Degradable and Thermosensitive Microgels with Tannic Acid as the Sole Cross-Linker

Poly(N-isopropylacrylamide) (PNIPAM)-tannic acid (TA) microgels were successfully prepared via surfactant-free emulsion polymerization (SFEP) at 70 °C in aqueous solution using N-isopropylacrylamide (NIPAM) as the monomer and a natural polyphenol macromolecule, TA, as the sole cross-linker. The cross-linking network of the PNIPAM-TA microgels was confirmed to contain both physical cross-linking structures formed via hydrogen-bonding interactions between TA and PNIPAM chains and chemical cross-linking structures formed via capturing the radicals of propagating polymer chains by catechol and pyrogallol groups of TA. Furthermore, TA was applied to modify the surface of hydrophobic Fe₃O₄ nanoparticles, leading to hydrophilic Fe₃O₄@TA composite nanoparticles, which were successfully used as the cross-linker to fabricate PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels. The obtained PNIPAM-TA and PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels had a uniform spherical shape with a relatively narrow size distribution and exhibited thermosensitive behavior and pH-tunable degradation. The PNIPAM-TA microgels were stable in the pH range of 1.3-11.1 but underwent complete degradation with pH above 11.4. The PNIPAM-Fe₃O₄@TA hybrid microgels were partially degraded at pH values of 1.3 and 2.1, stable in the pH range of 3.1-11.1, and underwent complete degradation at pH above 11.4. The partial degradation of PNIPAM-Fe₃O₄@TA organic-inorganic hybrid microgels under strong acidic conditions was attributed to the disintegration of Fe₃O₄ nanoparticles. The complete degradation of both microgels at pH above 11.4 was attributed to the hydrolysis of ester groups of TA under strong alkali conditions.