Gelation Behavior of Thermo-Responsive Poly(ethylene oxide) and Poly(propylene oxide) Multiblock Polycarbonates

Multiblock polycarbonates were facilely synthesized by covalently binding poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) blocks, using triphosgene as coupling agent. The resulting multiblock polycarbonates exhibited thermo-responsive gelation behavior. A pronounced viscosity increase was observed when the temperature increased, which was traced to the formation of a network of micelles. With further heating, the viscosity began to drop, which reflected breaking down of the network. Viscoelastic properties of the multiblock polycarbonates in aqueous solutions were probed by dynamic oscillatory techniques. At low frequencies, the systems showed a typical liquid-like behavior and at high frequencies they behaved as elastic materials. Data also showed that the network formed by polycarbonates with less EO/PO composition ratio evolved and strengthened, thus exhibited more elastic behavior. Temperature sweep results revealed that the aqueous solutions of polycarbonates underwent a sol-gel-sol transition with increasing temperature. By properly regulating the EO/PO composition, gelation occurred upon increasing the temperature to 37C, which made it a promising candidate for an injectable drug delivery system.