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College Park, Maryland      June 6 - 10 , 2004

T3-B5 (5:00 PM): Neutron Spin Echo Study of the Local Dynamics in a Polymer Micellar Solution

Hasan Yardimci (Department of Physics & Astronomy, Johns Hopkins University), Brian Chung, James Harden (Department of Chemical Engineering, Johns Hopkins University), Robert Leheny (Department of Physics & Astronomy, Johns Hopkins University)

We investigate the local dynamics of aqueous solutions of the triblock copolymer polyethylene oxide - polypropylene oxide - polyethylene oxide (Pluronic F108) by neutron spin echo (NSE). In solution the pluronic self-associates into spherical micelles composed of cores of polypropylene oxide (PPO) with corona of solvated polyethylene oxide (PEO). Depending on concentration and temperature, which affect the conformation of the corona chains and hence influence the interactions between micelles, the micelles order in either a liquid or crystal state. In particular, as the PEO becomes less hydrophilic with increasing temperature, collapse of the corona drives a melting transition on heating. Our small angle neutron scattering measurements on Pluronic F108 solution demonstrate the changes in the structure on heating from room temperature associated with the formation of micelles and their ordering into a crystal followed by melting of the crystal upon further heating.

The micellar dynamics as probed in NSE show a clear systematic dependence on concentration and temperature as the phase boundaries between polymer solution, micelle crystal and micelle liquid are traversed. However, these local dynamics are surprisingly insensitive to phase behavior and macroscopic rheology. For example, the relaxation time of a 21 % micellar solution changes only by a factor of approximately four across the melting transition near 80 C. Analysis of the wave-vector dependence across the transition over the range of 0.02 < Q < 0.12 -1 indicates the specific local micellar motions probed by NSE.

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