Friction and wear of polyzwitterionic brush-grafted surfaces*

Inspired by the hydrophilic biomacromolecules adsorbed to the surface of articular cartilage, we investigate the tribology of betainized poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes grafted on soft poly(dimethyl siloxane) (PDMS) and hard silicon substrates. These polyzwitterions are known to provide excellent lubricity due to their ability to retain water in their side groups. Surfaces are hydrolyzed to introduce surface silanol groups which serve as an attachment point for a trichlorosilane initiator. After initiator deposition, PDMAEMA is grown from the surface via atom transfer radical polymerization (ATRP) and then betainized to render it zwitterionic. We measure the steady state and transient friction coefficients of the substrates as a function of brush film thickness and substrate elasticity. Our initial results suggest that use of soft, deformable substrates such as PDMS yields increased wear resistance for brush films due to a reduction in the contact pressure below the yield point of the polymer brushes. Furthermore, repetitive transient contact appears to result in increased wear resistance, with the total shearing time required for noticeable wear to occur increasing by an order of magnitude.