基于自由体积理论的沥青分子模型黏度预测

Viscosity estimation of model asphalt based on free volume theory

沥青的黏度特性由化学结构决定.然而,目前在分子层面上对沥青动态黏度机理的认识并不清晰.本文采用分子动力学(molecular dynamic,MD)模拟,从自由体积的角度研究了沥青的黏度.基于一定范德瓦耳斯(van der Waals,vdW)半径下的自由体积预测,计算得到了沥青模型黏度与温度之间的关系.结果表明,沥青的动态黏度与自由体积有关;沥青模型的自由体积随vdW半径、沥青模型类型等参数变化,随vdW半径的减小而增大;单组分模型的自由体积明显大于多组分模型.利用不同半径的自由体积对沥青的动态黏度进行了预测,当vdW半径为1.39?时计算出来的自由体积预测的沥青模型黏度平均偏差最低.此外,不同沥青模型的平均偏差不同,四组分模型的平均偏差最小.黏度测试结论与模拟结果吻合较好,说明MD结果是可靠的.本文有助于理解沥青黏度的微观机理及其与分子结构间的关系.

Temperature dependent viscosity determines the viscoelastic properties of asphalt.However,the understanding of dynamic viscosity of asphalt is ambiguous,especially at the molecular level.In this study,molecular dynamic(MD)simulations were used to help understand the viscosity from the viewpoint of free volume.The temperature dependence of viscosity for asphalt model was computed based on the free volume estimation with certain vd W radius.The first step of conducting molecular simulations is to build a molecular model.The second step of the simulation was to reach the target density with the minimum energy level.The Materials Studio(MS)was employed to calculate interatomic interactions in the systems and conduct the MD computation of the molecular models.The Atom Volumes&Surfaces tool was used to evaluate the free volume of the asphaltic model,which is on the side of the atom volume surface free of atoms.The vd W surface that intersects with the vd W radius of the atoms in the structure was applied in the creation of free volume.The specified solvent probe radius and Connolly probe radius were introduced based on the evaluation of molecular morphology of asphaltic molecules.According to free volume,the fraction of space that can accommodate the jump unit of molecules determines the viscosity.The morphology of asphaltic molecules was complex thus the size of jump unit was not unique.The radial distribution function(RDF)was introduced in this study to estimate the size of probe radius.The solvent probe radius determines the distance of the solvent surface from the structure.Connolly probe was used in calculating the atom volume field.Results showed that the dynamic viscosity of asphalt was related to the free volume and thus was predictable by free volume theory.The free volume of asphaltic model varied with parameters including vd W radius and asphaltic model types.The free volume increased with the decrease of vd W radius.The free volume of single component model was obviously larger than that of multiple components models.The FFV%of four components model was obviously lower than that of single component model,which was due to the molecular morphology of asphaltic models.The molecular morphology of four components model expressed more diversity because it contained multiple function groups comparing with single component model.Therefore,the four components model could occupy more free volume and shows lower FFV%.The free volume with different radius was used to predict the dynamic viscosity of asphalt.The free volume with vd W radius of 1.39?showed the lowest average deviation for asphaltic models.Furthermore,the average deviation varied with different asphaltic models.The four components model expressed the lowest average deviation.The results also reflected the dependence of the viscosity on the probability that a molecule would obtain sufficient energy to overcome attractive forces and on the probability that a fluctuation in the local density would produce a hole of sufficient size so that the moving molecule can jump.Moreover,the good agreement between the rheological experiments and simulation results demonstrated that the MD results were reliable.This paper helps understand microscopic picture of asphaltic viscosity and its relaxation mechanisms with temperature.