The drive towards better fuel economy through lower viscosity engine oils places increased importance on reducing friction in the so-called boundary regime, where direct solid-solid contact can lead to increased wear and friction. Intriguingly, functionalized olefin copolymers (FOCP) containing a small fraction of polar, aromatic groups are found to reduce wear and improve the efficiency of real engines in addition to improving their viscosity stability vs. temperature. We have utilized a suite of surface-sensitive techniques to elucidate the molecular-level origins of this friction reduction.  Quartz crystal microbalance with dissipation monitoring measurements demonstrate that FOCP forms viscoelastic layers at steel-oil interfaces with significantly higher irreversibly adsorbed masses compared to equivalent non-functionalized olefin copolymers (OCP). These adsorbed FOCP layers display a complex, non-monotonic relationship between bulk FOCP concentration and nanoscale friction measured by colloidal probe atomic force microscopy (AFM). In addition, intermediate concentrations of FOCP are significantly more lubricious than polymer-free oil (PAO4) or equivalent OCP solutions. To understand this behavior, we performed quasi-static normal force AFM measurements, which showed that the adsorbed FOCP function as a repulsive, steric barrier upon approach of the probe, and induces a long-range adhesion upon its retraction. The magnitude and range of both repulsion and adhesion increase monotonically with increasing FOCP concentration. Bulk rheology measurements indicate that the polar functional groups form associative bonds which promote stronger repulsive and adhesive interactions with increasing bulk FOCP concentration by increasing the adsorbed mass and the number of adhesive interpolymer bridges respectively. The importance of association between bulk and surface FOCP is further highlighted by the reduction in friction and adhesion between adsorbed FOCP layers when non-associative OCP replaces the bulk FOCP.  Together, these results demonstrate that the steric barrier reduces friction while adhesive bridges increase it, with competition between both processes leading to the observed non-monotonicity.<br/>

Molecular mechanisms of complex boundary lubrication behavior for multifunctional associative polymer viscosity modifiers

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.