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College Park, Maryland      June 6 - 10 , 2004

MP26: Nucleation of Phase Separation in Polymer Blends

T.J. Rappl (Department of Chemical Engineering, University of California, Berkeley), N.P. Balsara (Department of Chemical Engineering, University of California, Berkeley; Material Sciences Division, and Environmental Energies Technology, Lawrence Berkeley National Laboratory)

The work presented is intended to supplement the material covered in the talk given by Nitash Balsara concerning nucleation of phase separation in polymer blends. In this work, neutron scattering was employed to investigate the nature of the structures formed during the onset of homogeneous nucleation, specifically, how the critical nucleus size varies with quench depth. Time-resolved small angle neutron scattering profiles were obtained from off-critical blends within the metastable regime of the phase diagram, revealing that the critical nucleus size decreases with increasing quench depth; this contradicts the theoretical prediction of Cahn and Hilliard that the critical nucleus size should diverge at the spinodal. Recent simulations (Chandler) of nucleation in the 3D Ising model are in agreement with our experimental data.

Additionally, within the metastable regime of a given blend we observed a line of demarcation below which phase separation could not be observed during experimental time scales via a direct quench. This line of demarcation corresponds well with the pseudospinodal, or limit of metastability, proposed by Z.-G. Wang. A series of double quench experiments were performed, wherein the system was studied consecutively at two separate locations within the metastable regime. A comprehensive comparison of the results obtained from direct quench and double quench experiments will be presented. Of note, this double quench method permitted the study of nucleation below the limit of metastability and also revealed the dissolution of small structures (created while aging below the limit of stability) upon the reduction of quench depth. This dissolution of small structures lends credibility to our notion of the critical nucleus size as measured by time-resolved small angle neutron scattering.

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