Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S10: Charged and Ion-Containing Polymers |
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Sponsoring Units: DPOLY Chair: Charles Sing, UIUC Room: 269 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S10.00001: The effect of salt on the morphologies of compositionally asymmetric block copolymer electrolytes Whitney Loo, Jacqueline Maslyn, Hee Jeung Oh, Nitash Balsara Block copolymer electrolytes are promising for applications in lithium metal solid-state batteries. Due to their ability to microphase separate into distinct morphologies, their ion transport and mechanical properties can be decoupled. The addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt to poly(styrene)-block-poly(ethylene oxide) (SEO) has been shown to increase microphase separation in symmetric block copolymer systems due to an increase in the effective interaction parameter ($\chi _{\mathrm{eff}})$; however the effect of block copolymer compositional asymmetry is not well-understood. The effect of compositional asymmetry on polymer morphology was investigated through small and wide angle X-ray scattering (SAXS/WAXS). The effective Flory-Huggins interaction parameter was extracted from the scattering profiles in order to construct a phase diagram to demonstrate the effect of salt and compositional asymmetry on block copolymer morphology. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S10.00002: Effects of ion size and charge asymmetry on the salt distribution in polyelectrolyte blends and block copolymers Ha-Kyung Kwon, Kenneth R. Shull, Jos W. Zwanikken, Monica Olvera de la Cruz Polyelectrolytes have received much attention as potential candidates for rechargeable batteries, membrane fuel cells, and drug delivery carriers, as they can combine the electrochemical properties of the charged components with the mechanical stability and biocompatibility of the polymer backbone. The role of salt in determining the bulk and interfacial behaviors of polyelectrolytes has been of particular interest, as the miscibility has shown to depend significantly on salt identity and concentration. Recent studies employing the SCFT-LS method have shown that ionic correlations can enhance phase separation in polyelectrolytes and can induce selectivity in neutral solvents. Here, we extend the theory to investigate the role of salt in strongly correlated polyelectrolytes. We find that in lamellae-forming block copolymers, the addition of monovalent, symmetric salt can lead to a decreased lamellar spacing due to increased selectivity of the salt. When multiple electrostatic interactions are introduced via size and valency asymmetry in the salt pair, the bulk phase behavior and salt distribution across interfaces are significantly altered, as size and charge mismatch can transform the charge ordering seen in monovalent, symmetric salts. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S10.00003: Molecular Dynamics Simulations of Ion-Doped Microphase Separated Diblock Copolymers Youngmi Seo, Jonathan R. Brown, Lisa M. Hall The effects of ion doping on microphase separated block copolymers are crucial to understand for transport applications such as battery electrolytes or fuel cell membranes. Prior experiments and theories have observed interesting trends, e.g. ions generally increase effective $\chi $, broaden the domain interface at high loadings, and significantly change the order-to-disorder transition point. To provide a molecular level understanding of these trends and further information about ion dynamics, in this study, we perform molecular dynamics (MD) simulations using a generic coarse-grained model. We capture the selective ion solvation in one polymer microphase by adding an 1/r$^{\mathrm{4}}$ term to the intermolecular potential to account for the charge induced dipole effect between cations and A monomers. The model was validated by comparing with experimental domain spacing and density profile results. We find that as ions are added, the lamellar interface becomes sharper at first, then broadens with further ion loading, and finally forms a cylindrical morphology. We also observe that the interfacial broadening is retarded as the associative interaction between cations and A monomers or the ion-ion interaction strength is increased. These observations are compared to the results from fluids density functional theory (fDFT) which uses a similar model. We analyze ion dynamics in the model systems and discuss the impacts of ion selectivity and other variables on transport. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S10.00004: Fluids Density Functional Theory of Salt-Doped Block Copolymers Jonathan R. Brown, Lisa M. Hall Block copolymers have attracted a great deal of recent interest as potential non-flammable, solid-state, electrolyte materials for batteries or other charge carrying applications. The microphase separation in block copolymers combines the properties of a conductive (though mechanically soft) polymer with a mechanically robust (though non-conductive) polymer. We use fluids density functional theory (fDFT) to study the phase behavior of salt-doped block copolymers. Because the salt prefers to preferentially solvate into the conductive phase, salt doping effectively enhances the segregation strength between the two polymer types. We consider the effects of this preferential solvation and of charge correlations by separately modeling the ion-rich phase, without bonding, using the Ornstein-Zernike equation and the hypernetted-chain closure. We use the correlations from this subsystem in the inhomogeneous fDFT calculations. Initial addition of salt increases the domain spacing and sharpens the interfacial region, but for high salt loadings the interface can broaden. Addition of salt can also drive a system with a low copolymer segregation strength to order by first passing through a two phase regime with a salt-rich ordered phase and a salt-poor disordered phase. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S10.00005: Ionic Groups Enhanced Microphase Separation of Diblock Copolymers Jing Zong, Dong Meng Placement of ionic functionality on block copolymers, if properly designed, is known to generate more uniform nanoclusters and enhanced microphase separation. While recent experiments have demonstrated the effectiveness of this strategy, understanding from a computational point of view would reveal insightful information for systematic development and optimization of the approach. Here we apply our newly developed particle-field hybrid simulation formalism to study the microphase separation of a symmetric diblock copolymer (DBC) with the two blocks connected by an ionic junction group. In our simulations electrostatic interactions are taken into account explicitly, while the mean-field approximation is invoked for Flory-Huggins interactions. The method offers advantages in efficiently resolving both mesoscale self-assembled structures and structures due to strong correlations of electrostatic interactions. We show that inclusion of a single ionic group at the junction is able to drive DBC from the disordered into ordered state, agreeing with experimental observations [1]. Segregation is further enhanced by inclusion of multivalence ions as the result of the ``bridging effect''. Distribution of ions, while concentrated within the lamellar interfacial domains, also exhibit highly ordered in-plane structures. For multivalence case, charge neutrality analysis suggests existence of alternating positively-negatively charged layers in direction normal to lamellar interfaces. [1] ACS Macro Lett. 2015, 4, 1332-1336 [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S10.00006: Structure and Dynamics Ionic Block co-Polymer Melts: Computational Study Dipak Aryal, Dvora Perahia, Gary S. Grest Tethering ionomer blocks into co-polymers enables engineering of polymeric systems designed to encompass transport while controlling structure. Here the structure and dynamics of symmetric pentablock copolymers melts are probed by fully atomistic molecular dynamics simulations. The center block consists of randomly sulfonated polystyrene with sulfonation fractions $f \quad =$ 0 to 0.55 tethered to a hydrogenated polyisoprene (PI), end caped with poly(t-butyl styrene). We find that melts with $f \quad =$0.15 and 0.30 consist of isolated ionic clusters whereas melts with $f \quad =$ 0.55 exhibit a long-range percolating ionic network. Similar to polystyrene sulfonate, a small number of ionic clusters slow the mobility of the center of mass of the co-polymer, however, formation of the ionic clusters is slower and they are often intertwined with PI segments. Surprisingly, the segmental dynamics of the other blocks are also affected. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S10.00007: Anomalous phase behavior of ionic polymer blends and ionic copolymers Victor Pryamitsyn, Ha-Kyung Kwon, Johannes Zwanikken, Monica Olvera de la Cruz The development of DHEMSA approximation and its application for the ionic polymer blends and ionic copolymers has resulted in the prediction of ``inverted'' phases in ionic diblock-copolymers. In a contrast with uncharged diblock copolymers, ionic-neutral diblock-copolymers at high electrostatic coupling ($\Gamma$) and a low fraction of the ionomer component ($f$) form phases where the minority phase goes into the outside matrix and the majority phase forms inside cylinders. We have found that such behavior is related to the phase segregation of ionic polymer blends at high $\Gamma$ where neutral polymer can coexist with a blend of neutral and charged components. We combined the DHEMSA approximation with Helfand-Tagami theory to calculate the interfacial tension $\gamma$ between coexisting phases of ionomers. Knowledge of $\gamma$ allows us to use strong stretching theory and evaluate the whole phase diagram as a function of $f$, $\Gamma$, $\chi$, and the degree of polymerization $N$. In addition to the commonly formed lamella, sphere, and cylinder phases in diblock copolymers, we found ``inverted'' cylinder and sphere phases. 3D SCFT modeling then confirmed the existence of the ``inverted'' bi-continuous phases between lamella and the inverted cylinder phases. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S10.00008: Effect of Carboxyl Groups on Ionomer Properties from Molecular Dynamics Simulations Janani Sampath, Lisa M. Hall Ionomers are polymers with a small fraction of charged groups covalently bound to the non-polar polymer backbone, used in packaging and other applications. We consider ionomers and counterions with no solvent, in which the aggregation of ions significantly impacts overall material properties. Prior work established a useful coarse-grained model for fully neutralized acetic acid based ionomers, which include COO$^{\mathrm{-}}$ and Na$^{\mathrm{+}}$ but no COOH groups. To better model typical experimental systems that are only partially neutralized, we use additional ``sticker'' groups that represent COOH. These stickers are similar to uncharged monomers but with adjusted Lennard-Jones interaction strengths with each other and with ionic groups. Sticker-sticker interactions are radially symmetric (in contrast to true hydrogen bonding), however, aggregate morphologies obtained using the sticker-based model are in good agreement with prior atomistic simulation results. We analyze the structure (including scattering profiles) and dynamics of partially neutralized ionomers with differing chain architectures. Rheological properties such as viscosity and stress relaxation obtained from equilibrium stress fluctuations will also be discussed. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S10.00009: Synthesis and Properties of a Precision Sulfonated Trimethylene-Styrene Polyelectrolyte. Justin Kennemur, William Neary, Michele Bohlmann, Aaron Kendrick We recently reported successful ring-opening metathesis polymerization of 4-phenylcyclopentene to afford a precision ethylene-styrene type copolymer with a phenyl branch at exactly every fifth carbon along the backbone following mild hydrogenation of the backbone olefins. ( \underline {http://dx.doi.org/10.1002/marc.201600121} ) Compared to polystyrene, this material shows a markedly reduced glass transition temperature ($T_{\mathrm{g}} \quad \approx $ 17 \textdegree C) and remains amorphous. We have now extended the function of this polymer via sulfonation of the phenyl branches to produce a precision polyelectrolyte with an ionic charge spacing at every fifth carbon along the chain. The reduced yet precise charge density coupled with the low $T_{\mathrm{g}}$ of the native material translates into a variety of properties that are unique to this system and potentially useful as an addition to the limited set of available polyelectrolyte materials. Synthetic aspects in addition to thermal and mechanical properties will be discussed. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S10.00010: Evaporation of Solutions Containing Charged Polymers: A Molecular Dynamics Study Chengyuan Wen, Shengfeng Cheng Electrostatic interactions lead to rich behavior of solutions containing charged polymers different from that of neutral counterparts. The evaporation of the solvent from a polymer solution composed of a polar solvent, charged polymer chains, counterions, and salts is studied via large-scale molecular dynamics simulations. In our computational scheme, polymers are modeled as bead-spring chains containing neutral and charged beads. The solvent consists of dimers of oppositely charged beads connected by chemical bonds. Counterions are explicitly included as mobile single beads to make the whole system neutral. The morphology of the charged polymer chains at different volume fractions before evaporation and the structure of the resulting film after evaporation are compared and the effects of evaporation rate are clarified. We also add salts to the solution and study their influence on the polymer morphological evolution during evaporation. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S10.00011: Effect of Pendant Side-Chain Sterics and Dipole Forces on Short Range Ordering in Random Polyelectrolytes Chinomso Nwosu, Tara Pandey, Andrew Herring, Edward Coughlin Backbone-to-backbone spacing in polymers is known to be dictated by the length of the pendant side-chains. Dipole forces in random polyelectrolytes lead to ionic clusters with a characteristic spacing that can be observed by SAXS. Repulsion due to side-chain sterics will compete with dipole forces driving cluster formation in random polyelectrolytes. A model study on short range order in anion exchange membranes (AEMs) of quaternized P4VP-ran-PI is presented. Quaternization of P4VP with alkyl bromides having different numbers of carbons, C$_{n}$Br, introduces pendant side-chains as well as charges. X-ray scattering performed on PQ4VP-ran-PI(C$_{n}$Br) show that when n$<$5 the dipole forces dominate leading to the formation of ionic clusters. However, when n$>$4, the chains remain separated due to sterics, forming a distinct backbone-to-backbone spacing morphology. For n=3, both dipole clustering and backbone spacing can coexist. Crosslinking of the isoprene units increased the coexistence window from n=3 to n=6. Impedance measurements show that a maximum conductivity of 110mS/cm was obtained for PQ4VP-ran-PI(C$_{3}$Br). A discussion on short range order due to competition, or counter balancing, of steric repulsion and dipole forces will be presented. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S10.00012: Field-theoretic simulations of correlation effects in charged polymers Jing Cao, Robert Riggleman Dielectric properties of inhomogeneous soft materials have attracted great deal of attention over the last several decades. Polyelectrolyte block copolymers can demonstrate more rich features comparing to its ``simple'' homogeneous counterpart since the packing between bound-ions and surrounding counterions are complicated as well as the local dielectric constant can vary significantly between different domains. However, in the standard field-theoretical approach of treating charged systems, a uniform dielectric constant has been assumed through the system and packing effects are challenging to incorporate, leading to qualitative differences between theoretical predictions and experiments. We present here a molecularly statistical field theory model of inhomogeneous diblock copolymers with ionic junctions. We studied the tendency of phase separation due to strong correlations between bound-ions and counterions by using mean-field approximation and complex Langevin simulation technique. In addition, the results from our field-theoretical model were compared with the results from molecular dynamics simulations and experiments. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S10.00013: Dielectric Effects on Polyelectrolyte Adsorption Studied by a Local Monte Carlo Algorithm Jian Jiang, Zhen-Gang Wang We study the effects of dielectric discontinuity on polyelectrolyte (PE) adsorption and double layer properties using an improved local Monte Carlo algorithm originally proposed by Maggs and Rossetto [A. C. Maggs and V. Rossetto, \textit{Phys. Rev. Lett.} \textbf{88}, \textbf{196402} (2002)]. We distinguish between two different mechanisms for PE adsorption, one due to electrostatic attraction, and the other due to van der Waals type attraction. For both systems, as a result of image charge repulsion, the threshold for PE adsorption is increased in terms of the surface charge density and the strength of the van der Waals attraction. For electrostatically driven adsorption, we find nearly all the monomers of the PE are captured by the surface when the surface charge exceeds the adsorption threshold. For adsorption driven by van der Waals attraction, only partial monomers are adsorbed onto the surface--the adsorbed chain has many loops extending away from the surface. The addition of salt is shown to have opposite effects on the adsorption threshold and the amount of adsorption in these two cases. In addition, due to counterion condensation on the PE backbone, the adsorption of the PE chain is accompanied by a concurrent significant adsorption of the counterions. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S10.00014: Polyelectrolyte brushes on dielectric surfaces Hanne Antila, Erik Luijten When chains of charged polymers are grafted to a solid surface, a polyelectrolyte (PE) brush results. These types of PE assemblies have a wide range of applications ranging from fuel cells and switchable electrodes to drug delivery. Many of these applications stem from the ability of PE brushes to respond to external stimuli: the brush properties can be tuned, for example, by varying electric field, PE grafting density, pH, salt concentration or salt valency. Accordingly, deciphering the brush behavior under different conditions has been a subject of considerable experimental, theoretical, and computational research efforts. However, the effect of the dielectric properties of the substrate on the PE brush has received much less attention. We use coarse-grained molecular dynamics simulations to show how varying the dielectric mismatch between the solvent and the substrate can significantly affect the brush. We demonstrate how tuning this mismatch can either diminish or enhance the effects of other control parameters, such as pH, on the brush properties. Furthermore, we investigate how dielectric properties of the substrate affect the brush, and the ion distribution and mobility within the brush, when the brush is exposed to an electric field. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S10.00015: Counterion adsorption and desorption rate of a charged macromolecule Yu Shi, Jingfa Yang, Jiang Zhao The rate constant of counterion adsorption to and desorption from a synthetic polyelectrolyte, polystyrene sulfonate (PSS-), is measured in aqueous solution by single molecule fluorescence spectroscopy. The results show that both adsorption and desorption rate of counterions have strong dependence on polymer concentration, salt concentration as well as the molecular weight of polyelectrolytes. The results clearly demonstrate that the contribution of electrostatic interaction and the translational entropy to the distribution of counterions of a polyelectrolyte molecule. The information is helpful to the understanding of polyelectrolyte physics. [Preview Abstract] |
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