Adsorption properties of poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) at a hydrophobic interface: influence of tribological stress, pH, salt concentration, and polymer molecular weight

The adsorption properties of a graft copolymer of poly(ethylene glycol) (PEG) with a polycationic backbone, namely, poly( l-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG), onto nonpolar, hydrophobic PDMS surfaces from aqueous solution and the lubrication properties of the self-mated sliding contacts of PDMS surfaces modified with PLL- g-PEG have been investigated. Whereas PLL- g-PEG is spontaneously attracted to negatively charged surfaces as a result of the polycationic PLL backbone, the collective interaction of (CH 2) 4 hydrocarbon moieties on the lysine units in the PLL backbone with nonpolar, hydrophobic surfaces also enables the adsorption of PLL- g-PEG onto hydrophobic surfaces such as PDMS. The adsorption and lubrication properties of PLL- g-PEG have been investigated by varying the aqueous solution parameters, such as pH (2, 7, and 12) and KCl concentration (0, 0.01, 0.1, and 1 M) as well as the length of the PLL backbone of the copolymer (20 vs 375 kDa). In the absence of tribological stress, the adsorption of PLL- g-PEG onto PDMS surfaces was mainly governed by the KCl concentration, whereas the role of pH or the molecular weight of the copolymer was of relatively minor importance; for all pH values, the adsorbed mass decreased with increasing KCl concentration. Under tribological stress, however, a clear dependence of the lubrication properties of PLL- g-PEG on all of the studied parameters, including pH, KCl concentration, and backbone molecular weight, was observed. The adsorption strength of PLL- g-PEG on PDMS surfaces, rather than the adsorbed mass itself, appeared to be the most critical parameter in determining the lubrication properties.