CRITICAL MOLECULAR-WEIGHT FOR BLOCK COPOLYMER REINFORCEMENT OF INTERFACES IN A 2-PHASE POLYMER BLEND

The effect of a small addition of poly(styrene-b-2-vinylpyridine) block copolymer on the microstructure and mechanical properties of phase-separated blends of polystyrene (PS) and poly(2-vinylpyridine) (PVP) was investigated by transmission electron microscopy on thin films cast from solution. Our experiments allow us to separate the effect of the bcp addition on the morphology from its effect on the strength of the interface between the two phases. A series of block copolymers with a PS degree of polymerization > 390 and a PVP degree of polymerization varying between 68 and 570 were added to the blends. Although we expect in all cases significant segregation of the block copolymer to the interface and a corresponding decrease in the interfacial tension, only those copolymers whose PVP block degree of polymerization was above 200 were found to be effective in preventing failure at the interface. This critical value corresponds roughly to the molecular weight between entanglements M(e), suggesting therefore that at least one average "entanglement" between the PVP block and the PVP homopolymer is necessary to have good stress transfer at the interface.