A particulate molecular model in which the solvent particles are considered explicitly is developed for studying the linear viscoelasticity of nanocolloidal suspensions using molecular dynamics simulations. Nanocolloidal systems of volume fractions ranging from 0.10 to 0.49 are studied. The hydrodynamics in these model systems are governed by interparticle interactions. The volume fraction dependence of the relative zero shear viscosity exhibited by this molecular model is consistent with that reported in the literature experiments and simulations. Over the range of frequencies studied, the relative dynamic viscosity values follow the same qualitative trend as that seen in the literature experiments. The time-concentration superposition (TCS) principle is successfully applied to construct the viscoelastic master curves that span nine decades of frequency in the case of the elastic modulus and more than four decades of frequency in the case of the loss modulus. The TCS principle was observed to fail at high volume fractions that are near the glass transition concentration; this finding is consistent with the literature experimental and simulation observations. The volume fraction dependence of the shift factors used in the construction of the viscoelastic master curves is in good quantitative agreement with that of the viscosity of the nanocolloidal systems. Our results demonstrate that molecular simulations in conjunction with an explicit solvent model can be used to quantitatively represent the viscosity and the viscoelastic properties of nanocolloidal suspensions. Such particulate models will be useful for studying the rheology of systems whose properties are governed by specific chemical interactions.
Although bead microrheology experiments are routinely used to characterize the viscoelasticity of complex matter, their simulation analog—probe rheology molecular simulations—has been scarcely used since the system characteristics required for its robust implementation are not established in the literature. We address this issue by analyzing an active probe rheology simulation setup consisting of a probe particle that is subjected to an external oscillatory force and a harmonic trapping force. We identify a set of eight conditions of the system properties that must be satisfied for the successful implementation of the probe rheology technique in molecular simulations. Among these criteria, the two most important are as follows: (1) The spring force constant for the trapping force should be sufficiently large such that the peak in the Fourier transform of the probe displacement occurs at the same frequency as that of the applied force. (2) System parameters should be chosen such that the magnitude of the external force used to drive the probe motion should be comparable to the magnitude of the hydrodynamic friction force experienced by the probe particle in the viscoelastic medium. Furthermore, a scaling relation that can be used to determine the frequency at which inertial effects set in for a given probe size is also established. The validity of our procedure is demonstrated by applying it to determine the viscoelastic properties of a weakly entangled polymer melt system.
Cyclic topologies or loops in crosslinked networks have negative effect on mechanical and functional properties. In this study, an epoxy resin diglycidyl ether of bisphenol A (DGEBA) crosslinked by a hardener 4,4-diaminodiphenyl methane (DDM) with various cyclic topologies were simulated to find correlations between the mechanical/shape memory properties (i.e., glassy/rubbery elastic modulus, shape recovery ratio, and recovery stress) and cyclic topologies (i.e., number of total loops, numberof defective loops, etc.). The effect of cyclic topology on shape memory properties was more significant than its effect on mechanical properties, altering recovery stress by more than 25% on average. After analyzing several topological fingerprints such as total number of loops, number of defective loops and number of higher order loops, we found that the effect of cyclic topology on the mechanical/shape memory properties of the systems can be best understood by the fraction of hardeners reacted with 4 distinct epoxy molecules (tetra-distinctly-reacted (TDR) hardeners). By increasing the number of TDR hardeners, the network is closer to ideal, resulting in an increase in the number of higher order loops and a reduction in the number of defective loops, which in turn leads to an increase in rubbery elastic modulus andshape recovery ratio to a lesser degree, but a substantial increase in recovery stress. These results suggest that utilization of experimental techniques such as semibatch monomer addition, which leads to a more expanded and defect-free network, can result in a simultaneous increase in both shape recovery ratio and recovery stress in thermoset shape memory polymers (TSMPS). Moreover, topology alteration can be used to synthesize TSMPs with improved recovery stress without significantly increasing their stiffness.
The probe rheology simulation technique is a technique for measuring the viscosity of a fluid by measuring the motion of an inserted probe particle. This approach has the benefit of greater potential accuracy at a lower computational cost than other conventional simulation techniques used for the calculation of mechanical properties, such as the Green–Kubo approach and nonequilibrium molecular dynamics simulations, and the potential to allow for sampling local variations of properties. This approach is implemented and demonstrated for atomistically detailed models. The viscosity of four different simple Newtonian liquids is calculated from both the Brownian motion (passive mode) and the forced motion (active mode) of an embedded probe particle. The probe particle is loosely modeled as a nano‐sized diamond particle: a rough sphere cut out of an FCC lattice made of carbon atoms. The viscosities obtained from the motion of the probe particle are compared with those obtained from the periodic perturbation method, and good agreement between the two sets of values is observed once the probe‐fluid interaction strength (i.e., εitalicij in the pair‐wise Lennard–Jones interaction) is two times higher than their original values, and the artificial hydrodynamic interactions between the probe particle and its periodic images are accounted for. The success of the proposed model opens new opportunities for applying such a technique in the rheological characterization of local mechanical properties in atomistically detailed molecular dynamics simulations, which can be directly compared with or help guide experiments of similar nature.
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