Mechanistic insight into morphological transition of Pluronic aggregates in aqueous solution via tuning hydrogen bonding strength

Abstract

Hydrogen bonding interaction has been increasingly recognized as a viable tool for manipulating the morphology of amphiphilic block copolymers aggregates in selective media. In this regard, mechanistic understanding on the impact of the strength of such an interaction on the morphological transition of the aggregates is of crucial importance for rational design targeting functional assemblies, yet still remains poorly understood. By performing multiple experimental techniques and molecular dynamics simulations (MDS), we here demonstrate how the morphology of Pluronic P123 micelles evolves by introduction of three octane derivatives (i.e., n-octanol, n-octylamine and n-octanoic acid) capable of forming distinct hydrogen bonding strength with P123 in aqueous solution. It is found that the P123-n-octanol mixture exhibits spherical aggregates as P123 alone. However, upon adding n-octylamine, the P123 micelles undergo sphere-short rod-long worm transition in both concentration and temperature-dependent manners. Interestingly, n-octanoic acid brings about a series of morphological transition from sphere, long worm to unilamellar vesicles as the concentration increases and the formed vesicles are fairly stable in the tested temperature range (20–60 °C). The nature of the additives acting on the copolymer aggregates was probed by 2D-NMR and MDS. Although both of the hydrophobic and hydrogen bonding interactions between the additives and block copolymers affect the architecture of the copolymer aggregates, the diverse strength of the latter generating from the functional groups (viz. –OH, –NH2 and –COOH) of the additives with the ether oxygen atoms on P123 backbone results in distinct location of the three additives in the micelles, consequently leading to varying micellar morphologies. This work enriches our knowledge of hydrogen bonding driven morphological transition of amphiphilic block copolymers aggregates, and as a result the fabrication of a desired nano-material through fine-tuning the hydrogen bonding strength can be straightforwardly envisaged.