One pot preparation and evaluation of self-heqling polyurethane coatings

RIVERO GUADALUPE; NGUYEN LE-THU T.; HILLEWAERE XANDER; DU PREZ FILIP E.

Simposio; Macro Young Researchers' Meeting 2014; 2014

capability under mild temperature conditions will be presented. The material design combines the main advantages of several cutting edge topics in a complex functional system, but achievable in a straightforward, one-pot process. Diels-Alder thermoreversible covalent bonds were introduced as crosslinkers into a shape-memory polyurethane material, improving thereby the material mechanical properties. The healing strategy applied is based on a synergetic combination of Diels Alder covalent thermo-remendability with the simultaneous assistance of a shape memory agent to favor the crack closure. When a crack occurs, Diels-Alder bonds will preferentially break, regenerating free furan/maleimide functional groups. The shape memory effect favors the crack closure upon heating, resulting in a reformation of the reversible crosslinking bonds. In this way, the concept can be adapted as ?Diels Alder based shape memory assisted selfhealing? (DASMASH). The preparation method involves an initial Diels-Alder reaction among a commercial bismaleimide and a diol with a pendant furan moiety, easily synthesized on large scale. After the incorporation of polycaprolactone (PCL) as shape memory switching segments, the polyurethane formation reaction takes place in the same reaction vessel. The concept was proven and the healing performance upgraded after studying the role of each component and testing different compositions. The hard segment content does not only affect the physical/structural parameters such as crystallinity, toughness and transparency but also the self-healing characteristics in accordance with the resulting crosslinking degrees. Diels-Alder moieties are then proved to be the main responsible factor for the polyurethane thermo-remendability. Polyurethanes heal at 50°C after mechanical damage induced by either the application of a large tensile deformation, macro and instrumented micro scratches. The PCL melting provides enough mobility to recover the shape in a couple of minutes, enabling the progressive Diels-Alder bonding reformation in this same condition. On-line FT-IR monitoring allowed the structural and kinetic description of the system reversibility for numerous cycles. Furthermore, mechanical recovery was accomplished after multiple cycles of large deformation. Complete disappearance of microscratches was also achieved. The results were not only confirmed by by an optical inspection with scanning electron microscopy but also by the comparison of the cross-section profiles before and after healing with confocal microscopic mapping.