Strain sensitive polymer composite material

Composite materials were synthesized by direct polymerization of PP on the surface of natural or synthetic graphite particles. The method ensures grafting of polymer to a part of the filler particle surface, while the other part remains open to physical contact between the particles, the resulting properties of the material becoming favourably different from previously known composites. The coefficient of strain sensitivity K = Delta R/R sub epsilon (R is the resistance of the sample, epsilon is the tensile strain) was measured for different concentrations of filler at different temperatures. There is a broad maximum of K around the percolation threshold (4.5 vol.% for natural graphite) with a peak value of 100-150, which is much higher, compared to conventional wire resistors. A slight hysteresis is observed at unloading due to plasticity of the matrix. The hysteresis disappears when the temperature is lowered by 20-50 deg C, or epsilon is < 1%, but the previous high value of K remains. Below the glass transition temperature K is very low. The results are explained by the rearrangement of current carrying chains in loading-unloading cycles. The temperature dependence of resistance is presented, thermal conductivities were calculated for different models and compared with experimental values.