Phase Separation of Oligomeric Polystyrene-Polybutadiene Blends As Studied by Excimer Fluorescence

The binodal curve and phase separation behavior of 826 MW polystyrene (PS)/2500 MW polybutadiene (PB) blends have been studied by optical density measurements and fluorescence spectroscopy, respectively. The equilibrium compositions from the binodal curve and the Gelles-Frank two-phase model, which relates the composition of phases generated during spinodal decomposition to the ratio of excimer to monomer fluorescence intensity, I_E/I_M, were used to follow the time-dependent compositions of the phases during phase separations as well as the kinetics of phase separation. For 30 wt % and 60 wt % PS blends quenched at 32 °C, the change in I_E/I_Mduring phase separation was quite small, less than 10%, but nevertheless large enough to yield precise information. Cahn's theory of spinodal decomposition apparently describes adequately at least the first 3 min of phase separation, and the Gelles-Frank two-phase model predicts quantitatively I_E/I_Matequilibrium. Fluorescence results for the 30 wt % blend indicate that equilibrium is achieved in about 60 min; on the basis of the composition of the PS-rich phase the phase separation is about 90% complete in the first 13min. From the measured growth rate of decomposition, the Cahn-Hilliard diffusion coefficient is estimated to be of theorder -10_-12cm₂/s for both the 30 and 60 wt % PS/PB blends.