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.