Synthesis and solution behavior ofpH- and electrolyte-responsive polyzwitterions and polyelectrolytes in aqueous media
Zwitterionically modified polymeric systems incorporating carboxybetaine and/or sulfobetaine-type monomers were synthesized and the solution behavior of these systems thoroughly investigated. Three issues are addressed in order to assess the structure/property relationships of zwitterionic polyampholytes in regard to solubility and rheological behavior. The first issue investigated is the effect of varied numbers of methylene units between the anionic and cationic moieties of zwitterionic monomers on the solution behavior of the resulting polymers. Three carboxybetaine-type monomers have been synthesized in which the number of methylene units between charged centers is one, three, and five, respectively: 2-(2-acrylamido-2-methylpropyldimethylammonio)ethanoate, (AMPDAE); 4-(2-acrylamido-2-methylpropyldimethylammonio)butanoate, (AMPDAB); and 6-(2-acrylamido-2-methylpropyldimethylammonio)hexanoate, (AMPDAH). The carboxybetaine monomers were copolymerized with acrylamide in varying molar ratios by free radical polymerization in an aqueous medium. Solubility of the copolymers is dependent on both the amount of the carboxybetaine monomer incorporated and the number of methylene units between the charged centers. Viscometric studies were conducted as a function of polymer composition, concentration, added electrolytes, and pH. At high pH, the polyzwitterions show an enhancement in viscosity as the concentration of added electrolyte is increased. The copolymers also exhibit an enhancement in viscosity in deionized water as the pH is lowered due to protonation of the carboxylate groups which renders the polymer coil cationic. The second issue addressed is the effect of the nature of the anionic moiety of zwitterionic polymers on the solution behavior in aqueous media. Copolymers of 3-(2-acrylamido-2-methylpropyldimethylammonio)propane sulfonate (AMPDAPS) and AMPDAB, as well as the two homopolymers, were synthesized and characterized. The solubility and viscosity behavior of this series is dependent on polymer composition, concentration, pH, and nature of added electrolytes. Higher incorporation of AMPDAPS leads to lower solubility in aqueous media. The third issue examined is the effect of incorporating sodium acrylate (NaA) mer units into zwitterionically modified polymers based on AMPDAPS or AMPDAB. At high pH, these systems behave as polyelectrolytes because of the excess anionic charge provided by NaA. At low pH, NaA is neutralized, and the solution behavior of the polymers is governed by the nature of the zwitterionic mer unit. The systems based on AMPDAPS display polyampholytic behavior, while those based on AMPDAB display polycationic behavior. The synthesis, structural characterization, and solution behavior of water soluble copolymers of N-vinylformamide (NVF) with various anionic comonomers has been conducted. Reactivity ratios were determined and indicate that NVF forms highly alternating copolymers with the selected comonomers. The viscosity and phase behavior of the resulting copolymers are dependent on pH and electrolyte concentration of the aqueous medium.