We have used amido-amine functionalized carbon nanotubes (CNTs) that form covalent bonds with cross-linked epoxy matrices to elucidate the role of the matrix-filler interphase in the enhancement of mechanical and thermal properties in these nanocomposites. For the base case of nanocomposites of cross-linked epoxy and pristine single-walled CNTs, our previous work (Khare, K. S.; Khare, R. J. Phys. Chem. B 2013, 117, 7444-7454) has shown that weak matrix-filler interactions cause the interphase region in the nanocomposite to be more compressible. Furthermore, because of the weak matrix-filler interactions, the nanocomposite containing dispersed pristine CNTs has a glass transition temperature (Tg) that is ∼66 K lower than the neat polymer. In this work, we demonstrate that in spite of the presence of stiff CNTs in the nanocomposite, the Young's modulus of the nanocomposite containing dispersed pristine CNTs is virtually unchanged compared to the neat cross-linked epoxy. This observation suggests that the compressibility of the matrix-filler interphase interferes with the ability of the CNTs to reinforce the matrix. Furthermore, when the compressibility of the interphase is reduced by the use of amido-amine functionalized CNTs, the mechanical reinforcement due to the filler is more effective, resulting in a ∼50% increase in the Young's modulus compared to the neat cross-linked epoxy. Correspondingly, the functionalization of the CNTs also led to a recovery in the Tg making it effectively the same as the neat polymer and also resulted in a ∼12% increase in the thermal conductivity of the nanocomposite containing functionalized CNTs compared to that containing pristine CNTs. These results demonstrate that the functionalization of the CNTs facilitates the transfer of both mechanical load and thermal energy across the matrix-filler interface.

Contrary to thermosets, vitrimers adjust their topology upon heating without loss of network integrity. Here, the proposed simulation methodology utilizes coarse-grained molecular dynamics in conjunction with a Monte Carlo method to capture the network integrity and flowability of vitrimers at high temperatures. The model vitrimer shows two transition temperatures. In addition to the conventional glass transition temperature, the topology freezing temperature is detected from the volumetric and rheological data. In the glassy state, the mobility of the vitrimer and thermoset is identical, whereas increasing the temperature results in a diffusive behavior in the vitrimer. The rheological data capture the main feature of vitrimers, which is the terminal regime of the elastic modulus at low frequencies. The zero-shear viscosity of the model vitrimer follows an Arrhenius-like temperature dependence at temperatures above the topology freezing temperature. The horizontal shift factors obtained from collapsing the rheological data onto master curves also display the same temperature dependence. Simulations reveal that the lifetime of the exchangeable bonds determines the rheology and dynamics of these networks. When the rate of the deformation is higher than the rate of the bond exchange, the system behaves as a typical thermoset, while at lower rates, the vitrimer behaves as a viscous liquid.

Low-density "equilibrium" gels that consist of a percolated, kinetically arrested network of colloidal particles and are resilient to aging can be fabricated by restricting the number of effective bonds that form between the colloids. Valence-restricted patchy particles have long served as one archetypal example of such materials, but equilibrium gels can also be realized through a synthetically simpler and scalable strategy that introduces a secondary linker, such as a small ditopic molecule, to mediate the bonds between the colloids. Here, we consider the case where the ditopic linker molecules are low-molecular-weight polymers and demonstrate using a model colloid-polymer mixture how macroscopic properties such as the phase behavior as well as the microstructure of the gel can be designed through the polymer molecular weight and concentration. The low-density window for equilibrium gel formation is favorably expanded using longer linkers, while necessarily increasing the spacing between all colloids. However, we show that blends of linkers with different sizes enable wider variation in microstructure for a given target phase behavior. Our computational study suggests a robust and tunable strategy for the experimental realization of equilibrium colloidal gels.

A series of branched ionic liquids (ILs) based on the 1-(iso-alkyl)-3-methylimidazolium cation from 1-(1-methylethyl)-3-methylimidazolium bistriflimide to 1-(5-methylhexyl)-3-methylimidazolium bistriflimide and linear ILs based on the 1-(n-alkyl)-3-methylimidazolium cation from 1-propyl-3-methylimidazolium bistriflimide to 1-heptyl-3-methylimidazolum bistriflimide were recently synthesized and their physicochemical properties characterized. For the ILs with the same number of carbons in the alkyl chain, the branched IL was found to have the same density but higher viscosity than the linear one. In addition, the branched IL 1-(2-methylpropyl)-3-methylimidazolium bistriflimide ([2mC3C1Im][NTf2]) was found to have an abnormally high viscosity. Motivated by these experimental observations, the same ILs were studied using molecular dynamics (MD) simulations in the current work. The viscosities of each IL were calculated using the equilibrium MD method at 400 K and the nonequilibrium MD method at 298 K. The results agree with the experimental trend. The ion pair (IP) lifetime, spatial distribution function, and associated potential of mean force, cation size and shape, and interaction energy components were calculated from MD simulations. A quantitative correlation between the liquid structure and the viscosity was observed. Analysis shows that the higher viscosities in the branched ILs are due to the relatively more stable packing between the cations and anions indicated by the lower minima in the potential of mean force (PMF) surface. The abnormal viscosity of [2mC3C1Im][NTf2] was found to be the result of the specific side chain length and molecular structure.

Here we demonstrate through experiment and simulation the polymer-assisted dispersion of inorganic 2D layered nanomaterials such as boron nitride nanosheets (BNNSs), molybdenum disulfide nanosheets (MoS2), and tungsten disulfide nanosheets (WS2), and we show that spray drying can be used to alter such nanosheets into a crumpled morphology. Our data indicate that polyvinylpyrrolidone (PVP) can act as a dispersant for the inorganic 2D layered nanomaterials in water and a range of organic solvents; the effectiveness of our dispersion process was characterized by UV-vis spectroscopy, microscopy and dynamic light scattering. Molecular dynamics simulations confirm that PVP readily physisorbs to BNNS surfaces. Collectively, these results indicate that PVP acts as a general dispersant for nanosheets. Finally, a rapid spray drying technique was utilized to convert these 2D dispersed nanosheets into 3D crumpled nanosheets; this is the first report of 3D crumpled inorganic nanosheets of any kind. Electron microscopy images confirm that the crumpled nanosheets (1-2 μm in diameter) show a distinctive morphology with dimples on the surface as opposed to a wrinkled, compressed surface, which matches earlier simulation results. These results demonstrate the possibility of scalable production of inorganic nanosheets with tailored morphology.