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The relaxation behavior of highly entangled polybutadiene critical gels
Abstract
The stress relaxation behavior of critical gels originating from six highly entangled polybutadienes of low polydispersity with molecular weights from 18,100 to 97,000 g/mole were investigated. The polymers were vulcanized by a hydrosilation reaction which takes place nearly exclusively at the pendant 1,2-vinyl sites which are distributed randomly along the polybutadiene chain. The empirical BSW-spectrum was found to describe the relaxation behavior of the uncrosslinked precursor state. A characteristic parameter of the BSW-spectrum is the longest relaxation time of the precursor. Crosslinking increases this longest relaxation time even further but had little effect on the relaxation behavior in the entanglement and transition zones. The relaxation time spectrum of the material at the gel point (critical gel) was found to be well represented by the superposition of the BSW-spectrum and CW-spectrum with minor modifications to the intermediate time regime. The long time behavior follows a power law as described by the Chambon-Winter equation, G(t) = St$\sp{\rm -n}.$ n was found to be constant with a value close to 0.5 over a stoichiometric ratio range of 0.25 $<$ r $<$ 3.0. The gel strength (S) was found to scale with the precursor molecular weight as $\rm S \sim M\sbsp{w}{zn}$ where z is the exponent from the zero shear viscosity molecular weight relationship $\rm\eta\sb0\sim M\sbsp{w}{z},$ is commonly found to be z = 3.3-3.6. A simple empirical model combining first order kinetic analysis with the Flory-Stockmayer branching theory was developed to predict the gel time as a function of stoichiometric ratio and precursor functionality. In-situ rheological measuring techniques of gelation allow for accurate determination of the gel point. The kinetic model was found to agree well with experimental data.
Subject Area
Polymer chemistry
Recommended Citation
Derosa, Michael Edward, "The relaxation behavior of highly entangled polybutadiene critical gels" (1994). Doctoral Dissertations Available from Proquest. AAI9420619.
https://scholarworks.umass.edu/dissertations/AAI9420619