Glucose-induced sphere to ellipsoid transition of polyoxyethylene-polyoxypropylene block copolymer micelles in aqueous solutions

The structure of micelles formed by a representative polyoxyethylene-polyoxypropylene-polyoxyethylene amphiphilic block copolymer (Pluronic P105, EO₃₇PO₅₈EO₃₇) in aqueous glucose solution has been probed with the help of viscosity and small angle neutron scattering (SANS) measurements. In the presence of glucose (40wt%), the relative viscosities of the aqueous block copolymer solutions increase significantly (10-100 times) above 50°C for 1wt% and above 40°C for 8wt% block copolymer, whereas in the absence of glucose the relative viscosities remain almost constant. The macroscopic information obtained by viscosity is elucidated by structural information on the micelles obtained by fitting SANS data to a core-corona prolate ellipsoid form factor. The addition of glucose in aqueous EO₃₇PO₅₈EO₃₇ solutions results in micelle growth from sphere to ellipsoid (cigar-like) with major/minor radius ratio in the range 2-5. The onset of the sphere-to-ellipsoid shape transition discerned by SANS corresponds to the onset of the solution viscosity increase. In comparison, the shape of micelles formed in binary EO₃₇PO₅₈EO₃₇ - water solutions remains almost independent of temperature. The observed changes in the micelle structure can be attributed to enhanced dehydration of polyoxyethylene caused by glucose. These findings are relevant to waterborne formulations incorporating polyoxyethylene-containing amphiphiles.