Azobenzene-based thermoplastic polyurethanes with photothermal effects

The chemical structure of Az-PUs was characterized by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The resulting optimal polymer, synthesized by varying the molar ratios, with a 3:2:1 ratio for diisocyanate: polymer diol: azobenzene diol possesses a weight average molecular weight of183,900 g/mol, and a water contact angle of 106.3 ± 4.1o, along with tensile strength 6.9 ± 0.7MPa, Young’s modulus of 50.1 ± 5.3 MPa, elongation at break of 55.4 ± 0.3%, and energy at break of 3.6 ± 0.4 kJ/m3.

The polymer shows a heat release enthalpic change of 64.8 J/mol after ultra-violet (UV) irradiation (325 nm) for 5 minutes at 100 W. During UV irradiation, the photon energy is stored in the isomers of the molecule, which can be retrieved in the form of heat by a triggered release or a self-decay process. Density functional theory (DFT) calculations and UV-Vis absorption spectra were used to correlate modelling data with experimental data, providing insights into the electronic transitions, heat release and stability of the Az-PUs.

The DFT results indicated significant energy barriers (ΔG 219.2 − 299.6 kJ/mol) for isomerization. This aligns with the experimental IVUV-Vis spectra which show a decrease in the π→π* transition peak around 350 nm, indicating the conversion from the trans to the cis form of azobenzene. Notably, the inclusion of bulky substituents like methyl and ethyl groups significantly enhanced energy release.

Experimental data revealed that the Az-PU, containing a higher proportion of bulky azobenzene units with ethyl substituents (Az-PU 3(2)), recorded an energy release of 473 J/mol over 720 minutes. In comparison, the Az-PU with the same proportion of bulky azobenzene units containing methyl substituents (Az-PU 2(2)), recorded an energy release of 338 J/mol over the same period. Additionally, the mechanical and surface properties were evaluated, showing that Az-PU 3(2) exhibited superior hydrophobicity with a water contact angle of 108 ± 2.7º and the highest tensile strength of 10.1 ± 1.5 MPa.

This study provides a detailed insight into the interplay between polymer composition and solar energy harvesting efficiency, highlighting the critical role of azobenzene substituents and polyurethane segments in optimizing the material design for high-performance solar thermal applications. This type of smart recyclable polymers could be promising sustainable materials for developing efficient solar energy harvesting, storage and utilization.<br><br><i>Thesis embargoed until 31 July 2030.</i><br>