Mechanical behavior of alumina/poly(methyl methacrylate) nanocomposites

Alumina/poly(methyl methacrylate) (PMMA) nanocomposites were synthesized using 38 and 17 nm alumina nanoparticles. At an optimum weight fraction, the resulting nanocomposites display a room-temperature brittle-to-ductile transition in uniaxial tension with an increase in the strain-to-failure that averages 40% strain and the appearance of a well-defined yield point in uniaxial tension. Concurrently, the glass transition temperature (T-g) of the nanocomposites drops by more than 20 degreesC. The brittle-to-ductile transition is found to depend on poor interfacial adhesion between polymer and nanoparticle. This allows the nucleation of voids, typically by larger particles (similar to100 nm), which subsequently expand during loading. This void formation suppresses craze formation and promotes delocalized shear yielding. In addition, the reduction in T-g shrinks the shear yield envelope, further promoting this type of yield behavior. The brittle-to-ductile phenomenon is found to require both larger particles for void growth and smaller particles that induce the lowering of yield stress.