Relation between molecular structure of gels and friction properties under low load in water

Lubrication properties of hydrogels have been investigated using polyvinyl alcohol(PVA), polyvinyl alcohol methyl ether(PVME), polyacrylamide(PAAm) and N-isopropyl polyacrylamide(NiPPAm) in pure water under low loads. PVA showed good lubricity with low friction coefficient (about 0.1) at 5 to 20 mN of load. On the other hand, PVME had poor lubricity (friction coefficient was around 1). Similar results were obtained with PAAm and NiPAAm, that is, the former showed low friction coefficient but the latter showed higher friction coefficient. These results indicate that hydrophilic groups such as alcoholic OH in PVA and amide group in PAAm play an important role at the gel-slider interface for lubrication of hydrogels. The friction coefficient of PAAm gels decreased with keeping time in air. The friction coefficient of PAAm decreased by one order by incorporation of only one mole percent of acrylic acid to PAAm by copolymerization. From these results, carboxylic group, which is a hydrolysis product of amide, may act as a hydrophilic group in the gel. When 20 mole percent of acrylic acid was added to PAAm, friction coefficient increased to about one. At higher concentration of acrylic acid, amide groups can be coupled with carboxylic groups by an intramolecular hydrogen bonding, and then the copolymer surface became to be hydrophobic. It can be concluded from the results that water acts as a lubricant at slider-gel interface by interaction with hydrophilic groups on gel surfaces.