Copyright © 2007 John Wiley & Sons, Ltd. Poly(L-lysine)-graft-poly(ethylene glycol): a versatile aqueous lubricant additive for tribosystems involving thermoplastics Seunghwan Lee and Nicholas D. Spencer* ,† Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland ABSTRACT The adsorption and aqueous lubricating behaviour of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) have been investigated for tribopairs involving thermoplastic materials, including polypropylene, polyamide-6,6 and polyethylene. A major finding is that PLL-g-PEG adsorbs onto both hydrophobic, non-polar surfaces and hydrophilic, polar (negatively charged) surfaces from aqueous solution, and thus plays as a very unique and effective aqueous boundary lubricant additive for the sliding contact of thermoplastics against themselves as well as against many hydrophilic, polar materials, including metals (e.g. stainless steel) or ceramics (e.g. zirconia, ZrO 2 ). Copyright © 2007 John Wiley & Sons, Ltd. KEY WORDS: aqueous lubrication; thermoplastics; poly(L-lysine)-graft-poly(ethylene glycol) INTRODUCTION Thermoplastics have long attracted attention from the tribology community as alternative bearing materials to metals due to a few distinctive advantages, such as low cost, low density and high resis- tance to corrosion and oxidation. 1–6 In particular, their resistance to corrosion in aqueous environments allows for the use of water as a lubricant for thermoplatics. 1–6 Some early studies have shown that thermoplastics possessing hygroscopic properties, such as polyamides, exhibit dramatic lubrication effects in the presence of water, especially when they are slid against polar materials, such as glass. 3 Absorption of water and consequent swelling of these materials, however, can also cause unfavourable tribological effects, such as an increased wear rate compared to sliding under dry conditions. 1,2,5 On the other hand, most thermoplastics are not wetted by water due to their extreme surface hydrophobic- ity, 2 and thus are not effectively lubricated by water alone. Previous research on these issues has tended to focus on the improvement of the material properties themselves, e.g. synthesis of composite LUBRICATION SCIENCE Lubrication Science 2008; 20: 21–34 Published online 21 September 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/ls.50 *Correspondence to: Nicholas D. Spencer, Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland. † E-mail: nicholas.spencer@mat.ethz.ch