Effect of crosslinking on the microtribological behavior of model polymer brushes

Polymer brushes in good solvents are known to exhibit excellent tribological properties. We have modeled polymer brushes and their gels using a multibead-spring model and studied their tribological behavior via nonequilibrium molecular-dynamics (MD) simulations. Simulations of brush- against-wall systems were performed using an implicit solvent-based approach. Polymer chains were modeled as linear chains, randomly grafted on a planar surface. Quantities extracted from the simulations are the normal stress, shear stress and concentration profiles. We find that while an increase in the degree of crosslinking leads to an increase in the coefficient of friction, an increase of the length of crosslinker chains does the opposite. Effect of crosslinking can be understood in two ways: (i) there are fewer polymer chains in the outer layer as the degree of crosslinking increases to take part in brush-assisted lubrication, and (ii) crosslinked polymer chains are more resistant to shear than non-crosslinked ones

Study on the tribological properties of pHEMA hydrogels for use in artificial articular cartilage

The tribological properties of synthetic hydrogels based on poly (2-hydroxyethyl methacrylate) (pHEMA) were studied in a pin-on-disc equipment using stainless steel 316 L as disc counterface lubricated by distilled water. This work establishes the correlation between the crosslinking density, chemical changes and tribological properties of the pHEMA/poly (methyl methacrylate-co-acrylic acid) (75:25) blend using 10% (w/w) crosslinking agent and pHEMA/n-vinyl pirrolidone (10% (w/w)) blend with 0, 5% and 10% (w/w) of crosslinking agent. The tribological parameters investigated were the sliding speed (0.16 <= v <= 0.5 ms(-1)) and the contact pressure (2.4 <= p <= 5.5 MPa). The friction coefficient was continuously evaluated during each test and the wear rate was quantified by weight loss. The results showed that the hydrogel crosslink density and hydration are important factors to determine the wear behavior of hydrogels. The friction coefficient decreased with the increasing of the sliding speed from 0.16 to 0.50 ms(-1) (0.01 <= m <= 0.09 for v = 0.16 ms(-1) and 0.01 <= m <= 0.06 for v = 0.50 ms(-1)). The wear rate ranged from approximate to 10(-6) to 10(-5) gm(-1), depending on the interactions between crosslinking density of hydrogels, contact pressure and sliding speed. The dominant wear mechanisms were identified by Scanning Electron Microscopy. The most compliant hydrogels (0% (w/w) of crosslinking agent) presented adhesive wear as the main wear mechanism. As the crosslinking density of hydrogels increased, the capacity of absorption of water was reduced and the dominant wear mechanism became abrasion. (C) 2007 Elsevier B.V. All rights reserved.2654173226927