Two-Stepping: Sol-gel-gel transitions in mixed thermoresponsive polymer systems

Conventional thermoreversible gels exhibit a sol-gel transition upon modulation of temperature. These systems are typically comprised of block copolymers in which one block exhibits a lower critical solution temperature (LCST), triggering a solvophilic to relatively solvophobic switch to that moiety when heated. The systems, which include poly(ethylene oxide)(PEO)-b-poly(propylene oxide)(PPO)-b-PEO (“Poloxamers”) and poly(N-isopropylacrylamide) (PNIPAM)-based block copolymers, thus exhibit a single step in their rheological profile upon heating, switching from a predominantly dissipative response to an elastic one. It has been found that a mixed tertiary system of PNIPAM-PEO-PNIPAM and Poloxamer 407 displays an unconventional sol-gel-gel transition. The rheological behaviours of this system have been studied to demonstrate the rheological profiles of the sol, “Gel I” and “Gel II” phases, as well as the reversibility of the gelation. A mechanism is proposed for this process, learning from dynamic light scattering and small-angle neutron-scattering experiments in dilute and concentrated regimes, respectively.