Surface viscoelastic parameters of poly(( dimethylamino)ethyl methacrylate-methyl methacrylate) diblock copolymer solutions: pH dependence of the evolution of the equilibrium values

The dynamic interfacial properties of a diblock copolymer poly((dimethylami no)ethyl methacrylate-b-methyl methacrylate) have been obtained by surface quasi-elastic light scattering. The temporal evolution of the capillary wav e frequency and damping as well as the surface viscoelastic properties (sur face tension, dilational modulus, and dilational viscosity) of a freshly fo rmed air-water interface for a range of pH values (at fixed scattering vect or) has been investigated, The equilibration dynamics were dependent on the solution pH, but the equilibrium state was obtained by the time the interf ace was 12 h old. Tensiometry showed that a surface phase transition occurs between pH 7.0 and pH 6.0. The surface light scattering data clearly showe d the differences in temporal response in this same range of pH. For soluti ons with pH 5.0, the air-water interface is populated by unimers. For highe r pH solutions, micelles are the initial species at the surface, but these disaggregate for solutions with pH 6.0 and 6.5. The disaggregation process is faster for the solution with pH 6.5, and this is associated with resonan ce between the capillary and dilational modes enabling energy to be transfe rred to the dilational mode, accelerating the disaggregation of the micelle s.