Fabrication of Bulk Skinless Polyetherimide (PEI) Nanofoams

Abstract

Polymer nanofoams are classified as foams having pore size less than hundred nanometers. Several techniques to fabricate nano porous polymer morphologies have been developed. However, majority of them result in foams with a thin layer of un-foamed polymer on the surface. The un-foamed polymer region called the skin layer is typically around 10 microns thick which restricts the use of these foams for many applications such as filtration. Skinless open-celled nanofoams are a unique category of foams that have pore size on the order of tens to hundreds of nanometers and do not have any unfoamed solid polymer layer on the surface post processing. Potential applications for skinless nanofoams include filtration, catalysts, dielectronics and biological scaffolds. It has also been suggested that these nanofoams will have improved thermal and electric properties due to the open celled porous morphology and the absence of skin layer. In this study bulk skinless polyetherimide (PEI) nanofoams were fabricated using a novel two stage technique consisting of combined solid state and laser foaming. Initially, PEI samples embedded in a sacrificial polymer layer were saturated with supercritical carbon dioxide (CO2) as the blowing agent. The sacrificial layer ensures uniform gas concentration gradient at the surface during subsequent desorption step. A hot press and heated bath technique were used independently to foam the samples. Solid state foaming parameters — foaming time and number of sacrificial layers were varied to study their effect on the skin thickness of the nanofoams. The thickness of the sacrificial polymer layer had a direct effect on the skin thickness of the PEI nanofoams after foaming. It was observed that the skin thickness reduced by nearly 60% on an average due to the sacrificial layer. The solid state foaming process was followed by laser foaming using a CO2 laser with a wavelength of 10.6 μm to generate pores in the thin skin layer. The power intensity of the laser beam, the travel speed and the working distance had a direct effect on the laser pore formation process. It was found that a power intensity of 0.09 W, laser head travel speed of 50 mm/s and working distance of 4.5 cm were the most ideal conditions to form pores in a skin layer of approximately 10 micron thickness. The cross sections were observed using a scanning electron microscope to study the cell morphologies, pore size and the skin layer. This technique was found to produce skinless nanofoams with average porosity 78 % and smallest pore-size of 250 nm. To summarize, optimal parameters and processing conditions for bulk production of skinless PEI nanofoams are presented in this study.