closure. It was concluded that, replacing S3 by S6 in the formulation, the mobility increase. By the other hand, it is possible to include a trifunctional amine as chain extender (instead of a difunctional one, which decreased the mobility) and even keep the self-healing ability.
Another strategy was presented by Zhang et al. [75], proposing the use of C-ON bonds in alkoxyamine to promote the self-healing action in PU composites. Based on this idea, 5-hydroxy-2-(4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yloxy)-2-methyl pentanenitrile (CTPO) was proposed as a dynamic reversible molecule that contain a nitrile group bonded to C–ON moieties. Due to nitrile has the ability to absorb electrons and stabilize carbon radicals, C-ON bonds in CTPO can be activated at a lower temperature.
Figure 3.27 Chemical structure of bis[4-(3-hydroxypropyloxy)phenyl]disulfide (S3) and bis[4-(6-hydroxyhexoxy)phenyl]disulfide (S6) (Reprinted with permission from Nevejans et al. [69]).
Polyurethane was obtained through the copolymerization of CTPO with 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate (IPDI) and polyethylene glycol (PEG), where PEG represent the soft block, while IPDI and CTPO are the hard segments. Figure 3.28 presents a scheme of the healing mechanism. The self-healing efficiency was measured through notched impact test. The same sample was damaged and healed up to three times, obtaining an average healing efficiency of 94.6, 91.7 and 89.8% for the first, second and third time, respectively.
Hu et al. [76] grafted a hard segment of TPU with 2-ureido-4[1H]-pyrimidione (UPy), embedding the disulfide bonds in the principal chain. UPy groups exhibits a dimerization trend that includes four hydrogen bonds in a donor–donor–acceptor–acceptor array, with rapid kinetic and high dimerization energy.
The robust interaction between quadruple H-bonding of the UPy side groups acts as supramolecular crosslinking points allowing the improvement of mechanical properties in the TPU elastomer. The self-healing abilities was characterized by a dumbbell-shaped sample, who was fully cut in half and cured at 100 °C for 2 h. After the applied healing procedure, it was not possible to find the original crack, suggesting that the structural integrity was recovered.
Figure 3.28 Schematic diagram of the healing reaction in PU (Adapted with permission from Zhang et al. [70]).
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