Triggered by an continuous upsurge in household and industrial power consumption, one goal of modern science is to provide up-to-date recipes for efficient harvesting and storing of the so-called “green” energy. The latter encompasses electricity produced by solar, wind, geothermal, biogas, and low environmental impact hydroelectric sources. In this respect the conducting behavior of amorphous materials such as ionic liquids (ILs), glassy ionic conductors, and polymer electrolytes have been thoroughly investigated.
However, information on the strongly emerging class of materials formed by the polymerized ionic liquids (PolyILs) is sparse. This is largely due to the fact that the methods of synthesis for these compounds are still under development, so they are not yet commercially available. PolyILs are synthesized via the covalent bonding of monomeric units which contain IL fragments. As schematically depicted in Fig. 1(c), in these binary materials all ions of a given sign are attached to polymeric chains which create a framework (matrix) hosting the “freely” migrating counterions. Combining the benefits of ILs in terms of high charge density with those of polymers in terms of mechanical behavior, PolyILs are potential candidates for the next generation of materials in electrochemical devices.
Schematic representations of the structure of (a) ionic liquids, (b) polymer electrolytes, and (c) side-group polymerized ionic liqiuds.
Main goal
Comparing the behavior of several PolyILs with that of their monomeric precursors (ILs), the overarching goal of this project is to unravel the role played by covalent bonding on the structure and dynamics of highly concentrated electrolyte melts. In particular, this project will analyze the influence of polymerization on conductivity and other spectral characteristics provided by linear and non-linear dielectric spectroscopy, on the flow and viscoelastic behavior revealed by shear rheology, on the single-particle diffusivity probed by nuclear magnetic resonance, and on the calorimetric signature of the dynamical freezing in, associated with the glass transition in these materials. T
Our approach
The investigations will be carried out on samples synthesized in the groups of Dr. Alexei Sokolov from the University of Tennessee in Knoxville, USA and of Dr. Jiayin Yuan from the Clarkson University in New York, USA. A number of nine PolyILs are already at our disposal. Six of them have the positively-charged units located in the side-groups and contains different anions. For the other three, with their synthesis developed in 2017, the cationic units are part of the main-chain, have monomers of different lengths, implying different densities of cations along the backbone, and share the same type of anion. This morphological mélange allows one to investigate the impact that environmental polarity of cations and size of anions bear on the charge transport and on the relaxation behavior for these polymers. For reference purposes the structurally homologous ionic liquids, herein regarded as monomers, will also be investigated with the question in mind how much of the knowledge previously gained from the study of ILs can be transferred to the case of PolyILs.
As in most of the studies concerning conducting materials, the primary objective is to understand the microscopic mechanisms underneath their electric conductivity. In this respect the study of their dielectric properties is essential. For an unambiguous analysis of the ionic sub-diffusive regime and for a good overlap with the high-T habitat of nuclear magnetic resonance (NMR) diffusometry, our investigations will cover a frequency range extending up to GHz region. On the other side of the T-range, close to the dynamical freezing-in, the dielectric results are to be complemented by rheological or/and calorimetric investigations. For the particular case of PolyILs both these techniques can provide extremely valuable information. One important aspect is that only for the PolyILs with the cations attached to the polymeric backbones (i.e. the newly systemized ones) the local rearrangements of these charges take place on the timescale of the structural relaxation process. This is not the case for the side-groups charges. Since rheology is the workhorse technique for the study of polymer dynamics in general, we will employ this method to unravel the interplay between the relaxation behavior of fast, dynamically decoupled anions and that of the covalently-bonded matrix. Other two objectives are to analyze the thermodynamic signature of structural fluctuations (which correspond to those of the main-chain charges) and to significantly increase the dynamic range in which the elusive nature of this decoupling phenomenon can be studied for the proposed materials. Both goals can be achieved via scanning ac nano-calorimetry, a technique which has not been applied for PolyILs so far. With respect to its differential scanning (DSC) variant, the modern ac calorimetry allows for measurements at scanning rates up to 10 kHz and it is able to provide model independently the spectral shape and thus the rate of the calorimetric fluctuations in these complex materials.
Selected publications
A. Rivera, J. Santamaria, C. Leon, T. Blochowicz, C. Gainaru, E. A. Rössler, Temperature dependence of the ionic conductivity in Li3xLa2/3-xTiO3: Arrhenius versus non-Arrhenius, Appl. Phys. Lett. 82, 2425 (2003)
A. Rivera, T. Blochowicz, C. Gainaru, E. A. Rössler, Spectral response from modulus time domain data of disordered materials, J. Appl. Phys. 96, 5607 (2004)
C. Gainaru, W. Hiller, R. Böhmer, A dielectric study of oligo and poly (propylene glycol), Macromolecules 43, 1907-1914 (2010)
C. Gainaru, R. Böhmer, G. Williams, Ion sweeping in conducting dielectric materials, Eur. Phys. J. B 75, 209 (2010)
C. Gainaru, R. Figuli, T. Hecksher, B. Jakobsen, J. C. Dyre, M. Wilhelm, R. Böhmer, Shear-modulus investigations of monohydroxy alcohols: Evidence for a short-chain-polymer rheological response, Phys. Rev. Lett. 112, 098301 (2014)
M. Hofmann, C. Gainaru, B. Cetinkaya, R. Valiullin, N. Fatkullin, E. A. Rössler, Field-cycling relaxometry as a molecular rheology technique: Common analysis of NMR, shear modulus and dielectric loss data of polymers vs dendrimers, Macromolecules 48, 7521 (2015)
C. Gainaru, E. W. Stacy, V. Bocharova, M. Gobet, A. P. Holt, T. Saito, S. Greenbaum, A. P. Sokolov, Mechanism of conductivity relaxation in liquid and polymeric electrolytes: Direct link between conductivity and diffusivity, J. Phys. Chem. B 120, 11074 (2016)
M. Heres, Y. Wang, P. J. Griffin, C. Gainaru, A. P. Sokolov, Proton conductivity in phosphoric acid: The role of quantum effects, Phys. Rev. Lett. 117, 156001 (2016)
R. Kumar, J. P. Mahalik, V. Bocharova, E. W. Stacy, C. Gainaru, T. Saito, M. P. Gobet, S. Greenbaum, B. G. Sumpter, A. P. Sokolov, A Rayleighian approach for modeling kinetics of ionic transport in polymeric media, J. Chem. Phys. 146, 064902 (2017)