Determination of critical length scales and the limit of metastability in phase separating polymer blends

Liquid–liquid phase separation in polymethylbutylene/polyethylbutylene blends near the metastability limit was studied using small angle neutron scattering (SANS). Our objective was to study the relationship between quench depth and Rc, the lower limit for the length scale of the structures formed during the early stages of the phase transition (nuclei). During the early stage of phase separation, the SANS profiles merged at a time-independent critical scattering vector, qc. We discuss different methods for estimating qc, and present arguments for the scaling relationship, Rc∼1/qc. The theory of Cahn and Hilliard predicts that in metastable blends Rc increases with increasing quench depth, and diverges at the spinodal. In contrast, our experimental measurements showed Rc increases with decreasing quench depth, and the location of the point where Rc diverges lies between the binodal and the spinodal. Some aspects of our results are addressed in recent theoretical work of Wang and Wood [J. Chem. Phys. 117, 481 (2002)], wherein the effects of fluctuations on the binodal and spinodal curves in polymer blends are incorporated.