Charles E. Hewitt

Surface and bulk compositional characterization of plasma‐polymerized fluorocarbons prepared from hexafluoroethane and acetylene or butadiene reactant gases

The surface tension and surface and bulk composition of plasma-polymerized fluorocarbon films (PPFCs) prepared from hexafluoroethane (HFE) and either acetylene or butadiene reactant gases were determined. Increasing the HFE reactant gas content from 0 to 100% gave an increase in the amount of fluorine incorporated in the films and a shift to incorporation of more highly fluorinated species at the film surface, according to X-ray photoelectron spectroscopy (XPS ) data. Hydrogen levels in the films were determined by forward recoil spectrometry (FRS) and were shown to be inversely dependent on HFE concentration in the reactant gas feed and dependent on hydrocarbon co-reactant type. The compositional changes were mirrored by changes in the surface tension from 52 to 20 mN/m. XPS and surface tension results demonstrated that fluorine incorporation at the surface of the PPFCs is significantly reduced when butadiene, rather than acetylene, is used as a co-reactant gas with HFE. The differences are attributed to higher concentrations of hydrogen, which acts as a scavenger for reactive fluorine atoms and as an inhibitor in the CF x → CF x+1 reaction, and of carbon, decreasing the F/C ratio, when butadiene is used as the hydrocarbon source. Furthermore, potential changes in surface composition due to energetic ion bombardment are discussed. Three factors were suggested as strongly influencing the composition and the properties of the PPFCs: 1) the energy input into the plasma polymerization reaction, 2) the amount and type of fluorine scavenging reagent introduced with the HFE, and 3) the elemental composition of the reactant gases.

Compositions and surface energies of plasma-deposited multilayer fluorocarbon thin films

Abstract Multilayer fluorocarbon/hydrocarbon/silicon films were deposited by using a radio-frequency plasma discharge onto stainless steel substrates in order to produce coatings with progressively lower surface energies. Surface energy was varied through the use of reactant gas mixtures of hexafluoroacetone (HFA) and 5–100% acetylene or butadiene. The surface energy and surface and bulk composition of the films were determined by means of contact angle measurements, X-ray photoelectron spectroscopy (XPS), forward recoil spectrometry (FRS) and Rutherford backscattering spectrometry (RBS). Variations in HFA/hydrocarbon reactant gas mixtures used for deposition of the fluorocarbon layer demonstrated that films with surface energies as low as 19 mN m −1 could be obtained. Increasing the ratio of HFA to hydrocarbon in the feed gas mixture gave films with higher fluorine content and lower surface energy, with acetylene giving films with lower surface energy and higher fluorine content than butadiene. The surfaces of the films were shown to be richer in fluorine than the bulk. The surface and bulk compositional differences were attributed to lower levels of ion bombardment experienced by the film surface. The final composition and properties of the multilayer structure were suggested to result from a balance of the distribution of radical species in the plasma and the dose of energetic ion bombardment at the growing film surface. For the fluorocarbon layer, these factors were determined by the amount and type of hydrocarbon in the feed gas and by the plasma deposition conditions.

Stability of filled poly(dimethylsiloxane) and poly(diphenylsiloxane‐co‐dimethylsiloxane) elastomers to cyclic stress at elevated temperature

The response of aluminum oxide-filled poly(dimethyl siloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers, containing 3–24 mol % diphenylsiloxane, to cyclic stress at elevated temperatures (dynamic creep) was evaluated. The materials could be divided into two classes, based on their response to the application of cyclic stress: no or low-diphenylsiloxane content elastomers in which substantial creep and a decrease in crosslink density were observed, and high diphenylsiloxane content (16–24 mol %) elastomers that showed decreased creep with increasing diphenylsiloxane content and an increase in crosslink density. It was suggested that the phenyl groups stabilize the siloxane bond in the polymer backbone, decreasing the rate of chain scission reactions as the diphenylsiloxane content increases and stabilizing the elastomer against creep. The balance of chain scission, chemical crosslinking, and cyclic formation reactions varies depending on diphenylsiloxane content, giving rise to the differences in dynamic creep behavior. An activation energy of 12.9 kcal/mol was measured for dynamic creep of poly(16% diphenylsiloxane/84% dimethyl siloxane), suggesting that a catalyzed degradation mechanism was responsible. The primary catalysts of the degradation reactions are postulated to be the filler particles.