Steven D. Gardner

Surface characterization of electrochemically oxidized carbon fibers

High strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current to the fibers serving as an anode in 1% wt aqueous KNO3. Progressive fiber weight loss occurred with increasing extents of electrochemical oxidation. XPS studies (C 1s and O 1s) indicated that the oxygen/carbon atomic ratio rose rapidly to 0.24 as the extent of electrochemical oxidation was increased from 0 to 133 C/g and then remained almost constant as the extent of electrochemical oxidation rose to 10 600 C/g. Fitting the C 1s spectra demonstrated that the rise in surface oxygenated functional groups was mainly due to an increase in carboxyl (COOH) or ester (COOR) groups. An increase in the intensity of the O 1s peak (534.6–535.4 eV) after electrochemical oxidation corresponded to chemisorbed oxygen and/or adsorbed water. Electrochemical oxidation increased surface activity by generating more surface area via the formation of ultramicropores, and by introducing polar oxygen-containing groups over this extended porous surface. FT-IR spectra showed a broad peak at about 1727 cm−1 from C=O stretching vibrations of carboxyl and/or ketone groups, the relative intensity of which increased significantly with the extent of electrochemical oxidation. Post-oxidation heat-treatments in flowing nitrogen at 550°C for 30 min. caused further weight losses due to decarboxylation of carboxyl groups and other reactions in which oxygenated functions decomposed. These weight losses increased with the extent of electrochemical oxidation. This demonstrated that more oxygenated groups formed on the internal pore surfaces as pores increasingly penetrated deeper into the fibers with increased electrochemical treatment. Weight loss depended on the heat treatment temperature since different types of carbon–oxygen surface groups were formed during the electrochemical oxidations. Different functions have different abilities to decarboxylate or decarbonylate. The amount of Ag+ and NaOH uptake by electrochemically oxidized fibers rapidly decreased as the temperature of the post heat treatment increased to 550°C. Beyond 550°C the progressive decrease in Ag+ adsorption and NaOH uptake continued at a slower rate and approached 0 μmol/g after heating to 850°C. Conversely, after heat treatment I2 adsorption showed a marked increase as the treatment temperature was raised. Thermal decomposition of carbon–oxygen complexes within the pore structure leads to a lower hydrophilicity of the pore surface. The extensive micropore surface area generated by electrochemical oxidation becomes more accessible to I2 as CO2 and CO evolve. Very narrow pores (<10 A diameter) blocked by hydrogen bonding and oxygenated functions become more open. XPS analyses illustrated that the surface oxygen content decreased significantly after heat-treating to 550 or 850°C and was lowest after the 850°C treatment.

Chemical modification of carbon fiber surfaces by nitric acid oxidation followed by reaction with tetraethylenepentamine

Amino groups react rapidly with both isocyanates and epoxides. Thus, to prepare carbon fibers which might exhibit enhanced adhesion to both polyurethanes and epoxy resin matrices, attempts were made to introduce a high surface amine concentration onto high-strength carbon fibers (derived from PAN) by nitric acid oxidation followed by reaction with excess tetraethylenepentamine (TEPA). Fibers were oxidized with concentrated (70%) nitric acid at 115 °C (20, 40, 60 and 90 minutes) to generate surface acidic functions, primarily carboxyl and phenolic hydroxyl groups. These oxidized fibers were then reacted with TEPA at 190–200 °C to introduce surface-bound amino functions onto the surface via amide functions. The amide functions formed at the surface to graft TEPA to the surface. TEPA does not react with hydroxyl functions so hydroxyls remain on the surface. Decarboxylation of surface carboxyl groups was minor during the TEPA grafting. The quantity of surface-bound acidic and basic functions on these modified fiber surfaces was measured by NaOH and HCl uptake experiments. Then, methylene blue and metanil yellow dye adsorption experiments were employed to provide a measure of the surface area and both the surface density and steric availability of surface acidic and basic functions. The dye adsorption gave lower values of acidic and basic functional group surface concentrations than did NaOH/HCl uptake measurements. The number of acidic groups increased continuously with increasing oxidation times but after an initial jump in acidic groups per 100 A2 at the start of the oxidation, the density of the surface acidic was approximately constant. Instead, the surface area increased, which accounted for the increase in total acidic groups. Nitrogen BET measurements were also performed and the BET surface areas were compared to those derived from dye adsorption experiments. Similarly, both HCl and metanil yellow adsorption measurements led to a value of 5.6 basic (amine) groups per 100 A2 after reaction with TEPA. The results were consistent with an increase in surface area during nitric acid oxidation. About 52% of the surface acidic functions, which were generated during nitric acid oxidations of 20–60 minutes, had reacted with TEPA. An average of 2.6 amino groups were introduced for each carboxyl group consumed in the reaction with TEPA. Thus substantial looping of TEPA occurs where more than one amino function of TEPA has formed an amide bond at the surface. The average ratio of the total amino groups introduced to the total amount of acidic groups present after nitric acid oxidation was about 1.35.

Nitric acid oxidation of carbon fibers and the effects of subsequent treatment in refluxing aqueous NaOH

Abstract Nitric acid oxidation effectively created acidic functional groups on PAN-based high-strength carbon fibers. The acidic capacities of carbon fibers increased with oxidation time. Typically, 60 minutes of oxidation created 60 μeq acidic functional groups on each gram of carbon fibers. Nitric acid oxidation also caused tensile strength decreases and fiber weight losses. By treating fibers in refluxing aqueous NaOH after nitric acid oxidation, a weakly bound layer of partially oxidized graphitic fragments was removed and this caused higher weight losses. This treatment also removed a source of interference in later analyses. The effects of aqueous NaOH treatment after oxidation on the mechanical properties (tensile strength, Izod IS, IFSS, and ILSS) of fibers and their composites were also investigated.

Oxygen plasma and isobutylene plasma treatments of carbon fibers: Determination of surface functionality and effects on composite properties

High strength, PAN-based carbon fibers were treated with oxygen plasmas and isobutylene plasmas. The effects of exposure time, plasma power and gas pressure on the quantity of acidic functional groups introduced onto the fiber surfaces were examined. NaOH uptake measurements provided a quantitative determination of the surface acidic functions. Plasma treatments were able to generate a three-to-five-fold increase in the number of acidic functional groups per 100 A2. Methylene blue (MB) adsorption measurements, when used with NaOH uptake values, provided an estimate of the surface density of the acidic functions (functions/100 A2) that was independent of direct surface area measurements. This method was compared to the direct use of NaOH uptake values with nitrogen BET measurements of the surface area to give the number of acidic functions per 100 A2. These methods were compared for both oxygen plasma-oxidized and nitric acid-oxidized carbon fibers. The largest quantity of acid functions (16 μeq/g fiber) was obtained after 4 minutes in an oxygen plasma at 50 W. MB adsorption decreased while NaOH uptake increased as exposure time to oxygen plasma increased implying some decrease in surface area occurred. Surface area measurements (BET), however, showed no change in surface area over a 10 minute exposure to a 200 W oxygen plasma (all surface areas were in the range of 0.62 to 0.75 m2 g−1). These results are discussed in terms of possible effects of surface roughness, dye configuration or multiple dye layers. The rate of depositing polyisobutylene polymer residues on a flat glass surface was greater than that achieved on carbon fiber surfaces where shadowing effects could have existed within a tow. Oxygen plasma treatment improved the interfacial shear strength (determined by single filament fragmentation tests using an epoxy resin). The interlaminar shear strength (three-point bending) of carbon fiber/epoxy composites (Vf = 0.57) increased 28–29% using oxygen plasma-treated fibers while the Izod impact strength was unchanged. Plasma-polyisobutylene coated fibers (~ 650 A thick coating) exhibited a 37% increase in Izod impact strength when incorporated into epoxy matrix composites but the interlaminar shear strength decreased by 21% versus composites prepared with as-received fibers.

Adsorption of precious metal ions onto electrochemically oxidized carbon fibers

Electrochemically oxidized carbon fibers (ECF) adsorbed a prodigious amount of Ag+ in contrast to oxygen plasma and nitric acid treated carbon fibers. The amount of adsorbed Ag+ reached 3700 μmol/g after 5652 C/g of electrochemical oxidation. This value approaches the 4050 μmol/g of Ag+ which adsorbed onto steam-activated Kenaf-based carbon (with a surface area of 1284 m2/g determined by N2/BET) under the same adsorption conditions. ECF oxidized to 9540 C/g adsorbed more than its own weight of Ag+ (12 608 μmol/g). These fibers exhibited a surface area of 115 m2/g (CO2–DR). Two different reactions occurred during Ag+ adsorption. These reactions were ion exchange adsorption between Ag+ and acidic functions (carboxyl) and redox adsorption between Ag+ and reducing functions such as catechol groups on these electrochemically oxidized fibers (ECF). The redox capability was expressed by the reaction electric potential (E) using the Nernst equation. High resolution XPS C 1s spectra of ECFs (level of oxidation 5300 C/g), before and after Ag+ adsorption, showed that the carbon atoms present in phenolic, alcohol or ether groups and those present in carbonyl or quinone groups increased after Ag+ adsorption. X-ray diffraction and X-ray photoelectron spectroscopy (XPS) Ag 3d spectra of the ECF showed that adsorbed Ag+ was reduced to Ag0 after both Ag+ adsorption and subsequent post-heat treatment of the fibers under N2 at 550°C for 30 min. Only about one-third as much Au3+ adsorption occurred versus the extent of electrochemical oxidation as was observed for Ag+. This ratio matches the requirement that three electrons are required to convert Au3+ to Au0 versus one to convert Ag+ to Ag. High resolution angle resolved XPS (ARXPS) Pd 3d and Pt 4f spectra show that there are two different Pd oxidation states and three different Pt oxidation states present after adsorption of Pd2+ and Pt2+ onto ECF. The peak areas as a function of take off angle showed that substantial amounts of Pd0 and Pt0 are present in addition to Pd2+ and Pt2+ and Pt4+ on the outermost surface regions of oxidized fibers.

Reactivities of amine functions grafted to carbon fiber surfaces by tetraethylenepentamine. Designing interfacial bonding

Abstract PAN-base carbon fibers were oxidized with 70% nitric acid at 115 °C to introduce surface carboxyl, hydroxyl and other oxygenated functions. Subsequent reactions of these surfaces at 190–200 °C with tetraethylenepentamine (TEPA) generated amide bonds at carboxyl and ester sites thereby grafting TEPA and introducing primary and secondary amine groups onto the fiber surfaces. The reactivity of these surface-bound amine groups was evaluated in reactions with the small model reagents: phenyl isocyanates, 1,6-diisocyanatohexane, acetic anhydride and styrene oxide. Larger model reagents including isocyanate-terminated prepolymers and a series of epoxy resin prepolymers (MW 378–1000) were also used. The reactivity of the surface carboxyl and hydroxyl functions (introduced by nitric acid oxidation) was also examined in reactions with phenyl isocyanate. About 30% of these acidic functions were able to react with phenyl isocyanate. After grafting TEPA to the fibers, acetic anhydride and phenyl isocyanate reacted with 60% and 52%, respectively, of the surface-grafted amine groups. The larger isocyanate prepolymers reacted with only 2.5 to 7.8% of the amino groups and three epoxy resins, with molecular weights of 378, 500 and 1000, reacted with 15, 7.8 and 5.3% of the surface amino groups, respectively. The grafting efficiencies to surface amino groups (Ge) and the molecular weights of the reagent being grafted fit the relationship Ge = KMna where K = 3.07 × 103 and a = −0.848 for isocyanates, whereas K = 2.57 × 104 and a = −1.25 for the epoxy compounds.

XPS/ISS Investigation of Carbon Fibers Sequentially Exposed to Nitric Acid and Sodium Hydroxide

Type II carbon fibers have been oxidized to various extents in nitric acid, both with and without subsequent exposure to aqueous sodium hydroxide, and the resulting surface composition has been determined using angle-resolved x-ray photoelectron spectroscopy (ARXPS) and ion scattering spectroscopy (ISS). As-received fibers (unsized and commercially treated) contain substantial amounts of oxidized carbon, most of which is confined within the outermost 10-15 A of the surface. Fiber treatment in 70% nitric acid at 115°C for 20-90 min alters the O/C atomic ratio as well as the depth distribution of oxidized carbon species. Evidence is presented suggesting that the fibers are oxidized primarily within the outermost surface region when the nitric acid exposure time is <40 min. As nitric acid treatment time is increased to 60 and 90 min, the fiber subsurface region near the maximum XPS sampling depth (60-100 A) appears to be increasingly oxidized relative to further oxidation at the outermost surface region. This subsurface region is mostly the outer few graphitic layers which are exposed by pitting and roughening of the surface as oxidation continues. Exposing the nitric acid-oxidized carbon fibers to aqueous sodium hydroxide removes oxidized fragments from the outer fiber layers and consequently alters the depth profile of carbon/oxygen chemical groups. In most cases, after fiber exposure to sodium hydroxide, the relative O/C atomic ratio decreases by ∼10%, consistent with the removal of oxidized fiber fragments. Furthermore, ISS reveals that sodium hydroxide exposure enriches the outermost fiber layers with sodium, the concentration of which increases with increasing extent of prior nitric acid oxidation.

A spectroscopic examination of carbon fiber cross sections using XPS and ISS

Abstract Multiple transverse sections of type II, PAN-based carbon fibers (unsized and commercially treated) were investigated using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). No previous XPS/ISS spectra of carbon fiber cross sections have been reported. The data are compared to angle-resolved XPS (ARXPS)/ISS spectra acquired from external (longitudinal) fiber surfaces in order to assess the experimental technique and aid spectral interpretation. The spectra are consistent with nitrogen and oxygen concentration profiles that decrease overall from the fiber surface inward to the fiber core. Oxygen is present primarily as COH within the fiber cores, with increasing amounts of CO and COOH groups near the fiber skin. The spectra also reveal the presence of zinc, sodium and calcium within the fiber cores.

Residual Thermal Stresses in Filamentary Polymer-Matrix Composites Containing an Elastomeric Interphase

A three-phase micromechanical model based on the method of cells is for mulated to characterize residual thermal stresses in filamentary composites containing an in terphase between the fiber and the matrix. This is the first such study to incorporate a true three-phase version of the method of cells. The models performance is critically evaluated using data generated from other micromechanical models. Subsequently, a parametric study is performed to quantify the residual stresses in two hypothetical graphite fiber/epoxy matrix composites: one containing an elastomeric interphase whose Youngs modulus is less than that of the fiber and the matrix and one incorporating an interphase whose Youngs modulus is intermediate with respect to the fiber and the matrix. The data correlate the residual ther mal stresses in the fiber, interphase and matrix as a function of the interphase thickness and fiber volume fraction within each model composite. The study makes a broad assessment of the stress-attenuating characteristics that each interphase imparts to the graphite/epoxy com posites. Over the range of variables considered, properly dimensioning the elastomer inter phase leads to a more favorable reduction of residual thermal stress.

Polymeric composite materials incorporating an elastomeric interphase: A mathematical assessment

Abstract A mathematical model based upon the method of cells is extended in order to describe three-phase composite materials containing an interphase. A parametric study is performed wherein the effective properties of the composites are determined as a function of interphase material properties, interphase thickness and fiber volume fraction. The simulation is designed in particular towards describing the behavior of model composites which incorporate elastomeric polymers as an interphase to bond carbon fibers chemically to a polymeric matrix. Two hypothetical interphases are considered: one whose properties are representative of elastomers (Youngs modulus less than fiber and matrix) and one whose properties are intermediate with respect to the fiber and matrix. The two cases provide a broad assessment of how the interphase properties influence the effective composite properties.