Hydrophobic attraction of pyrene-end-labeled poly(ethylene glycol) in water and water-methanol mixtures

Fluorescence stationary-state measurements of pyrene-end-labeled poly(ethy1ene glycols) (Py-PEG-Py) of three different molecular weights are presented. In aqueous solution the excimer to monomer intensity ratio, Ie/Im, in the intramolecular excimer region decreases as the polymer molecular weight is increased. For Py-PEG-Py of PEG weight-average molecular weight of 4800, the Ie/Im does not scale according to the Wilemski-Fixman theory based on a diffusion-controlled end-to-end cyclization process. Surprisingly, this Py-PEG-Py(4800) phase separates. We believe that these observations are due to a hydrophobic attraction between pyrene groups. The addition of methanol reduces the hydrophobic attraction by the preferential solvation of methanol molecules around the pyrene group.