Structural, dynamic, and viscoelastic properties of complex materials using molecular dynamics simulations

The molecular structure and the intermolecular interactions govern the rheological properties of complex fluids such as polymers, molecular glasses, and gels. Molecular dynamics (MD) simulations can be used as a powerful tool to decipher the connection between the structure and rheological properties of these fluids. In this regard, we have used MD simulations to study volumetric, structural, dynamic, and rheological properties of three different complex fluid systems namely, dilute solutions of polymer chains of different architectures, polymer modified asphalt, and hydrated polyacrylate gel.

Effect of polymer chains of different architectures on the size, shape, and intrinsic viscosity of linear, comb, H, and star--shaped polymers was investigated using a coarse--grained model for the systems. A method analogous to the experimental procedure was utilized to determine the intrinsic viscosity ($ [\eta] $), and values of $ [\eta] $ were determined for all the chain architectures as a function of the chain length. Results demonstrated that the Fox--Flory relationship that is usually applied to linear chains, can also be used to relate the size of a chains of different architectures to their intrinsic viscosity.

The volumetric, structural, dynamic, and rheological properties of neat and polymer modified asphalts were studied. The model systems showed a glass transition temperature ($ T_{g} $) that was consistent with the experimental value. The asphaltene molecules in neat asphalt were found to aggregate. The aggregation of asphaltene molecules increased with a decrease in the temperature, and the inclusion of a styrene-butadiene rubber (SBR) polymer increased the degree of aggregation. The reciprocal of the diffusion coefficient of different constituents of neat and polymer modified asphalts followed the Vogel--Fulcher--Tammann (VFT) equation at temperatures above the $ T_{g} $ of the system. The shear viscosity, moduli, and creep compliance of asphalt were also determined at different temperatures. It was found that the rheological property values at different temperatures could be collapsed onto a master curve using time--temperature (TTS) superposition principle. In this regard, the normalized relaxation time determined from the shift factors of moduli of atactic polypropylene modified asphalt showed a deviation from the VFT equation functional form at temperatures below the $ T_{g} $ of the systems.

The structure of the solvent molecules affects their dynamics in polymer gels. Hydrogen bonding analysis showed that the penetrant molecules tend to interact with the polyacrylate gel network at a low solvent content. A dynamic coupling between the solvent molecules (water and ethanol) and polyacrylate gels with varying the degree of hydrophilicity was found using MD simulations. In particular, the results showed that polymer chains govern the dynamics of the water and ethanol molecules at very low solvent content. On the other hand, as the solvent concentration increased, the mobility of the water and ethanol molecules was enhanced due to the formation of solvent clusters in the system. The dynamic heterogeneity analysis for the systems with low solvent content showed that the mobility of water molecules and polymer chains are closely correlated due to hydrophilic interactions. On the other hand, this coupling for dynamics of ethanol molecules was weak, and mobility of these molecules was governed by $ T_{g} $ of the polymer.