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: S0(q), the initial structure factor, St(q), the terminal structure factor, and R(q), a kinetic parameter that indicates the time scale for the transformation from S0(q) to St(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, qc. At small quench depths, qc is obtained because R(q)→0 as q→qc. At deeper quenches, qc is obtained because St(q)→S0(q) as q→qc. Scattering characteristics at q