Concentration-Driven Reversible Morphological Transitions by the Self-Assembly of Crystalline-Coil Polyphosphazene-b-Polystyrene (PP-b-PS) Block Copolymers.

Abstract

In this study, we demonstrate that varying the initial concentration of the poly-[bis(trifluoroethoxy)phosphazene]-b-poly(styrene) (PTFEP₅₅-b-PS₅₀) block copolymer enables the formation of diverse, well-defined nanomorphologies through self-assembly in tetrahydrofuran (THF). By adjusting the copolymer concentration, spherical micelles (0.1 mg/mL), bicontinuous micelles (0.33 mg/mL), toroidal micelles (2.0 mg/mL), cylindrical micelles (10 mg/mL), and vesicles (50 mg/mL) were successfully prepared. This simple methodology, using a single solvent and no additives, allowed for the investigation of morphological transformation mechanisms. Intermediate structures, such as "flower-like" morphologies (0.66 mg/mL), large compound micelles (LCMs, 1.0 mg/mL), and perforated micelles (1.5 mg/mL), were identified, revealing transitions between bicontinuous and toroidal morphologies. In situ studies captured the opening of toroidal structures into cylindrical micelles, while "octopi-like" structures were observed at intermediate concentrations between cylindrical and vesicular architectures. Wide-angle x-ray diffraction (WAXD) analysis showed that PTFEP blocks in the nanostructure cores are amorphous at low concentrations (< 2 mg/mL) but exhibit increasing crystallinity at higher concentrations (> 2 mg/mL), transitioning from toroidal to cylindrical and vesicular morphologies. This work highlights the ability to control the self-assembly of PTFEP-b-PS, generating a wide range of nanomorphologies by modulating PTFEP crystallinity through concentration adjustments. The unprecedented structural diversity of this system underscores its potential for designing advanced nanostructured materials.