Micellar self-assembly, bridging and gelling behaviour of two reverse triblock poly(butylene oxide)–poly(ethylene oxide)–poly(butylene oxide) copolymers with lengthy hydrophilic blocks

Triblock polyethyelene oxide–polybutylene oxide-based block copolymers overcome some of the synthetic drawbacks of commercially available Pluronic block copolymers as well as providing a more hydrophobic environment to solubilise poorly aqueous-soluble compounds. These facts can be exploited to use this class of copolymers as efficient drug delivery nanocarriers (A. Cambon et al., Int. J. Pharm., 2013, 445, 47–57), and their interactions with biologically relevant entities and biological performance should be regulated by the nature, conformation and state of the copolymeric chains. For this reason, in this work we investigated the self-assembly process of two of these reverse triblock poly(butylene oxide)–poly(ethylene oxide)–poly(butylene oxide) block copolymers, BO8EO90BO8 and BO20EO411BO20, to obtain a clear picture of their self-assembly behaviour in order to correlate it with their biological performance. As a consequence of their particular structure, BO20EO411BO20 possesses a rich rheological behavior characterized by the formation of flower-like micelles (ca. 10 to 30 nm in size) and intermicellar bridging at low copolymer concentrations, as shown by atomic force microscopy and rheology data. Conversely, BO8EO90BO8 displays a behaviour more similar to that observed for diblock EOmBOm and direct triblock EOmBOnEOn copolymers, with single non-associated micelles at low concentrations, and a flow behaviour typical of mesoscopic ordered cubic structures. Strikingly, the relatively wide Poisson distribution of the copolymeric chains makes the present copolymers behave also rather distinctly to conventional associative thickeners.