Session R35: Electric Polarization and Polymer Physics

How to Define Electric Potential in a Polarized Polymer Electrolyte Why is it Important?

Polymer electrolytes comprise mobile ionic species, usually both cations and anions, but are electronic insulators. The definition of electric potential is not trivial as the case of the metallic current collectors wherein the presence of excess electronic charge can readily be sensed by a voltmeter. Newman’s concentrated theory is built on potential measured using an electrode that undergoes a reversible electrochemical reaction. We apply this theory to polymer electrolytes that are examined under a constant direct current (dc). The concentration gradients in the electrolyte can be calculated without invoking the potential. For practical applications, however, it is important to know if the potential drop necessary to drive the current is within the range afforded by the chosen electrodes. We have worked through these relationship for homopolymer electrolytes. We are currently working on establishing these relationships in block copolymer electrolytes. We hope to present these results at the meeting.   

The impact of chemical modification on charge injection at metal/polyolefin interfaces

The process of charge injection at metal/polymer interfaces is crucial to many areas of research and technology, such as organic light emitting and harvesting devices, high-voltage capacitors and cables. In this work, we study charge injection at metal/polymer interfaces for two polymers commonly used in high-voltage applications, namely polyethylene (PE) and polypropylene (PP). Using first-principles electronic structure methods, we compute charge injection barriers at model aluminium/PE and aluminium/PP interfaces. We show that the introduction of polar chemical groups (e.g., -COOH, -CH2Cl, and -CHO) in the polymer chains at the interface can tune the intrinsic charge injection barrier significantly. We take into account of thermal disorder by averaging over a large ensemble of interface structures obtained from first-principles molecular dynamics trajectories. Our results suggest the possibility of rational design of metal/polymer interfaces via localised chemical modification.   

Field-theoretic study of salt-induced order and disorder in a polarizable diblock copolymer

We study a salt-doped polarizable symmetric diblock copolymer using a recently-developed field theory that self-consistently embeds dielectric response, ion solvation and van der Waals (vdW) attractions via the attachement of classical Drude oscillators and/or fixed dipoles to the constituent fluid elements. This field theory can be directly simulated via the complex Langevin sampling technique, requiring no approximations beyond the phenomenology of the underlying coarse-grained model. We measure the shift in the order-disorder transition with salt-loading in our simulations and observe rich non-monotonic behavior in which ion solvation competes with dilution and charge screening effects to determine whether the ordered or disordered phase is stabilized. At low salt concentrations, the salt behaves as a selective solvent, localizing into the high-dielectric domains and stabilizing the ordered phase. At high salt concentrations, however, the salt localization vanishes due to charge screening effects, and the salt behaves as a nonselective solvent that screens vdW attractions and stabilizes the disordered phase. Our results raise questions regarding the conditions under which it is appropriate to ignore the screening effect of the ion cloud in theories of salt-doped polymers.   

Nanostructure and Local Dynamic Effects on Ionic Conductivity of Polymer-Grafted Nanoparticles in Ionic Liquids

In this talk, I will present our recent results about structures and dynamics of poly(methyl methacrylate) (PMMA)-grafted iron oxide nanoparticles in (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) (HMIM-TFSI) / solvent mixtures. The dynamics of HMIM⁺ cation is measured with QENS experiments and conductivity of grafted particles in HMIM-TFSI is obtained from impedance spectroscopy. Higher diffusivity and conductivity were achieved when grafted particles are dispersed in HMIM-TFSI/acetonitrile mixture than in methanol. These results reveal that acetonitrile solvates HMIM-TFSI and better dispersion of grafted particles leads to better mixing between PMMA and TFSI^- anions. The ion-dipole interactions between TFSI^- and PMMA self-dissociate HMIM-TFSI, thereby increase the number of free cation carriers. The dynamic measurements combined with conductivity results suggest that grafted particles dispersion and conductivity increases with the solvation and with the interactions between high graft density PMMA and TFSI^-. Particles form strings in methanol and yield lower conductivity. In conclusion, both solvation and solubility of grafted particles in ionic liquid/solvent mixtures determine the mobility of ions in well-defined particle dispersions.   

Capacitance of films containing polymerized ionic liquids

Electrode-polymer interfaces, which encompass adsorbed layers of polymers, dictate
many of the properties of thin films such as capacitance, electric field experienced by
polymers and charge transport. To develop an understanding of electric field-induced
transformations of electrode-polymer interfaces, we have studied electrified interfaces of
an imidazolium based polymerized ionic liquid (PolyIL) using combinations of broadband
dielectric spectroscopy (BDS), specular neutron reflectivity and simulations based
on the Rayleigh dissipation function formalism. We demonstrated that the Rayleigh
dissipation function formalism provides key insights into charge storage, which include
information about capacitance of interface, thicknesses of absorbed and diffuse layers.
Overall, the camel-shaped dependence of the capacitance on applied voltage was
obtained which originated from responses of absorbed layer to applied
voltages. Furthermore, the diffuse layer contribution to the capacitance should decrease with
applied voltage (V ) as V^-1/2, which is a direct consequence of local incompressiblity/crowded nature of
the PolyIL melts.   

Comparing Stockmayer Fluid Simulation and Experiment: Ion Solvation with Permanent Dipoles

The solvation of ions in polymer membranes has been studied over many decades. Nevertheless, an understanding of the solvation mechanism involves various computational challenges. This is primarily because molecular simulations for ion solvation that can simultaneously consider both atomic and molecular length scales are still significantly limited, even in cases of non-polymeric solvents. In this study, we developed a coarse-grained Stockmayer fluid simulation to address this issue, treating solvent molecules as soft-core spheres with permanent dipole moments. In this talk, we validate our model concept by considering monovalent and divalent ions dissolved in various non-polymeric solvents, such as water and methanol. Despite model simplicity, the results of our simulations show striking agreement when compared with experimental data for the free energy and enthalpy of ion solvation. We will also discuss that the primary contribution to the solvation energy arises mainly from the first and possibly second solvation shells near the ions.   

Enhancing the Dielectric Breakdown Strength of Solid-State Polymer Capacitors by Chain End Manipulations

The need for high power density, flexible and light weight energy storage devices requires the use of polymer film-based dielectric capacitors. Theoretically, it has been shown that chain ends contribute adversely to electrical breakdown, resulting in low energy density in polymer capacitors. In this work, we enhanced the energy density of polymer capacitor by using well-ordered high molecular weight block copolymer (BCP), in which the chain ends are segregated to narrow zones. Cyclic homopolymers (no chain ends) and linear homopolymers having chemistry-controlled chain ends also show enhanced breakdown strength, resulting in higher energy density as compared to the linear counterparts. These novel insights into manipulating chain end distribution such as in BCPs and with molecular topology to increase the energy density of polymers will be helpful for fulfilling next-generation energy demands.   

Dendrimer Approach toward High Permittivity Polymer Dielectrics for Electrical Energy Storage

Polymers which display high dielectric permittivity (DP) and low dissipation factor (DF) are important in the area of energy storage using film capacitors. Unfortunately, most polymers, even polar, exhibit fairly low DP, 2-4. It has been shown in this study that well dried films prepared from second generation dendrimer (D2) (T_g ~ 50^oC) synthesized using 2,2-bis(hydroxymethyl)propionic acid monomer (bis-MPA) demonstrate at room temperature a fairly high DP = 10.5 and low DF = 0.014 in the kHz range of frequencies. The dipole relaxation mechanism, which contributes to this unusual dielectric response was found to be linked to numerous hydroxyl groups situated in the periphery of dendrimer molecules. WAXS and computer molecular dynamics (MD) simulation all indicated that these highly mobile hydroxyl groups in bis-MPA D2 glassy films exhibit a very high level of dipole-dipole (OH-OH) correlation mediated by H-bonding. In fact, terminal hydroxyls readily form H-bonded ‘chain-like’ clusters of different lengths which can range from single H-bond associations to clusters containing tens of hydroxyls. This behavior explains why D2 exhibits a high dielectric constant.   

High κ polymers of intrinsic microporosity: A new class of high-temperature and low-loss dielectrics for microelectronic applications

High performance polymer dielectrics is a key component for power and printable electronics. In this work, organo-soluble polymers of intrinsic microporosity (PIMs) are reported for the first time to show desirable dielectric properties with high permittivity (or κ), high temperature capability, and low dielectric loss. Due to the polar sulfonyl side groups and rigid contorted polymer backbone, sulfonylated PIM (SO₂-PIM) exhibited an optimized balance between relatively high κ and low dielectric loss in a broad temperature window (up to 200 °C). For examples, its discharged energy density reached as high as 17 J cm^-3 with κ = 6.0. The discharge efficiencies were 94% at 150 °C/300 MV m^-1 and 88% at 200 °C/200 MV m^-1. Furthermore, its application as high-κ gate dielectrics in field effect transistors (FETs) is demonstrated. With the bilayer SO₂-PIM/SiO₂ gate dielectric, the InSe FETs exhibited a significantly improved electron mobility in the range of 200-400 cm² V^-1 s^-1, much higher than 40 cm² V^-1 s^-1 for the bare SiO₂-gated InSe FET. This study indicates that highly dipolar PIMs with rigid polymer backbone and large free volume are promising as high performance, next generation polymer dielectrics.   

Phase equilibria in P(TrFE-VDF): conformation and chirality

Ferro- and piezo-electric materials near a phase boundary usually possess enhanced dielectric or piezoelectric properties. We have discovered a morphotropic phase boundary (MPB) in P(VDF-TrFE) polymers [1], the first for any organic material. We show that the MPB forms due to intrachain rather than interchain conformation competition between the planar all-trans and 3/1-helical conformations in single P(VDF-TrFE) chains [2]. Our ab-initio calculations reveal low energy barrier electric dipole moment rotations between the nearly energetically degenerate chain conformations, which result in the piezoelectric enhancement near MPB. In particular, chirality plays a crucial role in the formation of a helical conformation and the relative stability of chain conformations strongly depends on the spatial arrangement of chirality centers. Our results explain the experimental data and the near degeneracy of multiple phases in the P(VDF-TrFE) material. They also provide a guideline for designing new polymers with MPB from a molecular point of view.
[1] Y. Liu, A. Haibibu, B. Zhang, W. Xu, W. Lu, J. Bernholc, Q. Wang, Nature 562, 96 (2018).
[2] Y. Liu, B. Zhang, A. Haibibu, W. Xu, Z. Han, W. Lu, J. Bernholc, Q. Wang, J. Phys. Chem. C 123, 8727 (2019).   

Ionic and Local Electric Polarization Effects in Polymers

The fundamental role of electric polarization in polymer physics is discussed via two examples. The first discusses purely ionic polarization effects in single ion conductors that are based on polymerized ionic liquids (PILs) (collaboration with the group of Ulli Scherf, Wuppertal). We employ a series of PILs with a polythiophene backbone bearing imidazolium salts with butyl, hexyl, octyl, and decyl side groups and several counteranions. PILs bearing the polythiophene backbone are unique as they can simultaneously conduct electronic charge and ions at nanometer length scales. Dielectric spectroscopy measurements performed as a function of temperature and pressure revealed that ionic conductivity is controlled by the balance between ion diffusion and ion complexation. The former is favored by the small ion size, the presence of ion channels and the decoupling from the backbone dynamics. On the other hand, ion complexation is controlled by ion size, the dielectric constant and charge delocalization. We propose a stick and jump model for ion motion in PILs.
The second example explores the local electric polarization effects in polymer blends (collaboration with M. Kappl, H.-J. Butt, Mainz). The design of multi-component materials for nanotechnology requires knowledge of the local composition. Hence, it is essential to develop techniques that can probe the local composition at the nanoscale. Local dielectric spectroscopy (LDS) is based on the electrostatic interaction between a conductive atomic force microscopy (AFM) probe in close proximity with the sample supported by a flat conductive substrate. Here, LDS is employed to analyze the dynamic heterogeneity in miscible blends as a function of film thickness. In thin films, phase segregation occurs and the kinetics of phase demixing was studied using as a probe the change in local composition. These results open new possibilities for studying interdiffusion and adhesion at polymer-polymer interfaces.