Graduate Seminars"The Gel Effect (Autoacceleration) In Free Radical Polymerization: Its Molecular Origin"Professor John Torkelson THURSDAY, JANUARY 23, 1998 1175 BENEDUM HALL - 11:00 a.m.The gel effect in free radical polymerization, seen as autoacceleration or reaction runaway, has been of industrial and scientific interest for over fifty years. The gel effect can result in several outcomes. As the polymerization is typically very exothermic, there may be reactor explosion. Less severe outcomes include ruining the reactor by producing vitrified polymer or producing polymer of much higher molecular weight than desired. For decades, researchers have been trying to determine the molecular scale origin of the gel effect. While it is known that the feature responsible for autoacceleration is a slowing down of the diffusion-controlled termination reaction involving two growing free radical chains, the molecular scale processes responsible for this have been unclear. For fifteen years the most commonly accepted explanation has been that at a certain conversion the polymer chains become entangled, changing the mode of diffusion of growing radical chains, severely slowing termination. With basic experiments, we have shown that the gel effect is present under conditions where entanglements are impossible, proving that entanglements are not the cause of the gel effect. Other incorrect conclusions were drawn previously in part because the nonisothermal nature of the reactions during autoacceleration was not taken into account. By doing polymerizations under isothermal conditions, even during autoacceleration, and by comparing the polymer concentration dependence of the termination rate parameter with that of short and long polymer chain self-diffusion, we determine that the gel effect is associated with the polymer concentration dependence of the diffusion of short chains involved in termination. This work illustrates well the validity of Occam's Razor, in that the simplest explanation is often the best, even for problems which have been the subject of study for many years. |
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