Tunable phase transition behaviors of pH-sensitive polyaspartamides having various cationic pendant groups

New pH-sensitive graft copolymers based on poly(2-hydroxyethyl aspartamide) (PHEA) were prepared by attaching various cationic monomers, such as 4-(aminomethyl)pyridine (PY), 1-(3-aminopropyl)imidazole (IM), and N-(3-aminopropyl)dibuthylamine (BU), as pH-sensitive units and octadecylamine (C₁₈) as a hydrophobic segment on poly(succinimide). Phase transition of each copolymer solution occurred at a vicinity of the pK a value of the cationic groups, and their insoluble pH ranges were broadened as the feed amount of pH-sensitive moieties was increased. Depending on the cationic grafts having different pK a values, the pH ranges where the copolymer became insoluble could be tuned. Copolymers PHEA-g-C₁₈-PY, PHEA-g-C₁₈-IM, and PHEA-g-C₁₈-BU exhibited phase separations in solutions at pH ranges of 4~6, 6~8, and 9~12, respectively. These polymers have the unique feature of their pH sensitivity profiles being identified to three regimes. Under low pH conditions (below pK a ), the polymer solution is transparent. At medium pH (around pK a ), polymer precipitation occurred in solution. At pH > pK a , the polymer solution is gradually dissolved again.