Effects of Polymer Modification on Properties and
Microstructure of Model Asphalt Systems
- Publication date
- Publisher
Abstract
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 <i>g</i>(<i>r</i>) 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 [<i>g</i>(<i>r</i>) <1]
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