Rheological Images of Dynamic Covalent Polymer Networks and Mechanisms behind Mechanical and Self-Healing Properties

Abstract

In our previous work [Macromolecules2010, 43, 1191–1194], we synthesized dynamic covalent cross-linked polymer gels through condensation of acylhydrazines at the chain ends of poly(ethylene oxide) (A2) and aldehyde groups in tris[(4-formylphenoxy)methy]ethane (B3) and reported reversible sol–gel transition and self-healing properties of the gels. For those dynamic gels, this paper examines the gelation kinetics and rheological behavior in pre- and postgelation stages and discusses the molecular mechanism underlying the mechanical and self-healing properties. The results showed that the condensation reaction before the critical gelation point can be treated as the pseudo-second-order reaction. The scaling exponent n (=0.75) for the frequency dependence of the complex moduli at the critical gel point, the exponent γ (=1.5) for the concentration dependence of the viscosity in the pregel regime, and the exponent z (=2.5) for the concentration dependence of the equilibrium modulus in the postgel regime were found to not exactly obey the relationship for covalent gels, n = z/(z + γ), possibly because of the dynamic nature of the gels. The terminal relaxation of the dynamic gels at high temperature (125 °C) accorded with the Maxwellian model, as often observed for transient associating networks. In contrast, at low temperature (25 °C) where this transient network reorganization was essentially quenched in a time scale of experiments (∼50 s), the uniaxial stress–strain behavior of the gel was well described by the classical model of rubber elasticity σeng = G(λ – 1/λ2) up to 300% stretch (as similar to the behavior of usual gels chemically cross-linked in a swollen state). Ultimately, the gel cut into two pieces was found to exhibit self-healing under ambient conditions in 8 and 24 h, respectively, when the edges of those pieces were coated and not coated with acid (catalyst for dynamic covalent bond formation).