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 interfa ce between immiscible homopolymers was studied using an asymmetric double c antilever beam (ADCB) test. In this geometry, one can change the thickness of the homopolymer substrates and thus control the ratio of shear to tensil e stress intensity (the mixity) at the crack tip. Polystyrene (PS) and poly methylmethacrylate (PMMA) were the constituent homopolymers and PS/PMMA ran dom 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 copolym er present at the interface. A minimum in G(c) occurred at a thickness rati o of the PS to PMMA. beams of about 1.2. For both lower and higher thicknes s ratios, extra energy was dissipated in subsidiary crazes. The crack was f ound to propagate between the layer of random copolymer and the PS on a pat h 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 mo lecular weight increased from 160 000 to 450 000.