Session E54: Dielectric Relaxation and Charge Transport of Soft Materials

Morphology and Conductivity in Sulfonated Polyphenylenes

Proton conduction in hydrated, proton-conducting polymer membranes is highly affected by hydration level and membrane morphology. Here we examine morphology and dynamics in a promising proton-conducting polymer, a sulfonated Diels-Alder polyphenylene (SDAPP). We performed atomistic molecular dynamics (MD) simulations on a series of SDAPPs, systematically varying the degree of sulfonation and water content to determine their effect on the nanoscale structure, particularly for the hydrophilic domains formed by the ionic groups and water molecules. The static structure factors calculated from simulation are in good agreement with X-ray scattering data. NMR and impedance spectroscopy measurements show that the proton conduction mechanism evolves from being dominated by vehicular transport at low hydration and sulfonation levels to including a significant contribution from the Grötthuss mechanism at higher hydration and sulfonation levels.   

Phase Transition and Dynamics in Imidazolium-Based Ionic Liquid Crystals through a Metastable States

The phase transition behavior and dynamics of ionic liquid crystals, 1-methyl-3-alkylimidazolium tetrafluoroborate with various alkyl chain lengths, were investigated by X-ray scattering, differential scanning calorimetry, optical microscopy, and dielectric relaxation spectroscopy to elucidate the mechanism of their structural and phase changes. A metastable phase was found to appear via a supercooled smectic phase on cooling. In the metastable phase, disorder in the smectic phase is partially frozen; thus, the phase has order higher than that of the smectic phase but lower than that of the crystalline phase. During the subsequent heating process, the frozen disorder activates, and a crystalline phase appears in the supercooled smectic phase before entering the smectic phase. The relationship between the phase behavior and the dynamics of charge carriers such as ions is also discussed.   

Revealing the fine features of charge transport mechanism in ionic glass-forming liquids by dielectric spectroscopy

By examining the fine features of the dielectric spectra of a variety of ionic glass-forming materials, including ionic liquids and poly(ionic liquids), we show that the derivative of their real permittivity progressively broadens on the low-frequency side when the glass transition is approached from above. This phenomenon, ubiquitous and yet difficult to ascertain in the widely-used conductivity or modulus representation, is reminiscent of the spectrum broadening in supercooled dipolar liquids. Our finding provides direct evidence of heterogenous dynamics in ac conduction of ionic glasses and calls for a reconsideration of the classical models that predict a steep, Debye dispersion at low frequencies.   

Dielectric Relaxation and Conductivity in Polyelectrolyte Systems

Using Electrical Impedance Spectroscopy, we have monitored the dynamical relaxations of polyelectrolytes in concentrated solutions and in charged materials. Here, we explore the effects of chain length, ionic strength, cross-link density, and pH on the dynamics of charged macromolecular assemblies. We also report experimental results on the ionic conductivity of these systems. This research builds our understanding of the parameters necessary for tuning functional properties of polyelectrolyte-based systems, and motivates further work in logically designing electrically-responsive materials.   

Dielectric Spectroscopy on Choline Chloride based Deep Eutectic Solvents

Deep eutectic solvents (DES) have caught the attention of the scientific community in recent years and are discussed as a promising new class of ionic liquid analogues [1]. Prominent examples are mixtures based on choline chloride (ChCl) with various hydrogen-bound donors like, e.g., glycerol [2]. Although dielectric spectroscopy is an ideal tool to investigate ionic-conductivity mechanisms, until now only a few DES were studies by this method. In this talk, we present dielectric spectroscopy data for three ChCl based DES, covering a broad frequency and temperature range. Our evaluation focuses on the often neglected molecular reorientational dynamics and its correlation with the ionic mobility, as also found in ionic liquids [3].

[1] E. Smith, A. Abbott, and K. Ryder, Chem. Rev. 114, 11060 (2014). [2] A. Faraone, D. V. Wagle, G. A. Baker, E. Novak, M. Ohl, D. Reuter, P. Lunkenheimer, A. Loidl, and E. Mamontov , J. Phys. Chem. B 122, 1261 (2018). [3] P. Sippel, S. Krohns, D. Reuter, P. Lunkenheimer, and A. Loidl, Phys. Rev. E, in press (arXiv:1806.05000).

  

Molecular Dynamics Simulations of Ion Transport in High Transference Number Polyelectrolytes for Li-Ion Batteries

Conventional liquid electrolytes for Li-ion batteries suffer from low Li⁺ transference numbers, which limit mass transport in porous electrodes and thus reduce the battery’s energy density and rate capability. It has been proposed that replacing traditional Li-ion battery salts with lithium-neutralized polyanions dissolved in solution could be a means to increase the Li⁺ transference number while only modestly sacrificing ionic conductivity. While initial experimental studies have demonstrated the promise of this approach, rational design of optimal polyelectrolytes requires more fundamental, atomistic-level understanding of the ion transport mechanisms in these systems. To this end, we use classical molecular dynamics simulations to investigate the behavior of poly(allyl glycidyl ether-lithium sulfonate); this polyion has been thoroughly characterized experimentally for this application, enabling validation of the computational model. By characterizing the Li⁺ diffusion mechanism as well as ion aggregation behavior in the system, we elucidate the atomistic phenomena that most strongly govern experimentally-measured conductivity and transference numbers.   

Temperature dependent vibrational modes of Ammonium Nitrate in terahertz regime

Ammonium Nitrate (AN), often used as an ingredient for making improvised explosives, does not show any spectral feature at room temperature. We employed terahertz time domain spectroscopy (THz-TDS), a non-invasive technique, in the range of 0.30 THz to 2.50 THz to measure the effective dielectric properties of AN embedded in a polytetrafluoroethylene (PTFE) binder in the temperature range 5K to 300K. We extracted the dielectric constants of AN using effective medium theories and fitted them using Lorentz dispersion model combined with Huang equation. We identified six low frequency vibrational modes of pure AN at low temperature (<240 K). These lattice vibrations in the low frequency range agree reasonably well against our first principles Density Functional Theory (DFT) simulation of molecular AN, that reveals four low frequency vibrational modes within 3THz.   

Low-frequency dielectric response of a periodic array of charged spheres in an electrolyte solution

We study the low-frequency dielectric response of charged spheres arranged in a cubic lattice and immersed in an electrolyte solution. We focus on the influence of the out-of-phase current in the ionic charge neutral regime. In the thin double-layer limit, we use Fixman's boundary condition at the outer surface of the double layer to capture the direct interaction between the electric field and the flow of the ions. For periodic conditions, we combine the methods developed by Lord Rayleigh and by Korringa, Kohn and Rostoker. When the charged spheres occupy a very small volume fraction, smaller than one percent, our solution becomes consistent with the Maxwell Garnett mixing formula together with the single particle polarization response, as expected, because inter-particle interactions become less prominent in the dilute limit. By contrast, the inter-particle interaction, computable in the periodic geometry, greatly alters the dielectric response when charged spheres occupy even a slightly higher volume fraction, as low as two percent. Our results imply that the signature of the dielectric response of a system consisting of densely packed charged spheres immersed in the electrolyte can differ drastically from a dilute suspension.
  

Low-frequency dielectric response of charged oblate spheroidal particles immersed in an electrolyte

We studied the low-frequency polarization response of an oblate spheroidal particle with surface charge immersed in an electrolyte. Because the charged spheroid attracts counterions which form an electric double layer around the particle, the dielectric response involves an interplay between the motion of these counterions and the spheroidal shape of the particle. For two different counterion distributions in the thin electric double-layer limit, we obtained analytic expressions for the polarization coefficients to the first nontrivial order in frequency. We find that the polarization response normal to the symmetry axis depends on the total amount of charge carried by the oblate spheroid, while that parallel to the symmetry axis is suppressed when there is less charge on the edge of the spheroid. We then show that a dilute suspension of charged particles has an enhancement in the low-frequency dielectric response which is suppressed by high ion concentrations in the electrolyte. Both higher aspect ratios and smaller sizes of the spheroids lead to a stronger dielectric enhancement. The characteristic frequency associated with the dielectric enhancement scales inversely with the particle size, and it has a weak dependence on the shape of spheroid.