Temperature-quench phase separation and thermal convection in polystyrene solutions: Ultrasound and optical results

Nonequilibrium phenomena in solutions of polystyrene (PS) in the poor solvent diethyl malonate are studied with pulsed ultrasound at 5 MHz and polychromatic light. The time evolution of the attenuation and velocity of ultrasound and the attenuation of transmitted light were measured simultaneously during phase separation after rapid temperature changes (T quench) through the coexistence curve. Solution concentrations both below and near the critical values were investigated, and the monodisperse-PS sample molecular weights were 35 and 900 k. All of the parameters measured changed dramatically during phase separation and showed some unusual features. Up to an hour after a T quench, the samples were reversible upon T jump. For substantially longer times, only the acoustic velocity recovered, indicating that network softening (and swelling) occurs first in polymer mixing. The acoustic attenuation was higher during the opalescent time after T quench due to sound scattering from the larger nucleating droplets, whose growth and dissipation rates could thus be monitored. Under a temperature gradient and far from phase separation, solutions experienced instabilities in the convective flow which was sensitively detected as slow modulations of the acoustic attenuation. For smaller driving heat flux, the large modulations were nearly sinusoidal with ∼5 min period, and drops of dye traced out a single toroidal convective cell in this small-aspect-ratio sample. At higher heat flow, the oscillatory convection became very irregular.