EFFECTS OF A DIBLOCK COPOLYMER ON ADHESION BETWEEN IMMISCIBLE POLYMERS .2. PS-PMMA COPOLYMER BETWEEN PPO AND PMMA

Measurements have been made on the effects of a thin layer of polystyrene-poly(methyl methacrylate) diblock copolymer on the toughness of the interface between poly(methyl methacrylate) and poly(phenylene oxide). For a wide range of copolymer molecular weights, provided the interface is not saturated with copolymer, the interfacial toughness is a function of just the areal density SIGMA of the copolymer molecules, independent of their molecular weight. The toughness was found to vary with SIGMA2 in accordance with the predictions of a recent model of crazing failure where it is assumed that G is a measure of the energy to form the crack tip craze and the craze fails when the force per molecule at the crack tip (in the craze) equals the molecular scission force. SIMS experiments confirmed that the diblock copolymer organizes at the interface and that interface fracture breaks the copolymer molecules near their junction points. The highest obtainable toughness for any given molecular weight diblock was found to itself exhibit a maximum when considered as a function of molecular weight. This maximum occurred at a molecular weight of about 80 K. The obtainable toughness decreased as the copolymer molecular weight was increased from 80K because the saturation value of SIGMA decreased with increasing molecular weight. As the molecular weight was decreased from 80K, both pull-out and crowding effects become more evident, so again the toughness decreases from the value at 80K.