Preparation of Gradient Polyurethane and Its Performance for Flexible Sensors.

Flexible sensors are prone to the problems of slow recovery rate and large residual strain in practical use. In this paper, a polyurethane functional composite with a gradient change in elastic modulus is proposed as a flexible sensor to meet the recovery rate and residual strain without affecting the motion. Different hard and soft segment ratios are used to synthesize a gradient polyurethane structure. The conductive percolation threshold was obtained between 45 wt% and 50 wt% of flake silver powder. Both gradient polyurethane and gradient polyurethane composites demonstrated that gradient materials can increase the recovery rate and reduce residual strain. The gradient polyurethane composites had a tensile strength of 3.26 MPa, an elastic modulus of 2.58 MPa, an elongation at break of 245%, a sensitivity coefficient of 1.20 at 0-25% deformation, a sensitivity coefficient of 11.38 at 25-75% deformation, a rate of recovery of 1.95 s at a time, and a resistance to fatigue (over 1000 cycles at a fixed strain of 20% showed a stable electrical response). The sensing performance under different cyclic strain frequencies was also investigated. The process has practical applications in the field of wearable skin motion and health monitoring.

Synthesis of Controllable Superparamagnetic Nano Fe3O4 Based on Reduction Method for Colloidal Clusters of Magnetically Responsive Photonic Crystals.

In this paper, we designed and investigated a reduction-based method to synthesize controllably monodisperse superparamagnetic nano Fe3O4 colloidal clusters for magnetically responsive photonic crystals. It was shown that the addition of ascorbic acid (VC) to the system could synthesize monodisperse superparamagnetic nano Fe3O4 and avoided the generation of γ-Fe2O3 impurities, while the particle size and saturation magnetization intensity of nano Fe3O4 gradually decreased with the increase of VC dosage. Nano Fe3O4 could be rapidly assembled into photonic crystal dot matrix structures under a magnetic field, demonstrating tunability to various diffraction wavelengths. The nano Fe3O4 modified by polyvinylpyrrolidone (PVP) and silicon coated could be stably dispersed in a variety of organic solvents and thus diffracted different wavelengths under a magnetic field. This is expected to be applied in various scenarios in the field of optical color development.