Tannic Acid Mediated Suppression of PNIPAAm Microgels Thermoresponsive Behavior

Tannic acid (TA) complexation with poly(N-isopropylacrylamide) (PNIPAAm) microgels changed their morphology and temperature responsiveness, depending on TA concentration and pH (below the TA pK(a)). Complexes prepared with a low TA content had higher low critical solution temperature than pure PNIPAAm microgels as a consequence of the hydrophilic character of TA; however, above a concentration threshold, TA physically cross-links the polymeric network, altering their morphology and suppressing the thermodynamically driven PNIPAAm coil-to-globule transition. DRIFT spectral analysis indicated that within PNIPAAm-TA complexes hydrogen bonds were established between PNIPAAm amide and TA phenolic (C=O center dot center dot center dot H-O and N-H center dot center dot center dot O-H) and ester (N-H center dot center dot center dot O-C) groups. At pH 4, H-bonding was more diverse and extensive than at pH 7; hence the complexes thermoresponsive behavior was altered at lower TA contents for the acidic pH. Above the TA pK(a), H-bonding is destabilized and the complexes recovered their spherical morphology and the ability to respond to temperature stimulus, thus demonstrating a reversible process with pH.