pH-dependent self-assembly: micellization and micelle-hollow-sphere transition of cellulose-based copolymers.

pH-Sensitive micelles are usually made from double-hydrophilic block copolymers which can dissolve molecularly in water in a certain pH range and aggregate spontaneously upon an appropriate change in the pH value. Significant advances in this field have been achieved in recent years.[1] Polymeric hollow spheres have great potential for the encapsulation of large quantities of guest molecules. Different approaches, such as by the use of block copolymers as precursors,[2] as well as colloidal particles[3a,b] or liposomes[3c,d] as templates, have been developed to obtain such nanocapsules. Our research group has produced micelles in which no chemical bonds connect the core and the shell,[4] thus enabling hollow spheres to be obtained by simple dissolution of the inner component of the micelles.[4a–c] However, these approaches are mostly irreversible, that is, once the hollow structure forms, the core–shell micelles cannot be reformed. Hydroxyethylcellulose (HEC) has been extensively studied as a nonionic and water-soluble cellulose ether (Scheme 1).[5] However, studies on the self-assembly of HEC are limited, despite its biocompatibility and biodegradability. Here we report the self-assembly of HEC-graftpoly(acrylic acid) (HEC-g-PAA) in water, which was prepared by free-radical graft polymerization of acrylic acid from HEC backbones (Table 1). The results demonstrate its micellization and the transition between micelles and hollow spheres; both processes were found to be pH-dependent and reversible. Figure 1 shows the relative scattering intensity I/I0 of the solutions of two HEC-g-PAA copolymers, CAA-1 and CAA2, as a function of the pH value (I0 is the intensity of the starting solution at pH 13.5). The two solutions exhibit a