Development of spiropyran bonded bio-based waterborne polyurethanes for mechanical-responsive color-variable films

Waterborne polyurethanes (WPUs) use water as the solvent to reduce volatile organic compounds emissions and are widely increased because they are environment friendly. Vegetable oil-based polyol is a potential environmental protection material and can replace petroleum-based polyols for WPUs synthesis. In this work, we synthesized new mechanical-responsive soybean oil (SBO)-based WPUs, and their corresponding responsive behaviors were regulated by the flexibility and rigidity of the polymer chains. Soybean oil polyols (MESOs) were firstly synthesized from soybean oils, which were used to synthesize waterborne polyurethanes (SP-MSPUs). Spiropyran (SP) as a mechanophore was bonded to polyurethane chains, and the color of polyurethane films would be changed under the action of stress, which shown the stress-sensitive behavior of polyurethane membrane. The double hydroxyl spiropyran diol (SP2) and the control single hydroxyl spiropyran (SP1) were synthesized and bonded into the SP-MSPU molecular chains, respectively. The SP-MSPU polymer films appeared a translucent light yellow in visible light, and the purple color of polyurethane films was observed under stress tension. Soybean oil polyols with triglyceride bonds are implanted into the polymer to regulate the flexibility and rigidity of the films. From SP- MSPU-115 to SP-MSPU-172 of the polymer samples, the stress required ranged from 1 MPa to 4 MPa when the polymerization tensile deformation was 200%. At the same time, under the same deformation, the UV absorption of polyurethane films synthesized from the same polyol (MESO-134) showed a trend of enhancement with the increase of the molar ratio of NCO and OH in polymer chains. Therefore, excellent mechanical response behavior of a solid SP-MSPU membrane can be obtained. This mechanical responsive SP-MSPU would be used in a variety of applications, such as multi-responsive probe materials, decorative coatings, and anti-falsification technology.