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The temperature variation of the 11/13 ratio of pyrene vibrational fine structure has been studied in aqueous poly(ethy1ene oxide)-block-poly(propy1ene oxide)-block-poly(ethy1ene oxide) (PEO-PPO-PEO) copolymer solutions, in bulk PEO, PPO, and various organic solvents. Linear decreases were observed in organic solvents and in mixtures ofbulk PEO and PPO. The data in aqueous PEO-PPO-PEO copolymer solutions reveal three distinct solution conditions: at low temperatures, the linear decrease with temperature parallels that for water; over a small intermediate temperature range there is a sharp decrease in the ratio, attributed to the formation of micelles; and at higher temperatures, a linear decrease is again observed. Values in this region are strongly dependent on the PPO block and are consistent with the representation of micelles with a core dominated by PPO. These decreasing values for the intensity ratio in the latter region indicate a less polar environment for pyrene and were considered to be a linear combination of the temperature effects on the polarity observed in bulk poly(ethy1ene oxide) and poly(propy1ene oxide) homopolymers. The composition of the micelle core was estimated using these data and indicated an increase of poly(ethy1ene oxide) as the polymer becomes more hydrophilic. In addition, the size of the micelles and their aggregation numbers were determined using light scattering and pyrene luminescence decay studies, respectively. These data showed an increase of the aggregation number with temperature while the hydrodynamic radius remained constant and were interpreted in terms of dehydration of the PEO block.

The micellar microviscosity afforded by Pluronic and Tetronic poly(ethylene oxide)-polyCpropylene oxide) block copolymer aqueous solutions has been investigated by fluorescence and NMR spectroscopy. Comparison is made with bulk polyCpropylene oxide) (PPO) samples of different molecular weights. The microviscosity in Pluronic PEO-PPO-PEO copolymer micelles is much larger than that observed in conventional surfactant micelles and depends strongly on the size of the hydrophobic PPO block: the larger this block, the higher the viscosity. Above the critical micellar temperature (CMT), as temperature increases, the microviscosity decreases. However, this decrease is not as important as that observed in bulk PPO. Hence, the relative microviscosity, defined as the ratio of the two observed phenomena, increases. This suggests structural transformation of the micelles resulting in a core becoming more and more compact as temperature increases. Such results have been confirmed by NMR studies that showed broadening of the PPO peak and relatively constant spin-lattice relaxation time, T\, with increasing temperature while the PEO signal remained relatively sharp with an exponential increase in T\. In addition, solubilization of benzene in Pluronic copolymer micelles as detected by NMR indicated that benzene partitions preferentially in the core of the micelle constituted mainly of PPO.

We correlate the results of pulsed-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) measurements of solvent self-diffusion coefficients in binary poly(methyl methacrylate) (PMMA) mixtures with laser interferometry-fluorescence quenching (LIFQ) measurements of PMMA film dissolution for a series of identical ketone and ester solvents. Within a homologous solvent series, the self-diffusion coefficients as well as film dissolution rates decrease in tandem with increasing solvent molecular size and cross-section. When self-diffusion coefficients are compared, little difference is found between esters and ketones of normally identical molecular size. However, systematic differences in film dissolution rates are evident when comparing ketone and ester solvents. These differences are eliminated when film dissolution rates for esters and ketones are compared on the basis of the solvent free volume parameters extracted from the PGSE NMR self-diffusion coefficient data using Fujita's free volume theory.

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