Production and characterization of carbon-silica nanocomposites from the pyrolysis of polyacrylonitrile in a porous silica matrix

Carbon-silica nanocomposites are fabricated from the pyrolysis of polyacrylonitrile (PAN), which has been photopolymerized in the similar to7 nm voids of porous Vycor glass. The effect of the pores on the pyrolysis process was investigated by thermal analysis. Temperatures at which key processes in the pyrolysis of bulk PAN occur are not significantly changed in the porous constraints of the Vycor, although prepolymer (nitrile cyclization) formation (similar to270 degreesC) was split into two exotherms. The carbon that forms in the pores was found to exist as graphitic nanoparticles. In contrast to bulk pyrolyzed PAN, little amorphous carbonaceous material is formed when carbonization is carried out in the pores. Moreover, the graphitic nanoparticles were oriented in the pores with the 002 graphitic planes aligned from crystallite to crystallite. A mechanism for this orientation is proposed on the basis of how the polymer aligns in the pores when the monomer is photopolymerized. Heating to 1100 C closed the pores around the carbon particles, creating a dense transparent black glass.