 would like to extend my thanks to Mrs. D. S. Duckworth of Thornton Research Centre for her assistance with literature searches, and to Drs. A. Dilks and E. Kay for providing manuscripts prior to publication. 

Yasuda and N. Inagaki, Purazuma Jugo, Tokubetsu Toronkai, 1979,239. 

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The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

A more complicated but very variable process (iii) was the pulsed plasma-initiated polymerization or copolymerization. Here, the desired monotype functional groups are provided by the monomer, which are polymerized in the pulsed plasma. The art in producing such 50 nm thick monotype functionalized polymer coatings lays in carrying out the plasma process under as mild conditions as possible to avoid fragmentation of monomers and to preserve and remain the functional groups of the respective monomers. 

Depolymerization of PBS in fluorocarbon plasmas results in system-dependent etching rates which are difficult to reproduce. This is due to plasma-initiated polymerization of the gaseous degradation products which competes with the degradation reaction. 

Modification of Textile Fibers. The reaction of hydrophobic chemicals with textile fibers offers the possibUity of permanent repeUency without alteration of the other physical properties of fibers. However, the disadvantages caused by complex processing, and resultant higher costs of carrying out chemical reactions on fiber in commercial textile plant operations, have limited the commercial appHcations. The etherification and esterification of ceUulose have been most effective in terms of achieving durable water repeUency (32,33). Radiation grafting of reactive repeUents onto fibers has been studied as a potential commercial process (34,35), as has modification by plasma polymerization of gas monomers or plasma initiated polymerization of Hquid monomers (36). 

Plasma-Initiated Polymerization and Copolymerization of Liquid Vinyl Monomers... 

Monomers Polymerizable by Plasma Initiation. Polymerization data for all of the vinyl monomers utilized in this study are summarized in Table 1. As shown previously, methyl methacrylate is readily polymerizable (, ). Methacrylic acid (MAA) and acrylic acid (AA) are polymerized immediately upon exposure to the plasma. Because the resulting polymers are insoluble in their monomers, the products are precipitated out and conversion is low despite prolonged post-polymerization. However, if water is now added as solvent, polymerization becomes homogeneous and high conversions can be readily achieved with post-polymerization. For example, after a 15 second plasma initiation period more than 80% yield was obtained for a 75% aqueous solution of MAA. The molecular weight, determined by intrinsic viscosity measurements, was found to be 4.5 X 10 gm/mole. 

When solid monomers of acrylamide (AM) and methacrylamide (MAM) were subjected to plasma initiation, only trace amounts of insoluble polymers were obtained. Now if aqueous solutions of AM and MAM were used in the plasma initiated polymerization high conversions were again achievable upon post-polymerization. Polyacrylamide was found to be completely soluble, whereas polymethacrylamide forms a gel. It is of interest to note that no poljnner is formed with post-polymerization alone without plasma initiation... 

We have recently shown that solid state polymerizations can also be carried out via plasma initiation ( ). In that work, 1, 3,5-trloxane and 1,3,5,7-tetraoxane were used as monomer crystals. Highly crystalline polyoxymethylene were obtained using either monomer. However, if the monomers were dissolved in an appropriate solvent, such as cyclohexane, then no polymer was formed with plasma initiation. These observations are the reverse of those for AM and MAM, where plasma initiated polymerizations in solution appear to proceed satisfactorily during homogeneous post-polymerization periods, but not in the bulk crystalline state. The unresolved question is then if water molecules may have dissociated in the plasma in highly active radical species, perhaps OH or H, to promote efficient initiation. 

Figure 1. Monomer—copolymer composition relationship for the plasma-initiated copolymerization of methyl methacrylate with styrene. Plasma-initiated polymerization (%) NMR, (x) elemental analysis. Thermal polymerization (O) NMR, (Aj elemental analysis, (—) theoretical curve, tmma = 0.46 =...

Figure 3. NMR spectra of poly(methyl methacrylate-co-methacrylic acid) obtained by (1) plasma-initiated polymerization and (2) thermal polymerization in 0.2% deuterated pyridine.

Characterization of Crystalline Poly(trioxane) and Poly(tetraoxane) Obtained through Plasma-Initiated Polymerization... 

Single crystals of TOX and TEOX of the approximate dimensions of 1 mm X 10 mm were prepared by sublimation under reduced pressure. Radiation-initiated polymerizations of these crystals were carried out by y-ray pre-irradiation (1 MR) at room temperature in air. They were subsequently post-polymerized at 55°C for TOX, and at 62, 81 and 105 C for TEOX ( ). Plasma initiated polymerizations were conducted b sealing the crystals in a glass ampule after degassing at 10 - 10 torr. A glow discharge was initiated in... 

SAXS pattern of radiation-polymerized PTOX in Figure 3A shows only a sharp equatorial scattering, but no meridional scattering. Those for PTOX obtained by plasma initiated polymerization (samples PTOX-20-1P and PTOX-40P shown in Figures 3B and 3C) are similar. In the case of PTEOX, those polymerized through radiation initiation but post-polymerized below 80°C (sample PTEOX-12)... 

Figure 1. Scanning electron micrograph of polytrioxane (Sample PTOX-40P) obtained by plasma-initiated polymerization...

Sub-crystal fractions in PTOX from both radiation and plasma initiated polymerizations were determined from the (100) reflections and summarized in Table 2. Their values are somewhat, but not substantially, lower for the plasma samples than for the radiation samples. However, since the amount of subcrystal fraction depends on both the temperature and the yield, meaningful comparisons between these two types of samples are difficult on the basis of these rather limited data. 

Figure 8. Differential scanning calorimetry thermograms of polytrioxane and polytetraoxane obtained by plasma-initiated polymerization in the solid state...

The twenty chapters included in this volume can be conveniently divided into the following groups review plasma polymerization of hydrocarbons plasma polymerization of fluorocarbons plasma polymerization of organometallic systems plasma-initiated polymerization and applications of plasma polymerization. Though the emphasis of this Symposium is on the fundamental aspects of plasma polymerization, we should not lose sight of the fact that it is the potential applications of this technique that has stimulated the efforts in basic research. Potential applications for plasma-polymerized films include membranes for reverse osmosis, protective coatings for optical components, and insulating layers for semiconductors. 

Non-thermal plasma can be applied not only for the stepwise polymerization discussed earlier, but also for effective stimulation of more conventional chain polymerization processes. Plasma-initiated polymerization of methyl methacrylate (MMA) with production of practically important polymer poly methyl methacrylate (PMMA) is a good example of such... 

The following are typical characteristics of enzyme immobilization by use of plasma-initiated polymerization ... 

Plasma-initiated polymerization of grafting can be carried out by using polymerizing gases and precursors like fluorocarbons, hydrocarbons and silicone containing monomers (Fig. 3.1). Carrier gas plays important roles in these plasma-surface interactions, and usually inert gas like helium or argon is used as carrier gases [27]. 

A plasma polymerization is a gas plasma initiated and/or propagated conversion of a low-molar-mass compound into a polymer. The plasma-initiated polymerization has also been called a plasma-induced polymerization and the plasma propagated polymerization is sometimes named a plasma state polymerization. The mechanism of the former is a conven-... 

In 1978 Shen and Bell [36] developed a technique called plasma-initiated polymerization in which the plasma was used to initiate conventional polymerization of liquid vinyl monomers, resulting in soluble linear polymers with high or even ultrahigh molecular weights [37]. Moreover, we found that if the polymerization conditions were set up appropriately, the polymer molecules would be aligned and packed more regularly, even leading to the formation of crystallized products [38]. 

Osaka, Y, and Shen, M., Plasma-initiation polymerization, in Plasma Polymerization, ACS Symp. Ser. No. 108 (M. Shen and A. T. Bell, eds.), based on a symposium sponsored by the ACS Division of Polymer Chemistry at the 176th Meeting of the American Chemistry Society, Miami Beach, September 15-16, 1978, American Chemical Society, Washington, DC, 1979, pp. 253-261. 

Johnson, D. R., Osada, Y, Bell, A. T., and Shen, M., Studies of the mechanism and kinetics of plasma-initiated polymerization of methyl methacrylate. Macromolecules, 14, 118-124 (1981). 

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