A Surprise from 1954: Siloxane Equilibration Is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism

Tetramethylammonium silanolate-initiated ring-opening copolymerization of octamethylcyclotetrasiloxane (D4) and bis(heptamethylcyclotetrasiloxanyl)ethane (bis-D4) renders cross-linked network polymers that contain ethylene bridges and active silanolate end groups. These "living" reactive anionic species are not neutralized by ambient atmosphere exposure (are stable to water, oxygen, CO2) and promote thermally activated equilibration among different network isomers and cyclic oligomers. The cross-link density of these living networks can be controlled by the ratio of D4:bis-D4, and the density of active chain ends is determined from the initiator:monomer ratio. We report that samples prepared with particular ratios of initiator:D4:bis-D4 can be cut with a sharp knife, even into two pieces, and can heal by siloxane equilibration to restore the original strength of the silicone sample. Fracture toughness measurements were carried out and revealed complete (mechanical) healing. Broken and healed samples generally failed in locations other than the initially cracked region. We call attention to publications and patents from the 1950s that suggest that this self-healing behavior was likely obvious 60 years ago.