Chain dimensions and surface characterization of superhydrophilic polymer brushes with zwitterion side groups

Zwitterionic polymers constitute a unique class of polyelectrolytes which have not been studied systematically because of the difficulty in their controlled synthesis and precise physicochemical characterization. The salt-concentration dependence of the chain dimensions and swollen brush structures of polyzwitterions, namely poly(2- methacryloyloxyethylphosphorylcholine) (PMPC) and poly[3-(N-2- methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS), in aqueous solutions of various ionic strengths was characterized by static light scattering, dynamic light scattering, atomic force microscopy (AFM), neutron reflectivity (NR), contact angle measurements, and macroscopic friction tests by sliding a glass ball under a load of 0.49 N. The hydrodynamic radius R _H of PMPC was independent of NaCl concentration, whereas the R _H of PMAPS markedly increased with the ionic strength. AFM and NR measurements also showed the independence of NaCl concentration of the swollen thickness of the PMPC brush in aqueous solution and significant changes in the swollen thickness of the PMAPS brush in aqueous NaCl solution. Both PMPC and PMAPS brushes showed oil detachment behavior in water and aqueous NaCl solutions. The PMPC brush had a significantly low friction coefficient (0.02-005) at a sliding velocity of 10^-2 to 10^-1 m s ^-1 in water even under a high normal pressure of 139 MPa.