Nanogel-like UCST triblock copolymer micelles showing large volume expansion before abrupt dissolution

This paper reports the synthesis and study of a novel ABA-type triblock copolymer, of which the terminal A-blocks comprise hydrophilic poly(N,N-dimethylacrylamide-co-7-acryloyl-4-methylcoumarin) (PDMA) and the central B-block comprises UCST thermosensitive poly(acrylamide-co-acrylonitrile) (PAAm-co-AN). Although the formation of micelles of this triblock copolymer at temperatures below the UCST of the PAAm-co-AN block is not surprising, the micelles, surprisingly, behave like crosslinked thermosensitive nanogels over a wide temperature range and exhibit a large expansion and contraction in volume on heating and cooling, respectively, as long as the solution temperature remains below a critical point at which abrupt and complete dissolution of the micelles occurs, which results in dissolved polymer chains in the unimer form. A comprehensive study of this peculiar behavior by means of dynamic light scattering, small-angle X-ray scattering, fluorescence spectroscopy, solution transmittance, viscosity and solvent substitution showed that the thermosensitive increase in micelle volume prior to dissolution is mainly due to swelling owing to the entry of water as a result of an increase in hydration inside the particles.