THE EFFECT OF ENTANGLEMENTS ON THE RHEOLOGICAL BEHAVIOR OF POLYBUTADIENE CRITICAL GELS

We investigated the stress relaxation behavior of critical gels origin ating from six nearly monodisperse, highly entangled polybutadiene mel ts of different molecular weight from 18 000 to 97 000 g/mole. The pol ymers were vulcanized by a hydrosilation reaction which takes place ne arly exclusively at the pendant 1,2-vinyl sites distributed randomly a long the polybutadiene chain. The BSW-spectrum represents the relaxati on of the initial uncrosslinked precursor. A characteristic parameter is the longest relaxation time of the precursor. Crosslinking increase s this longest time even further. Surprisingly, the relaxation spectru m of the precursor is not altered much by the crosslinking except for an additional long time behavior. At the gel point (critical gel), thi s long time behavior is self-similar. It follows the typical power law as described by the Chambon-Winter gel equation, G (t) = St(-n), in t he terminal zone. The critical relaxation exponent was found to be clo se to n = 0.5 over a range of stoichiometric ratios and for all precur sor molecular weights analyzed. A new scaling relationship was found b etween the gel stiffness, S, and the precursor molecular weight of the form: S approximately M(w)zn where exponent Z from the zero shear vis cosity-molecular weight relationship, eta0 approximately M(w)z, is com monly found to be z = 3.3-3.6.