Electrical and mechanical investigation of silver-polyurethane composite film under applied pressure

Today, there is a growing demand for flexible, lightweight, inexpensive conductive materials in the electronic industry. Many scientific studies have focused on conductive polymer composites (CPCs) because CPCs are ideal materials for electronic applications such as wearable sensors, smart fabrics, flip-chips and switching devices. This research investigates the electrical and mechanical properties of micro silver polyurethane (Ag-PU) composite films with and without segregated structures. Ag-PU composite films were prepared by adding micro silver particles (<3.5 m) into the thermoplastic polyurethane (PU) matrix using the solution mixing method followed by spin-coating or casting techniques. This method led to a segregated network of silver microparticles with concentrations from 0 to 10 vol.%. Then, to achieve a uniform dispersion of the filler in the polymer matrix, the silver solution was ultra-sonicated prior to addition to the PU solution. Ag-PU composite films with uniform distribution of the silver were prepared at the same silver concentration as the segregated composite. Both composites were characterized using an optical microscope and SEM. The electrical conductivity of the composites was measured using a developed set-up with a digital multimeter. Both composites, with or without the segregated network of silver, showed electrical conductivity only through thickness when compressed under vertical pressures. For the segregated structures, the spin-coated samples witnessed a jump in conductivity (percolation threshold) at 1.2 vol.% silver, whereas the casted counterparts showed this increment at higher concentrations, at around 6.6 %.vol of filler. The maximum conductivity value for these composites was about 4×10-3 S/m. The percolation threshold and conductivity values of the composite film with uniform distribution of silver were lower than those of the segregated composite. Mechanical properties were also analysed using a tensile test based on the D882 standard. Generally, tensile strength and elongation at break decreased as filler concentration increased for both composites. The electrical performance of fabricated composites was numerically simulated using the finite element method based on the representative volume element model (FE-RVE) using MSC-Digimat software for the first time. This simulation technique enabled to simulate the electrical behaviour under applied pressure for the agglomerated and uniform dispersed microstructure of Ag-PU composite.

Keywords: electrical conductivity, micro silver, percolation threshold, simulation, RVE