Self-assembly of the Thermosensitive and pH-Sensitive Pentablock Copolymer PNIPAMx-b-P(tBA-co-AA)90-b-PPO36-b-P(tBA-co-AA)90-b-PNIPAMx in Dilute Aqueous Solutions

A series of pH-sensitive and thermosensitive amphiphilic pentablock copolymers, PNIPAM(x)-b-P(tBA-co-AA)(90)-bPPO(36)-b-P(tBA-co-AA)(90)-PNIPAM(x) (x-DH) with different lengths x of PNIPAM blocks (x = 58, 95, and 111) and various degrees of hydrolysis of PtBA blocks (DH approximate to 10, 60, and 100%), were prepared and self-assembled into micelles in dilute aqueous solutions. By dynamic light scattering (DLS) and UV-vis spectrophotometry characterizations, at pH = 2, x-DH micelle aqueous solutions show an irreversible thermosensitive process due to the hydrogen (H) bonding between PNIPAM blocks and protonated AA groups. At pH = 12, x-DH micelle aqueous solutions show a reversible thermosensitive process when the DH is low (10%) but are thermoinsensitive when the DH is high (similar to 60 and 100%) due to the electrostatic repulsion between deprotonated AA groups. The morphologies and structures of x-DH micelles are directly visualized by cryogenic transmission electron microscopy (cryo-TEM) at 20 degrees C. Under acid conditions, with increasing hydrophilicity, the transformation of x-DH micelles from large compound micelles (LCMs) with a bent corona, through LCMs with extended corona, core-corona irregularities, core-shell-corona spheres, coexistence of core-shell-corona spheres and core-corona cylinders, core-corona cylinders, and coexistence of core-corona spheres and core-corona cylinders, to core-corona spheres is identified. Under alkaline conditions, the deprotonated AA groups introduce electrostatic repulsion into micelles, and four nanostructures of x-DH micelles, i.e., LCMs with an uneven surface and inhomogeneous core, core-corona spheres, core-corona cylinders, and core-shell-corona spheres, are observed.

Automated summary

A series of pH-sensitive and thermosensitive amphiphilic pentablock copolymers were self-assembled into micelles with different morphologies and structures, exhibiting varying thermosensitive behavior and nanostructures under different pH conditions.