DIRECTIONAL PHASE-SEPARATION OF A BINARY POLYMER MIXTURE DRIVEN BY A TEMPERATURE-GRADIENT

Phase separation of the binary polymer mixtures poly(2-chlorostyrene)/poly(vinyl methyl ether) (P2CS/PVME) with a lower critical solution temperature was induced by using a temperature gradient. The critical point of the mixture was set between the two ends of the temperature profile. The interface which separates the miscible region from the thermodynamically unstable region of the samples was dragged toward the low temperature side by increasing the gradient. By using phase-contrast optical microscopy and digital image analysis, it was found that the mixture exhibits inhomogeneous structures with different characteristic length scales, indicating that the temperature gradient has affected the thermodynamic instabilities of the mixtures. The characteristic wavelengths of the structures along the gradient are smaller than those in the perpendicular direction. Toward the high temperature side, this structural anisotropy becomes less significant. These experimental results are qualitatively in agreement with the computer simulation reported recently by Furukawa [Physica A 180 (1992) 128] for the phase separation of binary mixtures under inhomogenous quenching conditions. These preliminary results suggest that polymer mixtures undergoing phase separation induced by a temperature gradient can provide a physical system to study the wavelength selection processes under conditions far from thermodynamic equilibrium.