Novel Pyrolytic Carbon Membranes Containing Silica:  Preparation and Characterization

A new class of pyrolytic carbon membranes containing silica (C−SiO 2 ) has been prepared by the pyrolysis of copolyimides consisting of two phases and characterized. To control micropores in the C−SiO 2 membrane, the polyimide copolymer precursors were synthesized via two building blocks that consisted of a carbon-rich phase and a silicon-rich phase. The carbon-rich block consisted of pyromellitic dianhydride and oxydianiline, and the silicon-rich block was oligomeric organosiloxane. These C−SiO 2 membranes were characterized by FT-IR spectroscopy, X-ray diffraction, ESCA, FE-SEM, AFM, and TEM. These analyses revealed that the C−SiO 2 membranes have an asymmetric structure in which the top surface consists of a SiO 2 -rich phase in a continuous carbon matrix and the bottom surface is mainly a carbon-rich phase. In a molecular probe study using small molecules (He, O 2, N 2, and CO 2 ) having sizes from 2.6 to 3.64 Å, the C−SiO 2 membranes exhibited an outstanding molecular sieving capability, together with a high gas permeability. Furthermore, the gas permeation behavior of the C−SiO 2 membrane was very similar to that of the precursor, because the initial morphology of the precursor was well-kept by the use of thermostable components (carbon and silicon) in all blocks after the pyrolysis. The present study provides important information, namely, that the main geometry of the carbon precursor determines the micropore structure and the separation capability of the final pyrolytic carbon membranes.