Probing Sol–Gel Matrices and Dynamics of Star PEG Hydrogels Near Overlap Concentration

Abstract

Although numerous studies have been conducted to describe the gelation of multi-arm star polymers, reports on the relationship among rheological quantities, overlap concentration (c*), and microstructures have still remained insufficient. Here, we examine the sol–gel dynamics of hydrogels formed by 4-arm poly(ethylene glycol) (PEG) near c* based on dynamic scaling theory. We investigated the evolution of viscoelastic modulus (storage modulus G′ and loss modulus G″) with reduced gelation time (τ) and the normalized extent of crosslinking (ϵ), and a divergent dependence was observed near c*. A general expression of the Hill equation was employed to evaluate the complex modulus spectra and critical relaxation exponent (Δ) at the gel point (i.e., G′ ∼ G″ ∼ ωΔ), providing a way to access such a critical exponent, regardless of how fast the gelation occurs. Besides, the dynamic scaling exponent with ϵ shows high sensitivity to the pre-gel clusters and post-gel networks. Moreover, two-dimensional time–frequency viscoelastic mapping indicates that the hydrogel formed at c* shows higher homogeneity than those away from c*, and inhomogeneity of the local cluster density would contribute to the large-scale fluctuation in rheological quantities during gelation.