Light-responsive polyurethanes: classification of light-responsive moieties, light-responsive reactions, and their applications

Abstract

Stimuli responsiveness has been an attractive feature of smart material design, wherein the chemical and physical properties of the material can be varied in response to small environmental change. Polyurethane (PU), a widely used synthetic polymer can be upgraded into a light-responsive smart polymer by introducing a light-sensitive moiety into the polymer matrix. For instance, azobenzene, spiropyran, and coumarin result in reversible light-induced reactions, while o-nitrobenzyl can result in irreversible light-induced reactions. These variations of light-stimulus properties endow PU with wide ranges of physical, mechanical, and chemical changes upon exposure to different wavelengths of light. PU responsiveness has rarely been reviewed even though it is known to be one of the most versatile polymers with diverse ranges of applications in household, automotive, electronic, construction, medical, and biomedical industries. This review focuses on the classes of light-responsive moieties used in PU systems, their synthesis, and the response mechanism of light-responsive PU-based materials, which also include dual- or multi-responsive light-responsive PU systems. The advantages and limitations of light-responsive PU are reviewed and challenges in the development of light-responsive PU are discussed.

Fig. 1. (a–h) The chemical structure of photosensitive moieties.

Fig. 2. (a–f) The light-induced reactions.

Scheme 1. The reaction scheme for the synthesis of the photomechanical elastomer PME.

Fig. 3. (a) The PME film was cut into two pieces and held together for 5 min at 23 °C. The film can then be stretched to 100% strain without breakage. Reproduced with permission. Copyright 2019, John Wiley and Sons, and (b) the H-bonding in the PU structure of the thermoplastic PU-based PME with light-responsive and self-healing properties. The hashed bonds show the H-bonding.

Fig. 4. The chemical structure (a) dye-bonded BWPU and (b) dye-bonded BWPU-coated cotton fabric.

Fig. 5. The schematic illustration of shape memory of the bilayer film consisting of AZO-PU and EVA composites upon UV/Vis/NIR light irradiation (red: AZO-based PU layer and blue: EVA composite with photothermal filler layer).

Scheme 2. The reaction scheme for the synthesis of SCLCPU(AZO) and SCLCPU(AZO)-N.

Fig. 6. The chemical structure of (a) EHAB and (b) PCL-based SPU and (c) illustration of the light-responsive triple shape-memory effect (SME) in thermalstretching, as well as UV and NIR light irradiation.

Fig. 7. Schematic illustration of the bending mechanisms for 5CAZ/UCNP/SMPU films upon UV/vis/NIR irradiation. Reprinted with permission. Copyright 2019, Elsevier.

Scheme 3. The reaction scheme of the preparation of BHPU/TABA.

Fig. 8. Schematic illustration of the mechanism of photo-thermal staged-responsive shape memory properties in the molecular structure. Reprinted with permission. Copyright 2019, Elsevier.

Scheme 4. The synthesis route of Azo11.

Fig. 9. (a) Synthesis route of the AZO-PU polymeric dye. Reprinted with permission. Copyright 2018, Elsevier and (b) reaction scheme of the AZO moiety under UV light irradiation and acid condition.

Scheme 5. (a) Host-guest interaction of AZO. Reprinted with permission from L. Peng, S. Liu, A. Feng and J. Yuan, Mol. Pharm., 2017, 14, 2475–2486. Copyright 2017, American Chemical Society and CD under light irradiation and (b) the reductant breaks the disulfide bond and the crosslinkers are dissociated.

Fig. 10. The digital photos of stretch sensitive SP-WPU film. Reprinted under terms of the CC-BY license. Copyright 2017, Royal Society of Chemistry.

Scheme 6. The reversible photoinduced [2 + 2] cycloaddition reaction of coumarin moieties.

Fig. 11. Optical microscopy of coumarin-PU during the filling process. Reprinted with permission. Copyright 2016, Elsevier.

Fig. 12. Illustration of the core crosslinked polyurethane micelles for the intracellular release of anticancer drugs triggered by the acidic microenvironment inside the tumor cell. Adaptation under terms of the CC-BY license. Copyright 2020, International Journal of Molecular Sciences.

Fig. 13. The schematic illustration of the self-healing process of the PUAn films.

Scheme 7. (a) The photocleavage reaction of o-nitrobenzyl and (b) the reaction of o-nitrosobenzaldehyde and primary amine.

Scheme 8. The LBCL was photocleaved under 365 nm and further initiated light-induced methacrylate polymerization.

Fig. 14. (a) Reaction scheme of the photocleavage of o-NB and (b) schematic illustration of the formation and structural change of the light-responsive PU nanoparticles loaded with Nile Red as the model drug.

Ki Yan Lam

Choy Sin Lee

Mallikarjuna Rao Pichika

Sit Foon Cheng

Rachel Yie Hang Tan