Long Time Dynamics and Viscoelasticity of Soft Matter Systems from Molecular Simulation

Soft matter systems include a wide range of substances with properties between those of conventional liquids and solids. Their distinctive properties make them useful for many applications in the field of biotechnology, food science, energy storage, etc. Molecular dynamics (MD) simulations provide the unique ability to make a connection between the molecular structure and the end-use properties of these materials.

The first part of this work addressed the issue of availability of a short time scale in molecular simulations. The time-temperature superposition principle (TTS) was used to extend the timescale of molecular simulations of imidazolium-based ionic liquids. By employing TTS principle, properties such as dynamic moduli, shear viscosity, and mean squared displacement of cations and anions were collapsed onto a master curve using a single set of shift factors. The results of this work demonstrate the exciting ability of TTS to overcome the large timescale disparity between simulations and experiments which will enable the use of molecular simulations for quantitatively predicting the rheological property values at frequencies of practical interest.

In the second part, the effect of chemical interactions and molecular topology on small molecule dynamics was studied in three different polymeric systems. The effect of polymer chain topology on the separation of a dilute ethanol-water mixture was studied by focusing on polyacrylate membranes. Four different polyacrylate polymers that differ in chain length and degree of hydrophilicity were studied. The results of this study showed that water mobility in the polymer membrane is governed by hydrogen bond formation with polymer chains, while ethanol dynamics is governed by the free volume in the system.

MD simulations were also used to investigate the dynamics of salt and water within NF270 membranes under varying salt concentrations, focusing on calcium chloride (CaCl2) and sodium chloride (NaCl) solutions of different concentrations. The mobility of water and salt molecules in the membrane was quantified by calculating their mean squared displacement within the membrane. Our results show that the difference in water mobility in different salt solutions can be attributed to hydrogen bond formation between water molecules and between water molecules and polymer chains.

The structural and dynamic properties of lithium cation (Li+) and bis(trifluoromethane sulfonyl)imide anion (TFSI-) in poly (ethylene oxide) matrix were examined in bulk PEO-LiTFSI electrolyte and in the presence of a graphite surface using molecular dynamics simulations. Our findings suggest that the presence of graphite surface does not affect the molecular structure around Li+ ions. Results also show that the dynamics of the ions and ether oxygen is hindered near the graphite surface compared to the region away from the graphite surface.

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