| Pyrograf III™ fibers (PR-19-PS, Applied Sciences, Inc.) with 100–300 nm diameters and ∼10–100 μm lengths were used with a low viscosity aliphatic epoxy resin (Clearstream 9000, Clearstream Products, Inc.) to produce composites. The VGCFs were oxidized in 69–71 wt% nitric acid (115°C) for various times (10 min to 24 h) to modify the surface to enhance fiber/matrix adhesion. Remarkably, little fiber weight loss was detected even after 24 h of oxidation. Composites containing 19.2 volume percent (29.4 weight percent) VGCFs were prepared. Their flexural strengths and flexural moduli were obtained. The flexural strengths did not increase using oxidized VGCFs. Fiber surfaces were characterized using N2 BET, CO2 DR, XPS, SEM, TEM and base uptake measurements. Increasing the oxidation time produced only small initial increases in surface area up to a limit. Significant surface oxygen was present before oxidation and the amount increased initially, though not continuously, with nitric acid oxidation. |
