Surface tension of polymer mixtures and copolymers

Contact angles θ of liquids of different polarity were measured on a series of mixtures of solid high polymers and a series of copolymers. The mixtures were composed of an alternating poly(ethylene‐co‐maleic anhydride) (EMA), and its addition product with n‐octadecylamine, poly(ethylene‐co‐N‐n‐octadecylmaleamic acid) (EOM). The co‐polymers were composed of the same monomeric units as the mixtures. The surface tension γs of EOM, calculated from θ by the Good‐Fowkes‐Owens‐Wendt treatment, decreased slightly with increasing molecular weights and then reached a limiting value. Plots of γs against EOM concentration indicated large negative excess surface tension of the units with lower surface tension, EOM, in both series studied. For the mixture series, γa first sharply decreased with the EOM concentration; then it reached a limiting value, the γs of pure EOM, at a very low EOM concentration. This indicates phase separation of the two polymers, and the thickness of a monomolecular surface layer was calculated from these data. For the copolymers, γs varied logarithmically with the EOM concentration. Throughout the whole concentration range, the data fitted the equation developed by Belton and Evans for ideal mixtures.