Adhesion of polymer interfaces reinforced with random and diblock copolymers as a function of geometry

The reinforcement effect of a thin layer of random copolymer at the interface between immiscible homopolymers was studied using an asymmetric double cantilever beam (ADCB) test. In this geometry, one can change the thickness of the homopolymer substrates and thus control the ratio of shear to tensile stress intensity (the mixity) at the crack tip. Polystyrene (PS) and polymethylmethacrylate (PMMA) were the constituent homopolymers and PS/PMMA random copolymers were used. The energy dissipated in propagating the crack, G(c), (the fracture energy of the interface), depends strongly on the degree of asymmetry in the ADCB sample, as well as the amount and type of copolymer present at the interface. A minimum in G(c) occurred at a thickness ratio of the PS to PMMA. beams of about 1.2. For both lower and higher thickness ratios, extra energy was dissipated in subsidiary crazes. The crack was found to propagate between the layer of random copolymer and the PS on a path that was quite independent of the sample geometry. Hence, the subsidiary crazes had no effect on the position of final failure. The effectiveness of the random copolymer as a toughening agent was found to increase as its molecular weight increased from 160 000 to 450 000.