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

We report m this article that a 20.0 wt % aqueous solution of a thermo- and light-sensitive hydrophilic block copolymer, poly(ethylene oxide)-b-poly(ethoxyl(ethylene glycol) acrylate-co-o-nitro-benzyl 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 isotropoc gel at 33 degrees C. The tel 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 incresing temperature to 50 degrees 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 degrees 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.