Determination of interaction parameters in a bottom-up approach employed in reactive dissipative particle dynamics simulations for thermosetting polymers

The limitations in previous dissipative particle dynamics (DPD) studies confined simulations to a narrow resin range. This study refines DPD parameter calculation methodology, extending its application to diverse polymer materials. Using a bottom-up approach with molecular dynamics (MD) simulations, we evaluated solubility parameters and bead number density governing nonbonded interactions via the Flory-Huggins parameter and covalent-bonded interactions. Two solubility parameter methods, Hildebrand and Krevelen-Hoftyzer, were compared for DPD simulations. The Hildebrand method, utilizing MD simulations, demonstrates higher consistency and broader applicability in determining solubility parameters for all DPD particles. The DPD/MD curing reaction process was examined in three epoxy systems: DGEBA/4,4'-DDS, DGEBA/MPDA and DGEBA/DETA. Calculations for the curing profile, gelation point, radial distribution function and branch ratio were performed. Compared to MD data for DGEBA/4,4'-DDS, the maximum deviation in secondary reactions between epoxy and amine groups according to DPD simulations with Krevelen-Hoftyzer was 14.8%, while with the Hildebrand method, it was 1.7%. The accuracy of the DPD curing reaction in reproducing the structural properties verifies its expanded application to general polymeric material simulations. The proposed curing DPD simulations, with a short run time and minimal computational resources, contributes to high-throughput screening for optimal resins and investigates mesoscopic inhomogeneous structures in large resin systems.

The effect of hydration on the stability of ionic liquid crystals: MD simulations of [C14C1im]Cl and [C14C1im]Cl·H2O

The thermal range of the stability of Ionic Liquid Crystal (ILC) phases of imidazolium ILCs, and the type of the mesophase itself are affected by several molecular structural features, the two prominent ones being the alkyl chain length and the counter-anion. Hydration is also very important: monohydrate samples of 1-alkyl-3-methylimidazolium halides have a higher clearing point and a wider thermal range of the stability of the ionic smectic phase, compared with the analogous anhydrous sample. To understand the reasons, at a microscopic level, for such increased stability due to hydration, we run classical Molecular Dynamics (MD) simulations of a typical ionic liquid crystal, 1-tetradecyl-3-methylimidazolium chloride, and of its monohydrate form. We tested a full-charge non-polarizable force field and a scaled-charge version having the total charge of the ions scaled by a factor of 0.80. Comparison of the structural and dynamic properties with available experimental data reveals that the scaling of the charge by a factor of 0.80 results in a good agreement between simulated and experimental data and it sheds light on the microscopic mechanism responsible for the increased stability of the monohydrated phase. A hydrogen-bond network between water and the chloride anion is established in the ionic layer which increases the stability of the ionic layer; this in turn increases the nano-segregation between the ionic and hydrophobic layers which eventually produce an increased order of the alkylic layer as well.

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

The impact of mesoscale organization on dynamics and ion transport in binary ionic liquid mixtures is investigated by broad-band dielectric spectroscopy, dynamic-mechanical spectroscopy, X-ray scattering, and molecular dynamics simulations. The mixtures are found to form distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components. For instance, it is shown that the mesoscale morphologies in ionic liquids can be tuned by mixing to enhance the static dielectric permittivity of the resulting liquid by as high as 100% relative to the neat ionic liquid components. This enhancement is attributed to the intricate role of interfacial dynamics associated with the changes in the mesoscopic aggregate morphologies in these systems. These results demonstrate the potential to design the physicochemical properties of ionic liquids through control of solvophobic aggregation.