Molecule relaxation and viscosity in model asphalt mixture systems
Mixtures resembling asphalts were studied using equilibrium molecular dynamics to understand the contributions of individual molecules to overall viscosity. Model asphalts were devised based on solubility and polarity, using one or more compounds to represent each of the four typical classifications of molecules found in real asphalts, i.e., asphaltene, polar aromatic, naphthene aromatic, and saturate. Relaxation dynamics were assembled using rotational correlation functions of aromatic ring normal vectors and end-to-end vectors. Typical relaxation behavior was estimated using modified KWW fits of the simulation results. A Vogel-Fulcher temperature dependence of the relaxation time was observed, and mixture viscosity over a range of temperatures was then estimated using a high-temperature direct calculation (via the Green-Kubo method) and the Debye-Stokes-Eistein relationship. The same methods were applied to both unmodified and polymer modified model asphalt mixtures. Viscosity results at room temperature agreed reasonably with literature values. The mechanism of how polymer chains modify asphalt viscosity was also analyzed. The long-term results could supply guidance for designing new functional polymers for use in road pavements. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).