Temperature Gradients Induce Phase Separation in a Miscible Polymer Solution

Phase separation occurred up to 20 °C above the coexistence temperature in a polymer solution (polystyrene-polybutadiene-dioctylphthalate) to which small temperature gradients ( $\approx 2°\mathrm{C}$) were applied. Before convection began, spinodal-like patterns with characteristic spacing that grew in proportion to time elapsed persisted for times up to hours. The cause appears to be thermally driven concentration gradients normal to the surface, large enough to induce phase separation parallel to the surface, although temperatures throughout the mixture exceeded the thermodynamic coexistence temperature.