Mesogen-Driven Formation of Triblock Copolymer Cylindrical Micelles

Abstract

Evidences were gathered to support mesogen-driven formation of cylindrical micelles from BCF and ACF triblock copolymers. Here A, B, C, and F denote poly(acrylic acid), poly(tert-butyl acrylate), poly(2-cinnamoyloxylethyl methacrylate), and the liquid crystalline poly(perfluorooctylethyl methacrylate) block, respectively. At room temperature (21 °C) in all tested solvents that were selective for the A or B blocks, three of the four copolymers with various compositions formed exclusively cylindrical micelles possessing an F core, a C shell, and an A or B corona. Our further analyses indicated that the F core chains were almost fully stretched, and the C shell chains were compressed relative to their unperturbed dimensions. These abnormal chain packing motifs suggest that the need to form a liquid crystalline F phase in the cores dictated micelle formation and prevailed over the needs of the shell chains to achieve their normal stretched conformations. A subsequent wide-angle X-ray scattering study of the dried cylindrical micelles confirmed smectic A phase formation for the F blocks at room temperature. The smectic A to isotropic phase transition upon raising temperature was detected by a differential scanning calorimeter for the dry cylindrical micelles and by 19F NMR for the solvated micelles. This smectic A to isotropic phase transition was accompanied by a morphological transformation from cylindrical micelles at room temperature to other morphologies at 70 °C. More interestingly, this cylinder to vesicle conversion could be cycled repeatedly by temperature cycling for one ACF sample.