Michael L. Greenfield

Use of Molecular Dynamics to Investigate Self-Healing Mechanisms in Asphalt Binders

The fatigue-cracking life of an asphalt mixture measured in the laboratory is generally a small fraction of the fatigue-cracking life observed in the field. One of the reasons for this large difference is the self-healing property of asphalt binders. Self-healing is a process that reverses the growth of fatigue cracks during rest periods between load applications. A thorough understanding of the healing mechanism is required to accurately model and predict the influence of healing on the fatigue-cracking life of asphalt mixtures. Previous studies have used experimental evidence to demonstrate a correlation between chemistry of asphalt functional groups, such as chain length and branching, and healing measured in asphalt binders. One of the mechanisms of healing is the self-diffusion of molecules across the crack interface. This paper demonstrates the use of molecular simulation techniques to investigate the correlation of chain length and chain branching to self-diffusivity of binder molecules. The findings reported in this paper are consistent with observations reported in previous studies and expand on the understanding of the relationship between molecular architecture, self-diffusivity, and self-healing properties of asphalt binders.

Relaxation time, diffusion, and viscosity analysis of model asphalt systems using molecular simulation

Molecular dynamics simulation was used to calculate rotational relaxation time, diffusion coefficient, and zero-shear viscosity for a pure aromatic compound (naphthalene) and for aromatic and aliphatic components in model asphalt systems over a temperature range of 298-443 K. The model asphalt systems were chosen previously to represent real asphalt. Green-Kubo and Einstein methods were used to estimate viscosity at high temperature (443.15 K). Rotational relaxation times were calculated by nonlinear regression of orientation correlation functions to a modified Kohlrausch-Williams-Watts function. The Vogel-Fulcher-Tammann equation was used to analyze the temperature dependences of relaxation time, viscosity, and diffusion coefficient. The temperature dependences of viscosity and relaxation time were related using the Debye-Stokes-Einstein equation, enabling viscosity at low temperatures of two model asphalt systems to be estimated from high temperature (443.15 K) viscosity and temperature-dependent relaxation time results. Semiquantitative accuracy of such an equivalent temperature dependence was found for naphthalene. Diffusion coefficient showed a much smaller temperature dependence for all components in the model asphalt systems. Dimethylnaphthalene diffused the fastest while asphaltene molecules diffused the slowest. Neat naphthalene diffused faster than any component in model asphalts.

Molecular modelling and simulation of asphaltenes and bituminous materials

Molecular level calculations targeting the behaviour of asphaltenes, resins and bitumens are reviewed with an objective of relating molecular structure and interactions to macroscale physical, chemical and mechanical properties. The review discusses molecular dynamics simulations of asphaltenes and bitumens and briefly summarises structure elucidation, quantum mechanics, coarse graining and thermodynamic model approaches. The molecular architecture of asphaltenes in simulations plays an important role, with continental and archipelago asphaltenes showing different packing tendencies in isolated systems and solutions. Alkyl side chains bonded to fused aromatic rings interfere with multiple ring stacking layers. Diversity of molecular structure increases asphaltene aggregate formation via alternatives that improve packing. Strongly bent aromatic rings are vacuum simulation artefacts and are absent when enough molecules can solvate unbent fused aromatic rings. Bitumen simulations provide methods to estimate the influence of asphaltenes on dynamic properties. Future needs include methods to incorporate chemically specific parameters into modelling approaches that address larger length and longer timescales.

Rotational relaxation times of individual compounds within simulations of molecular asphalt models

The dynamical properties of a complex system incorporate contributions from the diverse components from which it is constituted. To study this relationship in a multicomponent system, relaxation times based on rotation autocorrelation functions in molecular dynamics simulations were analyzed for molecules in two sets of unmodified and polymer-modified model asphalt/bitumen systems over 298–473 K. The model asphalt systems were proposed previously to approximate the chemical and mechanical properties of real asphalts. Relaxations were modeled using a modified Kaulrausch–Williams–Watts function and were based on the third Legendre polynomial of normal vector time correlation functions for aromatic species (asphaltene, polar aromatic, naphthene aromatic). Both the end-to-end vector and the longest axis eigenvector of the radius of gyration matrix were used for time correlation functions of chain molecules (C22, polystyrene). Decreases in temperature induced large increases in relaxation time consistent with ...

Analysis of FT-IR spectroscopic data: The Voigt profile

Abstract A novel spectral analysis technique is presented for the determination of peak areas from spectra in which there are multiple overlapping peaks. The technique involves a combination of Fourier spectral analysis and profile modeling. Although the technique presented could be applied to many different types of problems (analysis of UV spectra or chromatographic results) it is applied here to the problem of determining, from Fourier transform infrared (FT-IR) spectra, the peak areas which correspond to monomeric and hydrogen-bonded species. For FT-IR spectra it is found that the Voigt profile is better than either the Lorentzian or Gaussian profiles. Use of the Voigt profile leads to more accurate and descriptive spectral peak parameters and, in turn, to more accurate determinations of concentrations of monomeric and hydrogen-bonded species. The technique is demonstrated using the carbonyl peaks (monomeric and hydrogen-bonded) in the acetone/methanol/carbon tetrachloride system.

Coupling of Penetrant and Polymer Motions During Small-Molecule Diffusion In a Glassy Polymer

Abstract Multidimensional transition-state theory was used to simulate methane jump motions in glassy atactic polypropylene at 233 K in the limit of small methane concentrations. Transition states were found with respect to both penetrant and polymer degrees of freedom, using all generalized coordinates associated with atoms interacting with the methane penetrant. Animations followed the multidimensional reaction coordinate for three different jumps. The jump mechanism involved polymer atoms retracting to form a channel, followed by penetrant motion through the channel. Methyl groups within 4 A of the penetrant transition state location were displaced by 0.9 A on average, while carbon atoms and methyl groups further than 9 A from the penetrant transition state location were displaced by less than 0.2 A. The energy profiles along the diffusion path differed considerably among all jumps simulated, and the jump rate did not correlate simply with changes in particular types of degrees of freedom. Jumps for wh...

Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives

The molecular-level function of model and commercial friction modifier additives in lubricants of the type used at the wet clutch interface in automatic transmissions has been studied using a surface forces apparatus (SFA) modified for oscillatory shear. The nanorheological properties of tetradecane with and without a model friction modifier additive (1-hexadecylamine) were examined in the boundary lubrication regime and compared to a fully-formulated automatic transmission fluid (ATF). 1-Hexadecylamine adsorbed as a single layer on the sliding surfaces, reduced the static frictional force and the limiting shear stress, and eliminated the stick–slip transition that exists in pure tetradecane. The ATF, which contains commercial-grade friction modifiers, showed nanorheological properties similar to those observed for tetradecane containing 0.1–0.2 wt% 1-hexadecylamine.

Molecular dynamics simulation study of model friction modifier additives confined between two surfaces

We have used molecular dynamics simulations to investigate the molecular energetics and orientation of surfactant‐like model “friction modifier” (FM) additives with and without hydrocarbon solvent and confined between idealized atomistic surfaces. The normal load, fluid layer thickness, and additive surface concentration dependencies agree favorably with those measured experimentally for model fluids using a surface forces apparatus. The simulations predicted either a single or multiple free energy wells with increasing surface separation, depending on the FM concentration. With no solvent added, pure FMs showed oscillations in normal pressure and free energy with increasing surface separation; stable states corresponded to successive layers of FM molecules being expelled from the region between the adsorbed films. In the case of FM/hydrocarbon solution, only a single stable position was found. The equilibrium structure was also found to depend on the head group structure of the FMs.

Hydrogen-bonding competition in entrainer cosolvent mixtures

Abstract In chemical separation processes such as supercritical extraction, the use of an entrainer cosolvent can dramatically improve selectivity and yield. Ideally, an cntrainer cosolvent should solvate only the desired solute, pulling it from the feed. Prospective entrainers therefore are chosen for their hydrogen bonding tendency. But not all cosolvents are effective entrainers, and an entrainer that is effective for one application may not be effective for others. The effectiveness of a cosolvent as an entrainer can be limited by competition among various hydrogen bonding species in the mixture. In this paper, experiment and theory are presented for hydrogen bonding in entrainer cosolvent mixtures. Concentrations of monomelic and hydrogen bonded species are determined using FTIR spectroscopy and these data are modeled using the Associated Perturbed Anisotropic Chain Theory (APACT). Liquid solvents with hydrogen bonding properties similar to those of supercritical fluids are used. Using APACT, it is s...

Simulation of small molecule diffusion using continuous space disordered networks

Disordered networks were created and kinetic Monte Carlo simulations were conducted in order to assess the effects of jump network connectivity on the diffusion coefficient. Off-lattice jump networks were created using reverse Monte Carlo, with an objective function defined by agreement to specified inter-site connectivity, inter-site (jump path) distance and consecutive jump angle distributions. Both Gaussian and Poisson distributions of connectivity were applied, with average connectivities spanning a range appropriate for small molecules within a variety of polymers. Distance and angle distributions were taken from earlier work on jump networks in polypropylene. At short times, anomalous diffusion with a range of exponents n ≥ 0.4 was found over the domain of average connectivities. At longer times, diffusion was normal for connectivities above the percolation threshold, while particles were trapped for lower connectivities. The percolation threshold was slightly higher for Gaussian distributions of connectivity than for Poisson distributions. The diffusion coefficient increased linearly for connectivities well above the threshold, with slightly faster diffusion occurring for Gaussian distributions of connectivity.