Stimuli-responsive ampholytic terpolymers of N-acryloyl-valine, acrylamide, and (3-acrylamidopropyl)trimethylammonium chloride: Synthesis, characterization, and solution properties

Low-charge density ampholytic terpolymers composed of acrylamide (AM), (3-acrylamidopropyl)trimethyl ammonium chloride (APTAC), and N-acryloylvaline were prepared via free-radical polymerization in 0.5 M NaCl to yield terpolymers with random charge distributions. Sodium formate (NaOOCH) was employed as a chain transfer agent during the polymerization to suppress gel effects and broadening of the molecular weight distribution (MWD). Terpolymer compositions were determined by ¹³C NMR spectroscopy. Terpolymer molecular weights (MWs) and polydispersity indices (PDIs) were obtained via size exclusion chromatography/multi-angle laser light scattering (SEC-MALLS). Intrinsic viscosity values determined from SECMALLS data using the Flory-Fox relationship were compared with those determined by low-shear dilute solution viscometry and found to be in good agreement. SECMALLS experiments allowed examination of radius of gyration-MW (R _g-M) relationships and the Mark-Houwink-Sakurada intrinsic viscosity-MW ([η]-M) relationships for terpolymers. The R _g-M and [η]-M relationships indicated little or no excluded volume effects under SEC conditions indicating that the terpolymers were in near theta conditions in an aqueous buffer solution. Potentiometric titration experiments were performed in deionized (DI) water. These studies revealed that the apparent pK _a of the AMVALTAC terpolymers increases with increasing VAL content. The solution properties of low-charge density ampholytic terpolymers have been studied as functions of solution pH, ionic strength, and polymer concentration. The charge-balanced terpolymers exhibit polyampholyte behavior at pH values ≥ 6.5. As solution pH is decreased, these charge-balanced terpolymers become increasingly cationic due to the protonation of the VAL repeat units. Charge-unbalanced terpolymers generally exhibit polyelectrolyte behavior, although the effects of intramolecular electrostatic interactions (e.g., polyampholyte effects) on the hydrodynamic volume are evident at certain values of solution pH and salt concentration. The solution behavior of the terpolymers in the dilute regime correlates well with that predicted by various polyam pholyte solution theories.