A facile approach to fabricate dual-responsive smart natural fibre sensors

Smart textiles with the characteristics of lightweight, flexibility, deformability, and breathability have been around for some decades. Although natural fibres have these qualities naturally, the intelligence of natural fibres is still a challenge. This study provides a facile approach to prepare dual-responsive natural cellulose and wool fibres based on reversible acidichromic and photoisometric water-soluble polymeric dye. An acidichromic and photoiscomeric waterborne polyurethane reactive dye (PUD) with the chromaticity, further reactive groups, and waterborne macromolecular structure was first designed and subsequently synthesized via a diazo coupling reaction between a polyurethane precursor (PUP) and a para-ester diazonium salt (PDS). The synthesized PUD with the characteristics of a polyurethane backbone, azobenzene chromophore, β-sulfatoethylsulfone reactive groups, and waterborne sodium sulfate groups was applied in dying of natural fibre products including cotton, ramie and wool fibres with an excellent colour fastness. The PUD possesses a thermal stability and solvent resistance. The synthesized PUD and its dyed natural fibres displayed the acidichromic and photoisomeric behaviours. The maximum absorption peak of the PUD aqueous solution shifted from 442 nm to 503 nm due to the structural transition changing from diazo to hydrazone when the pH values were adjusted from 7 to 1. Meanwhile, the colour of PUD aqueous solution and its dyed cotton, ramie and wool fibres reversibly changed from yellow to red upon pH changing from 7 to 1. With being exposed to 365 nm UV light, the trans-isomers in PUD slowly transformed into cis-isomers with an isomerization rate of 7.68 × 10−5 s−1. The stimuli-responsive polyurethane reactive dye can be a promising candidate for the design of a wearable and flexible natural fibre sensors aiming for monitoring the changes of environment. Furthermore, the conductive and rheological behaviours of PUD aqueous solution exhibited a promising application in the preparation of smart textile sensors via ink-jet printing technique.