Rajesh Khare

Effect of Carbon Nanotube Functionalization on Mechanical and Thermal Properties of Cross-Linked Epoxy–Carbon Nanotube Nanocomposites: Role of Strengthening the Interfacial Interactions

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 Youngs 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 Youngs 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.

Molecular simulation and continuum mechanics study of simple fluids in non-isothermal planar couette flows

The behavior of simple fluids under shear is investigated using molecular dynamics simulations. The simulated system consists of a fluid confined between two atomistic walls which are moved in opposite directions. Two approaches for shear flow simulations are compared: in one case, the sheared fluid is not thermostatted and only the confining walls are maintained at a constant temperature, while in the other, a thermostat is employed to keep the entire mass of the sheared fluid at a constant temperature. In the first case the sheared fluid undergoes significant viscous heating at the shear rates investigated, consistent with experimental observations and with theoretical predictions. Most simulations to date, however, have used the second approach which is akin to studying a fluid with infinite thermal conductivity. It is shown here that results for transport coefficients are significantly affected by the thermostat; in fact, the transport properties of the fluid determined using the two methods exhibit a...

Rheological, thermodynamic, and structural studies of linear and branched alkanes under shear

The rheological, thermodynamic, and structural behavior of linear and branched alkanes in simple shear is investigated using nonequilibrium molecular-dynamics simulation of united atom model fluids. Our results for the zero-shear viscosity of pure linear alkanes as well as mixtures of alkanes are in reasonable agreement with experiment. Simulation results for intermediate molecular weight linear alkanes indicate that the simple models employed here are capable of describing the pressure and temperature dependence of the viscosity. More importantly, our calculations indicate that addition of short, flexible branches to alkanes leads to a viscosity enhancement of a factor of 2 or more, thereby offering interesting possibilities for formulation of lubricants with specific properties.

Effect of Carbon Nanotube Dispersion on Glass Transition in Cross-Linked Epoxy–Carbon Nanotube Nanocomposites: Role of Interfacial Interactions

We have used atomistic molecular simulations to study the effect of nanofiller dispersion on the glass transition behavior of cross-linked epoxy-carbon nanotube (CNT) nanocomposites. Specific chemical interactions at the interface of CNTs and cross-linked epoxy create an interphase region, whose impact on the properties of their nanocomposites increases with an increasing extent of dispersion. To investigate this aspect, we have compared the volumetric, structural, and dynamical properties of three systems: neat cross-linked epoxy, cross-linked epoxy nanocomposite containing dispersed CNTs, and cross-linked epoxy nanocomposite containing aggregated CNTs. We find that the nanocomposite containing dispersed CNTs shows a depression in the glass transition temperature (Tg) by ~66 K as compared to the neat cross-linked epoxy, whereas such a large depression is absent in the nanocomposite containing aggregated CNTs. Our results suggest that the poor interfacial interactions between the CNTs and the cross-linked epoxy matrix lead to a more compressible interphase region between the CNTs and the bulk matrix. An analysis of the resulting dynamic heterogeneity shows that the probability of percolation of immobile domains becomes unity near the Tg calculated from volumetric properties. Our observations also lend support to the conceptual analogy between polymer nanocomposites and the nanoconfinement of polymer thin films.

New forcefield parameters for branched hydrocarbons

A new set of united-atom force field parameters is proposed for simulating the phase equilibria of branched alkanes. These parameters complement the already existing set of Nath, Escobedo, and de Pablo revised (NERD) [Nath et al., J. Chem. Phys. 105, 4391 (1998); Nath and de Pablo, Mol. Phys. 98, 231 (2000)] force field parameters. The proposed force field is used to study vapor–liquid equilibria for various isomers of alkanes up to C8. Results of simulations are found to be in good agreement with available experimental data.

Percolation of immobile domains in supercooled thin polymeric films

We present an analysis of heterogeneous dynamics in molecular dynamics simulations of a polymeric film supported by an absorbing surface. Using a bead-spring model for polymers, we show that slow, immobile beads occur throughout the film, with the probability of their occurrence decreasing with distance from the substrate. Still, enough immobile beads are located near the free surface to cause them to percolate in the direction perpendicular to the substrate, at a temperature near the glass transition one. This result is consistent with a recent theoretical model of glass transition.

Conformation and diffusion behavior of ring polymers in solution: a comparison between molecular dynamics, multiparticle collision dynamics, and lattice Boltzmann simulations.

We have studied the effect of chain topology on the structural properties and diffusion of polymers in a dilute solution in a good solvent. Specifically, we have used three different simulation techniques to compare the chain size and diffusion coefficient of linear and ring polymers in solution. The polymer chain is modeled using a bead-spring representation. The solvent is modeled using three different techniques: molecular dynamics (MD) simulations with a particulate solvent in which hydrodynamic interactions are accounted through the intermolecular interactions, multiparticle collision dynamics (MPCD) with a point particle solvent which has stochastic interactions with the polymer, and the lattice Boltzmann method in which the polymer chains are coupled to the lattice fluid through friction. Our results show that the three methods give quantitatively similar results for the effect of chain topology on the conformation and diffusion behavior of the polymer chain in a good solvent. The ratio of diffusivities of ring and linear polymers is observed to be close to that predicted by perturbation calculations based on the Kirkwood hydrodynamic theory.

Molecular Topology and Local Dynamics Govern the Viscosity of Imidazolium-Based Ionic Liquids.

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.

Molecular simulation and continuum mechanics investigation of viscoelastic properties of fluids confined to molecularly thin films

A combination of molecular dynamics simulations of oscillatory shear flow and continuum mechanics is used to investigate viscoelastic properties of materials confined to molecularly thin films. The atoms of the simple liquid interact via a repulsive Lennard-Jones potential. The chain molecules are modeled as strings of similar spheres connected via finite extensible nonlinear elastic springs. The fluid is confined between two surfaces composed of identical spheres that are moved to simulate oscillatory flow. In order to mimic experiments, the temperature is controlled by coupling the wall atoms to a heat bath, and the viscoelastic properties are obtained via an analysis using continuum mechanics. Both simple and polymeric fluids exhibit linear viscoelastic behavior under typical simulation conditions, although inertial effects play an important role in determining the flow behavior. Simple fluids display a smooth transition from liquidlike to solidlike behavior when confined to molecularly thin films, whe...

On the calculation of transport properties of polymer melts from nonequilibrium molecular dynamics

This note presents an analysis of the viscosity of linear and branched polymer melts at low shear rates and compares the results of several different nonequilibrium molecular dynamics methods. The new, more precise results presented here indicate that earlier calculations of the zero shear viscosity of linear polymers are consistent with Green–Kubo estimates. For branched chains, however, these results suggest that earlier extrapolation of viscosities to zero shear rate overpredicted the effect of branching.