Atomistic simulations using a combination of classical force field and density-functional theory (DFT) show that carbon atoms remain essentially ${\mathrm{sp}}^2$ coordinated in either bent tubes or tubes pushed by an atomically sharp atomic-force microscope (AFM) tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.

Most ionic liquids are known to be hygroscopic to varying degrees, and that can be detrimental or useful depending upon the application in question. Water can accumulate slowly over hours or days to saturation levels corresponding to the humidity level. When designing or deploying a new ionic liquid it is important to be able to estimate its maximum moisture absorbing ability at the temperature and pressure of its operation. With this goal in mind we have carried out computational studies on three ionic liquid systems based on [BF₄]⁻, [PF₆]⁻, and [Tf₂N]⁻ anions and 1-alkyl-3-methyl-imidazolium ([C_nmim]⁺) cations within an implicit solvent formalism. For highly hygroscopic systems like [C_nmim][BF₄] we find that non-iterative calculations with single water molecules can lead to significant underestimation of the maximum moisture content, while iterative calculations can result in miscibility behavior qualitatively different from experimental observations. On the other hand, the inclusion of small hydrogen-bonded water-clusters up to an appropriately chosen size is shown to yield better quantitative agreements with experimentally observed water uptake. Additionally, such calculations appear consistent with a number of thermodynamically interesting phase behaviors, including limited-solubility to full-miscibility transitions as a function of temperature and as a function of the alkyl chain length of the imidazolium cation. For hydrophobic systems like [C_nmim][PF₆] and [C_nmim][Tf₂N] the computed solubility (for each n) is found to have a smooth convergence behavior as a function of the largest cluster-size considered with the results for the larger clusters being close to that obtained by iterative calculations with single water molecules.

Modulus softening and permanent set in filled polymeric materials due to cyclic loading and unloading, commonly known as the Mullins effect, can have a significant impact on their use as support cushions. A quantitative analysis of such behavior is essential to ensure the effectiveness of such materials in long-term deployment. In this work we combine existing ideas of filler-induced modulus enhancement, strain amplification, and irreversible deformation within a simple non-Gaussian constitutive model to quantitatively interpret recent measurements on a relevant PDMS-based elastomeric cushion. We find that the experimental stress-strain data is consistent with the picture that during stretching (loading) two effects take place simultaneously: (1) the physical constraints (entanglements) initially present in the polymer network get disentangled, thus leading to a gradual decrease in the effective cross-link density, and (2) the effective filler volume fraction gradually decreases with increasing strain due to the irreversible pulling out of an initially occluded volume of the soft polymer domain.

Pair association in Ionic Liquids is an important quantity that affects many of their physical and chemical properties. However, the association constant is a complex function of the component ions as well as of the solvent environment, and no single theory can compute or predict it with quantitative accuracy. In this work we analyze infinite-dilution association data from a number of recent conductance measurements, and develop a linear model correlating the association constant with two relevant interaction energies, i.e., (1) the dielectrically screened Coulomb attraction and hydrogen bonding between ion-pairs, and (2) the ion solvation energy, which in turn takes into account solvent-specific interactions like hydrogen-bond acidity/basicity and hydrophobic/hydrophilic interactions. The results reveal the unique nature of water as a solvent in that it affects ionic association in ways qualitatively different from other common solvents.