Correlation between mechanical stiffness and thermal transport along the solid framework of a uniaxially compressed polyurea aerogel

• Uniaxial compression of a polyurea aerogel
• Mechanical stiffness and thermal conductivity measured as a function of density
• Impact of micro-structural changes on macroscopic properties
• Correlation between mechanical stiffness and solid thermal conductivity
• Different sensitivity to the curvature of struts and to the homogeneity of network

A polyurea aerogel with a density of 0.027 g/cm3 was uniaxially compressed to a maximum density of 0.400 g/cm3. The micro-structural effects of the uniaxial compression were analyzed by scanning electron microscopy, small angle X-ray scattering and nitrogen sorption experiments. The thermal conductivity was determined as a function of density with a guarded hot plate apparatus. The macroscopic mechanical stiffness was characterized by quasi-static compression testing. Analysis of the experimental data reveals that when the polyurea aerogel is subjected to a mechanical load, the solid backbone structure provides thermal pathways that are not simultaneously contributing to supporting the external mechanical load. At low densities, the mechanical stiffness of the compressed aerogels is correlated to solid thermal conductivity in a square relationship, since micro-structural deformation is dominated by bending mechanism. At high densities, however, the exponent describing the correlation decreases from 2 to 1, as network elements are compressed linearly. To allow for a structural decoupling of mechanical and thermal properties, the microscopic homogeneity of the network structure and the curvature of the network elements are identified as key parameters.