We present molecular dynamics simulations of the air-liquid interface for three room temperature ionic liquids with a common anion: bis(trifluoromethylsulfonyl) imide ([Tf(2)N]), and imidazolium-based cations that differ in the alkyl tail length: 1-butyl-3-methylimidazolium ([C(4)mim]), 1-hexyl-3-methylimidazolium ([C(6)mim]), and 1-octyl-3-methylimidazolium ([C(8)mim]). The CHARMM type force field is used with the partial charges based on quantum calculations for isolated ion pairs. The total charge on cations and anions is around 0.9e and -0.9e, respectively, which somewhat mimics the anion to cation charge transfer and many-body effects. The surface tension at 300 K is computed using the mechanical route and its value slightly overpredicts experimental values. The air-liquid interface is analyzed using the intrinsic method of Identification of the Truly Interfacial Molecules. Structural and dynamic properties of the interfacial, sub-interfacial and central layers are determined. To describe the structure of the interface, we compute the surface roughness, number density and charge density profiles, and orientation ordering of the ions. We further determine the survival probability, normal and lateral self-diffusion coefficients, and re-orientation correlation functions to characterize the dynamics of the cations and anions in the layers. We found a significant enhancement of the cation density and preferential orientation ordering of both the cations and anions at the interface. Overall, the surface of the interfacial layer is smoother than the surface of the sub-interfacial layer and the roughness of both the interfacial and sub-interfacial layers increases with the increase of the length of the cation alkyl tail. Finally, the ions stay considerably longer in the interfacial layer than in the sub-interfacial layer and dynamics of exchange of the ions between the consecutive layers is related to the distinct diffusion and re-orientation dynamics behavior of the ions within the layers.

The paper describes the pH-dependent selfassembly of a diblock copolymer, poly(2-vinylpyridine)-blockpoly(ethylene oxide), P2VP−PEO in aqueous media using computer simulations. We employed the dissipative particle dynamics (DPD) method and found that the copolymer with electrically neutral (i.e., deprotonated) or very low-protonated P2VP blocks form multimolecular spherical core−shell micelles with insoluble P2VP cores in neutral and alkaline solutions, while protonization (ionization) of P2VP blocks exceeding 25% provokes the dissociation of micelles in single chains in acidic media. The finding that a clearly pronounced transition occurs in a restricted pH region slightly above pK A ap (where pK A ap is the apparent dissociation constant of the conjugated acid P2VPH + ) is in good agreement with the experimental data (Macromolecules 1996, 29, 6071−6073). This suggests that (i) the tested model is reasonable and the mutual relationship between the parameters used in the soft repulsive and electrostatic potentials was set appropriately and (ii) the DPD method is a suitable simulation technique for studying phenomena accompanied by pronounced changes in the global properties of complex polymer and polyelectrolyte systems. Although we studied the behavior of only one specific system, the simulation yields a generic pattern for the pH-dependent self-assembly of copolymers containing one neutral water-soluble block and one annealed (weak) polyelectrolyte block with fairly hydrophobic backbone.

Mixing microphase-separating diblock copolymers and nanoparticles can lead to the self-assembly of organic/inorganic hybrid materials that are spatially organized on the nanometre scale. Controlling particle location and patterns within the polymeric matrix domains remains, however, an unmet need. Computer simulation of such systems constitutes an interesting challenge since an appropriate technique would require the capturing of both the formation of the diblock mesophases and the copolymer-particle and particle-particle interactions, which can affect the ultimate structure of the material. In this work we discuss the application of Dissipative Particle Dynamics ( DPD) to the study of the distribution of nanoparticles with different degree of functionality and volume fraction in a lamellar microsegregated copolymer template. The DPD parameters of the systems were calculated according to a multi-step modelling approach, i.e., from lower scale (atomistic) simulations. The results show that positioning and ordering of the nanoparticles, as well as the dimensions of the block domains depend on covering extent and volume fraction, in full agreement with experiments. The overall results provide molecular-level information for the rational, a priori design of new polymer-particle nanocomposites with ad hoc, tailored properties

We present a dissipative particle dynamics (DPD) study of scaling behaviour for three polymer models. The scaling behaviour is explored for the conformational and dynamic properties of unentangled polymer melts. DPD employs a beadspring model together with an aggressive coarse-graining to represent polymers at the mesoscale. The first model studied utilises a simple soft repulsion potential for the bead-bead interactions together with a harmonic spring potential to connect beads into a polymer chain. The second model differs from the first model by replacing the harmonic spring with a finitely extensible nonlinear elastic spring. The third model uses realistic coarse-grain potentials for the bead-bead, spring and bending interactions based on the iterative Boltzmann inversion procedure and it corresponds to a mesoscopic model of polyethylene. We systematically vary the chain length and spring constant (in the case of the first and second models), and simulate the conformational properties such as the end-to-end distance or radius of gyration, and dynamic properties such as the centre-of-mass self-diffusion coefficient or viscosity. The scaling of the conformational and dynamic properties with chain length (scaling laws) is compared with the Rouse theory, which is considered as a standard theory for unentangled polymer melts. The comparison shows that simulated scaling laws typically agree with the Rouse scaling laws for the DPD polymer models with more than 10 DPD beads. For the shorter DPD polymers, deviations from the Rouse theory exist and become significant for the dynamic properties, especially for the viscosity of the polymer melts.