Kinetics of the molar mass decrease in a polyurethane melt: a rheological study

Dynamic melt viscometry was used to study the kinetics of molar mass decrea se in the molten liquid state of a polyurethane elastomer based on 4,4'-dip henyl-methane diisocyanate and 1,4-butanediol. It was found that at constan t temperatures chosen between 180 and 220 degreesC the molar mass of the po lyurethane melt decreased continuously until a thermal equilibrium was reac hed. The rates as well as the degree of decrease strongly depend on tempera ture and time. Based on a kinetic model, the temperature dependence of the zero shear viscosity was shown to be controlled not only by the molecular m obility but also by the change in molar mass. The true activation energy of how (131 kJ/mol) could be calculated after separation of the molar mass co ntribution. The rate constants and the activation energies of urethane form ation and dissociation were also determined. (C) 2001 Published by Elsevier Science Ltd.