Micelles of Gradient vs Diblock Copolymers: Difference in the Internal Structure and Properties.

We performed computer simulations to reveal a difference in internal structures of micelles formed by AB gradient copolymers and equivalent diblock copolymers in a selective solvent. In contrast to distinct core-shell structure of the diblock copolymer micelles (DCM), the soluble and insoluble monomer units are less segregated in the gradient copolymer micelles (GCM). Furthermore, the concentration of the soluble units in the GCM has a maximum at the core-corona interface. The maximum is a consequence of loop formation near the interface due to the broad distribution of the insoluble units along the chain and their assembly into the core of the micelle. As a result, the interfacial area per one gradient copolymer chain is larger than the area of the diblock copolymer, and the aggregation number of the GCM is smaller. Worsening of the solvent quality (increase of attraction between the insoluble groups) enlarges the aggregation number of the DCM. On the contrary, the aggregation number of the GCM practically does not change. Furthermore, the corona of the GCM becomes less swollen because more and more insoluble units join to the core and aggregate in the corona upon solvent worsening. In other words, the GCM become smaller. Such behavior is known as a "reel in" effect detected for gradient copolymer micelles at temperature elevation.39.