The special characteristics of ionic amphiphilic diblock copolymers, which consist of hydrophilic ionic chain and hydrophobic segment, and their self-assembling behavior such as micelle and monolayer formation, will be described.
Very strange and newly found characteristics of ionic amphiphilic diblock copolymer with short hydrophobic chain is non-surface activity. These polymers form micelles in water due to their amphiphilic nature, which can be confirmed by light scattering and X-ray, neutron small-angle scattering. However, the surface tension of their solution does not decrease with increasing polymer concentration. In addition, the solution shows less foam formation. These observations mean that these polymers are non-surface active. Non-surface activity with micellization is out of common sense of surface and interface science. The non-surface activity depends on salt concentration (Fig.1) and hydrophobic chain length. The essential factor of this curious characteristics is thought to be the balance of hydrophobicity of the polymer and electrostatic repulsion by image charge effect at the air/water interface. The critical micelle concentration (cmc) showed very unique dependence on hydrophobic chain length and added salt concentration, which could be interpreted by simultaneous micellization and adsorption at the water surface.
Micelle Nanostructure and Extremely High Stability against Salt Addition2)
The nanostructure of the micelles in water was investigated by SAXS, SANS and dynamic light scattering (DLS). The micelle showed sphere/rod transition by block length change and salt concentration, which could be explained by the concept of the critical packing parameter. The correlation between corona thickness change and shape transition was clearly confirmed by SANS. However, what surprising was so high stability against salt addition. The micelle was stable at 1M NaCl condition, which is out of common sense again of colloid science.
Monolayer and Polyelectrolyte Brush Formation at the Air/Water Interface.3)
The polymers with long hydrophobic block form monolayer on the water surface when spread. The nanostructure and transition of polyelectrolyte brush under the water surface were investigated by X-ray and neutron reflectivity. It was confirmed that the brush layer shows transition between "carpet-only" and "carpet+brush" structures as a function of brush density, hydrophobic chain length and added salt concentration. The critical brush density (cbd), the critical salt concentration, where transition occurs, were quantitatively estimated. The salt concentration dependence of cbd was different for strong and weak acid brushes, which means different brush formation mechanism.
1) P.Kaewsaiha, K.Matsumoto, H.Matsuoka, Langmuir, 21(22), 9938 - 9945 (2005).
2) P.Kaewsaiha, K.Matsumoto, H.Matsuoka, Langmuir, 23(18), 9162-9169 (2007).
3) P.Kaewsaiha, K.Matsumoto, H.Matsuoka, Langmuir, 23(13), 7065-7071 (2007) and refs therein