Self-healing 8-armed star-shaped epsilon-caprolactone oligomers dually crosslinked by the Diels-Alder and urethanization reactions

The reactions of a hydroxy-terminated 8-armed star-shaped epsilon-caprolactone oligomer (H8CLO), which was prepared by the ring opening polymerization of epsilon-caprolactone initiated by tripentaerythritol, with furfuryl isocyanate and (4-maleimidophenyl)isocyanate at the OH/NCO ratios of 1/1.5 and 1/1.2 produced furan-and maleimide-functionalized star-shaped oligomers (F8CLO and M8CLO), respectively. Furthermore, the same reactions except that the OH/NCO ratios are 1/0.75 and 1/0.6 produced half furan-and maleimide-functionalized star-shaped oligomers (H4F4CLO and H4M4CLO). By the furan/maleimide Diels-Alder (DA) reaction and/or OH/NCO urethanization reaction with 1,6-hexamethylene diisocyanate, a DA network (DA8CLO), polyurethane/DA network (PU4DA4CLO) and polyurethane network (PU8CLO) were synthesized. The differential scanning calorimetric analysis revealed that the endothermic peak temperature (113 and 143 degrees C) due to retro DA reaction of PU4DA4CLO was lower than that (157 degrees C) of DA8CLO. The conversion (87%) of DA reaction for PU4DA4CLO was a little higher than that (77%) for DA8CLO, and their DA conversions decreased with increasing annealing temperature over the range of 90 150 degrees C by the progression of retro DA reaction. The decrease of crosslinking density by the retro DA reaction was confirmed by the swelling test. When partially overlapped cut-films of DA8CLO, PU4DA4CLO and PU8CLO were stood at 70 degrees C for 24 h, the PU8CLO film was not self-repaired, but the DA8CLO and PU4DA4CLO films were self-healed. Especially, the self-repaired PU4DA4CLO film maintained the original shape during the treatment at 70 degrees C, while the self-repaired DA8CLO film considerably deformed by softening. (C) 2018 Elsevier Ltd. All rights reserved.