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The effect of molecular weight and polydispersity on the interfacial tensions between polystyrene and polybutadiene homopolymer is examined by using a pendant drop apparatus. The data show that the interfacial tension of the monodisperse homopolymer decreases linearly with temperature and increases with molecular weight. The interfacial tension becomes lower as the polydispersity of polystyrenes having the same number-average molecular weight increases. It is also found that the interfacial tensions of unimodal polystyrene mixtures are quite different from those of bimodal polystyrene mixtures. In the case of bimodal polymer systems, the lower-molecular-weight species is considered to migrate preferentially to the interface in order to reduce the free energy of the interface, which results in lowering the interfacial tension. From the slope of the plot of interfacial tension versus temperature, the interfacial entropy increases with the polydispersity, indicating that the higher polydispersity gives rise to more aggregation of the lower-molecular-weight species at the interface. |
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