ESR Study of the Plasma Polymerizations of Trimethylsilane and Methane

Electron Spin Resonance (ESR) was used to study, at the molecular level, the plasma polymerization of trimethylsilane (TMS) and methane. Direct ESR analysis of the plasma coated Al substrate required the use of a novel ESR technique. TMS plasma deposit on Al showed a single broad resonance line near g = 2.003. The signal was stable in vacuum and decayed on exposure to air, with a significant fraction persisting for days. Results show that this signal arises from silicon dangling bonds. Identical TMS signals were observed from films prepared by the DC cathodic or the AF glow discharge method but their decay rates were different. In contrast, the deposition of methane produced two distinct types of carbon-based signals depending upon the method of deposition. TMS or CH₄ films deposited by the DC cathodic method showed slow signals decay and high refractive indices value. While the use of Al as the substrate showed plasma-coating radicals, only substrate radicals were observed when PE was used as the substrate. The nature of radicals formed depends not only on the deposition method used but also on the substrate type.     

Surface modification of chlorobutyl rubber by plasma polymerization

Radio frequency (r.f.) plasma polymerization of vinylidene fluoride (CH₂CF₂) has been used to modify the surface properties of chlorobutyl rubber. FTIR-ATR spectra of the treated rubbers and transmission spectra of plasma polymer films on NaCl windows indicated that as power increased the F/H ratio decreased. SIMS tests supported the FTIR results, and showed that the decrease in the F/H ratio was due to a decrease in the amount of F and an increase in the amount of H in the plasma polymer. Sliding friction measurements showed a reduction in the coefficient of friction (μ) from 3.7 for the untreated rubber to values ranging between 0.4 and 1.9 for the plasma-treated rubbers. There did not appear to be any correlation between the coefficient of friction and plasma power or monomer flow rate, and the average coefficient of friction for the plasma-treated samples was 0.9, which was lower than a commercially used silicone oil treatment (μ = 1.1–1.3). Repetitive sliding friction tests showed that the plasma- and silicone oil treated-chlorobutyl rubbers had the similar lubricating lifetimes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1651–1660, 1997     

Analysis of plasma polymerization of allylamine by FTIR

The plasma polymerization of allylamine in an inductively coupled rf plasma reactor is analyzed by Fourier transform infrared spectroscopy. Comparison of the infrared spectra of the as-received monomer and the plasma polymerized film reveals a conversion of the primary amine in the monomer (? CH₂? NH₂) to an imine (? CH?NH) and a nitrile (C?N). Plasma polymerization of ethylenediamine yields the same results, suggesting that this polymerization scheme may be typical of primary amines. Increasing the plasma power seems to increase the proportion of nitrile groups in relation to the imine groups. The infrared spectra of the vapor phase polymerized monomer was similar to that of the substrate-grafted allylamine film implying a similar structure. Aging of this vapor phase polymer at 120°C for 1 h in vacuum and at 295°C for 15 min in an oxygen free environment reveals nitrile group reaction similar to that observed in polyacrylonitrile. Thermogravimetric analyses of the vapor phase polymers in a nitrogen atmosphere at 20°C/min demonstrated the thermal stability, with the polymer produced at a plasma power level of 50 W retaining 20% of its weight at 1000°C. This was better than the stability shown by the polymer produced at 150 W and is attributed to the ease of nitrile group polymerization in the former.     

Plasma polymerization of fluoromethanes

Methane and fluoromethanes (CH_nF_4−n, 1 ≤ n ≤ 3) were subjected to an rf glow discharge plasma. All the fluoromethanes (including methane) polymerized in the plasma and formed thin films. The deposition rate of the fluoromethanes depended on their monomer structure: CH₂F₂, of which the F/H ratio is unity, showed the greatest deposition rate. The elimination of H and F atoms as H—F was found to be a key factor for the polymerization of fluoromethanes. The chemical composition of the polymerized film, measured with X-ray photoelectron spectroscopy and glow discharge emission spectroscopy, was also found to be strongly dependent on monomer structure. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2043–2050, 1998     

Polymerization and degradation of polyacrylonitrile in gas discharge

The plasma polymerization of acrylonitrile and the plasma degradation of polyacrylonitrile have been studied mass-spectrometrically in high-frequency gas discharge at low pressure. The polymerization was found to proceed without decay to gaseous products. During the polymerization, the monomer partial pressure in the closed plasma-chemical reactor decreased exponentially. When treating polyacrylonitrile in argon plasma, the polymer decayed to C, H₂ and N₂; in oxygen plasma, the species were C, H₂, N₂, H₂O, CO and CO₂. Unlike the thermal decomposition, no traces of monomer and oligomers were detected among the products of plasma degradation.     

A study of surface dynamics of polymers. III. Surface dynamic stabilization by plasma polymerization

As demonstrated in Part II of this series of studies, the hydrophobic character of CF₄ plasma-treated Nylon 6 and poly(ethylene terephthalate) (PET) decay with time of water immersion, and the rate of decay can be used as a measure for the surface mobility of (substrate) polymers. The same method of using fluorine-containing moieties introduced by CF₄ plasma treatment as surface labeling is applied to investigate the influence of a thin layer of plasma polymer of methane applied onto the surface of those polymers. An ultrathin layer of plasma polymer provides a barrier to the rotational and diffusional migration of the introduced chemical moieties from the surface into the bulk of the film. The influence of operational parameters of plasma polymerization on the surface dynamic stability are examined by measuring the decay rate constants for (subsequently) CF₄ plasma-treated samples. The rate constant was found to decrease sharply with increasing value of plasma energy input manifested by J/kg monomer, and no decay was observed as the energy input reached a threshold value (about 6.5 GJ/kg for PET, about 7.0 GJ/kg for Nylon 6), indicating that unperturbable surfaces can be created by means of plasma polymerization.     

Some aspects of plasma polymerization of fluorine-containing organic compounds. II. Comparison of ethylene and tetrafluoroethylene

Plasma polymerizations of ethylene and tetrafluoroethylene are compared. In the plasma polymerization of ethylene and of tetrafluoroethylene, glow characteristics play an important role. Glow characteristic is dependent on a combined factor of W/F_m, where W is discharge power and F_m is monomer flow rate. At higher flow rates, higher wattages are required to maintain “full glow.” In the plasma polymerization of tetrafluoroethylene, simultaneous decomposition of the monomer competes with plasma polymerization. Above a certain value of W/F_m, decomposition becomes the predominant reaction, and the polymer deposition rate decreases with increasing discharge power. ESCA results indicate that the plasma polymer of tetrafluoroethylene that is formed in an incomplete glow region (low W/F_m) is a hybrid of polymers of plasma polymerization and of plasma-induced polymerization of the monomer. Polymers formed under conditions of high W/F_m to produce “full glow” are similar, regardless of the extent of decomposition of the monomer. They contain carbons with different numbers of F(CF₃, ? CF₂? , >CF? , >C<) and carbons bonded to other more electronegative substituents.     

Cationic polymerization of 1,4,6-trioxaspiro [4,4]-nonane

Cationic polymerization of 1,4,6-trioxaspiro [4,4]-nonane ( 1 ) with (CH₃)₃O⁺SbF₆^?(2) and CH₃OSO₂CF₃(3) initiators has been investigated. Although the observed rates of initiation and propagation are relatively slow, they consist of rapid reversible elementary reactions. In ¹H-NMR spectra, a broadening of the monomer signal was observed, indicating a fast exchange between “free” monomer and monomer engaged in the active species. The variety of orthoester bonds were observed in the polymer formed at the early stages of monomer conversion. The final polymer has, however, structure of a linear poly(ester–ether) including two subsequent ester or ether linkages. To account for these new facts, the mechanism of polymerization was proposed, consisting of a rapid reversible opening of one of the rings in the monomer molecule involved in the growing species, followed by the slower opening of the second ring with formation of the ester linkages. It appears that the rings originally present in the chains rearrange into the linear units intramolecularly.     

Plasma polymers from chloromethyltrimethylsilane and their modification with sodium azide

Plasma polymerization of chloromethyltrimethylsilane (CMTMS) was investigated by elemental analysis, infrared spectroscopy, and ESCA, and the modification of Cl substituents in the plasma polymers from CMTMS with sodium azide was discussed. CMTMS was plasma polymerized to yield filmy polymers. The polymer deposition rate was faster than that from tetramethylsilane containing no Cl atom. The plasma polymers from CMTMS were mainly composed of CH₃, CH₂, Si? CH₃, Si? O? Si, and Si? O? C groups with a small amount of C? Cl groups. The Cl substituents incorporated into the plasma polymers were capable of substitution reactions with azide groups to form azide polymers.     

Plasma polymerization of carbosilanes: Tetramethylsilane as a model monomer for reactivity study of silylmethyl groups

Plasma polymerization of tetramethysilane was studied in terms of file reactivity of silylmethyl groups. Two different plasma experiments have been carried out. The first was involved in trapping of low-molecular-weight conversion products of file monomer, and the second was used for deposition of polymer film. Gas chromatograplty/ mass spectrometry examination of file trapped material correlated with infrared analysis of the polymer film revealed that silymethyl groups undergo a variety of plasma reactions producing Si-Si, Si-CH₂-Si, Si-CH₂-CH₂-Si, SiH_x and SiEt units. The identified low-molecular-freight products show that the formation of SiH_x and Si-CH₂-Si units are predominant plasma reactions q/ file silylmetlml groups.     

Nafion-like thin film plasma-polymerized from perfluorobenzene/SO2 mixture

Plasma polymerization of perfluorobenzene/sulfur dioxide (PFB/SO₂) mixture was investigated by elemental analysis, infrared spectroscopy, and XPS. Plasma polymer films like Nafion were obtained from the plasma polymerization of PFB/SO₂ mixture. The plasma polymerization deposited fluoropolymers with sulfur moieties including sulfonate and sulfonic acid groups. The elemental composition, F/C, and S/C atomic ratio of the formed plasma polymers were 0.65–0.67 and 0.24–0.27, respectively, which were independent of the starting mixture composition. The polymers showed cation-exchange ability of which the capacity was 0.49 meq/g polymer, and initiated the polymerization of pyrrole.     

Molecular Tailoring of Surfaces: Pulsed Plasma Polymerization of Bromine Containing Monomers

The use of a variable duty cycle pulsed RF plasma is shown to provide film chemistry control during polymerization of saturated (CH₂Br₂) and unsaturated (CH₂=CHCH₂Br) bromine containing monomers. With both monomers, the degree of bromine atom retention in the films is observed to increase in a progressive fashion as the RF duty cycle employed during plasma polymerization is decreased. The film deposition rates, when expressed in terms of thickness per Joule of RF energy input, increase rapidly as the RF duty cycles are reduced. Additionally, the film morphology is observed to become increasingly smooth with decreasing RF duty cycles during deposition, as illustrated with the allyl bromide monomer. The film chemistry controllability of this study is demonstrated with monomers possessing the relatively weak C-Br bond. As such, the present work represents an important extension of the pulsed plasma polymerization approach to include retention of a labile bond during film formation. The introduction of reactive surface functional groups, at controlled densities, provides additional molecular tailoring possibilities via subsequent chemical derivatization processes.     

Long-lived polymer radicals. VI. Polymerization of N-methylmethacrylamide with formation of living propagation radicals

The polymerization of N-methylmethacrylamide (NMMAm) with azobisisobutyronitrile (AIBN) was investigated kinetically in benzene. This polymerization proceeded heterogeously with formation of the very stable poly(NMMAm) radicals. The overall activation energy of this polymerization was calculated to be 23 kcal/mol. The polymerization rate (R_p) was expressed by: R_p = k[AIBN]^0.63-0.68[NMMAm]^1?2.5. Dependence of R_p on the monomer concentration increased with increasing NMMAm concentration. From an ESR study, cyanopropyl radicals escaping the solvent cage were found to be converted to the living propagating radicals of NMMAm in very high yields (ca. 90%). Formation mechanism of the living polymer radicals was discussed on the basis of kinetic, ESR spectroscopic, and electron microscopic results.     

Glow discharge polymerization of tetramethyltin

Glow discharge polymerizations in the systems tetramethyltin (TMT), TMT/CH₄, TMT/C₂H₂ and TMT/N₂ were investigated and compared with those for methane and tetramethylsilane (TMS); some properties of the resulting polymers were examined. TMT is more easily polymerized in glow discharge than methane, and is deposited to as great an extent as TMS. Polymers prepared in these systems contain less carbon, nitrogen and oxygen than polymers from TMS; they consist mainly of CH₃, CH₂, CH, SnO and SnOSn groups. When nitrogen gas was mixed with TMT, amino groups were formed in the polymers. This formation arises from hydrolysis of SnN groups formed by interaction with TMT and nitrogen in glow discharge. This is distinctly different from the case of TMS. Surface energy, u.v. and visible absorption curve and thermal stability were measured. Adhesion between plasma films and polymer substrate is discussed.     

ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.     

Lifetime of living polymers in cationic polymerization. III. Living polymerization of isobutyl vinyl ether initiated by a cationogen/etalcl2 system in the presence of 1,4-dioxane as an added base

The concentration ([P^*]) and lifetime (half-life) of the propagating species were measured in the living cationic polymerization of isobutyl vinyl either initiated by the 1-(isobutoxy) ethyl acetate [CH₃COOCH (OiBu) CH₃]/ethylaluminum dichloride (EtAlCl₂) system in the presence of excess 1,4-dioxane in n-hexane at 0 to +70°C; the acetate serves as a cationogen that forms an initiating vinyl ether-type carbocation. The measurements were based on the end-capping reaction with sodiomalonic ester [Na^⊕?CH (COOEt)₂], which was shown to react rapidly and quantitatively with the living growing end. From the terminal malonate group of the quenched polymers, [P^*] was determined by ¹H-NMR spectroscopy. In contrast to its constancy during the polymerization, [P^*] progressively decreased with time after the complete consumption of monomer. The postpolymerization decay was first order in [P^*], and the lifetime (half-life) of the living end was determined from the decay rate constant. The lifetime increased on lowering polymerization temperature, decreasing EtAlCl₂ concentration, and increasing dioxane concentration. In particular, the “base-stabilized” living ends, generated by the CH₃COOCH (OiBu) CH₃/EtAlCl₂/dioxane system, turned out extremely stable at 0°C (half-life > 5 days in the absence of monomer).     

Cationic polymerization of styrene with electrochemically generated perchloric acid—II

The electroinitiated polymerization of styrene in LiClO₄-PC solutions has a living character due to the absence of termination. Marked side reactions were observed, limiting the yields. These reactions are mainly due to monomer oxidation by HClO₄ and polymer degradation at the anode. They can be minimized by increasing the monomer concentration. Conversion vs time curves show an induction period, an ascending linear portion and a descending portion. Kinetic treatment limited to the ascending portions shows that the polymerization is first-order with respect to monomer and HClO₄ concentrations. K_p values are considerably lower than those found in HClO₄C₂H₄Cl₂ and in HClO₄CH₂Cl₂, thus confirming that the influence of the solvent in these processes is not merely electrostatic.     

Spectroscopic study on chemical structure of plasma-polymerized hexamethyldisiloxane

A plasma-polymerized material was produced from hexamethyldisiloxane vapor by a glow discharge polymerization technique. Spectroscopic interpretation of the chemical structure of the polymerized hexamethyldisiloxane was studied by spectroscopic means such as IR, XPS, and NMR. The plasma polymer was barely soluble in the usual organic solvents, although it contained a small amount of the monomer and its oligomers. The IR spectrum indicated that the polymer consisted of Si? CH₃, Si? O, Si? CH₂, and Si? H groups. The surface of the polymer was found to retain structural units similar to the monomer from the XPS measurement. On the other hand, the ¹³C and ²⁹Si high-resolution, solid-state NMR measurements revealed that the plasma polymer was highly crosslinked with a variety of conformations and a number of O atoms surrounding a Si atom. Results from the XPS and NMR spectra suggested that the bulk of the polymer was more oxidized than the surface layer; Si atom was preferentially oxidized. A hypothetical chemical structure was proposed for the polymerized hexamethyl-disiloxane.     

Hydrophobic properties of plasma polymerized thin film gas selective membranes

Hydrophobic gas separation membranes were prepared by a plasma polymerization process. Thin films of about 400–500 nm thickness deposited on porous Al₂O₃ substrates represent the composite membranes investigated. The permeation properties of the composites were examined by the pressure increase and an isobar method. Depending on the precursor composition and the plasma polymerization parameters, it is possible to prepare membranes with Knudsen-like or solution-diffusion controlled separation factors. Low plasma polymerization energy densities and a mixture of silico- and fluoro-organic precursors result in water/methane separation factors as low as α_CH4^H₂O = 0.3 and high membrane permeabilities. Infrared analysis yields that the structure of the films is mainly determined by the silico-organic component. The fluoro-organic coprecursor causes a fluorination of methyl groups of the films as manifested by an infrared absorption band at 900-880 cm⁻¹.     

Real-time mid-IR monitoring of metathesis reactions by fiber optic FTIR spectroscopy

The real-time monitoring of metathesis reactions using a recently developed fiber optic transmission FTIR technique is reported in this paper. The ring-opening metathesis polymerization (ROMP) of 1,5-cyclooctadiene, the ring-closing metathesis (RCM) of 1,7-octadiene and the polymerization of phenylacetylene were investigated. The Schrock carbyne complex, Cl₃(dme)WCCMe₃, was used as the catalyst for these reactions. The phenylacetylene polymerization was also studied with WCl₆ as the catalyst. In the ROMP of 1,5-cyclooctadiene, monomer consumption was followed by monitoring the disappearance of the 1486 cm⁻¹ absorbance, characteristic of the CH₂ deformation vibration (δ_s CH₂) in the monomer. In the RCM of 1,7-octadiene, conversion data was obtained by monitoring the 1832 cm⁻¹ signal, which is an overtone of the wagging absorbance at 910 cm⁻¹ of the CH₂ end group in the monomer. Phenylacetylene polymerization was monitored by the disappearance of the ν -CCH stretch signal at 2110 cm⁻¹. Polymerization was much faster with the Schrock catalyst than with WCl₆, but similar conversions were reached in both reactions. Conversion data obtained by the IR technique agreed well with gravimetric product yields.