Stimuli-Induced Multiple Sol−Gel−Sol Transitions of Aqueous Solution of a Thermo- and Light-Sensitive Hydrophilic Block Copolymer

Abstract

We report in this article that a 20.0 wt % aqueous solution of a thermo- and light-sensitive hydrophilic block copolymer, poly(ethylene oxide)-b-poly(ethoxytri(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) (PEO-b-P(TEGEA-co-NBA)), can undergo multiple sol−gel−sol transitions in response to temperature changes and UV irradiation. The 20.0 wt % block copolymer solution was a free-flowing liquid at low temperatures. Upon heating above the lower critical solution temperature (LCST) of the thermo- and light-sensitive P(TEGEA-co-NBA) block, micellization occurred with the P(TEGEA-co-NBA) block associating into the core and the PEO block forming the corona. The solution turned into an immobile, optically isotropic gel at 33 °C. The gel exhibited a finite yield stress, and its dynamic storage modulus was essentially independent of frequency, suggesting that the gel was composed of cubic-packed micelles. With further increasing temperature to 50 °C, the gel flowed under its weight. This is because the PEO corona shrank at elevated temperatures, and thus the structural constraint among micelles was released. At 33 °C, we irradiated the micellar gel with 365 nm UV light. The o-nitrobenzyl group of NBA was cleaved, resulting in an increase in the LCST of the thermosensitive block and the dissociation of micelles. Consequently, the gel was transformed into a free-flowing liquid, which upon heating underwent sol−gel−sol transitions, similar to the solution before UV irradiation.