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

Abstract

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 H2O and D2O. 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 H2O and D2O and the influence of dye molecules on aggregation. The hydrodynamic radii of the diffusing species are larger in H2O than in D2O. The reason is seen in the stronger hydrogen bonds between EO units and D2O compared to those between EO units and H2O. 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.

Effects of unidirectional and mutual interactions between microstructures and azo dyes as “kinetic” probe molecules on cis→trans isomerization rate constants in aqueous P85 and F88 triblock copolymer solutions

“…We discussed the temperature of the maximum of D determined by means of PFG NMR measurements as cmT of "probe-free" samples [15,16]. Moreover, we have found that ln(bulk viscosity) vs. 1/T plots of 10% F88 solutions, and density vs. 1/T plots of solutions of ≥5% P85 and 10% F88, respectively, show also inflection points that can be assigned to micelle formation (see Figs.…”

“…We determined the cmT values of solutions without probe molecules from intersection points of linear parts of Arrhenius plots of self-diffusion coefficients (PFG NMR) [15,16], and bulk viscosity. Density data were also plotted against 1/T for better comparison and analyzed accordingly.…”

“…On increasing temperatures, the average self-diffusion coefficient D of the triblock copolymer molecules in aqueous solutions drops above the cmT when copolymer molecules associate in micelles, and their mobility is reduced [16]. We discussed the temperature of the maximum of D determined by means of PFG NMR measurements as cmT of "probe-free" samples [15,16].…”

“…b Estimated from the adjacent experimental data determined in [15,16], by means of extra-and inter-polation, respectively. c Contrary to P85, self-diffusion coefficients of 10% F88 molecules determined by PFG NMR measurements [15] monitor a significant temperature difference between the cmT in H 2 O and D 2 O.…”

“…Because of their large molecular mass (P85 > 4000), the random coil unimers existing at low temperature may solubilize small amounts of hydrophobic probe molecules otherwise insoluble in water. Our kinetic probe studies in aqueous solutions of PEO-PPO-PEO triblock copolymers [12,15,16] and solutions of vesicle-forming amphiphiles [17] were formerly directed to study discontinuous changes of rate parameters with temperature caused by discontinuous microstructure changes on increasing temperature (unimer-to-micelle transition, gel-to-liquid crystal transition). Coincidence between cmT or phase transition temperatures and characteristic temperatures of the rate parameters (unidirectional interactions) was expected, but not found in all cases.…”