Abstract:

Objective Flexible actuation materials with environmental stimulus responsiveness can respond to external stimuli and have corresponding actuation behaviors such as bending, deformation, rotation and contraction, which has great application prospects in the rehabilitation medicine, intelligent switches, artificial muscles, flexible robots and so on. However, the current problems, such as poor responsiveness, complex preparation process, high cost, and large pollution of stimulus sources, have greatly limited the development of the flexible actuation materials.

Method The polyvinyl alcohol(PVA)-ethylene(PE)/cellulose acetate butyrate (CAB) fibers were prepared by blending PVA-co-PE with CAB and then melting and extrusion. The PVA-co-PE nanofibers were obtained by separating CAB from composite fibers using acetone as solvent. Then, the obtained PVA-co-PE nanofibers were cut into pieces and put into a high-speed shear machine containing isopropyl alcohol aqueous solution (the PVA-co-PE nanofiber concentration was controlled at 3%), and a uniformly dispersed PVA-co-PE nanofiber suspension was formed by high-speed shearing for 2-3 min. Then, the obtained PVA-co-PE nanofiber suspension was evenly sprayed on the surface of PET substrate. After the solvent evaporated, the PET substrate was removed to obtain an independent PVA-co-PE nanofiber membrane. Then, the SiO₂ nanoparticles were dispersed in an aqueous solution with a concentration of 5%. Then, the dispersed SiO₂ nanoparticle dispersion solution was sprayed on the prepared PVA-co-PE nanofiber membrane. After drying at room temperature, the PVA-co-PE/SiO₂ composite actuation membrane was prepared. The microstructure characterization, contact angle properties, mechanical properties and actuation deform ability of the PVA-co-PE/SiO₂ composite actuation membrane were characterized.

Results First, the angle θ between the bending deformation of the PVA-co-PE/SiO₂ composite actuation membrane and the horizontal plane under the stimulation of moisture was taken as the maximum bending angle. It can be seen that the nanofibers stack layer by layer to form a disordered network structure, and with the decrease of the fiber diameter, the network structure of the nanofibers gradually decreased and became more uniform. In addition, the smaller the diameter of the PVA-co-PE nanofibers, the larger the specific surface area that can be provided, and the more conducive to the adhesion of SiO₂ powder on the surface of the PVA-co-PE nanofiber membrane. In addition, when the fiber diameter increased from 180 nm to 390 nm, the water contact angle on one side of the nanofiber membrane increased from 46.69° to 55.7°, and the water contact angle on the side of SiO₂layer increased from 10.9° to 45.9°. In other words, with the increase of nanofiber diameter, the hydrophilicity of both sides of the composite actuation membrane showed a decreasing trend. At the same time, the fiber diameter also had a great impact on the tensile properties of the composite membrane. As the diameter of the PVA-co-PE nanofiber increased from 180 nm to 390 nm, the tensile fracture stress of the PVA-co-PE/SiO₂ composite actuation membrane increased from 5.27 MPa to 6.94 MPa and the tensile strain increased from 3.33% to 8.99%. Then, the maximum bending angle and response speed of the composite membrane were characterized. The results showed that as the fiber diameter decreased from 390 nm to 180 nm, the maximum bending angle of the PVA-co-PE/SiO₂ composite membrane increased from 52.25° to 180°. At the same time, the increase of fiber diameter also affected the response speed of the composite membrane. With the decrease of fiber diameter, the response time of the composite actuation membrane decreased from 1.2 s to 0.7 s, and the bending angle of 180° was reached within 0.7 s. Additionally, the maximum bending angle of the PVA-co-PE/SiO₂ composite actuation membrane decreased with the increase of particle size of SiO₂ powder. As the particle size increased from 15 nm to 200 nm, the maximum bending angle decreased from 180° to 34°, and the response speed also gradually reduced from the initial 0.7 s to 1.2 s. Based on the excellent moisture stimulation response, the PVA-co-PE/SiO₂ composite actuation membrane was prepared into a bionic finger structure, which can induce the bending and stretching behavior similar to the human palm under external moisture stimulation.

Conclusion A PVA-co-PE/SiO₂ composite actuation membrane with an asymmetric structure was prepared by a simple spraying process. The tensile performance of the composite actuation membrane was greatly improved with the increase of fiber diameter, which can be attributed to the fact that the larger the fiber diameter, the larger the pores in the three-dimensional network structure of the nanofiber membrane, and the larger the relative slip space between the fibers, leading to increase in elongation at break. In addition, the hydrophilicity of the composite membrane also increased with the decrease of fiber diameter and powder particle size of SiO₂. This is because the smaller nanofiber diameter resulted in the dispersion effect of the powder, which in turn promoted rapid penetration and diffusion of water molecules on the membrane surface, and improvised the hydrophilicity. Finally, the PVA-co-PE/SiO₂ composite actuation membrane showed excellent actuation performance under external moisture stimulation. That is, the maximum bending angle of 180° can be reached within 0.7 s. This can be interpreted as the different hygroscopic response between the upper and lower layers, the PVA-co-PE/SiO₂ composite actuation membrane would have asymmetric hygroscopic swelling when stimulated by moisture, which led to the rapid and reversible deform behavior. Based on the rapid stimulation response and large-scale deformability of the composite actuation membrane, it has a great application prospect in the fields of intelligent control, artificial muscle and intelligent clothing.

Key words: smart material, polyvinyl alcohol-ethylene masterbatch, moisture response, asymmetric structure, flexible actuation membrane, rapid responsiveness, large scale bending deformation