Role of weak particle-matrix interfacial adhesion in deformation and fracture mechanisms of rigid particulate-filled poly(methyl methacrylate)

Abstract

Toughening glassy thermoplastics such as poly(methyl methacrylate) (PMMA) without sacrificing modulus and thermomechanical stability is a valuable but challenging objective. Rigid particulate fillers have been found to improve toughness of some polymers with complex dependence on matrix ductility, particle size, and particle-matrix interfacial adhesion. We tested the effects of both strong and weak interfacial adhesion on deformation and fracture of a model system comprising PMMA filled with monodisperse 1µm diameter silica spheres. Fracture energy GIC of PMMA was found to increase by over 50% when filled with 1 v% of weakly bonded particles, while the force observed during melt compounding increased by less than 15% and Young's modulus increased systematically with filler loading. However, GIC decreased with filler loading above 1 v%. This behavior is consistent with a modified Kinloch-type model considering localized shear banding and plastic void growth around debonded particles at the crack tip. The ability of the matrix to deform via shear yielding and plastic void growth was confirmed by digital image correlation measurement of volumetric strain in uniaxial tension. We have extended Kinloch's model to account for shortening of the crack tip craze by the particles, which reduces the intrinsic toughness and toughenability of the PMMA matrix. Particles with strong interfacial adhesion generally reduced toughness. The experimental and modeling results suggest weakly bonded particles with size on the order of the crack-tip craze width may provide optimum toughening of glassy thermoplastics.