Hard-to-soft transition-enhanced piezoelectricity in poly(vinylidene fluoride) relaxor-like secondary crystals activated by high-power ultrasonication.

Although high piezoelectric coefficients have recently been observed in poly(vinylidene fluoride--trifluoroethylene) [P(VDF-TrFE)] random copolymers, they have low Curie temperatures, which makes their piezoelectricity thermally unstable. It has been challenging to achieve high piezoelectric performance from the more thermally stable PVDF homopolymer. In this report, we describe how high-power ultrasonic processing was used to induce a hard-to-soft piezoelectric transition and improve the piezoelectric coefficient in neat PVDF.

Enhanced piezoelectricity from highly polarizable oriented amorphous fractions in biaxially oriented poly(vinylidene fluoride) with pure β crystals.

Piezoelectric polymers hold great potential for various electromechanical applications, but only show low performance, with |d | < 30 pC/N. We prepare a highly piezoelectric polymer (d = -62 pC/N) based on a biaxially oriented poly(vinylidene fluoride) (BOPVDF, crystallinity = 0.52).

Generalized model for the diffusion of solvents in glassy polymers: From Fickian to Super Case II.

The diffusion of small solvent molecules in glassy polymers may take on a variety of different forms. Fickian, anomalous, Case II and Super Case II diffusion have all been observed, and theoretical models exist that describe each specific type of behavior. Here we present a single generalized kinetic model capable of yielding all these different types of diffusion on the basis of just two parameters.

Enhanced ionic diffusion in ionomer-filled nanopores.

Coarse-grained simulations in the united-atom-model approximation are used to investigate confinement-induced morphological changes in Nafion-like ionomers. The system we study models a cylindrical pore in a hydrophobic matrix of supporting material with pore diameters that vary from 0.7 to 3.

Polyvinylidene fluoride molecules in nanofibers, imaged at atomic scale by aberration corrected electron microscopy.

Atomic scale features of polyvinylidene fluoride molecules (PVDF) were observed with aberration corrected transmission electron microscopy. Thin, self-supporting PVDF nanofibers were used to create images that show conformations and relative locations of atoms in segments of polymer molecules, particularly segments near the surface of the nanofiber. Rows of CF2 atomic groups, at 0.

Simulation study of poled low-water ionomers with different architectures.

The role of the ionomer architecture in the formation of ordered structures in poled membranes is investigated by molecular dynamics computer simulations. It is shown that the length of the sidechain L(s) controls both the areal density of cylindrical aggregates N(c) and the diameter of these cylinders in the poled membrane. The backbone segment length L(b) tunes the average diameter D(s) of cylindrical clusters and the average number of sulfonates N(s) in each cluster.

A simulation study of field-induced proton-conduction pathways in dry ionomers.

The morphological changes that can be induced in a dry ionomer by application of a strong electric field have been studied by means of computer simulation. The internal energy of the membrane at first slowly decreases with increasing field, but then rapidly increases after a certain threshold field is reached. This effect is interpreted as the reorganization of interacting head group dipoles in response to the external perturbation.

Simulation study of field-induced morphological changes in a proton-conducting ionomer.

A simulation study was made of the effects of strong electric fields on the morphology of a Nafion-like ionomer at various levels of hydration. The results of united-atom molecular-dynamics computations showed a self-organization of the side chain terminal groups into cylindrical clusters. The walls of these clusters contain the sulfonate dipoles, while the interior holds the majority of the water molecules.

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.

Predicted electric-field-induced hexatic structure in an ionomer membrane.

Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion-like ionomer by the imposition of a strong electric field. We observe the formation of structures aligned along the direction of the applied field. The polar head groups of the ionomer sidechains aggregate into clusters, which then form rodlike formations which assemble into a hexatic array aligned with the direction of the field.

Simulation study of the correlation between structure and conductivity in stretched nafion.

We have used coarse-grained simulation methods to investigate the effect of stretching-induced structure orientation on the proton conductivity of Nafion-like polymer electrolyte membranes. Our simulations show that uniaxial stretching causes the hydrophilic regions to become elongated in the stretching direction. This change has a strong effect on the proton conductivity, which is enhanced along the stretching direction, while the conductivity perpendicular to the stretched polymer backbone is reduced.

Role of electrostatic forces in cluster formation in a dry ionomer.

This simulation study investigates the dependence of the structure of dry Nafion-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 nonionic 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.

Internalization kinetics of the gonadotropin-releasing hormone (GnRH) receptor.

This study quantified the agonist-induced endocytotic and recycling events of the mammalian gonadotropin releasing hormone receptor (GnRH-R) and investigated the role of the intracellular carboxyl (C)-terminal tail in regulating agonist-induced receptor internalization kinetics. The rate of internalization for the rat GnRH-R was found to be exceptionally low when compared with G-protein coupled receptors (GPCRs) which possess a cytoplasmic C-terminal tail (thyrotropin-releasing hormone receptor (TRH-R), catfish GnRH-R (cfGnRH-R) and GnRH/TRH-R chimeric receptor). These data provide evidence that the presence of a functional intracellular cytoplasmic C-terminal tail is essential for rapid internalization of the studied GPCRs.