Fabrication and Thermal Conductivity Characterization of Polyetherimide Nanofoam

Abstract

A number of analytical models exist to predict the thermal conductivity of foams; however, they do not consider the effect of pore size on the effective thermal conductivity. It is speculated that foams with smaller pore sizes would have much lower thermal conductivity owing to the Knudsen effect. This study aims at fabricating polymer nanofoams with pore sizes on the level of nanometers and to characterize their thermal conductivity. Polyetherimide (PEI) foams were fabricated using solid state foaming. Process parameters such as saturation pressure and duration, desorption time and foaming temperature were varied to obtain foams with pore sizes ranging from a few hundred nanometers to two microns. The microstructures of the samples were characterized using scanning electron microscopy. Throughout the cross section of the foams, there exist regions with varying pore size and porosity. The effective porosity and thermal conductivity of the individual regions were determined based on a series model for effective thermal conductivity. It is confirmed that as the pore size is reduced while maintaining a fixed porosity, the thermal conductivity also decreases.