(248c) Engineering Lubricating Polymers Under Extreme Conditions Using Polymer Architecture

Polymeric lubricant additives are often engineered to perform a number of orthogonal functions including viscosity regulation, friction reduction and surface wear protection. It is increasingly recognized that macromolecular architecture can play a significant role in controlling these important functions, and recent studies have attempted to identify structure-property relationships in this regard. However, most lubricating oil applications involve flows at extremely high rates, rendering detailed, molecular-scale characterization under relevant conditions inaccessible using conventional methods. To overcome this limitation, we have developed novel devices and methods that enable scattering, rheology and surface force measurements in situ at shear rates exceeding 10⁶ s^-1. We have employed these methods to understand how combinations of polymer architecture and hybrid organic-inorganic chemistry affect key lubricant properties. Specifically, measurements on a well-controlled series of linear, branched and star architectures show that star polymers with inorganic cores represent an effective compromise between viscosity modification and friction reduction. Most importantly, we find that star polymers exhibit significantly greater mechanical stability at extreme shear rates relative to linear counterparts. In situ measurements allow this enhanced stability to be correlated directly with differences in chain deformation under flow at such high rates. Overall, our results highlight the importance of characterizing molecular properties and behavior at high shear rates to aid the rational design of polymer architectures for lubrication applications.