Thermodynamic Theory of Multiresponsive Microgel Swelling

Abstract

Using a thermodynamic theory, we present a systematic study of the behavior of microgels of N-isopropylacrylamide (NIPAm) and methacrylic acid (MAA) copolymers as a function of the temperature, pH, and salt concentration. These microgels swell with increasing pH; the onset of this transition displaces to higher pH values as the solution salt concentration decreases. The size of poly(NIPAm-co-MAA) microgels is a nonmonotonic function of the salt concentration; at constant pH, increasing the salt concentration drives particle swelling first, but a further increase of salinity leads to deswelling. Upon increasing the temperature, these microgels deswell and undergo a volume phase transition (VPT) around a characteristic temperature. This VPT temperature depends on the MAA content of the copolymer, the degree of cross-linking, the solution pH, and salt concentration. Changing these independent variables and/or design parameters are all means to modify the state of charge of the microgel at the VPT. The amount of the charge inside the polymer backbone controls the VPT temperature. Finally, we describe the absorption of two chemotherapeutic drugs, Daunorubicin and Doxorubicin, and evaluate the best conditions for incorporation into the multiresponsive microgels.