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.