The document discusses impact strength and toughening mechanisms of glassy polymers. It defines impact strength as the ability of a material to absorb energy without fracturing. Glassy polymers like PS and PMMA are brittle with low impact strength. The document outlines methods to toughen glassy polymers including rubber particle addition. Rubber particles increase toughness through mechanisms like crazing, shear yielding, and a combination. Crazing creates small cracks that absorb impact energy to prevent fracture. Shear yielding allows rubber particles to deform and absorb energy. Particle size and content affect toughening, with an optimum particle diameter of 1-2 micrometers. Properties of unmodified PS are compared to rubber-modified HIPS which has higher impact strength.