CONVECTIVE INSTABILITIES IN TRAVELING FRONTS OF ADDITION POLYMERIZATION

Traveling fronts of polymerization have been observed in unstirred solutions of methacrylic acid and benzoyl peroxide. Three types of convective instabilities have been observed. (1) The heat released by the exothermic reaction decreases the density of the reacting solution but changes in the composition tend to make the density increase. The net change in density is negative. Simple convection results, which causes a downward propagating front to remain perpendicular to the gravitational vector even as the tube is tilted. (2) However under some conditions of concentration and temperature, long slender "fingers" of polymer are observed to sink from the solid polymer front. The appearance of structures analogous to "salt fingers" in ocean layers of different temperatures and salinity are analyzed in terms of the theory of double-diffusive convection. Similarities with convection in directional solidification are considered. (3) Pulsating fronts have been observed which result in a striated material. The energy of activation of the fronts was determined and used to show that a convective instability instead of a pure thermal one is the cause of the pulsations.