Diblock bottlebrush polymer in a non-polar medium: Self-assembly, surface forces, and superlubricity

Michael C Stevens; Nicholas M Taylor; Xueying Guo; Hadeel Hussain; Najet Mahmoudi; Beatrice N Cattoz; Alice H M Leung; Peter J Dowding; Brian Vincent; Wuge H Briscoe

doi:10.1016/j.jcis.2023.12.056

Diblock bottlebrush polymer in a non-polar medium: Self-assembly, surface forces, and superlubricity

J Colloid Interface Sci. 2024 Mar 15:658:639-647. doi: 10.1016/j.jcis.2023.12.056. Epub 2023 Dec 11.

Authors

Affiliations

¹ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
² School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK; Syngenta, Jealott's Hill International Research Centre, Bracknell RG42 6EY, UK.
³ Diamond Light Source Ltd, I07 Beamline, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK.
⁴ ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, UK.
⁵ Infineum UK Ltd, Milton Hill Business and Technology Centre, Abingdon, Oxon OX13 6BB, UK.
⁶ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. Electronic address: wuge.briscoe@bristol.ac.uk.

PMID: 38134672
DOI: 10.1016/j.jcis.2023.12.056

Abstract

Whilst bottlebrush polymers have been studied in aqueous media for their conjectured role in biolubrication, surface forces and friction mediated by bottlebrush polymers in non-polar media have not been previously reported. Here, small-angle neutron scattering (SANS) showed that a diblock bottlebrush copolymer (oligoethyleneglycol acrylate/ethylhexyl acrylate; OEGA/EHA) formed spherical core-shell aggregates in n-dodecane (a model oil) in the polymer concentration range 0.1-2.0 wt%, with a radius of gyration R_g ∼ 7 nm, comprising 40-65 polymer molecules per aggregate. The surface force apparatus (SFA) measurements revealed purely repulsive forces between surfaces bearing inhomogeneous polymer layers of thickness L ∼ 13-23 nm, attributed to adsorption of a mixture of polymer chains and surface-deformed micelles. Despite the surface inhomogeneity, the polymer layers could mediate effective lubrication, demonstrating superlubricity with the friction coefficient as low as µ ∼ 0.003. The analysis of velocity-dependence of friction using the Eyring model shed light on the mechanism of the frictional process. That is, the friction mediation was consistent with the presence of nanoscopic surface aggregates, with possible contributions from a gel-like network formed by the polymer chains on the surface. These unprecedented results, correlating self-assembled polymer micelle structure with the surface forces and friction the polymer layers mediate, highlight the potential of polymers with the diblock bottlebrush architecture widespread in biological living systems, in tailoring desired surface interactions in non-polar media.

Keywords: Diblock bottlebrush copolymers; Friction; Lubrication; Polymer lubricant additives; Polymer self-assembly in non-polar media; Small-angle neutron scattering (SANS); Surface forces; X-ray reflectivity (XRR).