Hydrophobically modified acrylamide‐based polybetaines. I. Synthesis, characterization, and stimuli‐responsive solution behavior

Acrylamide‐based, hydrophobically modified (HM) polybetaines containing N‐butylphenylacrylamide (BPAM) and varying amounts of the sulfobetaine monomer 3‐(2‐acrylamido‐2‐methylpropanedimethylammonio)‐1‐propanesulfonate (AMPDAPS) or the carboxybetaine monomer 4‐(2‐acrylamido‐2‐methylpropyldimethylammonio)butanoate (AMPDAB) were synthesized by micellar copolymerization. The corresponding control (co)polymers lacking BPAM or betaine comonomers were also prepared. The terpolymers were characterized by 13C‐NMR and UV spectroscopy, classical light scattering, and potentiometric titration. Low charge density polymers contained 3.9–8.6 mol % betaine, whereas the high charge density systems contained 17–25 mol % betaine; the HM polymers contained up to 1.0 mol % BPAM as the hydrophobe. The weight‐average molecular weights of the polymers ranged from 4.19 × 105 to 1.29 × 106 g/mol, and most HM polymers exhibited negative second virial coefficients. The pKa of the carboxybetaine moieties was found to increase with increasing levels of hydrophobic and betaine comonomer incorporation. The response of aqueous polymer solutions to various external stimuli, including changes in solution pH and electrolyte concentration, was investigated using rheological analysis. The solution behavior of the polymers was characteristic of HM polyacrylamides and acrylamide‐based polyzwitterions. The high charge density HM polycarboxybetaine exhibited unusual solution behavior that can be explained in terms of electrostatic, hydrophobic, and hydrogen‐bonding associations.