Microviscosity in multiple regions of complex aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)

Aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO ₁₀₉-PPO ₄₁-PEO ₁₀₉) copolymers are nonionic surfactants that self-organize to form aggregate structures with increasing temperature or concentration. We have studied two concentrations over a range of temperatures so that the copolymers are in one of three microphases: unimers, micelles, or hydrogels formed from body centered cubic aggregates of micelles. Three different coumarin dyes were chosen based on their hydrophobicity so that different aggregate regions could be probed independently - water insoluble coumarin 153 (C153), hydrophobic coumarin 102 (C102), and the hydrophilic sodium carboxylate form of coumarin 343 (C343 ^-). Fluorescence anisotropy experiments provide detailed information on the local microviscosity. C153 experiences a fourfold increase in reorientation time and hence microviscosity with increasing temperature through the microphase transition from unimers to micelles. C102 also shows an increase in microviscosity with temperature but smaller in magnitude and with the microphase transition shifted to higher temperature relative to C153. C343 ^- shows only a slight sensitivity to the microphase transition. For any of the three coumarin probes, fluorescence anisotropies do not show any correlation with the microphase transition to form cubic hydrogels.