Lubrication Mechanism of Concentrated Polymer Brushes in Solvents: Effect of Solvent Quality and Thereby Swelling State

Abstract

The lubrication mechanism of concentrated polymer brushes (CPBs) exhibiting ultralow frictional property was investigated. The frictional force and hence the frictional coefficient μ between CPBs of polystyrene (PS) were measured as a function of shear velocity υ and degree of swelling. The degree of swelling was precisely controlled by varying the composition of solvent , which consisted of a mixture of toluene (good solvent for PS) and 2-propanol (nonsolvent for PS), from the brush highly stretched state (toluene rich) to glassy state (2-propanol rich). The μ data of the mixtures revealed two lubrication mechanisms, i.e., boundary and hydrodynamic lubrication. Boundary lubrication with μ values less dependent on shear velocity was observed both in ultralow (μ on the order of 10–4) and high frictional (μ on the order of 0.1) regimes. On the other hand, hydrodynamic lubrication was well described by the relation μ = β·υ α with α having an almost constant value of ca. 0.7. It was found that parameter β depended on the solvent composition and was scaled by the degree of swelling. It should be noted that the confronted polymer brushes interacted with each other even in this regime. Thus, CPBs in solvents may be employed as an efficient lubricating layer due to their unique features.

Friction Modifier Additives

“…One solution is to grow polymers on surfaces by polymerization from initiators pre-attached the surface, socalled "grafting-from". Very dense polymer films can be formed in this way and these show low friction in both high and low speed conditions [199,200]. Unfortunately this approach does not provide the replenishment process generally required of lubricant additives and is thus a surface treatment rather than being delivered and maintained by the lubricant.…”

Hairy nanoparticle assemblies as one-component functional polymer nanocomposites: opportunities and challenges

“…While Archer and Kim specifically reported HNP systems that were added at up to 45% w/w in a base oil solvent, reports of extremely low friction seen between polymer brushes in a collapsed state suggest that HNPs without a solvent may also be extremely effective as lubricants. [141] Voevodin et al [142,143] were the first to study the lubricating properties of neat HNPs, in particular at the contact surfaces of the switches on microelectromechanical systems (MEMS). Previous attempts to lubricate MEMS switch contacts involved self-assembled monolayers of thiolates, but these were found to be thermally unstable.…”

Synergies in lubrication

“…It is important to note that solvent present in the brush layer is essential for achieving low friction, as concluded from both theoretical considerations 19 and by experiments. [21][22][23] It should also be noted that the osmotic pressure from the counterions present in a polyelectrolyte brush layer counteracts compression and interpenetration, and this makes polyelectrolyte brushes more efficient in reducing friction than comparable brushes of uncharged polymers, as discussed by Zhulina and Rubinstein. 24 Thus, to obtain low friction between polymer-bearing surfaces it is important to reduce the interpenetration zone, which can be achieved by having a high graft density and good solvent conditions.…”