Improvement of surface lubricity of polymers and metals by a glow-discharge plasma cross-linking process

A plasma cross-linking process was employed to improve the surface lubricity of different types of biomaterials, including stainless steel (SS), nitinol, polyethylene and nylon. To investigate the influence of monomers containing double bonds on top-layer cross-linking of poly(ethylene oxide) compound (PEOC), five different monomers, N-trimethylsilyl-allylamine (TMSAA), ethylene, propylene, allyl alcohol and ethane, were used in the study to produce a cross-linked coating layer on sample surfaces. Before the plasma cross-linking, samples underwent plasma treatment followed by wet chemical coating. The plasma treatment consists of plasma etching in NH(3)/O(2), Tetramethylcyclo-tetrasiloxane (TMCTS) coating and TMSAA grafting. The wet coating process includes dip-coating in a solution of poly(oxyethylene)-compound bis(1-hydroxy-benzotriazolyl carbonate) (HPEOC), then dip-coating in a solution of PEOC. By application of plasma processing, HPEOC and PEOC wet coating to sample surfaces, the lubricity was increased by 83% compared to clean samples. The plasmas of TMSAA, ethylene, propylene and allyl alcohol, all containing a C=C double bond, produced a cross-linking layer on the PEOC surface. Consequently the surface lubricity was improved by 20% to 37% in comparison to no cross-linking. The favorable condition for plasma cross-linking was found to be high power and long time. Ethane plasma also reduced the pulling force although it has no double bond in the molecular structure, which indicated a thin plasma coating from saturated hydrocarbons deposited on HPEOC or PEOC surfaces could also cause cross-linking and improve lubricity. It was found that the TMSAA cross-linking also worked on HPEOC and HEPOC/PEOC, even though the prior plasma coating process was skipped.