| AVS 57th International Symposium & Exhibition | |
| Tribology Focus Topic | Wednesday Sessions |
| Session TR+NS+SS-WeA |
| Session: | Mechanical & Chemical Effects on Friction and Wear |
| Presenter: | Z. Zhang, University of Sheffield, UK |
| Authors: | Z. Zhang, University of Sheffield, UK A.J. Morse, University of Sheffield, UK S.P. Armes, University of Sheffield, UK A.L. Lewis, Biocompatibles UK Ltd., UK G.J. Leggett, University of Sheffield, UK |
| Correspondent: | Click to Email |
The frictional properties of surface grown zwitterionic polymer brushes: poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) have been characterized using friction force microscopy (FFM) in different liquid media.
1. Effect of molecular weight and solvent on the frictional properties
For brushes thicker than 192 nm, the coefficient of friction decreased with increasing brush thickness, while for brush layers with smaller thicknesses, the coefficient of friction varied little with molecular weight. It is suggested that water molecules bound to PMPC chains act as an interfacial lubricant; as brush thickness increases, the amount of bound water increases and the coefficient of friction decreases. This hypothesis is supported by comparative studies of the approaching parts of force-displacement plots acquired for PMPC brush samples with different molecular weights under water. In particular, it was found that thicker brushes exerted a greater repulsive force to the AFM probe. Gold-coated probes were used throughout this part to avoid any complication might be caused by tip surface chemistry. FFM has also been used to investigate the cononsolvency behaviour of PMPC. Friction force was measured for PMPC brushes with a dry thickness of 307 nm while immersed in alcohol/water binary mixtures with different compositions. A distinct increase was observed in the coefficient of friction at an ethanol-water ratio of 90:10, and a 2-propanol-water ratio of 70:30, but not for methanol/water mixtures. This result is attributed to the conformational change of the polymer brush, which induced the loss of hydration layer.
2. Influence of solvent and tip chemistry on the contact mechanics
To study the contact mechanics of tip-sample interactions in FFM, AFM probes were chemically functionalized by deposition of three different types of self-assembled monolayer of dodecanethiol (C11CH3) or mercaptoundecanoic acid (C10COOH), or cysteamine (C2NH2). In alcohol solvents, friction force acquired using acid- or CH3- functionalized tip has a linear relationship with the applied load, but nonlinear for amine-terminated tip. It is also noted that the coefficient of friction is highest in 2-proponal for all three types of probe, which again suggests that conformation of PMPC brush is one of the key factors. In aqueous medium, the friction-load relationships were nonlinear and characterized by the Derjaguin-Muller-Toporov model of contact mechanics. Coefficient of friction measured by amine-functionalized probes were greater than that of acid-functionalized probes, and than CH3- ones, which was attributed to the interaction between polymeric chains and probes.