Synthesis of Thermoresponsive Copolymers with Tunable UCST-Type Phase Transition Using Aqueous Photo-RAFT Polymerization

Currently, the phase transition of aqueous binary systems containing thermoresponsive (co)polymers, and exhibiting upper critical solution temperature (UCST), is exclusively investigated in dilute solutions, which can limit the knowledge of their UCST-type phase transition. Herein, a photo-RAFT polymerization approach, using acrylamide (AAm) and acrylonitrile (AN) as monomer models, is used to prepare well-controlled poly(AAm-co-AN) copolymers in situ in highly concentrated dispersions (60 wt%). The impact of the copolymer concentration and the chemical composition (as a variation of AN fraction in the copolymers) on the cloud point temperature (T-CP) are investigated using turbidity measurements. Importantly, the results show that upon increasing the polymer concentration, a sharp increase of T-CP up to a maximum point is observed, representing the UCST, before the decrease of T-CP at higher polymer concentrations. Finally, a model equation is developed to fit the UCST values of poly(AAm-co-AN), which can be useful to design new poly(AAm-co-AN) copolymers with a desired UCST for a specific application.