Copolymer vs interpenetrated polymer network microgels: The case of poly(N-isopropylacrylamide) and poly(acrylic acid)

Abstract

Microgels based on poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) were synthesized following two different procedures: random copolymerization of the two monomers, where a single network P(NIPAM-co-AAc) microgel is obtained made of the two repeating units, and polymer interpenetration that provides two separate polymer networks within a single microgel structure (PNIPAM-PAAc IPN). We demonstrate that the synthesis procedure has a profound impact on the resulting microgels as shown by investigating swelling, flow and thermal properties as well as the molecular mechanisms driving the microgel behaviour. To this purpose, a systematic study through Dynamic Light Scattering, Rheology, Calorimetry and Raman spectroscopy has been performed at different temperatures, pH values, and concentrations. The direct comparison between P(NIPAM-co-AAc) and IPN microgels at same PAAc content, same temperatures, and pH values shows a reduced or even absent temperature responsiveness of the microgel in the first case and a Volume Phase Transition, with a pH dependent swelling ratio, in the second one. The mutual interference or independence of PNIPAM and PAAc reflects in the different properties of copolymerized or interpenetrated microgels as deeply addressed in this work.