Characterizing the structure and properties of dry and wet polyethylene glycol using multi-scale simulations.

Simulation results for polyethylene glycol by employing a multi-scale approach combining mesoscopic and atomistic scales to characterize its structural, material and thermal properties in dry and wet environments are reported. After a meso-structure is created, DPD simulations with explicit hydrogen bond attraction are run for a molecular understanding of PEG oligomers. The meso-structure is analysed by the end-to-end distance and radius of gyration values, where we notice that water has an effect that makes the chains more flexible compared to the dry material, i.e., acts as a 'plasticizer' (as observed experimentally). Moreover, the helical formation of PEG chains is monitored by meso-scale simulations and a larger distribution of helical chain formation is found for wet PEG. Following the DPD simulations, a reverse-mapping algorithm is used to insert atomistic detail into the mesoscopic configuration in order to run atomistic molecular dynamics simulations to calculate material properties. The comparison of the elastic properties in dry and wet environments shows that PEG becomes less compressible and more elastic upon addition of water. Moreover, the estimated coefficient of thermal expansion is in good agreement with the experimental value of a lower molecular weight PEG.

Hydrogen bonding in DPD: application to low molecular weight alcohol-water mixtures.

In this work we propose a computational approach to mimic hydrogen bonding in a widely used coarse-grained simulation method known as dissipative particle dynamics (DPD). The conventional DPD potential is modified by adding a Morse potential term to represent hydrogen bonding attraction. Morse potential parameters are calculated by a mapping of energetic and structural properties to those of atomistic scale simulations. By the addition of hydrogen bonding to DPD and with the proposed parameterization, the volumetric mixing behavior of low molecular weight alcohols and water is studied and experimentally observed negative volume excess is successfully predicted, contrary to the conventional DPD implementation. Moreover, the density-dependent DPD parameterization employed provides the asymmetrical shapes of the excess volume curves. In addition, alcohol surface enrichment at the air interface and self-assembly in the bulk is studied. The surface concentrations of alcohols at the air interface compare favorably with the experimental observations at all bulk-phase alcohol fractions and, in consonance with experiment, some clustering is observed.

Hierarchical multi-scale simulations of adhesion at polymer-metal interfaces: dry and wet conditions.

We performed hierarchical multi-scale simulations to study the adhesion properties of various epoxy-aluminium interfaces in the absence and presence of water. The epoxies studied differ from each other in their hexagonal ring structures where one contains aromatic and the other aliphatic rings. As aluminium is unavoidably covered with alumina, a cross-linked epoxy structure near an alumina substrate is created and relaxed by performing coarse-grained simulations. To that purpose, we employ a recently developed parameterization method for variable bead sizes. For polymer-metal interactions, a multi-scale parameterization scheme is applied where the relative adsorption of each bead type is quantified. At the mesoscopic scale, the adhesion properties of different epoxy systems are discussed in terms of their interfacial structure and adsorption behavior. To further perform all-atom simulations, the mesoscopic structures are transformed into atomistic coordinates by applying a reverse-mapping procedure. Interface internal energies are quantified and the simulation results observed at different scales are compared with each other as well as with the available experimental data. The good agreement between observations from simulations and experiments shows the usefulness of such an approach to better understand polymer-metal oxide adhesion.

Surfactant formation efficiency of fluorocarbon-hydrocarbon oligomers in supercritical CO2.

We use dissipative particle dynamics simulations to explore the phase behavior and solution properties of ABCBA type model surfactants in near-supercritical CO(2) environment. We present design guidelines for functional surfactants with tunable properties. The block co-oligomers used in this study are made up of a CO(2)-phobic block having ethyl propionate and nine different types of ethylene monomers, flanked on either side by eight repeat units of fluorinated CO(2)-philic blocks. The most promising design block co-oligomer in the series is that with the longest CO(2)-phobic group in the ethylene monomers. For this particular oligomer, we systematically analyze the effect of concentration on the self-assembly behavior. Spherical micelles form in the 5%-65% volume fraction range for this oligomer, with the highest number of spherical micelles occurring at 45% surfactant in CO(2). When the volume fraction of the surfactant is increased from 70% to 85%, cylindrical micelles occur. We further investigate the effect of the length of the solvophilic fluorinated segments on self-assembly and find that stable micelles occur in a window of 8-14 repeat units. We find that the most critical contribution to stability is due to the mixing free energy between the chain tails residing in the outer layers and the interpenetrating molecules.