Date of Award
2007
Degree Type
Dissertation
First Advisor
Michael L. Greenfield
Abstract
Asphalt is a complex system. Based on solubility parameters, asphalt was classified into asphaltene, resin, and maltene. Using several compounds to represent each part and keeping carbon:hydrogen ratio and other elementary compositions close to real asphalts, three model asphalt systems were built for molecular simulations. Density, heat capacity, isothermal compressibility, thermal expansion coefficient, and bulk modulus were calculated at different temperatures. Glass transition temperature was analyzed based on physical property temperature dependence. Microstructures within model asphalt systems were calculated. Analyzing radial distribution functions between asphaltene and resin molecules and between asphaltene molecules, asphaltene aggregates and depletion of resin molecules surrounding asphaltene were found in some model asphalt systems. Packing between closest asphaltene pairs was analyzed by inter-molecular orientation calculation. Molecule bending was calculated by intra-molecular orientation. The viscosity-temperature dependence of model asphalts was investigated. Green-Kubo and Einstein methods were used to calculate viscosity at high temperatures. Molecular local dynamics were analyzed and rotation correlation functions at different temperatures were calculated; results were regressed using the modified Kohlrausch-Williams-Watts equation to calculate relaxation time. Based on the Debye-Stokes-Einstein theory, viscosities of the model asphalts at room temperature were estimated and semi-qualitative agreement between simulation results and reference was reached for naphthalene and model asphalts. Self-diffusion coefficients of components in model asphalts were calculated and related to molecule sizes and medium viscosity. One polystyrene molecule was added into the original asphaltene2 and AAA-1 model systems to build two modified systems. Viscosity was analyzed based on free volume. Reorientation correlation functions of a polymer in both modified systems were calculated. Inconsistent relaxation time temperature dependence of polymer was found from end-to-end vector and eigenvector correlation function analysis. With a polymer present, the relaxation time of small molecules increased; the packing between asphaltene molecules at high temperature was unchanged; the distribution between asphaltene molecules changed; small aromatic molecules packing distributions were unchanged; the diffusion rates of components decreased; the system density increased; the bulk modulus increased; the temperature dependence of isothermal compressibility and expansion coefficients decreased. Those understandings from model asphalts can help engineers to improve the performance of asphalt on pavement.
Recommended Citation
Zhang, Liqun, "Physical and mechanical properties of model asphalt systems calculated using molecular simulation" (2007). Open Access Dissertations. Paper 2214.
https://digitalcommons.uri.edu/oa_diss/2214
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