B. Jiang

A comparison of simple rheological models and simulation data of n-hexadecane under shear and elongational flows

The microscopic origins of five rheological models are investigated by comparing their predictions for the conformation tensor and stress tensor with the same tensors obtained via nonequilibrium molecular dynamics simulations for n-hexadecane. Steady-state simulations were performed under both planar Couette and planar elongational flows, and the results of each are compared with rheological model predictions in the same flows, without any fitting parameters where possible. The use of the conformation tensor for comparisons between theory and experiment/simulation, rather than just the stress tensor, allows additional information to be obtained regarding the physical basis of each model examined herein. The character of the relationship between stress and conformation is examined using model predictions and simulation data.

A reactive molecular dynamics study of the thermal decomposition of perfluorodimethyl ether.

Classical reactive molecular dynamics (RMD) simulation is used to model the thermal decomposition of perfluorodimethyl ether (CF(3)OCF(3)), which is relevant as a simple molecule containing the necessary architectural elements to study the chemical stability of perfluoropolyether lubricants. The RMD algorithm employs nonreactive interaction potentials for the reactants and products. The reactivity is implemented through a coarse-grained simulation algorithm, incorporating elements from both the quantum and macroscopic descriptions of the reaction. The RMD scheme maps the quantum mechanically determined transition state onto a set of geometric triggers. When a configuration matching those triggers is found in the RMD simulation, the reaction instantaneously occurs. A brief, local equilibration process stabilizes the configuration, and the simulation continues. Using two geometric triggers, the RMD simulation can describe quantitatively the temperature dependence of the thermal decomposition of CF(3)OCF(3), when compared to the quantum mechanical standard.

Comparison of rheological properties of short-chain perfluoropolyethers through simulation and experiment

Four short-chain perfluoropolyethers (PFPEs) with varied architectural modifications have been studied rheologically using non-equilibrium molecular dynamics simulation and experiment. An explicit-atom potential, which treats all fluorine atoms equivalently, is not capable of reproducing the experimentally observed trends in viscosity among the four compounds. Rather, the parameters of the potential governing the interaction of fluorine must reflect the proximity of the fluorine to the oxygen in the ether linkage. Defining four different types of fluorine atoms, we are able to reproduce the experimentally observed trends in viscosity among the four compounds. We examine the effect of this potential change on the structural properties as well.

Comparison of perfluoropolyethers and n-alkanes under shear via nonequilibrium molecular dynamics simulation

The structural, energetic and rheological properties of seven short-chain perfluoropolyethers (PFPEs) under planar Couette flow have been investigated through nonequilibrium molecular dynamics (NEMD) simulation. The full parameter set of a revised universal force field (A.K. Rappe et al., UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations, J. Am. Chem. Soc. 114 (1992), p. 10024) is presented for linear PFPEs, allowing for multiple types of fluorine atoms depending upon their local environment (B. Jiang et al., Comparison of rheological properties of short-chain PFPEs through simulation and experiment. Mol. Simul. 33 (2007), p. 871). The NEMD simulations quantitatively reproduce experimental zero-shear-rate viscosities for five PFPEs with varying molecular architectures. Rheological properties and structural variations of PFPEs are investigated as functions of flow strength, temperature and chain architecture. We find the following general relationships between PFPE architecture and viscosity: (i) longer chain lengths increase the viscosity, (ii) ether linkages in the backbone decrease the viscosity and (iii) longer (CF2) n units between ether linkages increase the viscosity. These effects are all explained in terms of chain flexibility. Additionally, we report the structural and rheological properties of four short-chain PFPEs with identical monomeric units but with different chain lengths using NEMD simulation of planar Couette flow. We explain the behaviour of the longer PFPEs due to the increased relative flexibility of longer chains over shorter chains. Finally, we provide a quantitative comparison of the structural and energetic properties of relatively rigid PFPEs and relatively flexible alkanes as a function of chain length. In general, alkanes respond to the flow field with a combination of alignment and extension. PFPEs respond with greater alignment but less extension. An increase in chain length enhances the degree of alignment at high shear rates and enhances the degree of extension at intermediate shear rates.

A Quantum Mechanical Study of the Decomposition of CF3OCF3 and CF3CF2OCF2CF3 in the Presence of AlF3

The effect of AlF3 on the decomposition of CF3OCF3 and CF3CF2OCF2CF3 is investigated using ab initio theory. Previous work by Pancansky et al. [Pacansky, J.; Waltman, R. J. J Fluorine Chem. 1997, 83, 41] showed that AlF3 significantly reduces the activation energy of the decomposition of CF3OCF3 due to the strong electrostatic interaction between the aluminum trifluoride and the reactant. In this work, a new transition-state structure and reaction mechanism have been identified for the decomposition of CF3OCF3 in the presence of AlF3. This new mechanism shows that AlF3 functions by accepting a fluorine atom from one carbon and simultaneously donating a fluorine atom to the other carbon. We show that the same pathway is obtained independently of the level of theory. The reaction rate, generated via statistical mechanics and transition-state theory, is 2-3 orders of magnitude higher for the new transition state when compared to that of the old one. The study was also performed for CF3CF2OCF2CF3 in order to ascertain the effect of chain length on the reaction mechanism and rate. We find that an analogous transition state, with lower activation energy, provides the lowest-energy path for decomposition of the longer chain.