Rheology, dynamics, and mechanisms of self-healing of polydimethylsiloxane dual-linked by ureido and dynamic imine...

For the self-healing polymer, the recovery of mechanical properties and self-healing mechanisms are significantly affected by the time and temperature dependences of physical and chemical bonding dynamics, or by a synergistic combination of the two. In this work, the self-healing dynamics of chains consisting of short polydimethylsiloxane (PDMS) blocks joined together at their ends through both hydrogen-bond-forming ureido and dynamic imine linkages are systematically investigated by shear and extensional rheology, in situ Fourier transfer infrared spectra (FTIR), and self-healing time of severed samples. Using synthesized polymers with different ratios of ureido and imine linkages and measuring rheology and dynamics of the self-healing process over a range of temperatures, it is shown that the self-healing time is comparable to characteristic relaxation time (the inverse of the cross-over frequency of G' and G″ near the terminal zone). Polymer relaxation and self-healing are significantly sped up by replacing ureido linkages with imine linkages, and increasing the temperature, both of which reduce the concentration of hydrogen bonds, according to FTIR. Extensional strain hardening disappears above a critical temperature, evidently because the hydrogen bonds between the ureido groups become too weak to act as robust physical cross-links between chains.