Photo-Cross-Linkable Coumarin-Based Poly(ε-caprolactone) for Light-Controlled Design and Reconfiguration of Shape-Memory Polymer Networks

Abstract

Photochemically cross-linked shape-memory polymer (SMP) materials have been achieved by functionalizing chain-ends of star-shaped poly(ε-caprolactone) (PCL) with 7-hydroxypropoxy-4-methylcoumarin followed by photodimerization of these end-groups. The kinetics of the network formation in function of the photosensitizer concentration has been studied by swelling experiments and rheology. Thanks to the design of a dedicated homemade mold, highly reproducible irradiation conditions have been achieved allowing to study the network formation and properties, especially the shapememory properties, in relation to the coumarin dimerization degree as determined by Raman spectroscopy. In optimized conditions, PCL-based SMP materials exhibiting high fixity and recovery have been achieved in remarkably short irradiation time, typically 5 min. In addition, the precise control of the network cross-link density with the irradiation time, so as the high stability of the formed networks toward temperature variations was also demonstrated allowing the fine-tuning of the network properties by the irradiation process. Finally, the reversible character of the coumarin dimerization under light irradiation of appropriate wavelength has been quantified by Raman spectroscopy. The dimer photocleavage allows the photoreconfiguration of the networks offering the ability to modify the "permanent" shape of the SMP material, while preserving the excellent shape-memory properties.

Programmed Multiresponsive Hydrogel Assemblies with Light‐Tunable Mechanical Properties, Actuation, and Fluorescence

“…The Fourier transform infrared (FTIR) spectrum for the nonirradiated gel (Figure D) shows a photolabile band at 1620 cm −1 due to CC stretching . Photocrosslinking (365 nm) caused coumarin photodimerization (Scheme B) and the CC stretching band absorbance decreased due to cyclobutane ring formation 19b. Subsequent photocleavage (254 nm) reformed the CC bonds (Scheme C) and the absorbance at 1620 cm −1 increased.…”

“…The relative absorbance decreased continually over 4 h of photocrosslinking but was mostly restored with 1 h of photocleaving (Figure S4C, Supporting Information). The slower rate for photocrosslinking is because this process involves a bimolecular reaction 19b…”

“…However, photocuring often involves small molecules, such as photoinitiators which may diffuse away from the point of construction or may be harmful to some cell types . Coumarin‐based polymers include a reversible photodimerizable group within the chains and have been included in gels 19a,20. Coumarins are intrinsically fluorescent and undergo photocrosslinking via a [2πs + 2πs] cycloaddition of the cyclobutane ring .…”

“…Coumarins are intrinsically fluorescent and undergo photocrosslinking via a [2πs + 2πs] cycloaddition of the cyclobutane ring . Coumarin‐based gels have been used for hydrogel object manufacturing20a and shape memory polymers 19a. In practice, the coumarin photodimerization is not completely reversible and we exploit this feature herein for construction of new multiresponsive gels.…”

Coumarins into Polyurethanes for Smart and Functional Materials

“…Since the implementation of this methodology, the first efforts focused on preparing polyurethanes carrying other photoreactive probes, such as anthracenes [123,124], cinnamates [125] and azo-benzenes [126]. On the other hand, coumarins have been inserted into other polymer systems such as PVA [127] or polyesters [128][129][130], achieving very encouraging results. However, it is not until the beginning of 2019 where the chemistry of coumarins was carried out within the polyurethane matrix in order to obtain shape memory materials [131,132].…”