Details

# Relaxation patterns of flexible polymers near the gel point

Abstract (Summary)
The complex rheological behavior of nearly critical gels of two high molecular weight flexible polymer precursor systems, vulcanizing polybutadienes above the entanglement molecular weight, $\rm M\sb{e},$ and endlinking polydimethylsiloxane below $\rm M\sb{e},$ was measured. The search for characteristic relaxation patterns resulted in a simple model which describes these patterns in a quantitative way with a minimum number of parameters. Both systems exhibit similar relaxation behavior in the terminal zone which changes dramatically during the crosslinking process. This pattern is represented by a modified Chambon-Winter gel spectrum with a relaxation exponent, n, which decreases with increasing degree of crosslinking. This spectrum is cut off at the longest relaxation time of the partially crosslinked polymer, $\rm\lambda\sb{max}.$ The entanglement and glass transition regions of the polybutadiene system are represented by the BSW spectrum. The entanglement and glass transition regions of the precursor polymer are less affected than the terminal relaxation by the gelation process, only the slope of the entanglement power law decreases due to polydispersity effects. The model parameters $\rm\lambda\sb{max}$ and G$\rm\sb{e}$ (equilibrium modulus) exhibit characteristic scaling behavior with respect to the distance from the gel point, $\rm\vert p-p\sb{c}\vert.$ A box-like contribution H$\sb{\infty}$ to the spectrum appears for samples beyond the gel point. The proposed model passes the self-consistency test by predicting the mechanical behavior (at different frequencies) as a function of the extent of reaction and other rheological observations during the sol-gel transition. The effect of large deformation flow on the crosslinking material was investigated. The samples were macroscopically destroyed at high shear rates and stresses which prevented the detection of any influence the flow might have on the gel time. No influence could be detected at smaller deformations or lower stress levels. Finally, the effect of changes in molecular weight distribution on time-resolved rheometry were studied for several different architectures. For the case of crosslinking polybutadiene, a critical mutation number of approximately 0.2 was found beyond which the experiment results in unreliable data.
Bibliographical Information: