Molecular weight dependence of non-surface activity for ionic amphiphilic diblock copolymers

The ionic amphiphilic diblock copolymers, (n-butyl acrylate)m-b-(styrene sulfonate)n, were synthesized by nitroxy radical mediated living radical polymerization with precise control of degree of polymerization and molecular weight distribution. Several polymers with different m and n values at m : n = 1 : 1 were obtained, and their non-surface activity was examined by surface tension experiments and foam formation observation. All the polymers with m = n = 7–50 were found to form micelles in aqueous solutions, but the polymers with a degree of polymerization larger than 20 showed non-surface activity while those less than 20 were found to be surface active. As with other polymers investigated previously, surface activity was observed when salts were added to the solution, even for polymers of a non-surface active nature in pure water solution. This means that non-surface activity is not only an electrostatic effect (image charge effect at the air/water interface) but also some kind of polymer effect. Non-surface activity is a specific characteristic only for polymers, and the border of “amphiphilic polymer” and “low molecular weight surfactant” is located around the degree of polymerization of about 20. This is proposed as a new definition of “polymer” from the view of surface science, at least, from the view of non-surface activity. The essential of this “polymer effect” can be attributed to the highly stable nature of polymer micelles in solution compared to adsorbed polymer at the air/water interface, which is highly destabilized by image charge repulsion.