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

In our previous work [Macromolecules 2010, 43, 1191-1194], we synthesized dynamic covalent cross-linked polymer gels through condensation of acylhydrazines at the chain ends of poly(ethylene oxide) (A(2)) and aldehyde groups in tris[(4-formylphenoxy)methy]ethane (B-3) 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 gamma (=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 + gamma), possibly because of the dynamic nature of the gels. The terminal relaxation of the dynamic gels at high temperature (125 degrees C) accorded with the Maxwellian model, as often observed for transient associating networks. In contrast, at low temperature (25 degrees C) where this transient network reorganization was essentially quenched in a time scale of experiments (similar to 50 s), the uniaxial stress-strain behavior of the gel was well described by the classical model of rubber elasticity sigma(eng) = G(lambda - 1/lambda(2)) 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).