Designed Interphase Regions in Carbon Fiber Reinforced Vinyl Ester Matrix Composites

Abstract

Fiber-reinforced polymer matrix composites are finding increased use in structural applications where long-term mechanical and environmental durability are key issues. This has prompted impetus for fundamental studies to determine relationships between the structure of fiber/matrix “interphase” regions and eomposite durability in both our laboratories and those of others. This study describes initial interphase and composite properties for a series of carbon-fiber-reinforced vinyl ester matrix composites wherein the liber sizing chemical structure has been varied within a series of ductile poly(hydroxyethcr)s. The mechanical properties of the sizings were similar. The primary difference between the systems studied was the amount of sizing-matrix interdiffusion and, hence, the inicrostructure of the interphase regions. A number of techniques were used to evaluate both the “macro and micro” properties of the composites as a function of interphase structure. These include atomic force microscopy and nanoindentation to map sizing matrix interdilfusion compositional gradients and the resulting mechanical properties across bi-layer films comprised of the fiber “sizings” and vinyl ester matrices. Micro-debond tests were carried out by the Korean group to probe adhesion between the “sized” libers and matrices. Fatigue cycling proved to be particularly useful in highlighting the influence of the tailored interphases on the durability of the carbon fiber/vinyl ester composites. Composite fatigue properties of AS-4 vinyl ester composites having a tough, ductile polyhydroxyether thermoplastic in the “interphase” region were improved dramatically (relative to using unsized fibers) with < 1 wt.% of the sizing in the composite. The thermoplastic sizing dissolved in the thermoset at the cure temperature, but microphase separated into ∼90 nm diameter inclusions during matrix curing. A gradient in chemical composition and morphology resulted, which moved outward from the fiber into the thermoset matrix. It is hypothesized that the unusually good fatigue resistance of composites from these materials can be at least partially attributed to this interphase gradient.