Dually responsive aqueous gels from thermo- and light-sensitive hydrophilic ABA triblock copolymers

We report in this article the synthesis of a series of thermo- and light-sensitive hydrophilic ABA linear triblock copolymers, poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate)-b-poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) (P(TEGEA-co-NBA)-b-PEO-b-P(TEGEA-co-NBA)), with various NBA molar contents and the study of sol–gel transitions of their aqueous solutions. The copolymers were prepared by ATRP from a difunctional PEO macroinitiator; the polydispersity indexes of all polymers were <1.2. We demonstrated that a 10.0 wt% aqueous solution of a triblock copolymer with a NBA content of 9.3 mol% in the thermosensitive blocks underwent a thermo-induced reversible sol–gel transition and a UV-triggered gel-to-sol transition at a constant temperature. The UV-irradiated solution again can undergo a temperature-induced sol–gel transition but at a higher temperature. The thermo-triggered sol–gel transitions stemmed from the formation of a 3-dimensional network with the dehydrated thermosensitive blocks being associated into hydrophobic cores and the central PEO block forming bridges among micelles. Exposure to UV light cleaved hydrophobic o-nitrobenzyl groups, raised the LCST of thermosensitive blocks, and thus triggered a gel-to-sol transition at a constant temperature. Rheometry was employed to study the sol–gel transitions and gel characteristics. In addition, we studied the effects of polymer concentration and NBA content on sol–gel transition temperature (Tsol–gel) and gel properties before and after UV irradiation. The Tsol–gel increased with the decrease of polymer concentration; the critical gelation concentration for the triblock copolymer with a NBA content of 9.3 mol% was between 3 and 4 wt% before UV irradiation and between 5 and 6% after photocleavage of o-nitrobenzyl groups. The change of Tsol–gel was in general larger for the copolymer with a higher NBA content after UV irradiation.