Dynamic viscoelasticity and critical exponents in sol-gel transition of an end-linking polymer

Abstract

Evolution of viscoelasticity in an end‐linking gel system has been investigated. Storage and loss dynamic shear moduli, G’(ω) and G‘(ω), are determined for angular frequencies ω over more than 2.5 decades near the sol‐gel transition. Gelation conditions are varied widely to obtain critical exponents which are independent of reaction rates and paths. A viscoelastic scaling exponent n at the gel point (G’∼G‘∼ωⁿ) has been observed as n=0.67–0.68. Reaction time dependent shift factors for relaxation time and compliance before the gel point are determined in creating modulus‐frequency master curves of G’ and G‘. Scaling of these shift factors yields critical exponents k and z for zero shear viscosity and steady state compliance as k=1.3 and z=2.7–2.8, respectively. On the other hand, the critical exponent for equilibrium shear modulus after the gel point is found to be 2.0–2.1, which is much smaller than the exponent for steady state compliance. It is suggested that links in the backbone mainly determine the critical exponent for the equilibrium modulus in chemical gelation. Besides the links in the backbone structure, blobs and dangling chains incorporated into the cluster are considered to contribute to the steady state compliance.