Surface forces and friction between Langmuir-Blodgett polymer layers in a nonpolar solvent

Nicholas M Taylor; Georgia A Pilkington; Tim Snow; Peter J Dowding; Beatrice N Cattoz; Andrew D Schwarz; Oier Bikondoa; Brian Vincent; Wuge H Briscoe

doi:10.1016/j.jcis.2023.09.146

Surface forces and friction between Langmuir-Blodgett polymer layers in a nonpolar solvent

J Colloid Interface Sci. 2024 Jan;653(Pt B):1432-1443. doi: 10.1016/j.jcis.2023.09.146. Epub 2023 Sep 26.

Authors

Nicholas M Taylor¹, Georgia A Pilkington¹, Tim Snow¹, Peter J Dowding², Beatrice N Cattoz², Andrew D Schwarz², Oier Bikondoa³, Brian Vincent¹, Wuge H Briscoe⁴

Affiliations

¹ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
² Infineum UK Ltd, Milton Hill Business and Technology Centre, Abingdon, Oxon OX13 6BB, UK.
³ XMaS, The UK CRG Beamline, European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, 38043 Grenoble, France; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
⁴ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. Electronic address: wuge.briscoe@bristol.ac.uk.

PMID: 37804612
DOI: 10.1016/j.jcis.2023.09.146

Abstract

Optimization of boundary lubrication by tuning the confined molecular structures formed by surface-active additives such as surfactants and polymers is of key importance to improving energy efficiency in mechanical processes. Here, using the surface forces apparatus (SFA), we have directly measured the normal and shear forces between surface layers of a functionalised olefin copolymer (FOCP) in n-dodecane, deposited onto mica using the Langmuir-Blodgett (LB) technique. The FOCP has an olefin backbone decorated with a statistical distribution of polar-aromatic groups, with a structure that we term as "centipede". The effect of lateral confinement, characterised by the surface pressure, Π_dep, at the air-water interface at which the LB films are transferred, was examined. Normal force profiles revealed that the thickness of the LB films increased significantly with Π_dep, with the film thickness (t > 20 nm) inferring a multi-layered film structure, consistent with the interfacial characterisation results from synchrotron X-ray reflectivity (XRR) measurements. The coefficient of friction, µ, between the LB films spanned two orders of magnitude from superlubricity (µ ∼ 0.002) to much higher friction (µ > 0.1) depending nonlinearly on Π_dep, with the lowest friction observed at the intermediate Π_dep. Molecular arrangement upon LB compression leads to the multilayer film with a structure akin to an interfacial gel, with transient crosslinking facilitated by the intra- and inter-molecular interactions between the functional groups. We attribute the differences in frictional behaviour to the different prevalence of the FOCP functional groups at the lubricating interface, which depends sensitively on the degree of compression at the air-water interface prior to the LB deposition. The LB films remain intact after repeated compression (up to pressures of 10 MPa) and shear cycles, indicating strong surface anchorage and structural robustness as a load-bearing and shear-mediating boundary layer. These unprecedented results from the friction measurements between LB films of a statistical copolymer in oil point towards new strategies for tailoring macromolecular architecture for mediating efficient energy dissipation in oil-based tribological applications.