Direct and indirect core–shell inversion of block copolymer micelles

Abstract

The core–shell inversion of micelles of diblock copolymers under environmental stimuli in solution is a basic and important subject in macromolecular self-assembly and is also a basis for some of the applications of the assemblies. In literature, the inversion kinetic studies for diblock copolymers switched by pH or temperature found that such inversion always took place indirectly, comprising steps of dissociation and re-assembly. In such an indirect process, molecularly dispersed chains as intermediates are inevitable, because in the middle range of pH or temperature, both blocks are soluble. It is remarkable to see that another pathway, i.e. direct inversion without any intermediates, which is favourable for many applications, has not been achieved yet on a bench scale. In this paper, a novel block copolymer of PNIPAm-b-PBOB (PNIPAm: poly(N-isopropylacrylamide); BOB: benzoboroxole) was studied, where the hydrophobic PBOB block containing sugar-responsive benzoboroxole groups turned hydrophilic at neutral pH and the temperature-sensitive block PNIPAm underwent the coil–globule transition around its LCST. By deliberately adjusting the program of imposing the triggers, i.e. sugar addition and pH change for PBOB block and temperature change for PNIPAm, we succeeded in controlling the inversion kinetics of the micelle of PNIPAm-b-PBOB (M-BOB) in water. Both indirect and direct inversion pathways are realized as desired. The direct inversion may be accompanied by micellar fusion or fission, or more or less proceeds in situ. The direct inversion was realized by treating the copolymer micelle M-BOB with fructose at 25 °C and neutral pH for a certain period to make the PBOB core swell, followed by a temperature increase, in which collision and fusion sometimes maybe involved. The temperature increase induced agglomeration of the PNIPAm block and moved inward, which can match the further interaction of PBOB with fructose turning hydrophilic and moving outward. As far as we know, this is the first time that these indirect and direct micelle inversions for the same diblock copolymer have been achieved on a bench scale.