Self-Healing of Unentangled Polymer Networks with Reversible Bonds

Abstract

Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory we study a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. We study the reaction kinetics of reversible bonds in this simple model and analyze the different stages in the self-repair process. The formation of bridges and the recovery of the material strength across the fractured interface during the healing period occur appreciably faster after shorter waiting time, during which the fractured surfaces are kept apart. We observe the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium (very low) density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess non-equilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface (formation of bridges) occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk.

Molecular dynamics studying on welding behavior in thermosetting polymers due to bond exchange reactions

“…Previously, Stukalin's et al 17 studied the surface welding of unentangled polymer networks. If we assume relatively higher bonding strength, the surface welding is dominated by hopping diffusion of exchangeable bonds, and the diffusivity D H is written as:…”

“…No matter which mechanism is chosen, CANs exhibit two features not found in traditional thermosetting polymers: malleability resulting from signicant stress relaxation, [14][15][16] and surface welding. 15,17 To further illustrate the process of BERbased surface welding effect, we drew a schematic graph as shown in Fig. 1.…”

A molecular dynamics study of bond exchange reactions in covalent adaptable networks

“…The developed sticky reputation theory demonstrated excellent capability in predicting the dynamics of reversible networks compared with the Gonzalez's theory, which typically exhibited several orders of magnitude deviation from the experimental data, especially when the material viscous behavior was obvious. Recently, Stukhalin and coworkers 28 developed a scaling theory to investigate the self-healing process in reversible networks, which was modeled by a system consisting of dangling chains with reversible stickers at one end attached to a permanently crosslinked network at the other end. They also found that the bond lifetime was the relevant time scale for the description of the stress relaxation in reversible networks.…”

Periodic “stick-slip” transition within a continuum model for entangled supramolecular polymers

“…9 bottom right), the velocity profile is again nearly linear meaning that the shear rate is again nearly homogeneous across the gap, and that all the material is now flowing in the direction of the applied shear. Note that f has recovered to higher values (more stickers attached), which indicates that the material has self-healed: it nearly recovered all the bonds it had before the rupture [75].…”