Developing structure and thermodynamic properties-consistent coarse-grained model for random copolymer systems

Self-assembly and structures of nanoscale double emulsion droplets through coarse-grained molecular dynamics simulations

Examples of self-assembled multiple emulsion droplets on the nanometre scale are very rare. In this work, we use coarse-grained (CG) molecular dynamics simulations to study the self-assembly of ternary mixtures consisting of water, n-heptane, and nonionic surfactant tetraethylene glycol monododecyl ether (C12E4). The water volume fractions studied are 1%, 3%, and 5%, respectively. Various nanoscale emulsions are obtained in a spontaneous process. When the water/surfactant volume ratio vm/s = 1.0/1.0, the obtained emulsion droplets are identified as oil-in-water-in-oil (O/W/O) double types, consisting of an oil core, an inner surfactant layer, a water layer, and an outer surfactant layer. The water molecules are distributed around the hydrophilic ends of the surfactants, while the hydrophobic ends of the surfactants wrap the oil cores and penetrate into the oil bulk. Hydrogen-bond interactions among water and the hydrophilic ends of the surfactants form cross-links that stabilize the double emulsion droplets. The sizes of all the oil cores inside the droplets are <6 nm in diameter, even with the highest water volume fraction of 5%. Both the concentration of free water molecules on the order of 10-6 mol/cm3 and the favourable energy change during emulsion formation indicate that the emulsion droplets are thermodynamically stable. In contrast, for vm/s = 1.0/5.5, no double emulsion but a simple water-in-oil emulsion was observed, with morphologies evolving from oblate to bicontinuous phases with an increase in the water volume fraction from 1% to 5%. Our coarse-grained molecular dynamics simulations provide valuable insight for the preparation of nanoscale double emulsions and the characterization of their structures.

Development of hybrid coarse-grained atomistic models for rapid assessment of local structuring of polymeric semiconductors

Decades of work in the field of computational study of semiconducting polymers using atomistic models illustrate the challenges of generating equilibrated models for this class of materials. While adopting a coarse-grained model can be helpful, the process of developing a suitable model is particularly non-trivial and time-consuming for semiconducting polymers due to a large number of different interactions with some having an anisotropic nature. This work introduces a procedure for the rapid generation of a hybrid model for semiconducting polymers where atoms of secondary importance (those in the alkyl side chains) are transformed into coarse-grained beads to reduce the computational cost of generating an equilibrated structure. The parameters are determined from easy-to-equilibrate simulations of very short oligomers and the model is constructed to enable a very simple back-mapping procedure to reconstruct geometries with atomistic resolution. The model is illustrated for three related polymers containing DPP (diketopyrrolopyrrole) to evaluate the transferability of the potential across different families of polymers. The accuracy of the model, determined by comparison with the results of fully equilibrated simulations of the same material before and after back-mapping, is fully satisfactory for two out of the three cases considered. We noticed that accuracy can be determined very early in the workflow so that it is easy to assess when the deployment of this method is advantageous. The hybrid representation can be used to evaluate directly the electronic properties of structures sampled by the simulations.

Laser-induced depletion of ultrathin PFPE lubricants using a quantitative coarse-grained model

Knowledge of laser-pulsed processing of ultrathin liquids on solid surfaces is of great importance in advancing the understanding of synthesis, characterization and applications of functional nanofilms. In this work, we performed molecular dynamics simulations coupled with a novel quantitative coarse-grained model to study laser-induced local heating and depletion behaviors of Fomblin Z2000 lubricants. It was found that the Fomblin lubricant films experience severe evaporation and thermodiffusion-related degradation under laser heating. The ultrathin lubricants inevitably lose their frictional resistance under extremely high temperatures and thermal gradients (i.e., up to 990 K and 65.4 K/nm) that are required for FePt media in HAMR systems. The factors that influence the local thermal depletion, such as laser power, spot size, and scanning velocity, were investigated as well. The results clearly show that the laser power has the greatest influence on the relative maximum temperature change and subsequently the severe lubricant film depletion during rapid laser heating, while the spot size and scanning velocity play a relatively weaker role in the laser-induced local thermal instability. The findings in this work are thus believed to provide molecular scale insights into laser-induced local heating and depletion of ultrathin lubricant films from a precise quantitative perspective.