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

Abstract

The reactions of a hydroxy-terminated 8-armed star-shaped ɛ-caprolactone oligomer (H8CLO), which was prepared by the ring opening polymerization of ɛ-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 °C) due to retro DA reaction of PU4DA4CLO was lower than that (157 °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 °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 °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 °C, while the self-repaired DA8CLO film considerably deformed by softening.