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The linear viscoelasticity of polydimethylsiloxane polymers near the gel point
Abstract
Rheology is a particularly useful tool for elucidating the nature of polymer structures. The polymer at the gel point, the critical gel, has a power-law relaxation modulus, G(t) = St$\sp{-n}$. This simple constitutive equation is an expression of structural self-similarity which evolves as a result of crosslinking. We tested the composition dependence of the critical gel properties--the strength S and the relaxation exponent n--by studying an array a precursor materials, with different chain length, stoichiometry, and diluent concentration. All materials studied exhibited power-law rheological behavior, however, the critical gel properties were found to depend on the precursor composition. Several end-linked polydimethylsiloxanes with various composition yielded n values between 0.18 and 0.92, which is almost the entire possible range (0 $<$ n $<$ 1), while the gel strength varied by over five decades. Series of experiments at different stoichiometry and dilution can be reduced with S = G$\sb0\lambda\sb0\sp{\rm n}$, where G$\sb0$ and $\lambda\sb0$ may be material characteristic constants. The rate of change of the dynamic modulus was found to scale as a power law of frequency. The exponent, termed the dynamic critical exponent, describes the evolution of mechanical properties. It appears to take on a universal value $\kappa$ = 0.21 $\pm$ 0.02. A simple constitutive equation in the form of a stretched exponential can model the evolution of mechanical properties very well.
Subject Area
Polymer chemistry|Materials science|Plastics
Recommended Citation
Scanlan, James C, "The linear viscoelasticity of polydimethylsiloxane polymers near the gel point" (1990). Doctoral Dissertations Available from Proquest. AAI9100539.
https://scholarworks.umass.edu/dissertations/AAI9100539