Session D16: Charged and Ion-Containing Polymers

Dynamic phase transitions in polyelectrolyte gels

When a polyelectrolyte gel is placed in an ionic solution, slight alterations in the environmental parameters can trigger enormous changes in the gel volume.  In some cases, the gel volume will change discontinuously, resulting in a volume phase transition.  Many studies of the volume phase transition focus on the equilibrium states of the gel or resolving the time dependence of the gel volume.  In this talk, I present a novel phase-field model of a polyelectrolyte gel that can be used to explore the evolution of the gel structure during the volume phase transition.  Numerical simulations reveal that the volume phase transition can occur via two routes.  In the first case, a swelling or deswelling front emerges from the free surface and propagates into the bulk of the gel.  In the second case, spinodal decomposition occurs alongside front propagation, resulting in a non-trivial distribution of ions that is coupled to the gel mechanics.  Our results also show that the electric double layer that forms at the gel-bath interface can play a key role in determining the internal structure of the gel.  If the Kuhn and Debye lengths are commensurate, then a localised mode of phase separation can emerge from the double layer and invade the gel, leading to a global breakdown of electroneutrality.  We conclude with a discussion of how these rich dynamics can be observed through experimentally measurable macroscopic quantities.   

Constraint release in entangled liquid coacervates made from oppositely charged polyelectrolytes

Mixtures of oppositely charged polyelectrolytes can phase separate to form a polymer rich coacervate phase, used in many technological applications and biological systems. Predicting coacervate dynamics is important for determining processing conditions and macromolecule transport in the coacervate. We developed a scaling theory predicting the dynamics of unentangled and entangled liquid coacervates made from polyelectrolytes of equal (symmetric) or unequal (asymmetric) linear charge density. We find high charge density polymers are dynamically coupled to the low charge density polymer even in the unentangled regime. We predict that entangled coacervates have multiple regimes depending on whether the high charge density polymer reptates along the tube formed by other high charge density chains or along a tube formed by low charge density chains. In the latter case, topological constraints imposed by the low charge density chains are dynamic, which modifies coacervate transport and rheology through a process called constraint release. In this work, we develop a scaling model to predict the effects of constraint release on entangled asymmetric coacervates, where the low and high charge density polymers have different conformations. We find that the regimes dominated by tube rearrangement are broader in asymmetric coacervates as compared to symmetric coacervates. Constraint release also weakens the concentration dependence of viscosity in regimes dominated by reptation of the high charge density chains.   

Ising density functional theory for the charge regulation of weak polyelectrolytes

Charge regulation of polymers is an important phenomena that is not well described by conventional methods owing to their mean-field approximations on electrostatic interactions.  As a result, the miss the intrachain correlations that lead to unique ionization behavior of polymers compared to their monomeric counterparts.  The pH-responsive behavior is advantageous for applications in smart systems to achieve targeted design.  We report a theoretical framework for weak polyelectrolytes by combining the polymer density functional theory with the Ising model for charge regulation.  The so-called Ising density functional theory (iDFT) provides an accurate description of the effects of polymer conformation on the ionization of individual segments and is able to account for both the intra- and inter-chain correlations due to the excluded-volume effects, chain connectivity and electrostatic interactions. Theoretical predictions of the titration behavior and microscopic structure of ionizable polymers are found in excellent agreement with experiment.  Furthermore, we demonstrate unique behavior that results from intrachain correlations that cannot be captured through conventional methods.   

Tunable ion-containing polymers from the facile copolymerization of functional polyethers

Complex coacervation phenomenon is driven by pairs of oppositely charged macroions and their self-assembly. Different parameters such as charge density, monomer sequence, polymer architecture, and counter-ions affect self-assembly of these polymers. In this work, we demonstrate a tunable polymer platform from the facile copolymer of functional epoxides that allows for control over each of these parameters independently. Statistical copolymers of propargyl glycidyl ether (PGE) and epichlorohydrin (ECH), with functional alkyne and chloromethyl groups respectively, were synthesized. Molecular weights up to 100 kg/mol with narrow distributions were achieved. Copolymer composition was varied by incorporating increasing ratios of PGE (20-80%) in the polymerization feed. Reactivity ratios were calculated as r_PGE= 0.69 and r_ECH= 1.43 using in situ ¹H NMR kinetic study which confirms the statistical nature of the copolymer. Various charge groups such as imidazole and sulfonate were tethered to the polymer chain via orthogonal chemistry through the chloromethyl and alkyne moieties. We controlled charge density by varying copolymer compositions. Therefore, we have demonstrated a facile approach to charged polymers which we will utilize in the future to study charged polymer self-assembly.   

Analysis of Dielectric Constant and Solvation Energy in Coarse-Grained Polymerized Ionic Liquid Simulation

Recent investigations into the dielectric properties of Ionic Liquids (IL) and Polymerized Ionic Liquids (PIL) indicate further theoretical studies are required for these materials. Some of these works suggest that polymerization of cations in ILs can result in an unconventional increase in the dielectric constant, the mechanism for which is not yet well understood. Additionally, it has been shown that the Born Solvation Energy can be quantitatively inaccurate for monovalent and divalent ions in inorganic non-polymerized solvents, so it is unclear if it can be used in Mean-Field theories for predicting IL and PIL behavior. We seek to probe these results with molecular dynamics simulation using the Stockmayer Fluid model, where all molecules are treated as Lennard-Jones spheres with permanent dipole moments and point charges. By varying ionic charge we analyze the dielectric constant, dipolar orientational order and correlation, and compare simulated solvation energy with the expected Born Solvation energy. A significant increase in the dielectric constant upon polymerization is found at low values of ionic charge, but the inclusion of local ion pairs into the calculation suggest large charges may show this as well. We also find that increases in the dielectric constant are generally associated with increases in order and correlation length. This suggests that the large increase in the dielectric constant results from increased order that occurs upon polymerization. Further, the energy per ion of the simulations can match that expected with the Born Solvation energy if scaled with an effected ionic radius, which is justified by our results from the radial distribution function.   

Overlap Concentration of Sodium Polystyrene Sulfonate in Solution

The overlap concentration c* of sodium polystyrene sulfonate in water is calculated using multichain atomistic and coarse grained (CG) simulations for a range of chain lengths. Fully atomistic molecular dynamics simulations are carried out for N = 32 to 192 monomers. The CG model was parameterized to match the end-to-end distance from the atomistic  simulations at small N and allows us to simulate much larger N. Treating the hydrophobic backbone by inclusion of attraction between monomers is an essential addition to the CG model. The simulation c* are in agreement with experimental data, yet at c*, the chains are not fully stretched even for N as large as 1200. This implies that none of the experimental systems are in the scaling regime, and to reach the scaling regime for NaPSS chains much longer than N=1200 are required.   

Impact of Sulfonation Degree and Nanoparticle Surface Chemistry on Ion Selectivity in Sulfonated Ionomer Nanocomposites

In this study, a series of sulfonated poly(ether ether ketone) (SPEEK) membranes were fabricated and the water and vanadium ion transport properties of the membranes were characterized. Specifically, the SPEEK nanocomposite membranes were prepared with degrees of sulfonation (DS) ranging from 60% to 80%, as well as nanoparticle (NPs) loadings ranging from 0 mass% to 10 mass% to elucidate how these design parameters affect the ion selectivity of these ionomer nanocomposites. The NPs were functionalized with both cationic and anionic surfaces such that they interact electrostatically with the fixed charges on the phenyl groups along the backbone of the SPEEK. The DS was observed to be well controlled by duration of the sulfonation reaction and was measured ¹H NMR. The permeability of vanadium through the ionomer nanocomposites was measured via ultraviolet-visible spectroscopy. Additionally, the through-plane proton conductivity of the membranes was measured. Furthermore, the NP dispersion state was characterized by transmission electron microscopy. Results from this investigation establish important processing-performance property relationships for sulfonated ionomer nanocomposites, helping to facilitate the development of novel, better performing ionomer membranes for VRFBs.   

High Li+ Conduction and Selectivity in Polymeric Ionic Liquid Electrolyte Complexes

Polymer electrolyte complexes (PECs) formed by blending oppositely charged polymeric ionic liquids (PILs) have been predicted to form electrostatically stabilized structures on the nanometer scale. We demonstrate that since solvent-free PECs can dissolve very high quantities of salt, they possess high ionic conductivity.  In this work, imidazoliums-tethered polysiloxane (polycations) and bis(trisfluoromethanesulfonyl)imides (TFSI)-tethered polyacrylate (polyanions) with varying charge densities are blended to form PECs.  PECs with an ionic charge on every monomer (100% charge density) are homogeneous due to the strong interpolymeric electrostatic interactions, solvate up to 50 wt% salt, and demonstrate high ionic conductivity (2×10⁻³ S/cm) and Li⁺ transference number (t₊ = 0.6) at 90 °C.  PECs with 10% charge density demonstrate a local correlation length of ca. 5 nm, while the addition of LiTFSI salt alters the structure to increase the correlation length by screening ionic interactions. This work guides our understanding of the structure and internal interactions of solvent-free charge-containing polymer complexes and the experimental design of such polyelectrolyte systems.   

Tapered block polymer electrolytes for lithium-ion batteries: enhancement of ion transport through the tuning of intra-domain structure

Tapered block polymers (TBPs) contain modified (e.g., gradient) monomer segment composition profiles at the chemical junction between two homogeneous blocks. In this work, we studied how the tuning of monomer segment and ion distributions (i.e., intra-domain structure) imparted enhanced ion transport in nanostructured polystyrene‑block‑poly(oligo-oxyethylene methacrylate) (PS-b-POEM) TBP electrolytes through a combination of X-ray reflectometry experiments and coarse-grained molecular dynamics simulations that included strong ion solvation effects. This combined experimental-computational approach revealed that in normal‑tapered electrolytes, the significant S/OEM covalent bonding and global stretching of chains across a domain (rather than the back-and-forth folding of chains across an interface) reduced local stretching throughout the domain and reduced the confinement of OEM/ions. Both of these effects improved ion transport. However, these factors were competing constraints in non- and inverse‑tapered electrolytes. The above-mentioned results provide important design parameters and synthesis/formulation avenues toward next‑generation nanostructured electrolytes.   

Orthogonal Phase Behavior in Polyzwitterionic Coacervate Systems

In this work, we experimentally characterize the liquid-liquid phase separation of a system composed of polyzwitterions and polyelectrolytes. These materials exhibit remarkable properties with respect to their solution behavior across a wide physicochemical parameter space. We relate pH, stoichiometry, and temperature dependencies on the phase behavior of our system to inherent properties of the constituent chains, and present a theory to explain observed experimental phenomenology. Finally, we use a physical simulation to demonstrate how our polymer system is uniquely suited to applications in the biomedical context of cargo retention and release.   

Modelling Coacervation in Polyelectrolyte – Surfactant Micelle Solutions

Self-assembly and solution behavior of oppositely-charged surfactant-micelles and polyelectrolytes have been studied for a variety of applications including pharmaceuticals and personal care products, as well as for understanding biomolecular condensates with significantly charged constituent molecules. These systems have been shown to undergo complex-coacervation, which is a liquid-liquid phase separation in solutions of oppositely-charged macromolecules. This phase separation results in a coacervate phase that is rich in charged macromolecules and the supernatant which is poor in charged macromolecules. The disparities in length scales and strong Coulombic interactions in these mixed macroion solutions can make modeling these systems computationally challenging. In this work, we use self-consistent field theory (SCFT) to model the thermodynamics of solutions containing worm-like surfactant-micelles interacting with oppositely-charged homopolyelectrolytes. The SCFT simulations are informed by Monte Carlo (MC) simulations of the surfactant-micelles as hard-sphere chains. In this work we show that local charge-charge correlations are a key component to phase separation in these systems. The hybrid SCFT / MC model presented can be further generalized to apply to other mixed macroion systems.   

Dynamics of Star Polyelectrolytes in Multilayer Assemblies

We report on the effect of the molecular architecture of poly(acrylic acid) (PAA) on its diffusion within electrostatic layer-by-layer (LbL) films with poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) at low pH. Ellipsometry measurements with dry films and films exposed to aqueous solutions both indicated faster mobility of star PAA. Specifically, in-situ ellipsometry revealed that the diffusion coefficient of an 8-arm star polyacid during penetration within wet pre-assembled LbL films was 4-fold higher than that of its linear counterpart. The difference in mobility of star and linear PAA was also evident in neutron reflectometry experiments, which showed fast intermixing of polymer layers in star PAA-containing films upon exposure to salt solutions. We suggest that higher mobility of star polymers results from a smaller number of contacts between the star and linear polymers, and support this conclusion by FTIR measurements of a lower ionization degree of assembled star PAA, and by isothermal titration calorimetry (ITC) studies of interpolymer binding in the solution that showed lower PDMAEMA-to-PAA ratio and higher dissociation constants for star PAA/PDMAEMA complexes, which indicated weaker binding of star PAA with the polycation.   

Wetting and Contact Angles of Complex Coacervates

Complex coacervates continue to be promising materials for a wide variety of applications, including encapsulation and bio-inspired adhesives. Despite the desire to exploit their ultralow interfacial tension and high wettability on solid surfaces, the interfacial behavior of complex coacervates remains relatively unexplored from both experiments and theory, particularly at the coacervate-solid interface. Here, we present the wetting behavior of coacervates using a mean-field theory. We focus on the effect of salt, polyelectrolyte-surface affinity, and electrowetting on the surface phase diagrams (wetting transitions) and prediction of contact angles.