Reinforcement of a Surfactant Boundary Lubricant Film by a Hydrophilic−Hydrophilic Diblock Copolymer

Abstract

The coadsorption from aqueous solutions of an anionic-neutral hydrophilic-hydrophilic diblock copolymer onto a mica-suported surfactant bilayer of a cationic oligomeric surfactant has been investigated. By using an atomic force microscope and a surface forces apparatus nanotribometer, we studied the resulting film morphology, the interactions between two coated surfaces, and the frictional properties of the boundary film. When the coated surfaces were compressed while being fully immersed in an aqueous surfactant solution, the hemifusion of the adsorbed surfactant bilayers could be easily induced. Noticeable friction forces could then be measured between the monolayer-coated surfaces. Coadsorbing poly(acrylic acid)-poly(acrylamide) diblock copolymer with the cationic surfactant changes the cohesion of the adsorbed layers. When the copolymer concentration is sufficiently high, the hemifusion instability of the adsorbed layers can be inhibited, considerably improving its lubricant properties.

Synergies in lubrication

“…Drummond and co-workers have explored this in some detail, using surfactants that adsorb onto the substrate surface mixed with a diblock copolymer to modify the lubrication performance. 44,48,79,80 In their initial studies, using a double-chained surfactant which in itself provided low friction and high load bearing capacity, it was found that coadsorption of a hydrophobically end-modified poly(ethylene oxide) compromised the lubrication performance. 48 In later studies oligomeric cationic surfactants were utilized, and the layers formed by these surfactants alone had relatively low load bearing capacity.…”

“…However, co-adsorption of a poly(acrylic acid)-poly(acrylamide) diblock copolymer significantly enhanced the cohesion of the adsorbed layer and thus increased the load bearing capacity. 79 It was also noted that there was an optimal copolymer concentration range that should not be exceeded in order to retain the excellent lubricity. 79,80 The structure of the surfactant played an important role in the enhancement of the load bearing capacity achieved by addition of the diblock copolymer.…”

“…79 It was also noted that there was an optimal copolymer concentration range that should not be exceeded in order to retain the excellent lubricity. 79,80 The structure of the surfactant played an important role in the enhancement of the load bearing capacity achieved by addition of the diblock copolymer. It was significantly enhanced by the poly(acrylic acid)-poly(acrylamide) diblock copolymer for the cationic trimeric and dimeric surfactants with C12 hydrocarbon chains and C3 spacers, whereas layers composed of the monomeric surfactant cetylammonium chloride and the co-polymer did not display a high load bearing capacity.…”

Effect of Surfactant Oligomerization Degree on Lubricant Properties of Mixed Surfactant-Diblock Copolymer Films

“…We have shown that the cohesion of the mix layer and its lubricant properties are dramatically improved by this coadsorption, probably because the PAA polyion interacts with several surfactant molecules creating a physical network on the substrate. We observed that the mix layers can be conditioned under shear to achieve a state of very good lubrication properties, after the incidence of one or several delamination transitions [12]. However, at high copolymer concentrations, we detected signs of mechanical weakness: the system presented a transition from a low friction force steady state to a high friction force steady state associated with the reduction of the thickness of the boundary layers [13].…”

“…In previous studies, we have reported that this copolymer coadsorbs into self-assembled layers of several cationic surfactants [12,13]. We have shown that the cohesion of the mix layer and its lubricant properties are dramatically improved by this coadsorption, probably because the PAA polyion interacts with several surfactant molecules creating a physical network on the substrate.…”