A delicate ionizable-group effect on self-assembly and thermogelling of amphiphilic block copolymers in water

A carboxyl-capped PLGA-PEG-PLGA block copolymer (PLGA: poly(d,l-lactic acid-co-glycolic acid), PEG: poly(ethylene glycol)) was synthesized, and its self-assembly and thermogelling behaviors in water were studied at different pH values. While the aqueous solution of the virgin PLGA-PEG-PLGA block copolymer with the composition in this paper did not undergo sol-gel transition, that of the end-capped derivative exhibited four macroscopic states, sol, turbid sol, physical gel, and precipitate, dependent upon temperature and pH. Especially between pH 4.5 and 5.0, a concentrated aqueous system underwent sol-gel-(turbid sol)-precipitate transitions upon increase of temperature. Complex of dissociation constant pK_a of weak-acid groups on the micelle surfaces was illustrated. Dynamic light scattering, hydrophobic dye solubilization, transmission electron microscopy, NMR, and potentiometric titration were used to examine the relationship between ionization of the end groups of the polymer chains on the molecular level, micellization of the amphiphilic block copolymers on the supermolecular level, and physical gelation on the macroscopic level. The present research reveals the complex of the multi-scale self-assembly of amphiphilic polymer ionomers in a selective solvent.