Ionic and Local Electric Polarization Effects in Polymers

The fundamental role of electric polarization in polymer physics is discussed via two examples. The first discusses purely ionic polarization effects in single ion conductors that are based on polymerized ionic liquids (PILs) (collaboration with the group of Ulli Scherf, Wuppertal). We employ a series of PILs with a polythiophene backbone bearing imidazolium salts with butyl, hexyl, octyl, and decyl side groups and several counteranions. PILs bearing the polythiophene backbone are unique as they can simultaneously conduct electronic charge and ions at nanometer length scales. Dielectric spectroscopy measurements performed as a function of temperature and pressure revealed that ionic conductivity is controlled by the balance between ion diffusion and ion complexation. The former is favored by the small ion size, the presence of ion channels and the decoupling from the backbone dynamics. On the other hand, ion complexation is controlled by ion size, the dielectric constant and charge delocalization. We propose a stick and jump model for ion motion in PILs.
The second example explores the local electric polarization effects in polymer blends (collaboration with M. Kappl, H.-J. Butt, Mainz). The design of multi-component materials for nanotechnology requires knowledge of the local composition. Hence, it is essential to develop techniques that can probe the local composition at the nanoscale. Local dielectric spectroscopy (LDS) is based on the electrostatic interaction between a conductive atomic force microscopy (AFM) probe in close proximity with the sample supported by a flat conductive substrate. Here, LDS is employed to analyze the dynamic heterogeneity in miscible blends as a function of film thickness. In thin films, phase segregation occurs and the kinetics of phase demixing was studied using as a probe the change in local composition. These results open new possibilities for studying interdiffusion and adhesion at polymer-polymer interfaces.