Model polyzwitterions based on polyacrylamide: Synthesis, characterization, and stimuli-responsive solution behavior in aqueous media
The fundamental structure-property-performance relationships of stimuli-responsive polyzwitterions (PZs) synthesized from all acrylamido monomers were studied using well-defined model systems. To enable meaningful comparisons between PZs of varying compositions, it was necessary to prepare model systems with non-composition-dependent degrees of polymerization ( DP ). A method of synthesizing polyacrylamide homopolymers with carefully controlled molecular weights (MWs) and MW distributions (M)WDs) using sodium formate as a chain transfer agent was developed and validated via size exclusion chromatography/multi-angle laser light scattering (SEC-MALLS) and low shear dilute solution viscometry. This method was subsequently used to control the DP s and MWDs of the model PZs synthesized for this study. Low charge density PZs, including ampholytic terpolymers composed of acrylamide, sodium 3-acrylamido-3-methylbutanoate, and (3-acrylamidopropyl)trimethylammonium chloride and carboxybetaine copolymers composed of acrylamide and 3-[(3-acrylamidopropyl)dimethylammonio]propionate, were prepared via free radical polymerization in 0.5 M NaCl to yield polymers with random ter- and comonomer distributions. Polymer compositions were obtained via 13 C NMR spectroscopy and residual counterion content was determined via elemental analysis for Na+ and Cl- . MWs and MWDs were determined via SEC-MALLS. Intrinsic viscosities determined from SEC-MALLS data and the Flory-Fox relationship were found to agree with intrinsic viscosities determined via low shear dilute solution viscometry. Data from the SEC-MALLS analysis was used to analyze the radius of gyration-MW ( Rg -M ) relationships and the Mark-Houwink-Sakurada (MHS) intrinsic viscosity-MW ([η]- M ) relationships for the model PZs. The Rg -M and [η]-M relationships and viscometric data revealed that under SEC conditions, the polybetaine copolymers exhibit more open, random coil conformations and greater polymer-solvent interactions than the polyampholyte terpolymers. The pH- and salt-responsive dilute solution viscosity behavior of the PZs was examined to assess the effects of polymer structure and composition on solution properties. At low values of solution pH, the model PZs behaved as cationic polyelectrolytes due to protonation of the carboxylate groups in each system; PZ behavior was elicited by raising the solution pH. The polyampholyte terpolymers were found to exhibit greater solution viscosities and more pronounced stimuli-responsiveness than the polybetaine copolymers due to stronger intramolecular interactions and increased chain stiffness. The dilute solution behavior of the polyampholyte terpolymers was found to correlate well with the behavior predicted by several polyampholyte solution theories.