Role of electrostatic forces in cluster formation in a dry ionomer

This simulation study investigates the dependence of the structure of dry Nafion$^{\tiny\textregistered}$-like ionomers on the electrostatic interactions between the components of the molecules. In order to speed equilibration, a procedure was adopted which involved detaching the side chains from the backbone and cutting the backbone into segments, and then reassembling the macromolecule by means of a strong imposed attractive force between the cut ends of the backbone, and between the non-ionic ends of the side chains and the midpoints of the backbone segments. Parameters varied in this study include the dielectric constant, the free volume, side-chain length, and strength of head-group interactions. A series of coarse-grained mesoscale simulations shows the morphlogy to depend sensitively on the ratio of the strength of the dipole-dipole interactions between the side-chain acidic end groups to the strength of the other electrostatic components of the Hamiltonian. Examples of the two differing morphologies proposed by Gierke and by Gebel emerge from our simulations.Comment: 39 pages, 18 figures, accepted for publicatio

Predicted field-induced hexatic structure in an ionomer membrane

Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion$^{\tiny \textregistered}$-like ionomer by the imposition of a strong electric field. We observe the formation of novel structures aligned along the direction of the applied field. The polar head groups of the ionomer side chains aggregate into clusters, which then form rod-like formations which assemble into a hexatic array aligned with the direction of the field. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexatic array of rod-like structures persists, and has a lower calculated free energy than the original isotropic morphology.Comment: 4 pages, 7 figure

Simulation Study of Sulfonate Cluster Swelling in Ionomers

We have performed simulations to study how increasing humidity affects the structure of Nafion-like ionomers under conditions of low sulfonate concentration and low humidity. At the onset of membrane hydration, the clusters split into smaller parts. These subsequently swell, but then maintain constant the number of sulfonates per cluster. We find that the distribution of water in low-sulfonate membranes depends strongly on the sulfonate concentration. For a relatively low sulfonate concentration, nearly all the side-chain terminal groups are within cluster formations, and the average water loading per cluster matches the water content of membrane. However, for a relatively higher sulfonate concentration the water-to-sulfonate ratio becomes non-uniform. The clusters become wetter, while the inter-cluster bridges become drier. We note the formation of unusual shells of water-rich material that surround the sulfonate clusters.Comment: 24 pages, 15 figure

DNA condensation and redissolution: Interaction between overcharged DNA molecules

The effective DNA-DNA interaction force is calculated by computer simulations with explicit tetravalent counterions and monovalent salt. For overcharged DNA molecules, the interaction force shows a double-minimum structure. The positions and depths of these minima are regulated by the counterion density in the bulk. Using two-dimensional lattice sum and free energy perturbation theories, the coexisting phases for DNA bundles are calculated. A DNA-condensation and redissolution transition and a stable mesocrystal with an intermediate lattice constant for high counterion concentration are obtained.Comment: 26 pages, 10 figure

Structural correlations in highly asymmetric binary charged colloidal mixtures

We explore structural correlations of strongly asymmetric mixtures of binary charged colloids within the primitive model of electrolytes considering large charge and size ratios of 10 and higher. Using computer simulations with explicit microions, we obtain the partial pair correlation functions between the like-charged colloidal macroions. Interestingly the big-small correlation peak amplitude is smaller than that of the big-big and small-small macroion correlation peaks, which is unfamiliar for additive repulsive interactions. Extracting optimal effective microion-averaged pair interactions between the macroions, we find that on top of non-additive Yukawa-like repulsions an additional shifted Gaussian attractive potential between the small macroions is needed to accurately reproduce their correct pair correlations. For small Coulomb couplings, the behavior is reproduced in a coarse-grained theory with microion-averaged effective interactions between the macroions. However, the accuracy of the theory deteriorates with increasing Coulomb coupling. We emphasize the relevance of entropic interactions exerted by the microions on the macroions. Our results are experimentally verifiable in binary mixtures of micron-sized colloids and like-charge nanoparticlesComment: 13 pages, 11 figure

Crystal Orientation and Temperature Effects on Double Hysteresis Loop Behavior in a Poly(vinylidene fluoride-<i>co</i>-trifluoroethylene-<i>co</i>-chlorotrifluoroethylene)-<i>graft</i>-Polystyrene Graft Copolymer

Recently, double hysteresis loop (DHL) behavior, which is advantageous for the high energy density and low loss dielectric application, was achieved in a poly­(vinylidene fluoride-<i>co</i>-trifluoroethylene-<i>co</i>-chlorotrifluoroethylene)-<i>graft</i>-polystyrene [P­(VDF-TrFE-CTFE)-<i>g</i>-PS­(14%)] graft copolymer due to the nanoconfinement effect. In this work, we continued to investigate the crystal orientation and temperature effects on the DHL behavior of this graft copolymer. Based on the electric displacement–electric field (D–E) study, crystal orientation had a profound effect on its electrical behavior. For the nonoriented sample, dielectric instead of ferroelectric behavior was observed. After uniaxial stretching, DHLs gradually developed in the oriented films upon increasing the extension ratio. For a fully stretched film, the DHL behavior was stable below 75 °C but gradually disappeared above 100 °C due to enhanced dc conduction and impurity ion migrational loss at elevated temperatures. After subtracting the dc conduction, D–E hysteresis loops from the ion loss were determined for the poling cycles below 100 MV/m. The hysteresis loss from ion migration under an applied field was closely related to ion concentration and diffusion coefficient, which were determined by broadband dielectric spectroscopy. Both parameters were used in a theoretical calculation to obtain hysteresis loops from ion migrational loss. By fitting the theoretical loops with those after dc conduction subtraction, ion mobility was found to be dependent upon both poling field and temperature. This study provides a quantitative understanding of the effects of impurity ions and dc conduction on dielectric and ferroelectric properties of polymers at elevated temperatures