Evolution of Linear Viscoelasticity across the Critical Gelation Transition

TL;DR

This paper develops a theoretical framework describing how linear viscoelastic properties evolve across the sol-gel transition, emphasizing the importance of continuity and symmetry at the critical point.

Contribution

It introduces a unified theoretical model that enforces continuity and symmetry of viscoelastic moduli at the gel point, deriving new scaling relations and bounds.

Findings

Dynamic moduli and their derivatives are continuous at the gel point.

The relaxation exponent n always exceeds the scaling exponent κ.

A new parameter C characterizes the evolution of storage versus loss modulus.

Abstract

In this work, we develop a rigorous theoretical framework for the evolution of linear viscoelastic properties across the sol-gel transition. More specifically, we derive general admissible expressions for the relaxation modulus and dynamic moduli as the critical gel state is approached from the pre-gel or the post-gel side. These expressions possess a generalized multi-mode series representation and recover the critical gel power law spectrum in the limit of vanishing distance from the gel point. We validate these expressions against the experimental data for various polymeric and colloidal systems. A central finding of the present work is the requirement of continuity of the dynamic moduli and their derivatives at the critical gel point, which imposes a profound physical constraint, necessitating the relaxation dynamics on both sides of the transition to be symmetric. This, in turn, leads to the hyper-scaling relation, which is a theoretical requirement rather than an empirical proposal. We further show that the critical relaxation exponent (n) always remains above the relaxation scaling exponent (\kappa), establishing a previously unrecognized lower bound on n. We also analytically estimate, for the first time, the parameter C that characterizes the relative evolution of the storage modulus with respect to the loss modulus as the critical state is approached. These results reveal that the symmetry, scaling, and hyperscaling properties of the sol-gel transition are all consequences of a single unifying physical requirement originating from the continuity of the linear viscoelastic properties at the critical gel point.