The rheology and gelation of bidisperse 1,4-polybutadiene

Abstract 1,4-polybutadiene (PB) of varying molecular weight, M , and varying polydispersity (obtained by mixing monodisperse PB) was characterized. As is well-known, some rheological properties are affected by both molecular-weight and polydispersity; however, others show a dependence on M , but not on molecular weight distribution (MWD). We find that the viscosity at constant weight-average M can be accurately accounted for from the ratio of the z- and weight average molecular weights. For M for which the steady-state recoverable compliance, J s , becomes invariant, no effect of polydispersity on J s was observed; this observation is contrary to the limited available literature. Gelation of the PB was also studied. In accord with previous work, the reaction time for network formation and the degree of crosslinking at the gel point both decrease significantly with increasing MWD. The only reliable measure of the gel point for polydisperse samples was from extrapolation of the soluble fraction of networks obtained for various states of cure. Different expressions describing the incipient network of crosslinking polymers were evaluated; these relations were found to be in quantitative mutual accord.