THE EVOLUTION OF VISCOELASTICITY NEAR THE GEL POINT OF END-LINKING POLY(DIMETHYLSILOXANE)S

The linear viscoelasticity of polymers near the gel point can be described by two scaling laws. The material at the gel point has a power-law linear viscoelastic relaxation modulus, and the relaxation exponent has been found to vary with the composition of the precursor materials, i.e., it is not universal for gelation. A second scaling law describes the evolution of the linear viscoelastic properties through the gel point. The rate of change of the dynamic mechanical modulus/viscosity is observed to scale as a power-law function of frequency. This power-law function defines a dynamic critical exponent, and this has been found to be independent of precursor composition for end-linking poly(dimethylsiloxane) polymers and equal to kappa = 0.21 +/- 0.02. This exponent may be a universal measure of gelation. The technique of Time Resolved Mechanical Spectroscopy is used to observe the evolution of linear viscoelastic properties of crosslinking polymers in situ in the rheometer. A stretched exponential relaxation modulus describes the evolution of mechanical properties in the vicinity of the gel point very well. The exponents which characterize the divergence of the zero-shear viscosity and the equilibrium modulus are not universal, since they are related to the relaxation exponent and the dynamic critical exponent.