Initial stages of phase separation in polymer blends near the limit of metastability

Liquid-liquid phase separation in polymethylbutylene/polyethylbutylene blends near the metastable limit was studied using small angle neutron scattering (SANS). In addition, the equilibrium thermodynamic properties of the blends were examined over a wide temperature and pressure range. The Flory-Huggins interaction parameter, χ, was measured by comparing static SANS profiles from single-phase systems with predictions based on the random phase approximation. The pressure dependence of the binodal temperature of one of the blends was experimentally determined from a series of dissolution experiments. The experimental binodal is in quantitative agreement with that computed using the Flory-Huggins theory without any adjustable parameters.; Quenching the blends from the single-phase region to deep into the metastable region of the mean-field phase diagram induced phase separation. During the early stage of phase separation in the blends, the time-resolved SANS profiles merged at a time-independent critical scattering vector, q_c. The critical size of the phase separated structures, R_c, formed during the early stages of phase separation is defined as R_c = 1/q _c. The theory of Cahn and Hilliard predicts that in metastable blends R_c increases with increasing quench depth, and diverges at the spinodal. The experimental measurements show that R_c increases with decreasing quench depth, and diverges between the binodal and spinodal. Some aspects of these results are addressed in recent theoretical work of Wang and Wood wherein the effects of fluctuations on the classical binodal and spinodal curves in polymer blends are incorporated.; The evolution of the structure factor was then examined using the Cahn-HilliardCook theory. This enables organizing the data in terms of three parameters that depend on scattering vector, q: S₀(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 S₀(q) to S_t(q). These parameters change systematically with quench depth. 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) → S₀(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 of R(q) and St(q).