Study of Structure Formation in Aqueous Solutions of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymers by Measuring Rate Constants of the Thermal Cis−Trans Isomerization of an Azobenzene Dye and Self-Diffusion of Copolymer Molecules

In aqueous solutions of poly(ethylene oxide) (PEO)−poly(propylene oxide) (PPO)−PEO triblock copolymers (Pluronics P85, F88, and L64), structure formation (micellization) on increasing temperature was followed by determination of rate constants k iso of the thermal cis−trans isomerization of 4,4‘-nitroanilinoazobenzene by means of flash photolysis in H 2 O and D 2 O. The kinetic solvent isotope effect k iso,H 2 O / k iso,D 2 O indicates that the azobenzene dye molecules are solubilized in a water-rich environment. From the nearly constant solvatochromic UV/vis absorption band maxima λ max of the dye, it is concluded that the S shape of the ln k iso vs 1/ T curves is mainly due to microviscosity changes on micellization. Critical micelle temperature values derived are in satisfactory agreement with those from self-diffusion coefficients of the copolymer molecules dependent on temperature determined by means of pulsed field gradient nuclear magnetic resonance measurements. The self-diffusion experiments allow conclusions on the size of the diffusing particles in H 2 O and D 2 O and the influence of dye molecules on aggregation. The hydrodynamic radii of the diffusing species are larger in H 2 O than in D 2 O. The reason is seen in the stronger hydrogen bonds between EO units and D 2 O compared to those between EO units and H 2 O. On gelation of 25% (w/v) F88 in water at 31° C, the bulk viscosity increases sharply but the microviscosity around the dye molecules does not.