Nanostructure and Transition of Polyelectrolyte Brush at the Air/Water Interface by X-ray and Neutron Reflectometory

Department of Polymer Chemistry, Kyoto University, Japan
â—‹Hideki Matsuoka

X-ray and neutron reflectivity techniques (XR and NR) are the unique technique for the in situ study on the nanostructure of monolayer on the water surface. In this study, the nanostructure and its transition of ionic amphiphilic diblock copolymer monolayer at the air/water interface by XR and NR. Although the monolayer was expected to consist of hydrophobic layer on the water and polyelectrolyte brush under the water, but it has been clarified that the nanostructure is not so simple by our systematic study; the monolayer consists of hydrophobic layer, polyelectrolyte brush layer and hydrophilic "carpet" layer just in between two layers. The existence of carpet layer is quite new finding, which was become possible by in situ reflectivity study. When the hydrophilic block length is short, only carpet layer is formed and no brush layer is formed under the hydrophobic layer. Also, even if hydrophilic chain is long enough, only carpet layer is formed if the brush density is low enough. By systematical investigation with changing the brush density, we have found that there is the "critical brush density" for the transition between "carpet-only" and "carpet + brush layer" structures. The critical brush density was found in the order of 10-1 chain per nm2 and it decreased with increasing hydrophilic chain length. Also, the nanostructure of the polyelectrolyte brush layer is expected to be very sensitive to salt addition. However, the nanostructure of strongly ionic polyelectrolyte brush layer was not influenced by salt addition up to 0.2 M NaCl condition. This is due to the very high effective ion concentration in the brush layer. Beyond 0.2M NaCl, the monolayer shrunk up and the brush chain also shrunk. Hence, there is also the "critical salt concentration" for the structural transition by salt addition. In addition, the added salt ions in the brush layer were squeezed out by the compression of the monolayer by LB trough. These phenomena could be interpreted by the relative ion concentrations inside and outside the brush layer.