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

Although high piezoelectric coefficients have recently been observed in poly(vinylidene fluoride-co-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 d31 in neat PVDF. After high-power ultrasonication for 20 min, a uniaxially stretched and poled PVDF film exhibited a high d31 of 50.2 ± 1.7 pm V-1 at room temperature. Upon heating to 65 °C, the d31 increased to a maximum value of 76.2 ± 1.2 pm V-1, and the high piezoelectric performance persisted up to 110 °C. The enhanced piezoelectricity was attributed to the relaxor-like secondary crystals in the oriented amorphous fraction, broken off from the primary crystals by ultrasonication, as suggested by differential scanning calorimetry and broadband dielectric spectroscopy studies.

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

Piezoelectric polymers hold great potential for various electromechanical applications, but only show low performance, with |d33 | < 30 pC/N. We prepare a highly piezoelectric polymer (d33 = -62 pC/N) based on a biaxially oriented poly(vinylidene fluoride) (BOPVDF, crystallinity = 0.52). After unidirectional poling, macroscopically aligned samples with pure ß crystals are achieved, which show a high spontaneous polarization (Ps) of 140 mC/m2. Given the theoretical limit of Ps,ß = 188 mC/m2 for the neat ß crystal, the high Ps cannot be explained by the crystalline-amorphous two-phase model (i.e., Ps,ß = 270 mC/m2). Instead, we deduce that a significant amount (at least 0.25) of an oriented amorphous fraction (OAF) must be present between these two phases. Experimental data suggest that the mobile OAF resulted in the negative and high d33 for the poled BOPVDF. The plausibility of this conclusion is supported by molecular dynamics simulations.

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. The principal determinant of the type of diffusion is observed to be a dimensionless parameter, γ, that describes the influence of solvent-induced swelling in lowering the potential barriers separating available solvent sites. A second parameter, η, which characterizes the effect of the solvent in reducing the potential energy of a solvent molecule when at rest at an available site, only influences the type of diffusion to a lesser extent. The theoretical analysis does not include any effects that are explicitly non-local in time, an example of which is the inclusion of polymer viscosity in the Thomas-Windle model; it thus represents a variant of Fick's second law utilizing a concentration-dependent diffusivity when η is small. To check the significance of time-delayed swelling, a simulation of a modified model was performed that contained a history-dependent term. The results were found to be very similar to those in the time-local model.

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 0.25 nm intervals, which marked the paths of segments of the PVDF molecules, were seen. The fact that an electron microscope image of a segment of a PVDF molecule depended upon the particular azimuthal direction, along which the segment was viewed, enabled observation of twist around the molecular axis. The 0.2 nm side-by-side distance between the two fluorine atoms attached to the same carbon atom was clearly resolved. Morphological and chemical changes produced by energetic electrons, ranging from no change to fiber scission, over many orders of magnitude of electrons per unit area, promise quantitative new insights into radiation chemistry. Relative movements of segments of molecules were observed. Promising synergism between high resolution electron microscopy and molecular dynamic modeling was demonstrated. This paper is at the threshold of growing usefulness of electron microscopy to the science and engineering of polymer and other molecules.

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 3.96 nm. Simulation results indicate a strong dependence of the equilibrium ionomer structures both on the pore diameter and on the sulfonate concentration in the pore. In the case of larger pores, the ionic clustering has the shape of a branched wire-like network oriented parallel to the pore axis. In the case of narrow pores, the ionic clusters occupy the pore center and exhibit strong density modulations both along the pore axis and across the pore diameter. The calculated diffusion coefficients for the ions indicate a sharp increase within the narrow pores. This finding is explained by ballistic-type ionic motion at shorter times and by the collective motion of ions in hydrophilic clusters. The influence of the hydrophobic walls on the distribution of ions and solvent molecules is discussed.

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 simple empirical formula is noted for the dependence of the number density of induced rod-like aggregates on the sidechain length L(s) within the parameter range considered in this study.

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. The resulting morphology contains continuous channels of hydrophilic material capable of facilitating proton conduction. Upon removal of the poling field, the system does not return to its original morphology, but retains the anisotropic structure of the poled material. The poled structure appears to be thermodynamically stable, as confirmed by calculations of the Helmholtz energy of the original and poled samples.

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. These cylindrical structures then align to form an hexatic array aligned along the direction of the applied electric field. The hexatic morphology persists after the removal of the field. A calculation by means of the Kirkwood coupling parameter method shows the Helmholtz free energy of the hexatic morphology of the poled membrane to be lower than that of the initial isotropic material, even in the absence of the applied field.

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. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexatic array of rodlike structures persists and has a lower calculated free energy than the original isotropic morphology.

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. In a humidified membrane, stretching also causes the perfluorinated side chains to tend to orient perpendicular to the stretching axis. This in turn affects the distribution of water at low water contents. The water forms a continuous network with narrow bridges between small water clusters absorbed in head group multiplets. In a dry membrane the side chains orient along the stretching direction.

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 nonuniform. The clusters become wetter, while the intercluster bridges become drier. We note the formation of unusual shells of water-rich material that surround the sulfonate clusters.

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. A series of coarse-grained mesoscale simulations shows the morphology 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 co-workers [J. Polym. Sci., Polym. Phys. Ed. 19, 1687 (1981); Macromolecules 15, 101 (1982); J. Membr. Sci. 13, 307 (1982)] and by Gebel [Fuel Cells 5, 261 (2005); Macromolecules 37, 7772 (2004)] emerge from our simulations.

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.