Abstract
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.
Original language | Unknown |
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Pages (from-to) | 612-621 |
Journal | Macromolecules |
Volume | 44 |
Issue number | 3 |
DOIs | |
Publication status | Published - 1 Jan 2011 |