Chemomechanical Properties of Electrospun Polyvinyl Acetate (PVAc) Fiber Mat and its Application in Water-Triggered Self-folding

Environmental-stimulus-triggered self-folding mechanisms have found promising applications in many engineering fields. Recently, a water-activated self-folding procedure has been designed by using the electrospun polyvinyl acetate (PVAc) fiber mat which contains high tensile residual stresses in the vitrified fibers during the spinning processes. The water permeation initiates plasticization of PVAc fiber mat and leads to a material shrinkage. When water diffusion starts at the top surface of a PVAc sheet, a shrinkage variation along the diffusion pathway forms a bending hinge on the sheet, which has been demonstrated in 3D origami design. To capture the water-triggered plasticization mechanism and chemomechanical coupling deformation compatibility, a consistent finite deformation viscoplastic model is developed for the PVAc fiber mat under coupled chemomechanical loading conditions. The residual stress and ‘fixed’ strain are modeled through the unrecoverable plastic strain in the PVAc fiber mat. As water permeates into the PVAc fiber mat, the induced increase in mixing entropy lowers the glass transition temperature of the material, and results in a gradual relaxation of the fixed viscoplastic strain. A non-Fickian diffusion model suitable for glassy material is adopted to capture the water permeation in the PVAc fiber mat. After calibrated and validated by a series of experiments, the proposed model is implemented in ABAQUS software to simulate the water-activated self-folding of PVAc sheet. The numerical example for a typical origami design suggests a promising engineering application prospect.