ИСТИНА

Polymer composites with non-uniform distribution of nanoscale filler in a matrix, also called as composites with segregated structure, are often characterized by very low percolation threshold and distinct piezoresistive properties. One of the types of composite materials with segregated structure are composites based on solid-state processable ultra-high-molecular-weight polyethylene UHMWPE, in which segregation can reach extreme degrees. This takes place due to inability of the filler particles to enter volume of the solid polymer, thus forcing them to stay on the boundaries of the polymer grains. To develop a complete understanding on how such type of the segregated structure responds to external stimulus, such as deformation, a multi-scale numerical modeling approach based on finite element method was proposed. Process of modeling was divided into two stages. At the first stage, a single representative element of the segregated structure was investigated. To play that role, a thin parallelepiped (layer) was chosen, with thickness value corresponding to such in real segregated structure of UHMWPE-based composite sample. In that volume, the filler particles were distributed uniformly, while realistically representing separate filler particles, i.e. accounting for ability of nanotubes to easily assume different conformations. With the help of external procedure for calculating the electrical conductance of the obtained distribution, taking into account that contact electrical resistances domination over resistance along the filler particle length between the contacts, the volume was deformed uniaxially at various magnitudes, while at each magnitude of deformation its electrical conductance was calculated. Electrical conductance was calculated for all main directions in of the layer. The layer deformation was also performed at different angles to the plane of the layer. Thus, a series of empirical laws was obtained, angle between direction of deformation and the layer’s plane, axis of the layer, magnitude of deformation and the electrical conductance value. At the next stage, a realistic volume element of composite with segregated structure was constructed numerically. To achieve that, focused ion beam scanning electron microscope was used, allowing to obtain several hundred of slices of small section of a UHMWPE-based composite sample. With the help of image processing software, a simplified surface mesh resembling the real distribution of the filler in the composite was created. The obtained mesh was successfully imported into FEA software. Each element of the mesh was characterized by conductivity change law, developed at the previous stage, relating the conductivity value in each direction of the element to orientation of the element relative to the future uniaxial deformation axis, and to a magnitude value of the uniaxial deformation. The mesh, representing the segregated structure made of the filler, was embedded into uniform parallelepipedal mesh, corresponding to the polymer matrix using embedded element technique. The obtained representative volume element (RVE) of the composite was uniaxially deformed, while to its opposite surfaces a unit potential difference was applied. In the course of deformation at series of steps the electrical current value, flowing through one of the selected surfaces was measured. The obtained results of the numerical simulation for the composite RVE was compared to the results, obtained experimentally for UHMWPE-based composite sample, to the results obtained numerically for a single element of segregated structure, and the results obtained experimentally for a thin layer of filler, located between two polymer plates. It was concluded, that complexity of segregation noticeably affects conductance response to deformation, and that developed numerical model can produce reliable results in predicting in. The reported study was funded by RFBR according to the research project 19-03-00369.