Fiber-reinforced polymer matrix composites are finding increased use in structural applications where long-term mechanical and environmental durability are kcy issucs. This has prompted impetus for fundamental studies to determine relationships between the structure of fiber/matrix "interphase" regions and composite 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 fiber sizing chemical structure has been varied within a series of ductile poly(hydroxyether)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 microstructure 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 intcrphase structure. These include atomic force microscopy and nano-indentation to map sizing/ matrix interdiffusion compositional gradients and the resulting mechanical properties across bi-layer films comprised of the fiber "sizings" and vinyl ester matrices. Microdebond tests were carried out by the Korean group to probe adhesion between the "sized" fibers and matrices. Fatigue cycling proved to be particularly useful in highlighting the influcncc of the tailored interphases on the durability ol' thc carbon fibcr/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 Georgia, USA, February 22-25, 1998. 395 Downloaded by [University of Otago] at 06:24 29 July 2015 396 M. A. F. ROBERTSON et al.(relative to using unsized fibers) with < 1 wt.Yo of the sizing in the composite. The thermoplastic sizing dissolved in the thermoset at the cure temperature, but microphase scparatcd 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 hypothcsizcd that the unusually good Fatigue resistance of composites from thcse materials can be at least partially attributed to this interphase gradient.

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