(592b) Understanding the Relationship between Molecular Architecture and Thermophysical and Rheological Properties of Perfluoropolyethers

Understanding the relationship between molecular architecture and thermophysical and rheological properties of perfluoropolyethers

  Abstract Perfluoropolyethers (PFPEs) form a class of lubricants, which are broadly applied in oxygen service, in aircraft instrument bearings, in reactive chemical environments, in vacuum pumps, in sealed-for-life electric motors, in computer hard drives, and as high-temperature greases. The relationship between molecular architecture and physical properties of PFPEs is crucial to design the high-performance PFPE lubricants with revised structure.  In this work, the following five short-chain PFPEs with different structures have been synthesized.  The chains vary four key architectural features including (1) monomer size (-CF₂O-, -CF₂CF₂O-, -CF₂CF₂CF₂O- , (2) chain length, and (3) end-group size.

Snapshot of five short-chain PFPE molecules

Previous work focused on the first four PFPEs and led to a revision of the Universal Force Field (UFF)¹ Model to allow for different types of fluorine atoms, depending upon their local environment,  which resulted in a quantitative description of the relationship between  the molecular architecture of the lubricant and the viscosity ². 

            In this contribution, we extend the work in two ways.  First, we have performed Equilibrium Molecular Dynamics (EMD) and Nonequilibrium Molecular Dynamics (NEMD) simulations to test the predictive capabilities of the new potential.  The additional compounds include (i) compounds with variation in monomer type and (ii) compounds of greater chain length.  For the longer chain PFPEs, we study the effect of chain length on viscosity in these systems, and how a longer chain deforms under a strong flow field. Bulk rheological properties of the perfluoropolyethers are investigated as functions of temperature, shear rate, and molecular structure. Structural properties such as the mean-square end-to-end chain lengths and radii of gyration are collected and analyzed as functions of shear rate and molecular architecture. Rheological properties from molecular simulation are compared to the experimental data.

            Second, experimentally determined PVT data, phase diagrams, heat capacities, critical properties, and viscosities are reported. Again, we examine the relationship between these thermodynamic properties and the molecular-level structure observed in the EMD simulations.  

Acknowledgements:

            This work is supported by Air Force Office of Scientific Research through contract # FA 9550-05-1-0342. The authors wish to acknowledge resources of the Center for Computational Sciences at Oak Ridge National Laboratory, which is supported by the Office of Science of the DOE under Contract DE-AC05-00OR22725.  

            (1)        Rappe, A. K.; Casevit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.

            (2)        Jiang, B., Crawford, N.J., Keffer, D.J., Edwards, B.J., Adcock, J.L., Mol. Simul. 2007, accepted .