The role of the soft phase in rigidity enhancements in a particulate composite

Filling a non-conducting soft matrix with a hard conducting phase leads to two critical transitions: a percolation threshold with a drastic increase in conductivity and a rigidity threshold marking a substantial increase in the elastic modulus. A metal-particulate polymer composite has interpenetrating phases with substantial rigidity beyond the rigidity threshold. Experimental results show that the Young's modulus of the composite is enhanced significantly when an entrapped incompressible polymer phase exerts high hydrostatic stresses in uniaxial compression at the rigidity threshold. Additional experiments beyond the glass transition temperature of the polymer reveal a substantial reduction in the modulus of the composite at the rigidity threshold, as the polymer compressibility is increased significantly, confirming the role of the soft phase in rigidity enhancements. The present investigation provides a potential new designing criterion for enhancing the Young's modulus of an interpenetrating composite structure, by utilizing an incompressible second phase and negative volumetric strain