Lubrication by charged polymers

Abstract

Long-ranged forces between surfaces in a liquid control effects from colloid stability to biolubrication, and can be modified either by steric factors due to flexible polymers, or by surface charge effects. In particular, neutral polymer 'brushes' may lead to a massive reduction in sliding friction between the surfaces to which they are attached, whereas hydrated ions can act as extremely efficient lubricants between sliding charged surfaces. Here we show that brushes of charged polymers (polyelectrolytes) attached to surfaces rubbing across an aqueous medium result in superior lubrication compared to other polymeric surfactants. Effective friction coefficients with polyelectrolyte brushes in water are lower than about 0.0006-0.001 even at low sliding velocities and at pressures of up to several atmospheres (typical of those in living systems). We attribute this to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments. Our findings may have implications for biolubrication effects, which are important in the design of lubricated surfaces in artificial implants, and in understanding frictional processes in biological systems.

Direct Measurement of Normal and Shear Forces between Surface-Grown Polyelectrolyte Layers

“…Additionally, it has been suggested that polymer brushes may play lubricating roles in certain biological contexts where cells bearing surfacegrafted polymers side past one another, such as in articular cartilage 8 and at the ocular surface [9][10][11][12] . Most relevant studies of polymer friction were carried out on neutral polymer brushes 2,[13][14][15][16] , with a few recent exceptions [17][18][19][20] . However, since surface-active biological polymers are typically electrically charged, biological lubrication would be better modeled by studying polyelectrolyte brushes.…”

“…Some previous surface force balance (SFB) studies of polyelectrolyte brushes have used brushes formed from diblock copolymers 17,[24][25][26] . In particular, the friction between polyelectrolyte brushes has been studied using a system of diblock copolymers adsorbed to a hydrophobized mica surface via a hydrophobic block, with a block bearing negatively-charged carboxylate groups forming the brush 17 .…”

Determination of the molar mass of surface-grafted weak polyelectrolyte brushes using force spectroscopy

“…In recent years, polymer brushes have been shown to be a useful class of materials for many medical and biological applications [13,14]. For example, custom synthetic polymers have great potential in drug delivery and molecular recognition [15], and tethering polymer chains onto surfaces can effectively reduce friction [16,17], and control adhesion [18][19][20].…”

Nanoscale Conformation and Compressibility of Cartilage Aggrecan Using Microcontact Printing and Atomic Force Microscopy

“…Using lateral force microscopy, the shear behavior of polymer layers can be also be studied with brush height obtained simultaneously; chemical grafting of the polymer may offer advantages over physical attachment for investigating nanomechanical behavior. 17 …”