Rotational relaxation times of individual compounds within simulations of molecular asphalt models

LQ Zhang and ML Greenfield, JOURNAL OF CHEMICAL PHYSICS, 132, 184502 (2010).

DOI: 10.1063/1.3416913

The dynamical properties of a complex system incorporate contributions from the diverse components from which it is constituted. To study this relationship in a multicomponent system, relaxation times based on rotation autocorrelation functions in molecular dynamics simulations were analyzed for molecules in two sets of unmodified and polymer- modified model asphalt/bitumen systems over 298-473 K. The model asphalt systems were proposed previously to approximate the chemical and mechanical properties of real asphalts. Relaxations were modeled using a modified Kaulrausch-Williams-Watts function and were based on the third Legendre polynomial of normal vector time correlation functions for aromatic species (asphaltene, polar aromatic, naphthene aromatic). Both the end-to-end vector and the longest axis eigenvector of the radius of gyration matrix were used for time correlation functions of chain molecules (C-22, polystyrene). Decreases in temperature induced large increases in relaxation time consistent with the Vogel-Fulcher-Tammann equation. The presence of a polymer slowed the decay of each correlation function to some extent. The product of relaxation time and diffusion coefficient revealed qualitative differences between larger and smaller molecules in the same system. These relaxation mechanisms remained coupled for small molecules, while the larger asphaltene and polymer molecules revealed significant slowdowns in rotation compared to translational diffusion at lower temperatures. Smaller values of the stretched exponential parameter beta for asphaltenes compared to smaller molecules suggested a broader range of relaxation times and were consistent with this distinction. Difficulties in converging polymer chain relaxation times are discussed in terms of fluctuations in the magnitude and orientation of the end-to-end vector and chain axis eigenvector. Viscosity results suggested by the Debye-Stokes-Einstein relationship are consistent with trends shown in the literature for true bitumen systems. (C) 2010 American Institute of Physics. doi: 10.1063/1.3416913

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