Ductility of a nanocomposite of glassy and rubbery polymers

A brittle glassy polymer can be made ductile by forming a nanocomposite with a rubbery polymer. This paper investigates a nanocomposite of poly(methyl methacrylate) (PMMA) and poly(ethyl acrylate) (PEA). Pure PMMA is a brittle glass, pure PEA is a rubber, and a PEA-PMMA nanocomposite is ductile. We fabricate the nanocomposite by swelling PEA with MMA monomer, followed by polymerizing MMA. We prepare nanocomposites of various weight fractions of PMMA and measure their properties, including modulus, yield strength, fracture strain, fracture strength, work of fracture, and toughness. Whereas bulk PMMA fractures at a strain of ∼0.05 by localizing inelastic deformation in crazes, the PEA-PMMA nanocomposite can be stretched several times its original length with homogeneous deformation. The nanocomposite separates into a glassy phase and a rubbery phase. For a nanocomposite of 45 % weight fraction of PMMA, atomic force microscopy shows that the two phases are bicontinuous and the phase size is at ∼20 nm. For the nanocomposite to undergo large deformation, the continuous glassy phase must accommodate. Our experiments exclude the mechanism that the glassy phase in the nanocomposite breaks into small pieces. Rather, the glassy phase in the nanocomposite is itself ductile. We discuss the molecular picture of this ductility.