Sheldon P. Wesson

Acid-base properties of carbon and graphite fiber surfaces

Carbon and graphite fiber surfaces representing a graded series in graphitic character were analyzed by inverse gas chromatography, programmed thermal desorption, and monofilament wetting. Lewis acids and bases were used as probes to determine the effect of electrooxidation and radio frequency glow discharge plasma on surface acid-base properties. Physical adsorption isotherms computed from chromatograms were analyzed with the CAEDMON (Computed Adsorption Energy Distribution in the MONolayer) algorithm to obtain histograms of surface area fraction versus adsorptive energy. Histograms from isotherms using the Lewis base show fiber surface energetics to be bimodal, with most of the surface presenting dispersion force or weak acid-base attraction to the adsorbates. A small surface area fraction of acid sites shows strong chemisorptive properties, and this area fraction increases with the degree of oxidation. Plasma treatment is effective at enhancing surface acidity, especially on high modulus graphite fiber...

Acid-base characteristics of silane-treated E glass fiber surfaces

Acid-base properties of E glass fiber surfaces treated with various commercial organosilane coupling agents were studied with angle dependent X-ray photoelectron spectroscopy (XPS), electrolytic thermodesorption analysis of water (ETA), inverse gas chromatography (IGC) and programmed thermal desorption (PTD). XPS analysis indicates that γ-aminopropyltriethoxysilane and γ-chloropropyltrimethoxysilane show some preference for inverted surface orientation. Monolayer isotherms using Lewis acids and bases as probe adsorbates show silane deposition to attenuate acid-base interaction between probe molecules and weak and medium strength sites on the substrate. Moisture thermodesorption analysis shows that the sorptive capacity for physically bound water was attenuated by all the silane treatments. Desorption polytherms using acidic and basic probes demonstrate that γ-aminopropyltriethoxysilane imparts strongly acidic and basic chemisorptive characteristics to the glass surface. Methyltrimethoxysilane imparts acid...

The Influence of Sizings on the Durability of High-Temperature Polymer Composites

To increase performance and durability of high-temperature composites for potential rocket engine components, it is necessary to optimize wetting and interfacial bonding between high-modulus carbon fibers and high-temperature polyimide resins. Sizings commercially supplied on most carbon fibers are not compatible with polyimides. In this study, the chemistry of sizings on two high-modulus carbon fibers (M40J and M60J, Toray) was characterized as was the chemistry of PMR-II-50 fluorinated polyimide resin. The carbon fibers were characterized using single filament wetting, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopic measurements. The polyimide matrix resins were coated onto glass filaments for characterization by wetting measurements. Surface energy components were obtained by wetting with nondispersive (methylene iodide), acidic (ethylene glycol), and basic (formamide) probes. A continuous desizing system that uses an environmentally friendly chemical-mechanical process was developed for tow level fiber. Composites were fabricated with fibers containing the manufacturers sizing, desized, and further treated with a reactive finish. Results of room temperature tests after thermal aging show that the reactive finish produces a higher strength and more durable interface compared to the manufacturers sizing. When exposed to moisture blistering tests, however, the better-bonded composite displayed a tendency to delaminate, presumably due to trapping of volatiles.

A computer model for wetting hysteresis 1. A virtual wetting balance

Abstract The modified Wilhelmy plate method is widely used to determine the wettability of solid surfaces by probe liquids. This is accomplished by monitoring the wetting force experienced by a solid suspended from a microbalance during immersion into and emersion from the fluid. In the absence of surface roughness, fluctuations in force observed as the liquid slides over the solid and hysteresis between the advancing and receding modes of measurement are caused by surface chemical heterogeneity. An independent analysis based upon the fundamental rules of stick-slip behavior of the moving contact line is developed to explain these effects quantitatively and to predict the hysteresis loop for a known surface. The extent of hysteresis and the amplitude and frequency of force fluctuations are shown to depend upon the nature of the heterogeneity and its spatial distribution. A detailed description of the model is given, followed by results of computer simulations performed on model surfaces. The results agree well with previous experimental work.

A computer model for wetting hysteresis 2. A virtual wettability scanning balance

Abstract We described previously a computer model for wetting hysteresis based on a theoretical analysis of the Wilhelmy wetting experiment. Applying this model to a number of virtual surfaces, we demonstrated that surface heterogeneity causes hysteresis in the wetting trace of an otherwise perfect solid. We now develop a model for the two-meniscus case to emulate liquid membrane wetting. This experiment scans a suspended solid specimen with a film of liquid contained in a loop attached to a vertical drive. After showing how variations in the size and proportion of surface energy domains affect the appearance of virtual wetting traces, we estimate the proportion of the surface covered randomly by two materials having different wettabilities, using real immersion and liquid membrane wetting data. The traces produced by the two models match the measured traces well. We are also able to mimic the effect of macroscale heterogeneity superimposed on surfaces featuring wetting hysteresis caused by microscale heterogeneity, specifically, fibers with non-uniform applications of finishing agents.

A computer model for wetting hysteresis. 3. Wetting behavior of spatially encoded heterogeneous surfaces

Abstract Heterogeneous surfaces are ofter modeled as a distribution of homogeneous “patches” of different energies. Direct testing of these models has not been possible, since the heterogeneous surfaces that have been studied are, at the start, ill-defined. We have synthesized tailored heterogeneous surfaces using a new combinatorial technique, designated PROLAPS. This method creates a mosaic of spatially encoded immobilized materials, i.e. each immobilized material is identified by its position on the support. The wetting behavior of glass vover slips on which compounds of different wettability have been synthesized in well-defined patterns is described. A model for wetting hysteresis was found to accurately mimic fluctuations in meniscus height as the wetting liquid encounters zones of alternating high and low solid surface energy. The changes in contact angle as the meniscus encounters transitions from regions of low to high surface energy and vice versa are not as abrupt in real life as in model data.

ACID/BASE Interactions with Silicon Carbide Fiber

Silicon carbide fiber surfaces were analyzed by programmed thermal desorption and inverse gas chromatography, using Lewis acids and bases as probe adsorbates to compare the effect of RF glow discharge plasma and thermal treatment on surface energetics. Changes in surface acid/base character were correlated with wetting data, surface titrations, and surface chemical composition deduced from x-ray photoelectron spectroscopy. Thermal treatment did not alter fiber surface energetics significantly. Plasma treatment rendered the surface more acidic and more basic. XPS and thermal desorption analysis indicate that the plasma removed: strongly adsorbed organic contaminants, exposing and activating the underlying glassy surface.