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Dielectric spectroscopy and rheology of polymeric and small-molecule electrolytes in the linear and nonlinear response regimes

Applicant Professor Dr. Roland Böhmer, since 8/2021
Subject Area Polymer Materials
Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Term from 2021 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 461147152
 
Final Report Year 2024

Final Report Abstract

The development of new technologies is increasingly focusing the attention of modern material science on the field of soft matter. Often, it is a specific combination of properties that renders a material interesting for certain applications. Useful battery electrolytes, for example, are required to combine high ionic conductivity with sufficient mechanical stability. In this project, we investigated the corresponding characteristics and their complex interplay mainly using broadband dielectric and conductivity spectroscopy as well as shear rheology for a series of low-molecular-weight and polymeric glass formers. In particular, methods were used and further developed which subject the materials under investigation to large electrical, mechanical, and thermal perturbations, as they often occur in practical applications. The focus of the now completed project was, however, less on technically relevant optimizations and more on the elucidation of the mechanisms that are active in the investigated materials. In particular, by means of nonlinear rheology it was possible to advance into the previously difficult-to-access regime of structural relaxation and to detect the intensively discussed "humps", here in the form of maxima in the cubic shear moduli. Transients, induced by large field and temperature changes, provided significant insights into the degrees of freedom relevant to the vitrification of the investigated substances. In order to precisely record physical aging phenomena of highly viscous materials, we were able to design a rheology-based Fourier temperature oscillation method and to successfully implemented it in the laboratory. The detailed theoretical analysis of this experiment led us to extend a widely employed model for the quantitative evaluation of differential calorimetry as well as of dilatometry. This generalization of the Tool-Narayanaswamy approach within the framework of a Wiener-Volterra series now provides a tool for the systematic description of material properties in the even stronger nonlinear range.

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