Transmission electron microscopy of 3F/PMDA-polypropylene oxide triblock copolymer based nanofoams

Transmission electron microscopy was performed on a polymeric nanofoam material, derived from a triblock copolymer composed of a fluorinated polyimide center block, 3F/PMDA (derived from pyromelletic dianhydride (PMDA) and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (3F)) and polypropylene oxide (PO) end blocks. The cast and imidized polymer exhibits a microphase-separated morphology consisting of PO microdomains within a polyimide matrix. The final nanofoam material is obtained by decomposing PO microdomains into low molecular weight products, which diffuse out of the polyimide matrix leaving nanometer length scale voids. Ruthenium tetroxide staining prior to microscopy was used to enhance the contrast between the 3F/PMDA matrix and the PO microdomains or voids, which permitted a more detailed view of the microstructure of both the foamed and unfoamed materials. From the power spectra of the micrographs, spatial correlation between the PO microdomains in the unfoamed material and between the voids in the foam were found. An interdomain separation distance of ca. 37 nm was observed. Analysis of the image yielded an average area of 411 nm² for the PO domains. The analysis indicated that the PO domains were oblong, having average major and minor dimensions of 35 and 12.5 nm, respectively. An autocorrelation of the image showed that the domain center of masses were positioned 41 nm apart, in close agreement with the domain spacing (ca. 37 nm) found as described above. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1067–1076, 1997