Production of bulk solid-state PEI nanofoams using supercritical CO sub(2)

In this paper, a process to introduce nanoscale cells in homogeneous polyetherimide (PEI) is presented. The nanofoams produced have a bulk porosity (void fraction) in the range of 2564%, with the average cell sizes in the range of 40100 nm. Uniform nucleation of cells throughout the volume of the PEI specimen was observed. Supercritical CO2 at 20 MPa was used as the blowing agent and the specimens were foamed in a hot press to ensure flatness for further processing and characterization. Sorption studies showed that at 20 MPa, PEI can absorb about 10% CO2 by weight and that a 1-mm thick specimen can reach an equilibrium concentration in approximately 100 h at 45 DGC. The effects of desorption time, foaming temperature, clamping pressure, and foaming time were investigated. Several nanoscale morphologies were observed through changes in the foaming temperature, which ranged from 165 to 210 DGC. In one experiment, it was found that when the clamping force is increased from 1 to 10 tons, the average cell size increased from 40 to 4000 nm or by a factor of 100. This points to the clamping force as an important process variable to control the nanostructure introduced in PEI. Optimal processing conditions for the production of defect-free nanofoams are presented.