Title

EFFECTS OF AN ELECTROPOLYMERIZED INTERLAYER ON THE MECHANICAL PROPERTIES OF GRAPHITE FIBER/EPOXY COMPOSITES

Date of Completion

January 1986

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

A uniform thickness, high molecular weight, acrylonitrile/methyl acrylate copolymer coating was applied onto graphite fiber surfaces through a batch electro-copolymerization technique. This technique was used to introduce a ductile interlayer of controlled composition and thickness between the composite fibers and matrix. Copolymers of methyl acrylate and acrylonitrile formed by this technique are random copolymers, with Tg's varying linearly with the copolymer composition from 14(DEGREES)C to 98(DEGREES)C. These copolymers have a fairly high number average molecular weight, in the range of 10('5) g/mol. Monomer reactivity ratios were determined, based upon free radical polymerization kinetics.^ Effects of the interlayer between the graphite fibers and epoxy matrix of a graphite/epoxy composite upon the composite mechanical properties were studied. The thickness and modulus of the interlayer and the bonding between the interlayer and the epoxy matrix were the three major parameters examined.^ It was found that interlaminar shear strength (ILSS) and impact resistance (IMPR) vary with the thickness of the interlayer, both showing the same general trend as the modulus of the interlayer changes. A thin interlayer, under 0.10 microns, improves the ILSS by relieving the thermal stress but also depresses the IMPR. An optimum thickness interlayer, between 0.1 and 0.15 microns, improves both ILSS and IMPR through stress relief and introducing shear failure during the fracture. At the maximum point, IMPR is improved about 30-40%, and ILSS is improved about 10-20% when compared with control samples. For interlayers thicker than 0.15 microns, ILSS and IMPR both decrease. The modulus of the interlayer does not show significant effects on the impact resistance due to high test speeds, all of the interlayers were glassy under impact conditions. The interlaminar shear strength decreases with the modulus owing to decreased stress transfer capability. Bonding between the interlayer and matrix, enhanced by acrylic acid, also improved the ILSS and IMPR. The single-fiber fragmentation test shows that good adhesion was achieved at both the fiber/interlayer and the interlayer/matrix interface.^ This study shows that it is possible to simultaneously improve both the interlaminar shear strength and impact resistance of composites by applying a suitable modulus and thickness interlayer at the fiber/matrix interphase. (Abstract shortened with permission of author.) ^