Does PNIPAM block really retard the micelle-to-vesicle transition of its copolymer?

The well-known coil-to-globule transition of poly(N-isopropyl acrylamide) (PNIPAM) at its LCST lasts as short as hundred of seconds with fully reversibility. However, for the PNIPAM-containing block copolymers, thermal transformation from micelles to vesicles caused by the conformation transition of PNIPAM took as long as several weeks, even at the temperatures much higher than the LCST, and without satisfactory reversibility. In the literature, this slow process has been attributed to the strong interchain hydrogen bonding in PNIPAM, which retards the transition. In this work, asymmetrically modified PNIPAM (Mw 10K), i.e. C12-PNIPAM-CA with a hydrophobic hydrocarbon chain -C12H25 (C12) at one end and a hydrophilic carboxyl group -COOH (CA) at the other, was prepared and found to form micelles with a core of the lightly associated hydrocarbon chains. When temperature is increased to the LCST of PNIPAM, the transformation from micelles to vesicles can be realized within 30 min, while the reverse process only takes a few minutes. Based on full monitoring of the transition process, it is proposed that the micelles serve as building blocks in constructing the vesicles via processes of combination, fusion, and etc., in which only local conformation adjustment of PNIPAM is involved. Therefore, reducing the restriction to the conformation change of PNIPAM chains, which is imposed by the micellar core, is one of the key factors in realizing the fast transition.

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