Thermally Induced Swellability and Acid-Liable Dynamic Properties of Microgels of Copolymers Based on PEGMA and Aldehyde-Functionalized Monomer

摘要

This paper describes a new class of smart microgels that possess entirely reversible thermally induced swellability and acid-liable dynamic properties in aqueous solution. To this end, 3-(4-formylphenoxy)-2-hydroxypropyl methacrylate (FPHPMA) was synthesized. RAFT polymerization of FPHPMA, RAFT random copolymerization of FPHPMA and poly(ethylene glycol) methacrylate (PEGMA), and chain extension RAFT polymerization of PEGMA using P(FPHPMA-ran-PEGMA) as a macromolecular chain transfer agent proceeded under visible light radiation at 25 °C. The results indicated the well-controlled polymerization of FPHPMA, comparable reactivity of FPHPMA and PEGMA, and living character of these polymerizations under such mild conditions. Thus, a range of well-defined copolymers of FPHPMA and PEGMA with different chain length, composition, or sequence were synthesized. The aldehyde groups of copolymers were stable against air oxidation in aqueous solution at 80 °C. These copolymers well dissolved in water at low solution temperature and exhibited a cloud point (CP) and a critical solution temperature for the formation of compressed aggregates (T_c) upon heating. The thermally induced phase transition of random copolymers was slightly influenced by chain length but significantly depended on the composition or concentration. Moreover, the sequence of copolymers exerted essential control over this thermally induced phase transition. Contrary to the previously reported PEGMA-based copolymers, these copolymers exhibited significant hysteresis in cooling process similar to what observed in poly(N-isopropylacrylamide) (PNIPAM). However, its isotopic solvent effect was contrary to what was observed in PNIPAM. In these thermally induced aggregates, the aldehyde groups could react with hexamethylenediamine or adipoyl hydrazide, leading to the formation of microgels. These microgels exhibited entirely reversible thermally induced swellability. This swellability could be finely tuned by adjusting the solution pH.