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

Abstract

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 (T_sol–gel) and gel properties before and after UV irradiation. The T_sol–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 T_sol–gel was in general larger for the copolymer with a higher NBA content after UV irradiation.