A New Method to Study Contributions of Polymer Fibers and Water Respectively to the Hydrogel Stress under Tension and Compression Using 3D Micro-Fiber Network Model

Abstract

The hydrogel can be regarded as a system comprising of a three-dimensional (3D) polymer network entrapping water in intermolecular space. Contributions of polymeric fibers and water to macrostresses of hydrogels are usually separated by fitting stress–strain relations from experiments. In this paper, we developed a new method to determine the two contributions by applying triaxial loads on a 3D micro-fiber network model, in which not only the uniaxial tension or uniaxial compression direction, but also the other two lateral directions are applied with external loads for keeping the unchanged volume of the fiber network model throughout the loading process. The lateral forces on the model can be regarded as the forces of water on the micro-fiber network at boundaries, since the lateral boundary conditions of hydrogels under uniaxial loading are completely free. With the calculation of the effective areas of polymeric fibers and water on the lateral surfaces, the hydrostatic pressure of water can be derived in each loading step, and further, the contributions of polymeric fibers and water to macrostresses of hydrogels in uniaxial loading direction can be determined quantitatively. The results show that in small deformation (the strain is less than about 10%), contributions of the water and polymeric fibers are of the same order of magnitude. In larger deformation, polymeric fibers are decisive in the contribution to stresses in tension, while water takes the dominant contribution in compression. Therefore, the difference in the contribution of water in tension and compression can explain the phenomenon of different initial elastic moduli of hydrogels in tension and compression. The method recommended in this paper can be used to learn the contributions of polymeric fibers and water in hydrogels with diverse fiber contents and even under multi-axial loading.