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 (C-18) 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-C-18-PY, PHEA-g-C-18-IM, and PHEA-g-C-18-BU exhibited phase separations in solutions at pH ranges of 4 similar to 6, 6 similar to 8, and 9 similar to 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.