(89f) Performance Mechanisms of Viscosity Modifiers Using Coarse-Grained Molecular Dynamics

Engine oils are constantly exposed to multiple drive cycles as well as diverse ambient conditions during non-operative periods, but their viscometric performance is expected to be uniform and robust across different conditions. For high entrainment speeds, temperatures, and pressures, viscosity must be high enough to contribute to wear prevention within elastohydrodynamic and mixed tribological regimes. However, the oil also must be thin enough to be pumpable at low temperatures to assist cold starting and have a high viscosity index to promote fuel-economy. This effect is commonly achieved through the introduction of viscosity modifiers (VMs), which are high molecular weight (MW) polymers. The traditional belief is that the performance mechanism of VMs follows the coil expansion model (Selby, ALSE Trans., 1, 68-81, 1958). As temperature increases, the solvency power of the oil increases causing the polymers to swell or uncoil, increasing viscosity. However, experimental evidence has suggested that this view may be inadequate for all VMs. While poly(alkyl methacrylate)s (PAMAs) show monotonic viscosity increases with temperature, other VMs (e.g., olefin copolymers (OCPs)) do not always show the same trend (Müller, Tribol. Int., 11, 189, 1978 and Covitch and Trickett, Adv. Chem. Eng. Sci., 5, 134, 2015).

Due to the macromolecular size of polymers, fully-atomistic simulations of VMs are too computationally intensive to study in dilute solution for MWs greater than about 2,000. We show that systematic coarse-graining allows us to accurately calculate structural properties of VMs with MWs used in engine oils (greater than 100,000). Using iterative Boltzmann inversion (Reith et al., J Comput. Chem., 1624, 24, 2003), we develop coarse-grained force-fields for OCPs and PAMAs, two commonly used VMs in the lubrication industry. Using coarse-grained molecular dynamics, we calculate the polymer’s radius of gyration and hydrodynamic radius in different organic solvents.