“Friction and Lubrication of Polymer Surfaces and Polyelectrolytes”
Nianhuan Chen
Doctoral Candidate
Chemical Engineering UCSB
Date:
Tuesday, June 15, 2004
Time: 4:00 p.m.
Place: Engineering II, Room 3361
ABSTRACT
Friction forces between two polystyrene (PS) films and between two polyvinylbenzyl chloride (PVBC) films were measured using a Surface Forces Apparatus (SFA) and are discussed in connection with the adhesion hysteresis between them. Friction forces as a function of applied load, sliding speed, and stop-start measurements were carried out at various surface conditions. The results show that (1) the friction force is correlated with the adhesion hysteresis, (2) both are determined mainly by the state of the polymer chains at or very near the surfaces, (3) both can be manipulated by adjusting the degree of crosslinking and the density of free ends at the surfaces, and (4) a progressive rearrangement of the molecule segments at the shearing interface occurs during continuous sliding over large distances. A fairly consistent and rational picture emerges in which polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.
Synovial fluid is believed to contain the molecules responsible for nearly frictionless lubrication of the joints. As a major component of synovial fluid, Hyaluronic acid (HA) is believed to play a role in joint lubrication. The normal and shear forces between surfaces containing free (non-absorbed), physically absorbed and chemically bound HA were measured using SFA. The results show that free and physisorbed HA fail as a lubricant when surfaces are pressed together and sheared due to the squeezing out of HA from between the surfaces. Covalently attached HA offers much better protection than physisorbed HA, but it too is pushed out as the surfaces are pressed together and sheared. On the other hand, HA layer can remain between the surfaces after crosslinking, and provides very good wear protection. Though HA does not appear to be responsible for all aspects of joint lubrication, it could well contribute to the protection of articular cartilage surfaces. This might be essential for the proper functioning of the joints when HA acts together with other bio-molecules.