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It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.

We present a model for dynamics of entangled networks made up of linear chains with many temporary cross-links. At times shorter than the lifetime of a cross-link such networks behave as elastic rubbers (gels). On longer time scales the successive breaking of only a few cross-links allows the chain to diffuse along its confining tube. The motion of a chain in this hindered reptation model is controlled by the concentration and lifetime of tie points. We calculate the self-diffusion coefficient and discuss the stress relaxation in terms of molecular parameters, including the chain length, the number of cross-linking groups per chain, and the lifetime and probability of formation of cross-links. We find good agreement with recent experiments by Stadler et al. on model thermoplastic elastomers.

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The implications of the entanglement concentration (C e) on the electrospinning process for a series of linear and branched poly(ethylene terephthalate-co-ethylene isophthalate) (PET-co-PEI) copolymers with weight-average molecular weights (M w) ranging from 11 700 to 106 000 g/mol and branching index values (g‘) from 1.0 to 0.43 were investigated. Analyzing the dependence of specific viscosity (ηsp) on concentration enabled the determination of the semidilute unentangled, semidilute entangled, and concentrated regimes for the PET-co-PEI solutions. Linear and branched copolymers were electrospun from semidilute unentangled, semidilute entangled, and concentrated solutions under identical conditions to determine the effects of concentration regime and molecular topology on electrospun fiber morphology. The dependence of the fiber diameter and morphology on the zero shear rate viscosity (η0) and normalized concentration (C/C e) was determined. For copolyesters with molecular weights well above the entanglement molecular weight, C e was the minimum concentration required for electrospinning of beaded fibers, while 2−2.5 times C e was the minimum concentration required for electrospinning of uniform, bead-free fibers. When the concentration was normalized with C e, the influence of chain length and topology on the electrospinning process was removed, and the fiber diameter universally scaled with the normalized concentration to the 2.6 power.

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A novel method is presented whereby the parameters quantifying the conductivity of an ionomer can be extracted from the phenomenon of electrode polarization in the dielectric loss and tan delta planes. Mobile ion concentrations and ion mobilities were determined for a poly(ethylene oxide)-based sulfonated ionomer with Li(+), Na(+), and Cs(+) cations. The validity of the model was confirmed by examining the effects of sample thickness and temperature. The Vogel-Fulcher-Tammann (VFT)-type temperature dependence of conductivity was found to arise from the Arrhenius dependence of ion concentration and VFT behavior of mobility. The ion concentration activation energy was found to be 25.2, 23.4, and 22.3+/-0.5 kJmol for ionomers containing Li(+), Na(+), and Cs(+), respectively. The theoretical binding energies were also calculated and found to be approximately 5 kJmol larger than the experimental activation energies, due to stabilization by coordination with polyethylene glycol segments. Surprisingly, the fraction of mobile ions was found to be very small, <0.004% of the cations in the Li(+) ionomer at 20 degrees C.

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