Fluctuation mediated phase separation in polymer blends near the limit of metastability

The evolution of the structure factor of off-critical polyolefin (polymethylbutylene/polyethylbutylene) blends quenched from the homogeneous state to states near the limit of metastability was studied by small angle neutron scattering. The Cahn-Hilliard-Cook theory was used to organize the data in terms of three time-independent parameters that depend on the scattering vector, q: S-0(q), the initial structure factor, S-t(q), the terminal structure factor, and R(q), a kinetic parameter that indicates the time scale for the transformation from S-0(q) to S-t(q). These three parameters change systematically with quench depth. Changes in the structure factor S(q,t) are only observed in the q values smaller than a critical scattering vector, q(c). At small quench depths, q(c) is obtained because R(q)-->0 as q-->q(c). At deeper quenches, q(c) is obtained because S-t(q)-->S-0(q) as q-->q(c). Scattering characteristics at q<q(c) such as scattering peaks or the lack thereof arise due to the interplay between R(q) and S-t(q). (C) 2002 American Institute of Physics.