pH-responsive polyzwitterions: A comparative study of acrylamide-based polyampholyte terpolymers and polybetaine copolymers

A comparative study of pH-responsive polyzwitterions (PZs) with polyampholyte or polybetaine architectures was conducted with well-defined model polymer systems. Low-charge-density PZs, including ampholytic terpolymers composed of acrylamide (AM), sodium 3-acrylamido-3-methylbutanoate, and (3-acrylamidopropyl)trimethylammonium chloride and carboxybetaine copolymers composed of AM and 3-(3-acrylamidopropyldimethyammonio)propionate, were prepared via free-radical polymerization in 0.5M NaCl to yield ter- and copolymers with random termonomer and comonomer distributions. Sodium formate was used as a chain-transfer agent during the polymerizations to eliminate the effects of the monomer feed composition on the degree of polymerization (DP) and to suppress gel effects and broadening of the molecular weight distributions. The polymer compositions were determined via C-13-NMR spectroscopy, and the residual counterion content was determined via elemental analysis for Na+ and Cl-. The molecular weights (MWs) and polydispersity indices (PDIs) were determined via size exclusion chromatography/multi-angle laser light scattering (SEC-MALLS); the polymer MWs ranged from 1.4 to 1.5 X 10(6) g/mol, corresponding to DPs of 1.6-1.9 X 10(4) repeat units, with all the polymers exhibiting PDIs less than or equal to 2.1. The intrinsic viscosities determined from SEC-MALLS data and the Flory-Fox relationship agreed with the intrinsic viscosities determined via low-shear dilute-solution viscometry. Data from the SEC-MALLS analysis were used to analyze the radius of gyration/molecular weight (R-g-M) relationships and the Mark-Houwink-Sakurada intrinsic viscosity/molecular weight ([eta]-M) relationships for the PZs. The Rg-M and [eta]-M relationships and viscometric data revealed that under size exclusion chromatography conditions, the poly[acrylamide-co-3-(3-acrylamidopropyldimethylammonio)propionate] betaine copolymers had more open, random-coil conformations and greater polymer-solvent interactions than the ampholytic poly[acrylamide-co-sodium 3-acrylamido-3-methylbutanoate-co-(3-acrylamidopropyl)- trimethylammonium chloride] terpolymers. The pH- and salt-responsive dilute-solution viscosity behavior of the PZs was examined to assess the effects of the polymer structure and composition on the solution properties. The polyampholyte terpolymers had greater solution viscosities and more pronounced stimuli-responsiveness than the polybetaine copolymers because of their stronger intramolecular interactions and increased chain stiffness. (C) 2004 Wiley Periodicals, Inc.