A generic methodology to study self-healing properties of thermo-reversible polymer networks (presentation)

Based on the reversible Diels-Alder (DA) reaction between furan and maleimide functional groups, an extrinsic self-healing material was synthesized for coating applications [1]. At elevated temperatures, the DA/retro-DA equilibrium is shifted towards the initial building blocks. This shift in equilibrium allows a temporary increase in local mobility, which is essential to seal damage. The recovery of initial properties takes place in a subsequent cooling by recombination of covalent bonds through the exothermic DA reaction.

Changing the spacer length in the furan functionalized compound leads to a flexible network design and tailor-made network properties with a variable cross-link density and glass transition temperature [1]. Based on these systems, a generic methodology was developed to study the self-healing properties of thermo-reversible networks.

The effect of temperature on kinetics and equilibrium of the reversible DA/retro-DA reaction, and also the effect of diffusion-control was measured (and modeled) by means of Fourier transform infrared spectroscopy, microcalorimetry and Modulated DSC. Both elastomeric and thermosetting reversible networks were investigated.

A maximum sealing temperature was determined to avoid unwanted side-reactions. In case of the DA networks studied, an irreversible homopolymerization of maleimide functional groups occurs above 120 °C. The flow behavior at elevated temperatures was characterized by dynamic rheometry in order to determine the gelation temperature (Tgel) of the reversible networks. It was shown that sealing of microscopic scratches is possible below Tgel, leading to the advantage for coatings that sufficient mechanical properties remain guaranteed during a thermal sealing/healing procedure. At low temperatures, the exothermic DA reaction was characterized by microcalorimetry and Modulated DSC proving the healing capacity of the networks and showing the repeatability of sealing/healing cycles in an acceptable temperature window. The mechanical properties in this temperature window were studied with dynamic mechanical analysis.

[1] Scheltjens G. et al., React. Funct. Polym. (2012), available online.