Session A33: Charged & Ion-Containing Polymers

Ionomer Self-assembly in Dilute Solution: a Coarse-grained Molecular Dynamics Study.

Self-assembly of semiflexible ionomer chains in dilute solution is studied by classical molecular dynamics (MD). Ionomer molecules consist of hydrophobic backbones, grafted with pendant side chains that are terminated by anionic headgroups. Coarse-grained MD simulations show the self-assembly of the semiflexible ionomer chains into cylindrical bundle-like aggregates. Bundles are comprised of a core of backbone chains surrounded by a surface layer of charged anionic headgroups and a diffuse halo of counterions. Parametric studies of bundle properties explored the role of backbone hydrophobicity, strength of electrostatic interactions between charged moieties, side chain content, and counterion valence. Expectedly, the size of bundles increases with backbone hydrophobicity. The aggregate size depends nonmonotonically on the value of the Bjerrum length. Increasing the grafting density of pendant side chains results in smaller bundles and the counterion valence exerts a strong effect on bundle size and counterion localization in the near-bundle area. Results are interpreted in terms of the interplay of the surface energy of hydrophobic chains and the electrostatic repulsion among the anionic headgroups. The findings are discussed within the context of experimental studies on the formation of rodlike structures in ionomer solution.   

Electrostatic Effect on the Solution Structure and Dynamics of PEDOT:PSS

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) is a popular material used in organic electronic devices as a conductor. It consists of PEDOT polycations complexed with PSS polyanions which are initially suspended in aqueous solution and eventually cast into a film. Various annealing and doping methods dramatically enhance PEDOT:PSS film conductivity. To understand the physical interactions at play, we explore structural and dynamic aspects of PEDOT:PSS solutions through scattering and rheology techniques. We highlight several aspects of the phase behavior of PEDOT:PSS, and the significance of electrostatic interactions.   

Ion Correlation and Transport in Polymer Electrolytes at Finite Salt Concentrations; Coarse-Grained Simulation Study

We present results from coarse-grained simulation for ion dynamics and structures in dry polymer electrolytes. To capture the thermodynamic, kinetic, and system-specific aspects of ion solvation and clustering, cation-monomer complexation is modeled via functionalized physical bonds whose functionality and lifetime vary due to local availability of binding monomers and competition with Coulombic interaction. By varying salt concentration, cation-monomer binding energy, dielectric constant, and maximal functionality of the physical bonds, we systematically study the growth of ion clustering activity as characterized by packing structures, and associated changes in electric conductivity via single-ion and collective charge mobility. Deviations from Nernst-Einstein predictions, and comparisons with existing experiments for concentration dependence of conductivity will be discussed.   

Explicit-ion Effects in the Coil-Globule Transition of Weak Polyelectrolytes

The first-order coil-globule transition in weak (annealed) polyelectrolytes involves a subtle balance of pH, charge strength, and solvation forces. In this work, we utilize a coarse-grain hybrid grand-canonical Monte Carlo and Molecular Dynamics approach to explore the free energetic topography of a model hydrophobic polybase [representing poly(2-vinylpyridine) (P2VP)] and explore the role of salt concentration/valency in influencing polyelectrolyte conformations using both an implicit Debye-Hückel and explicit salt approach. Our simulations reproduce the experimentally measured behavior for dilute annealed polyelectrolytes, and present a solid foundation for understanding pH responsive polyelectrolyte materials.   

The effect of ionic correlations on ion distribution across polyelectrolyte blend interfaces

Recent developments in high-density energy storage and generation devices have identified as polyelectrolyte blends and copolymers as suitable candidate materials for use in these applications, as they combine the low volatility and high flexibility of polymers with ion-selective conductivity of the charge-carrying backbone. It has been shown that in polyelectrolyte melts, where the dielectric constant is relatively low, ionic correlations can significantly reduce miscibility, inducing phase separation even at negative values of $\chi$N. At selected values of ionic coupling strengths, the polyelectrolyte blend exhibits a triple point, where coexistence is observed between phases with different concentrations and ordering of ions. When salt is added, the system undergoes re-entrant behavior as electrostatic effects are screened out. Using a hybrid of self-consistent field and liquid state theories (SCFT-LS), we investigate the distribution of ions across the interface in polyelectrolyte blends. We demonstrate that the inclusion of ionic correlations induces complex charge-dependent adsorption behavior at the interface, leading to changes in the interfacial width and miscibility of the blend.   

Effect of Charge Patterning on the Phase Behavior of Polymer Coacervates for Charge Driven Self Assembly

Oppositely charged polymers can undergo associative liquid-liquid phase separation when mixed under suitable conditions of ionic strength, temperature and pH to form what are known as `polymeric complex coacervates'. Polymer coacervates find use in diverse array of applications like microencapsulation, drug delivery, membrane filtration and underwater adhesives. The similarity between complex coacervate environments and those in biological systems has also found relevance in areas of bio-mimicry. Our previous works have demonstrated how local charge correlations and molecular connectivity can drastically affect the phase behavior of coacervates. The precise location of charges along the chain therefore dramatically influences the local charge correlations, which consequently influences the phase behavior of coacervates. We investigate the effect of charge patterning along the polymer chain on the phase behavior of coacervates in the framework of the Restricted Primitive Model using Gibbs Ensemble Monte Carlo simulations. Our results show that charge patterning dramatically changes the phase behavior of polymer coacervates, which contrasts with the predictions of the classical Voorn-Overbeek theory. This provides the basis for designing new materials through charge driven self assembly by controlling the positioning of the charged monomers along the chain.   

Connectivity and Excluded Volume Effects in Polymeric Complex Coacervates

Oppositely-charged polyelectrolytes in salt solutions can undergo phase separation to form complex coacervates. This charge-driven phase behavior is the basis for emerging motifs in self-assembly. Traditional uses for coacervates are in food and personal care products, while applications in technologies for drug delivery and sensory materials are being developed. One of the primary theories driving understanding of complex coacervates is the Voorn-Overbeek (V-O) theory, which is a precursor to more sophisticated field theories. We present both theory and simulation that provides an alternate picture of coacervates, specifically addressing the limitations of V-O. Our theoretical approach is based on PRISM, which is a liquid-state theory that specifically accounts for connectivity. This is compared with Monte Carlo-based simulations, which likewise provide a molecular picture of coacervation. We demonstrate that a combination of connectivity-based correlations and excluded volume has a profound effect on coacervation phase behavior, suggesting that favorable comparison of V-O to experiment benefits from a cancellation of errors. The influence of connectivity on coacervate phase behavior hints at new opportunities for molecular-based design in electrostatically-driven self-assembly.   

Salting-out and Salting-in in Polyelectrolyte Solutions

The phase behavior of polyelectrolyte (PE) solutions is governed by complicated interplay involving the mixing entropy, excluded volume, chain connectivity, and electrostatic interactions. Here we study the phase behavior of PE solutions in both salt-free condition and with added salt using a liquid-state (LS) theory based thermodynamic model. The LS model accounts or the hard-core repulsion by the Canahan-Starling equation of state, correlations due to chain connectivity by the first-order thermodynamic perturbation theory, and electrostatic correlations by the mean-spherical approximation. In comparison to the prediction from the well-known Voorn-Overbeek theory, the LS model predicts loop-type binodal curves in the salt-PE concentration diagram at temperatures slightly above the critical temperature of PE solution in salt-free case, consistent with the experimental study. The phase separated region shrinks with increasing temperature. Three scenarios of salting-out and salting-in phenomenon are predicted with addition of salts based, depending on the PE concentration.   

Influence of Higher Valence Ions on Flexible Polyelectrolytes Stiffness and Counter-ion Distribution

We investigate the influence of counter-ion valency on the flexibility of highly charged flexible polymer chains by molecular dynamics simulations that include both salt and an explicit solvent. A theoretical understanding of solutions of these molecules (e.g., DNA, RNA, and sulfonate polyestyrene) has been slow to develop due to the complex coupling between the polyelectrolyte conformation and the ionic species in solution due to their long range Coulomb interactions. As observed experimentally, we find that divalent counter-ions greatly reduce the chain persistence length, in comparison to monovalent counter-ions, an effect correlated with the tendency of the polyelectrolyte chain to become distorted by divalent counter-ions. We rationalize these results by with the substantial increase of counter-ion population at the interface with the polyelectrolyte, which not only leads to a more effective screening of the bare charge, but also leads to charge inversion in the trivalent counter-ion case. These conformational changes with counter-ion valency are also associated with a drastic increase of the number of contacts the counter-ions have at the interface with polyelectrolyte, an effect associated with polyelectrolyte chain “coiling” around the counter-ions.   

Understanding and Controlling Transitions in Polyelectrolyte Complex Materials

Polyelectrolyte complexation can be used in the self-assembly of a wide range of responsive soft materials ranging from dehydrated thin film and bulk solids to dense, polymer-rich liquid complex coacervates, and more complex hierarchical structures such as micelles and hydrogels. This responsivity can include swelling and dissolution, or liquid-to-solid transitions, typically as a function of ionic strength and/or pH. The patterning or presentation of charges and other chemical functionalities represents a powerful strategy for the design and manipulation of this type of responsiveness and the corresponding material properties. We utilize polypeptides and polypeptide derivatives as a model platform for the study of sequence and patterning effects on materials self-assembly. We also utilize rheology to understand the nature of the solid-to-liquid transition that has been observed in some systems. The goal of this systematic investigation of the effects of charge patterning is to elucidate design rules that facilitate the tailored creation of materials based on polyelectrolyte complexation with defined properties for a wide range of applications.   

Complexation of two oppositely charged polyelectrolytes

Existence of both polycation and polyanion in a solution will lead to complexation. We investigate both dynamics and the steady state solution for complexation of two oppositely charged polyelectrolytes in a salty solution. By use of Smoluchowski equation, both dynamic and steady-state properties of a complexation event occurring were studied.   

Morphology-induced low temperature conductivity in ionic liquids.

Ionic liquids exhibit nano-scale liquid crystalline order depending on the polymeric details of salt molecules. The resulting morphology and temperature behavior are key factors in determining the room temperature conductivity of ionic liquids. Here we discuss the phase behavior and related ionic conductivities of dry ionic liquids with volume fractions close to unity by using extensive molecular dynamics simulations. Temperature dependence, effective persistence length of tails, and excluded volume symmetry of amphiphilic ionic liquid molecules are investigated in large scale systems with short and long-range electrostatics. Our results suggest that by adjusting stiffness of the amphiphilic molecules and excluded volume interactions, lamellar or interconnected 3D phases can be obtained. Resulting phases have significant effects on the conductive properties. If there is no excluded volume asymmetry along the molecules, mostly lamellar phases with anisotropic conductivities emerge. If the excluded volume interactions become asymmetric, lamellar phases are replaced by interconnected phases consist of charged groups. Within temperature ranges that morphological phases are observed, conductivities exhibit low-temperature maxima in accord with experiments of ionic liquid-based liquid   

Structural Dynamics of Star-Shaped Weak Polyelectrolytes in Dilute Aqueous Solution

Weak polyelectrolyte (PE) bearing tunable charges along their backbones show great potential as ``smart'' polymer materials for diverse applications from drug delivery to energy storage. With the introduction of branched topology, the local counterion distribution in the vicinity to the polyelectrolyte segments becomes highly inhomogeneous. To experimentally investigate the interplay between structural dynamics and local electric environment of a branched polyelectrolyte, in this work we custom synthesized star-shaped poly(2-vinylpyridine) (P2VP) using reversible addition fragmentation chain transfer (RAFT) polymerization and labeled P2VP stars with pH-sensitive fluorophore precisely either in the center or periphery. By employing fluorescence correlation spectroscopy (FCS) with photon counts histogram (PCH) analysis, we observed gradual stretched-to-collapses conformational transition with increasing solution pH for both P2VP stars of different fluorophore labeling locations. However, the measured local pH, or local proton concentration, shows strong dependence of the fluorophore labeling locations. Higher electric potential yet lower ionization degree was observed in the core of P2VP star than that in the periphery. Ongoing work is carried out to examine the scaling behaviors of P2VP star sizes with varied number of arms, arm lengths and counterion concentrations in dilute aqueous solutions.   

Thermodynamics and Phase Behavior of Phosphonated Block Copolymers Containing Ionic Liquids

Charge-containing copolymers have drawn intensive attention in recent years for their uses in wide range of electrochemical devices such as fuel cells, lithium batteries and actuators. Particularly, the creation of microphase-separated morphologies in such materials by designing them in block and graft configurations has been the subject of extensive studies, in order to establish a synergistic means of optimizing ion transport properties and mechanical integrity. Interest in this topic has been further stimulated by intriguing phase behavior from charge-containing polymers, which was not projected from conventional phase diagrams of non-ionic polymers. Herein, we investigate thermodynamics and phase behavior of a set of phosphonated block copolymers. By synthesizing low-molecular weight samples with degree of polymerization (N) \textless 35, we observed order-disorder transition that enabled us to estimate effective Flory-Huggins interaction parameters ($\chi )$ by using random phase approximation. We further examined the systems by adding various ionic liquids, where noticeable increases in $\chi $ values and modulated microphase separation behavior were observed. The morphology--conductivity relationship has been elucidated by taking into account the segmental motion of polymer chains, volume of conducting phases, and the molecular interactions between phosphonated polymer chains and cations of ionic liquids.