Differences in Cationic andAnionic Charge DensitiesDictate Zwitterionic Associations and Stimuli Responses.

Zwitterionic materials have showntheir excellent performance inmany biological and chemical applications. Zwitterionic materialspossess moieties that own both cationic and anionic groups. The associationsamong zwitterionic moieties through electrostatic interactions playan important role in properties of zwitterionic materials. However,the relationship between the molecular structures and associationsof zwitterionic moieties are still not well understood. This workcompared thermal- and salt-responsive behaviors of sulfobetaine andcarboxybetaine polymers by examining their rheological propertiesas a function of temperature and their hydrodynamic sizes as a functionof salt concentration. Results showed that carboxybetaine polymersdo not exhibit stimuli responses as expected from the antipolyelectrolytebehavior of zwitterionic polymers as observed in sulfobetaine polymers.We studied and compared the associations among zwitterionic moietiesin these two zwitterionic polymers using molecular dynamic simulations.Simulation results show that the charge-density difference betweencationic and anionic groups determines the associations among zwitterionicmoieties, which are responsible for different stimuli responses ofcarboxybetaine and sulfobetaine polymers. [ABSTRACT FROM AUTHOR]