Solvent-induced immiscibility of polymer brushes eliminates dissipation channels

Abstract

Polymer brushes lead to small friction and wear and thus hold great potential for industrial applications. However, interdigitation of opposing brushes makes them prone to damage. Here we report molecular dynamics simulations revealing that immiscible brush systems can form slick interfaces, in which interdigitation is eliminated and dissipation strongly reduced. We test our findings with friction force microscopy experiments on hydrophilic and hydrophobic brush systems in both symmetric and asymmetric setups. In the symmetric setup both brushes are chemically alike, while the asymmetric system consists of two different brushes that each prefer their own solvent. The trends observed in the experimentally measured force traces and the friction reduction are similar to the simulations and extend to fully immersed contacts. These results reveal that two immiscible brush systems in mechanical contact slide at a fluid-fluid interface while having load-bearing ability. This makes them ideal candidates for tribological applications.

Shearing Nanometer-Thick Confined Hydrocarbon Films: Friction and Adhesion

“…For the shortest chain C20 strong oscillations occur in the pull-off force for v z = 0.1 m/s, and the wall-wall interaction prevail to larger surface separations than for v z = 10 m/s. This is due to the formation of a capillary bridge at the lower pull-off speed, and to (approximate) layering of the molecules in the capillary bridge [27]. Thus during pull-off the capillary bridge extends vertically by shrinking in width and adding more layers in the vertical direction [28].…”

On the friction and adhesion hysteresis between polymer brushes attached to curved surfaces: Rate and solvation effects

“…They are employed, for example to stabilize colloidal suspensions [3], in oil recovery [4], for protein analysis [5], as anti-fouling coatings [6,7] and as "smart" responsive systems [8], such as drug-delivery systems [9], nano sensors [10,11] and "pick-up and place" systems [12]. Especially promising is the utilisation of polymer brushes in a biomimetic approach as low-friction surface coatings [13][14][15][16][17][18][19][20], e.g., in artificial joints [21] or industrial applications [22].…”

“…In the last years, several methods have been developed to reduce [47] or even prevent [15] interdigitation of the polymers in opposing brushes. In one method [47], a modulated electric field is applied to tune the degree of overlap between polyelectrolyte brushes.…”

“…In one method [47], a modulated electric field is applied to tune the degree of overlap between polyelectrolyte brushes. In another method [15], the opposing brushes are chemically distinct such that each brush has its own preferred solvent, e.g., one hydrophilic and one hydrophobic brush, which are immiscible and thus do not interpenetrate. There is only a thin effective overlap zone due to long-wavelength thermal fluctuations of the interface [15].…”

“…In another method [15], the opposing brushes are chemically distinct such that each brush has its own preferred solvent, e.g., one hydrophilic and one hydrophobic brush, which are immiscible and thus do not interpenetrate. There is only a thin effective overlap zone due to long-wavelength thermal fluctuations of the interface [15]. Consequently, the friction in these immiscible systems can be more than two orders of magnitude lower than the friction for traditional, miscible systems [15].…”