Synthesis, self-assembly and photophysical properties of oligo(2,5-dihexyloxy-1,4-phenylene vinylene)-block-poly(ethylene glycol)

We describe the synthesis and characterization of a family of diblock copolymers with 5 units of a dihexyloxy-phenylenevinylene block (OHPV) connected to a series of poly(ethylene glycol) (PEG) chains of different average lengths (12, 45 and 115 PEG units: OHPV₅-b-PEG₁₂, OHPV₅-b-PEG₄₅, OHPV₅-b-PEG₁₁₅). All three polymers underwent self-assembly in ethanol, a good solvent for the PEG units, but poor for the OHPV segment. The nature of the structures formed depends sensitively on the length of the PEG block. OHPV₅-b-PEG₁₁₅ formed long fiber-like micelles of uniform width, whereas OHPV₅-b-PEG₄₅ formed fragile broad ribbons. We also obtained thin ribbons with OHPV₅-b-PEG₁₂ but they tend to fold and twist upon themselves. The structures obtained were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM), as well as by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). In addition, their photophysical properties were examined by UV-vis, steady state fluorescence and fluorescence decay measurements. The results of these experiments indicate that the OHPV groups pack differently in the fiber-like micelles of OHPV₅-b-PEG₁₁₅ than in the lamellar structures formed by OHPV₅-b-PEG₄₅.