Recent experimental results have demonstrated that polymeric ionic liquids doped with Li salts exhibit enhanced ionic mobilities and lithium ion transference numbers with increasing salt concentrations. In this study, we used atomistic molecular dynamics simulations on a model system of lithium salt-doped 1-butyl-3-methyl-imidazolium bistriflimide ionic liquids and poly­(1-butyl-3-methyl-imidazolium bistriflimide) electrolytes to identify the molecular mechanisms underlying such findings. Our results mirror qualitatively the experimental results on the influence of salt doping on the ion mobilities. Further, a surprisingly stronger dependence (coupling) between the lithium ion mobilities and polymer segmental dynamics is observed relative to the coupling between the anion diffusivities and polymer dynamics. We present results for ion coordination and hopping characteristics to rationalize such behaviors and identify the mechanistic origins of the properties of this emerging class of polymer electrolytes.

In this work, we used a molecular dynamics (MD) simulation approach to investigate the interfacial binding of boron nitride nanotubes (BNNTs) with poly[m-phenylenevinylene-co-(2,5-dioctyloxy-p-phenylenevinylene)] (PmPV), polystyrene (PS), and polythiophene (PT). Quantum partial charges of BNNT-polymer composites were determined by density functional theory (DFT) calculations and then included in MD simulations. The interaction energy between nanotubes and polymer molecules was computed, and the morphology of polymers stacked onto the surface of the nanotubes was investigated based on the dihedral angle (θ). Our results confirm that the interaction energy is strongly influenced by the specific monomer structure of polymer and nanotube radius, but the influence of temperature is likely negligible. Among the investigated polymers, PT possesses the strongest adhesion to the BNNTs, followed by PmPV and PS. Moreover, the comparison of our results for BNNT-polymer composities with those of the similar carbon nanotube (CNT)-polymer composites reveals that the BNNT-polymer interactions are much stronger, which is the most important result of this work. This finding is also in good agreement with recent experimental observations. The higher values of interaction energy of BNNT-polymer composites suggest that the BNNTs could be more efficient nanofillers than the CNTs for nanocomposite reinforcement applications.

Molecular dynamics (MD) simulations were performed to investigate the interfacial binding between the single-walled carbon nanotubes (SWCNTs) and conjugated polymers including polythiophene (PT), polypyrrole (PP), poly(2,6-pyridinylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PPyPV), and poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) (PmPV). The intermolecular interaction energy between SWCNTs and polymer molecules was computed, and the morphology of polymers physisorbed to the surface of nanotubes was investigated by the radius of gyration (R(g)) and the alignment angle (theta). The influence of nanotube radius and temperature on the interfacial adhesion of nanotube-polymer and R(g) of polymers was explored more. Our simulation results showed that the strongest interaction between the SWCNTs and these conjugated polymers was observed, first for PT, then PPy and PmPV, and finally PPyPV. Furthermore, we compared our results to the work by Yang and his co-workers (J. Phys. Chem. B 2005, 109, 10009). Our results show that the intermolecular interaction in our systems is strongly influenced by the specific monomer structure of polymer and nanotube radius, but the influence of temperature could be negligible. The high values of intermolecular interaction energy of such composites suggest to us that an efficient load transfer will exist in the interface between nanotube and heterocyclic conjugated polymer, which is of a key role in the composite reinforcement practical applications.

Ammonium-based protic ionic liquids (PILs) have shown promising physicochemical properties as proton conductors in polymer membrane fuel cells. In this work, molecular dynamics simulations are used to study the structural, dynamic, and transport properties of a series of tertiary ammonium triflate PILs. Nonpolarizable all-atom force fields were used, including two different models for the triflate anion. Previous simulation studies of these PILs have yielded poor results for transport properties due to unrealistically slow dynamics. To improve performance, polarization and charge-transfer effects were approximately accounted for by scaling all atomic partial charges by a uniform factor, γ. The optimum scaling factor γ = 0.69 was determined by comparing simulation results with available experimental data and found to be transferable between different ammonium cations and insensitive to both the temperature and choice of experimental data used for comparison. Simulations performed using optimized charge scaling showed that the transport properties significantly improved over previous studies. Both the self-diffusion coefficients and viscosity were in good agreement with experiment over the whole range of systems and temperatures studied. Simulated PIL densities were also in good agreement with experiment, although the thermal expansivity was underestimated. Structural analysis revealed a strong directionality in interionic interactions. Triflate anions preferentially approach the ammonium cation so as to form strong hydrogen bonds between sulfonate oxygen atoms and the acidic ammonium hydrogen. The ionic conductivity was somewhat underestimated, especially at lower temperatures. Analysis of conductivity data suggests that there is significant correlated motion of oppositely charged ions in these PILs, especially at short times. These results overall indicate that the transport properties of this class of PILs are accurately modeled by these force fields if charge scaling is used and properly calibrated against selected experimental data.

Protic ionic liquids (PILs) form through proton transfer from a Brønsted acid to a Brønsted base. In this work we use molecular dynamics simulation to study how PIL properties vary with χ, the extent of the proton transfer reaction. Three PILs are considered: N-propylammonium acetate, [N3][Ac], N-butylammonium acetate, [N4][Ac], and N, N-dimethylbutylammonium acetate, [N114][Ac]. In all cases density and viscosity increase with increasing χ, while diffusivities of all species decrease with increasing χ. In each PIL the ionic conductivity exhibits a maximum at intermediate χ due to competition between increasing ion concentration and decreasing ion mobility. Ionicity analysis suggests that strongly correlated behavior is present at all χ. Finally, we determine the χ for which the properties of each simulated PIL best agree with experimental data; these are χ = 0.86, 0.80, and 0.18 for [N3][Ac], [N4][Ac], and [N114][Ac], respectively. These results suggest that proton transfer is nearly complete in the primary ammonium PILs but not in the tertiary ammonium PIL, consistent with recent experimental observations. We propose that this difference is due to cooperative production of hydrogen bonds with increasing χ in the primary ammonium PILs, which does not occur in the tertiary ammonium PIL.