Ion Containing Polymers

Physical Chemical Characterization

Selected Publications

Wolfgang H. Meyer earned his Ph.D. at the University of Freiburg in Germany with a thesis about "Solid State Photopolymerization of Diolefins" in 1977. From 1978 to 1984 he was a researcher at the Laboratories RCA Ltd. in Zürich/Switzerland with electronically conducting polymers and high resolution photoresists as research fields. In 1984 he joined the MPI-P. His research topics are: Ion-containing polymers and physical chemical characterization of materials. Since August 2001 he acts as "Scientific Technical Coordinator" (WTK) of the institute and is responsible for the institute's technical and building service, the operational safety, the corporate communications, the information for the third party funds and the coordination of scientific activities.

Ion Containing Polymers

The combination of ions and polymers encompasses a large diversity of materials ranging from polyelectrolytes with salt-like properties over ion-containing networks with enormous swellability and polymer-salt mixtures with high ionic conductivity to phase-separated materials like ionomers. The relation between chemical structure, ionic charge density and distribution, supramolecular structure and physical chemical properties in ion containing polymers and network is the major interest of our research. A concurrent goal is the evaluation of relaxation phenomena, phase transitions and ion transport mechanisms.

While polyelectrolytes are "single-ion conductors" in which one type of ions is fixed to the polymer repeat unit, "polymer electrolytes" are solutions of inorganic salts in a polymer matrix in which both cations and anions can contribute to the conductivity.

New electrolytes for Li-ion batteries consist of cyclocarbonates tethered to organic polymers or other substrates. By this means the vapour pressure of the electrolyte can be minimized to reach almost zero in the high polymer case. This is important to avoid explosive damage of the batteries when heated to high temperatures for any kind of reason. This is also a precondition for large scale application of high power batteries in automobiles.
The cyclocarbonate containing phase should form a soft matrix in which Lithium salts are dissolved and in which the Lithium ions are highly mobil. In order to act as separator membrane, the soft conducting phase should be combined with a rather rigid phase in order to provide overall sufficient mechanical stability. This can be realized either in block copolymer systems with a soft block for good conductivity and a hard block for mechanical strength, or via blending the soft phase with a matrix phase or fillers to form the appropriate blends or composites.

A completely new approach for obtaining high proton conductivity in anhydrous polymers was elaborated by us in cooperation with the group of Dr. K. D. Kreuer from the MPI for Solid State Research in Stuttgart / Germany: The new proton conduction mechanism is based on an intermolecular proton transfer between protogenic functions like imidazole or phosphonic acid which are tethered to an organic polymer or other substrates. The protogenic groups may form a hydrogen bond network through which the protons are transported. The conduction mechanism also includes a structural reorganization of the hydrogen bonds with braking and forming processes and thus is called "structure diffusion". The details of proton transfer in such materials yet is not fully understood and may depend on the hydrophilic interactions of the protogenic groups, the hydrophobic interactions of the appendend molecules, and superstructures caused by it. In order to analyze the details various model systems are developed and investigated.

In traditional separator membranes for polymer electrolyte membrane fuel cells (PEMFC) the proton conductivity is based on the proton transport in the liquid phase of water. Typical systems are ionomers like Nafion®. These "perfluorosulfonic-acid" (PFSA) based materials lack sufficient proton conductivity at T>100°C, poor mechanical and chemical stability at high temperatures, a high methanol crossover and water drag. Multiblock-copolymers based on highly sulfonated polysulfones as hydrophilic blocks and polyarylenethersulfones as hydrophobic blocks undergo phase-separation when cast to films and exhibit membrane properties which are superior to the PFSA-based fuel cell membranes.
 

Physical Chemical Characterization

The focus in this project is on the dielectric characterization of polymers and precursors as well as on the investigation of their ion conduction mechanism.

The dielectric properties of polymers are relevant and serve to characterize their relaxation behaviour. Thus, in order to be electrically insulating, the polymer should exhibit low conductivity and low dielectric constant, while for conducting polymers the contrary is desired. These properties can be probed with our dielectric spectrometers (DS) in the frequency domain between 0.01 and 10⁷ Hz, and in the temperature range from liquid nitrogen-temperature to more than 600 °C. Molecular and segmental dynamics in polymers give rise for dielectric relaxations and can be characterized. Various DS-configurations allow us for relative humidity (RH) and temperature dependent measurements. This is especially important to characterize the conductivities of proton conducting materials under conditions which are similar to those present in operating fuel cells.