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 it 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,)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 1M) 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.