Influence of cleavage of photosensitive group on thermally induced micellization and gelation of a doubly responsive diblock copolymer in aqueous solutions: A SANS study

Abstract

In the present study, small-angle neutron scattering (SANS) was used to investigate the temperature and concentration dependence of micelle formation and growth of a thermo- and light-responsive hydrophilic block copolymer and the influence of removing photosensitive group. The water-soluble polymer poly(ethylene oxide)- b -poly(ethoxytri(ethylene glycol) acrylate- co - o -nitrobenzyl acrylate) (PEO- b -P(TEGEA- co -NBA) underwent sol-gel/gel-sol transitions in response to temperature changes in the range of 15–55 °C in a 25 wt% aqueous solution. The SANS data showed that individual chains existed at low temperatures and dilute solutions. Spherical micelles formed and packed into an ordered structure at elevated temperatures and high concentrations. Data fitting revealed that the micelle growth followed both micelle fusion and unimer insertion mechanisms after the critical micelle temperature was reached in moderately concentrated solutions whereas the fusion mechanism was dominant at higher temperatures. The UV exposure led to a 22% expansion in chain dimension at the unimer stage due to repulsive interactions between the ionized segments after the removal of the light sensitive group. The higher onset temperature of sol-gel transition after irradiation was mainly ascribed to the higher lower critical solution temperature (LCST). On the other hand, the lower gel-sol transition temperature upon UV exposure was due to the reduction in micelle volume fraction after the cleavage of o -nitrobenzyl group. These results correlated well with the dynamic rheological data. • SANS reveals the molecular mechanisms of polymer micellization and gelation. • The unimers stretch upon the removal of the light sensitive group. • The cleavage of the light sensitive group leads to a narrower gel zone. • The scattering data uncovers material distributions in micelles. • Micelle growth follows both micelle fusion and unimer insertion mechanisms.