Session A48: Charged and Ion-Containing Polymers

Influence of Plasticizer on Ion Aggregation in Single-ion Polymer Conductors

In this study, we add a miscible small molecule plasticizer to a polyester copolymer ionomer. The latter is synthesized from oligomeric polyethylene oxide (molecular weight = 600) separated by the lithium salt of 85 percent sulfonated dimethyl isophthalate units. Materials with different plasticizer contents are systematically investigated by FTIR spectroscopy, X-ray scattering and dielectric spectroscopy. Decreased Tg and a corresponding Increase in cation mobility are expected in these plasticized systems. Ionic conductivity depends on both the number of ions and their mobility, which in turn depends on the relationship between ion states (free ions, ion pairs and ion aggregates). With FTIR, we characterize and quantify the ionic structures in order to investigate how their ratio changes with plasticizer content. X-ray scattering reveals the change in ion aggregation. Further, dielectric spectroscopy is used to study ion conductivity and polymer dynamics of these materials.   

Atomistic molecular dynamics simulations of model ionomers

Ionomers are polymers that contain a small fraction of ionic groups. Due to their unique electrical properties, ionomers are being investigated as potential solid electrolytes in mobile battery applications. However, a lack of fundamental understanding of the relationship between ionomer chemistry, morphology and ion transport have hindered such efforts. To this end, we report atomistic molecular dynamics (MD) simulations of a model ionomer (polyethylene-co-acrylic acid) neutralized with different ions at various neutralization levels. The structure factor computed from the simulations is in good agreement with experimental X-ray scattering data, which provides strong validation of the simulation methods. Our simulations provide additional insight into the shape and size distribution of ionic clusters; in particular, we observe large networks of string-like clusters, and report quantitative features of these structures as a function of ionic group spacing in the polymer backbone, counterion type and neutralization level. We also investigate several features of ion transport in these systems. Since ion diffusion is slow relative to the time scales accessible to our simulations, we limit discussion to local, qualitative features of the ion transport mechanism.   

Weak polyelectrolytes grafted to nanoparticles and flat surfaces

The charge distribution on polyelectrolytes is a key factor which controls their conformation and interactions. In weak polyelectrolytes, this distribution is determined by a number of factors, including the solvent conditions and the local environment. We investigate this using grand canonical titration Monte Carlo simulations of a coarse grained polymer model. In this method, each polymer bead is able to change its ionization state based on its dissociation constant, the pH of the solution, and interactions with other particles in the system. We focus on a system of polymers with one end tethered to the surface of a nanoparticle and determine both the charge and the polymer conformation as the pH and solvent quality are varied. We compare the results to both a fixed charge model and to polyelectrolytes grafted to a flat surface.   

Large scale molecular dynamics study of polymer-surfactant complex

In this work, we study the self-assembly of cationic polyelectrolytes mediated by anionic surfactants in dilute or semi-dilute and gel states. The understanding of the dilute system is a requirement for the understanding of gel states. The importance of polyelectrolyte with oppositely charged colloidal particles can be found in biological systems, such as immobilization of enzymes in polyelectrolyte complexes or nonspecific association of DNA with protein. With the same understanding, interaction of surfactants with polyelectrolytes shows intriguing phenomena that are important for both in academic research as well as industrial applications. Many useful properties of PE surfactant complexes come from the highly ordered structures of surfactant self-assembly inside the PE aggregate. We do large scale molecular dynamics simulation using LAMMPS to understand the structure and dynamics of PE-surfactant systems. Our investigation shows highly ordered ring-string structures that have been observed experimentally in biological systems. We will investigate many different properties of PE-surfactant complexation which will be helpful for pharmaceutical, engineering and biological applications.   

Near Edge X-ray Absorption Fine Structure Studies of Cu Ion-Containing PAMAMOS Dendrimer Networks

There is continuing interest in the development of nanocomposites containing metal ions based on the use of dendrimers as the host matrix. One may utilize functionalized dendrimer interiors to complex the added constituent and serve as a template for the organization of the resulting nanoscale structures. Potential applications of include: catalysts, biotechnology, functional membranes, molecular sensors, etc. We report on recent results obtained using near edge X-ray absorption fine structure (NEXAFS) to characterize Cu(2+) covalent interactions in three-dimensionally cross linked dendrimer networks. These networks were made from radially-layered poly(amidoamine-organosilicon), PAMAMOS, dendrimers having generation 4 (G4) polyamidoamine (PAMAM) interiors surrounded by one layer of organosilicon, OS, exterior branch cells. Lower generation homologues (i.e., G1 through G3) were also examined on a more limited basis. The nitrogen and carbonyl moieties contained in the PAMAM dendrimer interior were shown to be highly interactive with metallic cations, in large measure because of the dendrimer geometry. Similar measurements on chemically similar but much less physically constraining hyperbranched polymers indicated very limited interaction with the amine and carbonyl moieties.   

Mean-Field Modeling of the Encapsulation of Weakly Acidic Particles in Polyelectrolyte Dendrimers

The unique architecture of dendrimers has attracted interest in a wide-variety of biomedical applications such as drug delivery. Dendrimers act as covalent micelles and have been shown experimentally to internalize hydrophobic molecules inside their cavities. Moreover, many drugs of low water solubility are weakly acidic and have been shown to form complexes with polybasic dendrimers, with the encapsulation ability being dependent upon the solution pH. Furthermore, the grafting of neutral water soluble chains such as polyethylene glycol (PEG) have shown to increase the encapsulation of poorly soluble drug molecules. In order to gain insight into the equilibrium behavior of drug-dendrimer complexes, we have developed and numerically solved a Self-Consistent Field Theory approach for both grafted and non-grafted annealed charged dendrimer molecules in the presences of drug molecules. Specifically, this work examines the effect of drug size, dendrimer generation, grafting chain length, and solution pH upon dendrimer encapsulation abilities.   

Structure-Property Relationships in Precise Acid-Containing Polymers

Acid-containing polymers have specific interactions that produce complex and hierarchical morphologies providing a remarkable combination of mechanical properties, namely being both dissipative and resilient. Despite the widespread industrial use of such materials, rigorous structure-property relationships remain elusive due to structural heterogeneity in the available copolymers. Recently, linear polyethylenes with pendent acid groups separated by a precisely controlled number of carbon atoms have been synthesized by acyclic diene metathesis (ADMET) polymerization. X-ray scattering shows that the molecular uniformity of these acid copolymers results in morphologies with nearly monodisperse acid aggregates and polyethylene crystals assembled in highly organized hierarchical structures. Taking advantage of these ordered morphologies to obtain well-defined mechanical data, we probe the elastic modulus, yield stress and second yield stress of precise acid-containing polymers as a function of the number of carbon atoms between acid groups, acid type (acrylic and phosphonic), and mono- or geminal acid functionalization.   

Block copolymer ion gels for gas separation

Carbon dioxide removal from light gases (eg. N$_{2}$, CH$_{4}$, and H$_{2})$ is a very important technology for industrial applications such as natural gas sweetening, CO$_{2}$ capture from coal-fire power plant exhausts and hydrogen production. Current CO$_{2}$ separation method uses amine-absorption, which is energy-intensive and requires frequent maintenance. Membrane separation is a cost-effective solution to this problem, especially in small-scale applications. Ionic liquids have recently received increasing interest in this area because of their selective solubility for CO$_{2}$ and non-volatility. However, ionic liquid itself lacks the persistent structure and mechanical integrity to withstand the high pressure for gas separation. Here, we report the development and gas separation performances of physically crosslinked ion gels based on self-assembly of ABA-triblock copolymers in ionic liquids. Three different types of polymers was used to achieve gelation in ionic liquids. Specifically, a triblock copolymer ion gel with a polymerized ionic liquid mid-block shows performances higher than the upper bound of well-known ``Robeson Plot'' for CO$_{2}$/N$_{2}$.   

Transport and phase behavior in anion-conducting diblock copolymers

Anion-exchange membranes can be used in various applications such as direct methanol fuel cells or devices for artificial photosynthesis. Consequently, these membranes have to conduct the anions efficiently, remain insoluble in water or methanol, and be impermeable to various gases. Block copolymers are good candidates to reach these aims. Their ability to self-assemble, particularly into bi-continuous phases, makes it possible to use one polycationic block that would conduct the anions, while a second neutral block can be designed as a structural block to insure the mechanical stability of the system. Our study focuses on the relationship between phase behavior and anion conductivity of diblock copolymers as a function of molecular weight, composition and cationic groups. We have chosen to use a model system made of styrene for the neutral block, and of chloromethylstyrene for the second block since it is easily cationizable with various functionalities, reacting easily with e.g. trimethylamine or n-butylimidazole. This model system is synthesized by nitroxide-mediated radical polymerization with molecular weights between 2 to 40 kg/mol and fractions of chloromethylstyrene in the range of 15-40 mol{\%}. The results obtained from small-angle X-ray scattering showed lamellar morphologies for most systems. The temperature-dependence of the conductivity was assessed by performing measurements on membranes that were either immersed in water or in a controlled atmosphere at 98{\%} of relative humidity.   

Polyelectrolyte Uptake by PEMs at High Salt

Upon a jump in salt concentration, a polyelectrolyte multilayer (PEM) constructed by the layer-by-layer process will swell, and in consequence, uptake from solution a large additional mass of the capping polyelectrolyte. Here, swelling and uptake are monitored in time by the quartz crystal microbalance with dissipation (QCM-D) method as a function of elevated salt concentration (0.75M$<$[NaCl]$<$2.5 M) during the uptake of poly(styrene sulfonate) (PSS, MW$\sim $70,000 g/mol) by poly(diallyldimethylammonium chloride)/PSS PEMs made at [NaCl]=0.5M. For [NaCl] less than $\sim $1 M, PSS adds only at/near the PEM surface, while for higher [NaCl], PSS fully permeates the PEM, contributing a PSS mass approaching, even exceeding, that already present; higher salt concentration leads to faster and greater PSS uptake. Above [NaCl]=1.0 M, uptake is diffusive, characterized by surprisingly large and sharply [NaCl]-dependent diffusion coefficients ($\sim $10$^{-14} - 10^{-12}$ cm$^{2}$/s). This uptake process opens a general opportunity for facile bulk and surface modifications of PEMs.   

Polyelectrolytes in Salt Solutions: Molecular Dynamics Simulations

We performed MD simulations of polyelectrolyte solutions in the presence of salt. Polyelectrolyte solutions were modeled as an ensemble of chains of charged LJ particles with explicit counterions and salt ions. Our simulations have shown that in dilute and semidilute polyelectrolyte solutions the electrostatic chain persistence length scales with ionic strength as $I^{-1/2}$. This is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing salt concentration as $R\propto I^{-1/5}$, which is in line with the scaling of the persistence length on the ionic strength, $l_{p}\propto I^{-1/2}$. In semidilute solution at low salt concentrations the chain size decreases with increasing polymer concentration, $R\propto c_{p}^{-1/4}$, while at high salt concentrations it is, $R\propto I^{-1/8}$. Our simulations confirmed that the peak position in the polymer scattering function scales with the polymer concentration in dilute polyelectrolyte solutions as $c_{p}^{1/3}$. In semidilute polyelectrolyte solutions at low salt concentrations the peak shifts towards larger values of $q^{\ast }\propto c_{p}^{1/2}$ while at high salt concentrations the peak location depends on the ionic strength as $I^{-1/4}$. The simulations confirmed a general scaling relations between a quantity $X(I)$ in salt solutions and corresponding quantity $X(I_{0})$ in salt-free solutions,$ X(I)= X(I_{0})(I$/$I_{0})^{\beta }$ with exponents; \textit{$\beta $}=-1/2 for persistence length $l_{p}$, \textit{$\beta $}=1/4 for solution correlation length, \textit{$\beta $}=-1/5 and \textit{$\beta $}=-1/8 for chain size $R$ in dilute and semidilute solution, respectively.   

Electrostatic persistence length of flexible polyelectrolytes: resolving the controversy between theory and experiments

The salt concentration dependence of the electrostatic persistence length $l_{e}$ of flexible polyelectrolytes has been the subject of extensive debate over the past two decades. Although theoretically a consensus has been reached regarding the correctness of the extension by Khokhlov and Khachaturian (KK) of the well-known Odijk--Skolnick--Fixman (OSF) theory to flexible polyelectrolytes, one crucial question remains: the strong disagreement between the OSF--KK prediction and various experimental observations. We present the results of extensive simulations of a flexible polyelectrolyte in solution to elucidate the origin of this discrepancy, and demonstrate that it originates from neglecting ionic excluded volume in the theoretical treatment.   

Layer-by-Layer Assembly of Charged Nanoparticles on Porous Substrates: Molecular Dynamics Simulations

We performed molecular dynamics simulations of multilayer assembly of oppositely charged nanoparticles on porous substrates with cylindrical pores. The film was constructed by sequential adsorption of oppositely charged nanoparticles in layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after completion of each deposition step. There is almost linear growth in the surface coverage and film thickness during the deposition process. The multilayer assembly also occurs inside cylindrical pores. The adsorption of nanoparticles inside pores is hindered by the electrostatic interactions of newly adsorbing nanoparticles with the multilayer film forming inside the pores and on the substrate. This is manifested in saturation of the average thickness of the nanoparticle layers formed on the pore walls with increasing number of deposition steps. The distribution of nanoparticles inside cylindrical pore was nonuniform with significant excess of nanoparticles at the pore entrance.   

Diffusion of Polyelectrolyte Chains within Multilayer Films

Using a series of polycations synthesized by atom transfer radical polymerization, we investigate the relative importance of the effects of hydrophobicity, polymer charge density, and steric hindrance to charge pairing on chain dynamics within polyelectrolyte complexes (PECs) and within polyelectrolyte multilayer (PEM) films. First, by applying fluorescence correlation spectroscopy (FCS), ellipsometry and fluorescence recovery after photobleaching (FRAP), we found that the dynamics of chain exchange within PECs is directly correlated with the mode (linear \textit{vs}. exponential) of PEM film growth. Second, through a combination of neutron reflectometry (NR) and FRAP techniques to the same PEM types, we found that diffusion of polyelectrolyte chains within multilayer films is highly anisotropic, with diffusion coefficients being 10$^{4}$-10$^{5}$ higher in a direction parallel to the substrate compared to that perpendicular. Chain mobility was also controlled by ionic strength of annealing solutions and steric hindrance to ionic pairing of interacting polyelectrolytes.\\[4pt] This work was supported by the National Science Foundation under Award DMR-0906474 (S.S.). Neutron measurements were performed at the Spallation Neutron Source at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the DOE under contract No. DE-AC05-00OR22725.