From thermoplastic polyurethane to covalent adaptable network via reversible photo-crosslinking of a biobased chain extender synthesized from caffeic acid

Abstract

An original segmented thermoplastic polyurethane (TPU) has been synthesized with a biobased aromatic chain extender prepared from caffeic acid (EC-diol). Some reference TPUs were also synthesized with aliphatic or aromatic chain extenders, for comparison. The TPUs were characterized by NMR, FTIR, TGA, DSC, DMA and tensile tests. 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. 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 [2 + 2] cycloaddition, leading to crosslinking. The photodimerization was followed by UV-vis and FTIR spectroscopies. 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. Photocrosslinking leads to significant changes in the physical properties of the material, with for instance an increased rigidity. When irradiated with UV light of shorter wavelength, the photodimerization is reversible, leading to decrosslinking. However, the reversibility is only partial. 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.