Effects of polymer modification on properties and microstructure of model asphalt systems
LQ Zhang and ML Greenfield, ENERGY & FUELS, 22, 3363-3375 (2008).
Physical properties and microstructures of computational model asphalts were investigated using molecular dynamics simulations in an all-atom framework. A new model asphalt is proposed that is targeted toward core asphalt AAA-1 of the Strategic Highway Research Program (SHRP) based on elemental composition and speciation. Individual compounds were chosen from the literature to represent asphaltene, polar aromatic, naphthene aromatic, and saturate, with interactions ranked using Hansen solubility parameters. The density and thermal expansion coefficient agreed better with experimental data than had predictions using earlier model asphalts. In addition, one polystyrene molecule with 50 repeat units was added into a ternary model asphalt from earlier work and the new six- component AAA-1 model system to analyze polymer modification effects. The expansion coefficient, isothermal compressibility, and their temperature dependence decreased with one polymer chain present, while density increased. Self-diffusion coefficients of each component in both model asphalts decreased upon including the polymer. To assess microstructure, radial distribution functions g(r) of asphaltene and simplified resin molecules were calculated at different temperatures. Asphaltene results changed with temperature and upon including one polymer; artifacts of initial configuration were found at lower temperatures. Radial distribution functions for pairs of resin-like molecules (dimethylnaphthalene, benzoquinoline, and ethylbenzothiophene) and for asphaltene-resin pairs retained similar shapes and first peak positions at different temperatures and when including the polymer. Results for unlike molecules indicated a depletion of resin g(r) < l immediately surrounding an asphaltene molecule, rather than the enrichment expected from standard "colloid model" descriptions, in which resins solubilize asphaltenes. Intermolecular orientations between closest asphaltene pairs in original and polymer modified systems were strongly peaked toward parallel packing and remained similar at several high temperatures. Orientations between asphaltenes and resins and among resins were weighted toward parallel, compared to random packing, both with and without a polymer and over a range of temperatures.
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