Fluorescence Probe and Pulsed Field Gradient NMR Study of Aqueous Solutions of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymer F88

Abstract

In aqueous solutions of the triblock copolymer F88 (EO96PO39EO96) micelle formation has been studied by means of dual fluorescence of the probe molecule 1,3-bis(1-pyrene)propane monitoring viscosity changes in its microenvironment. Mobility changes of the polymer molecules on micellization were followed by means of pulsed field gradient (PFG) NMR. These results complement a recent detailed study of cis−trans isomerization of azo dyes as “kinetic probes” for structure formation in aqueous solutions of F88 (Langmuir 2001, 17, 2907). Differences of the critical micellization temperatures determined by various methods, and in H2O and D2O as solvents, are discussed. Our results reflect the particular large content of polye(ethylene oxide) in F88 (80%) compared to other triblock copolymers of this type.

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.…”

“…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. It suggests that the stronger deuterium bonding compared to hydrogen bonding between D 2 O molecules and of D 2 O with the longer ethylene oxide chains of F88 contributes more effectively to the reduction of the mobility of the copolymer molecules in the case of F88 compared to P85, which have equal PPO chain length.…”

“…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.…”

“…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.…”

“…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.…”