Interfacial tension in binary polymer blends in the presence of block copolymers: Effects of additive MW

The effect of block copolymer addition on the reduction of the interfacial tension between two immiscible homopolymers is investigated as a function of the additive molecular weight and concentration for polystyrene/polyisoprene blends in the presence of polystyrene-block-polyisoprene copolymers using the pendant drop method. The interfacial tension decreases with the addition of small amounts of copolymer and reaches a plateau at higher copolymer concentration, in agreement with previous studies. However, the reduction of the interfacial tension is a nonmonotonic function of the copolymer additive molecular weight at constant copolymer concentration in the plateau region. As the additive molecular weight increases the interfacial tension reduction, \ Delta gamma \, goes through a maximum. This should be related with the increased tendency of micelle formation for high copolymer molecular weights, which is verified by small-angle X-ray scattering. The results are discussed in relation to theoretical predictions for polymer/polymer/copolymer mixtures.