This study thus shows an original TPU able to reversibly crosslink upon UV irradiation at different wavelength, but also points some limitations of this dissociative covalent adaptable network for developing thick materials, because of the well-known limited penetration depth of the UV light. However, the reversibility is only partial.
When irradiated with UV light of shorter wavelength, the photodimerization is reversible, leading to decrosslinking. Photocrosslinking leads to significant changes in the physical properties of the material, with for instance an increased rigidity. It is complete in about 2 h on thin films but is limited by the penetration depth of the UV radiation, and longer exposure is required to fully crosslink the TPU in the bulk. The photodimerization was followed by UV-vis and FTIR spectroscopies. Thanks to the peculiar structure of EC-diol, the corresponding TPU presents unsaturated esters as pendant chains, which are able to dimerize under UV irradiation by cycloaddition, leading to crosslinking. The TPU based on EC-diol presents the characteristic features of thermoplastic elastomers, with phase segregation between soft and hard segments, leading to remarkable mechanical and thermal properties. The TPUs were characterized by NMR, FTIR, TGA, DSC, DMA and tensile tests. Some reference TPUs were also synthesized with aliphatic or aromatic chain extenders, for comparison. An original segmented thermoplastic polyurethane (TPU) has been synthesized with a biobased aromatic chain extender prepared from caffeic acid (EC-diol).
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