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College Park, Maryland      June 6 - 10 , 2004

M2-B1 (10:30) AM: Searching for the Critical Nucleus in Phase Separating Polymer Blends (Invited)

N.P. Balsara (University of California, Berkeley; Lawrence Berkeley National Laboratory), T.J. Rappl (University of California, Berkeley)

The initial stage of liquid-liquid phase separation in metastable blends of high molecular weight polymers was studied by time-resolved small angle neutron scattering. The large size of the polymer molecules used in this study causes chain entanglement to occur, leading to extremely slow dynamics. This enables direct examination of the initial clustering of molecules prior to macroscopic phase separation. We find that the scattering profiles are time-independent for scattering vectors (q) larger than a critical scattering vector (qc). We demonstrate the relationship between the critical nucleus size (Rc) and the critical scattering vector. The dependence of Rc on quench depth is in qualitative disagreement with all available theories on phase separation kinetics.

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