Upper Critical Solution Temperature Layer-by-Layer Films of Polyamino acid-Based Micelles with Rapid, On-Demand Release Capability

Here, we report on upper critical solution temperature (UCST)-type polyamino acid-based block copolymer assembled in a layer-by-layer system with a natural polyphenol. The UCST-type block copolymer, polyvinylpyrrolidone- b -polyureido(ornithine- co -lysine) (PVP- b -PUOL), was synthesized via ring opening polymerization followed by postpolymerization functionalization with ureido groups. PVP- b -PUOL exhibited UCST behavior that was controlled by both molecular weight and ornithine-to-lysine ratio. Importantly, PVP- b -PUOL with a UCST of 33 °C assembled into block copolymer micelles below UCST and dissociated above UCST, yet this behavior was not repeatable in solution due to β-sheet formation and large-scale aggregation. To overcome this limitation, UCST micelles (UCSTMs) were deposited within layer-by-layer (LbL) films via hydrogen bonding with tannic acid (TA). Significantly, the assembled TA/UCSTM films stabilized the micelles from desorption while maintaining their morphology and prevented β-sheet formation even after continuous exposure to 40 °C for 7 days. Moreover, these films demonstrated repeatable swelling–deswelling for up to five temperature cycles from 25 to 40 °C. The thermo-switchable hydrophobicity of micellar cores was used for trapping of a model active molecule, pyrene, and its on–off temperature-controlled release, demonstrating the potential of TA/UCSTM films for controlled delivery of active molecules.