Structure of water in the vicinity of a zwitterionic polymer brush as examined by sum frequency generation method.

A zwitterionic poly(carboxymethylbetaine) (PCMB) brush was prepared on a fused quartz prism by the surface-initiated atom transfer radical polymerization (SI-ATRP) of CMB monomer. The conformation of PCMB brush and the state of water at the surface of the brush were examined using sum frequency generation (SFG) technique. The C-H stretching band of the brush, indicating the gauche defect of the brush, was affected by the contact medium such as dry nitrogen, water vapor-saturated nitrogen and liquid water. The water molecules at the PCMB-water interfaces were not largely oriented in comparison with the interfacial water of both bare and the ATRP-initiator-modified quartz prisms. The similar tendency was previously observed for water in the vicinity of water-soluble zwitterionic polymers and polymer thin films using Raman and attenuated total reflection (ATR) infrared spectroscopies, respectively. The electrical neutralization between neighboring positive and negative charges might diminish the electrostatic adsorption of water molecules to the vicinity of zwitterionic polymer brushes.