'Centipede' copolymer boundary lubricants at the solid-oil interface

: structure and interactions

Abstract

Statistical copolymers are widespread in industrial applications involving non-polar solvents, including as steric stabilisers and lubricant oil additives. Despite this, the molecular self-assembly and adsorption behaviour of statistical copolymers is relatively poorly understood relative to their block copolymer analogues. In this work, the adsorption behaviour, interfacial structure and lubrication mediated in a non-polar medium by a class of functionalised olefin copolymers (FOCPs) was studied using a range of solution and surface characterisation techniques. The FOCPs consist of a hydrophobic poly(ethylene)-poly(propylene) (PE-PP) backbone and a statistical distribution of polar groups bearing imide and aromatic functionality. Small-angle neutron scattering (SANS) measurements confirmed self-assembly of the FOCPs into large, network-like aggregates in n-dodecane. A combination of adsorption isotherm measurements and X-ray reflectivity (XRR) were used to investigate the adsorption behaviour of the FOCPs at the solid-oil interface which provided strong evidence for associative multilayer adsorption on top of a strongly adsorbed first layer, facilitated by interactions between the polar functional groups of adjacent FOCPs. Associative multilayering of the FOCPs under lateral compression at the air-water interface was investigated using XRR and dilational rheology, demonstrating the formation of an ‘interfacial gel’ that could be successfully transferred to solid substrates by Langmuir-Blodgett (LB) deposition for subsequent frictional studies in oil. The surface forces apparatus (SFA) was used to investigate the confined film structure and frictional behaviour of FOCP boundary films in n-dodecane, demonstrating significant differences in lubrication behaviour as a function of subtle changes in interfacial structure, resulting solely from the film deposition method (physisorption vs. LB deposition). These results shed light on the fundamental self-assembly and adsorption behaviour of statistical copolymers at interfaces, and on tuning the boundary lubrication performance of polymers in non-polar media, which is of key importance to future oil-based lubrication strategies.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	University of Bristol
Sponsors	Infineum UK Ltd
Supervisor	Wuge H Briscoe (Supervisor)