Synthesis of pH-responsive polyimide hydrogel from bioderived amino acid

A series of biobased polyimides bearing a structure derived from a predetermined tetracarboxylic dianhydride was synthesized. By ionizing the COOH group of the side chain with potassium hydroxide, four kinds of polyimides were solubilized in water, and the water-soluble polyimides were cast onto films over an aqueous solution, leading to higher optical transparency than that of non-water-soluble polyimides. 1H nuclear magnetic resonance measurements of the polyimides revealed no residual reactants from the polymerization process or side-chain modification. Partial crosslinking of the water-soluble polyimide chains by condensation of the carboxylate side chain with an amino acid-based diamine such as 4-aminophenylalanine or 4,4′-diamino-α-truxillic acid induced the formation of polyimide hydrogels. The remaining COOK groups of the obtained hydrogel were protonated/deprotonated by changing the pH, accompanied by reversible shrinking and swelling.