A pinboard by
Ha-Kyung Kwon

I am a PhD Candidate in the Olvera de la Cruz and Shull labs at Northwestern University


How can we design next-generation polymer batteries?

An ever-increasing demand for energy has driven the search for high-density energy storage devices that are portable, safe, and high functioning at a wide range of temperatures and pressures. Polymers, commonly known as plastics, represent a new class of lightweight, low-cost, and recyclable materials that can replace conventional rechargeable batteries, which are expensive, heavy, and flammable. Over the past decade, polymer battery has become an increasingly active area of research for space, military, and commercial applications. While significant progress has been made, increasing the efficiency of polymer batteries to competitive levels continues to be a challenge. The efficiency and lifetime of a battery is closely linked to the shape and size of the nanostructures formed in the polymer; however, the mechanism and physics behind the formation of these nanostructures are not very well understood. The goal of my research is understand the underlying physics to develop a comprehensive model for the phase behavior of charged polymer blends and block copolymers. In charged polymers, charge ordering can alter the miscibility and the macro- and micro-phase separation, which can influence the formation of the nanostructures and the distribution of ions within these structures. By properly incorporating the effects of ionic ordering, we have been able to develop a phase diagram that predicts experimentally observed phase separation in charged polymers. Specifically, we have found that charge ordering results in a non-linear trend in the miscibility, where the addition of weakly attracted ions promote miscibility but the addition of strongly attracted ions tend toward macro-phase separation. In cases where phase separation is caused by strongly attracted charged ions, increasing the charge concentration can drive the system toward mixing. In added salt ions, the effect of charge ordering manifests as an electrostatically tunable selectivity of the salt ions. This theory can be further extended to predict nanostructure formation and other relevant phenomena, to serve as a guideline for designing polymer batteries with optimized efficiency.


Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement

Abstract: The role of ion placement was systematically investigated in imidazolium bis(trifluoromethane)sulfonimide (ImTFSI) polymerized ionic liquids (PILs) containing pendant charges and charges in the backbone (sometimes called ionenes). The backbone PILs were synthesized via a facile step growth route, and pendant PILs were synthesized via RAFT. Both PILs were designed to have nearly identical charge density, and the conductivity was found to be substantially enhanced in the backbone PIL systems even after accounting for differences in the glass transition temperature (Tg). Wide-angle X-ray scattering (WAXS) revealed an invariance in the location of the amorphous halo between the two systems, while the anion–anion correlation peak was shifted to lower scattering wavevector (q) in the backbone PILs. This indicates an increase in the correlation length of ions and is consistent with charge transport along a more correlated pathway following the polymer backbone. Due to the linear nature of the backbone PILs, crystallization was observed and correlated with changes in conductivity. Upon crystallization, the conductivity dropped, and eventually, two populations of mobile ions were observed and attributed to ions in the amorphous and near-crystallite regions. The present work demonstrates the important role of ion placement on local structure and conductivity as well as the ability of backbone PILs to be used as controllable optical or dielectric materials based on crystallization or processing history.

Pub.: 15 Jul '16, Pinned: 14 Sep '17

Adsorption and encapsulation of flexible polyelectrolytes in charged spherical vesicles.

Abstract: We present a theory of adsorption of flexible polyelectrolytes on the interior and exterior surfaces of a charged vesicle in an electrolyte solution. The criteria for adsorption and the density profiles of the adsorbed polymer chain are derived in terms of various characteristics of the polymer, vesicle, and medium, such as the charge density and length of the polymer, charge density and size of the vesicle, electrolyte concentration and dielectric constant of the medium. For adsorption inside the vesicle, the competition between the loss of conformational entropy and gain in adsorption energy results in two kinds of encapsulated states, depending on the strength of the polymer-vesicle interaction. By considering also the adsorption from outside the vesicle, we derive the entropic and energy contributions to the free energy change to transfer an adsorbed chain in the interior to an adsorbed chain on the exterior. In this paper, we have used the Wentzel-Kramers-Brillouin (WKB) method to solve the equation for the probability distribution function of the chain. The present WKB results are compared with the previous results based on variational methods. The WKB and variational results are in good agreement for both the interior and exterior states of adsorption, except in the zero-salt limit for adsorption in the exterior region. The adsorption criteria and density profiles for both the interior and exterior states are presented in terms of various experimentally controllable variables. Calculation of the dependencies of free energy change to transfer an adsorbed chain from the interior to the exterior surface on salt concentration and vesicle radius shows that the free energy penalty to expel a chain from a vesicle is only of the order of thermal energy.

Pub.: 03 Jul '17, Pinned: 06 Sep '17

Electrostatic correlations and the polyelectrolyte self energy.

Abstract: We address the effects of chain connectivity on electrostaticfluctuations in polyelectrolyte solutions using a field-theoretic, renormalizedGaussian fluctuation (RGF) theory. As in simple electrolyte solutions [Z.-G. Wang,Phys. Rev. E 81, 021501 (2010)], the RGF provides a unified theory forelectrostatic fluctuations, accounting for both dielectric and charge correlationeffects in terms of the self-energy. Unlike simple ions, the polyelectrolyte self energydepends intimately on the chain conformation, and our theory naturally provides aself-consistent determination of the response of intramolecular chain structure topolyelectrolyte and salt concentrations. The effects of the chain-conformation on theself-energy and thermodynamics are especially pronounced for flexiblepolyelectrolytes at low polymer and salt concentrations, where application of thewrong chain structure can lead to a drastic misestimation of the electrostaticcorrelations. By capturing the expected scaling behavior of chain size from dilute tosemi-dilute regimes, our theory provides improved estimates of the self energy at lowpolymer concentrations and correctly predicts the eventual N-independenceof the critical temperature and concentration of salt-free solutions of flexiblepolyelectrolytes. We show that the self energy can be interpreted in terms of aninfinite-dilution energy μm,0(el) and a finite concentrationcorrelation correction μ(corr) which tends to cancel out the formerwith increasing concentration.

Pub.: 03 Mar '17, Pinned: 06 Sep '17

Anomalous Phase Behavior of Ionic Polymer Blends and Ionic Copolymers

Abstract: Nonionic diblock copolymers, with a highly asymmetric relative composition (f), microphase segregate into structures in which the minority component always forms cylindrical or spherical domains that are embedded in the majority component continuous matrix phase. Recently, a hybrid liquid state theory (called the DHEMSA approximation) and self-consistent field approach for ionic diblock copolymers have demonstrated the possible existence of “inverted” phases in which the minority ionic component forms the continuous matrix phase and the majority nonionic component forms cylindrical domains. We find that such anomalous behavior is closely related to the thermodynamics of phase segregation found in a blend of an ionic polymer and a nonionic polymer at the electrostatic coupling values typical of polymers in the molten state, at which nonionic and ionic polymers segregate into two partially miscible phases. This partial miscibility holds even across infinite molecular weights of the polymers. Such partial miscibility causes swelling of the minority component and a “switch” between minority and majority phases in ionic block copolymer melts. By combining the DHEMSA approximation with the Helfand–Tagami theory, we calculate the interfacial tension γ between coexisting phases of ionomers. The full phase diagram for ionomer blends and γ allows us to construct the phase diagram of block copolymers. In addition to the conventional microphases found in nonionic diblock copolymers, we find microphases with “inverted” cylindrical and spherical domains. We also predict an “inverted” phase at high values of f where the nonionic minority component becomes swollen by the ionic component and forms the matrix phase. Three-dimensional self-consistent field theory modeling confirms the existence of the “inverted” bicontinuous phases between the lamella and the inverted cylinder microphase regions of the phase diagram.

Pub.: 23 Jun '17, Pinned: 06 Sep '17