Polymer Brush Based Tribology

Abstract

Polymer chains with one of their ends grafted on a surface, stretch out in a good solvent to take a brush-like formation when the grafting density, \(\rho_{g} > 1/\left( {\pi R_{g}^{2} } \right)\), where Rg is the radius of gyration of a chain in a good solvent. The equilibrium height of such a polymer brush is larger than the unperturbed size (Rg) of the corresponding polymer chain in a bulk solution. Polymer brushes find applications in the fields of tribology, rheology, biology and colloid-stabilization. Polymer brush based tribology is a recent attempt to mimic glycoproteins based lubrication found in nature. Higher coefficients of friction are observed due to as perity contact when hard surfaces are brought in contact and sheared against each-other. In contrast, when polymer-brush bearing surfaces are brought in contact with each-other and sheared in the presence of a good-solvent, much lower coefficients of friction are observed. Due to entropic reasons opposing polymer brushes avoid inter-digitation even under high compression enabling development of a thin fluid film between the brushes. Such a formation helps polymer brushes to support relatively high applied normal load while the thin fluid film in-between helps in reducing the friction. Tribological behavior of polymer brushes can be tuned by changing the grafting-density (ρg), chain-length (Lc), chain-stiffness (Kb), solvent-quality and cross-linking of chains. Possibility of designing lubricant with specific tribological properties make polymer-brushes an interesting topic of research. In this chapter we will go through the effects of different polymer-brush architectures on the tribological behavior of polymer brushes.