Polyimide–Silica Gel Hybrids Containing Metal Salts: Preparation via the Sol–Gel Reaction

A facile preparation of polyimide–silica gel hybrids by the simultaneous in-situ formation of polyimides during the hydrolysis–condensation of tetramethoxysilane (TMOS) is reported here. The hydrolysis and condensation of TMOS was carried out in a solution of DMAc containing 5% LiCl, CaCl₂ or ZnCl₂ and the seven-membered cyclic polyimide intermediate. The seven-membered cyclic intermediates, precursors of polyimides, were derived from the low-temperature polycondensation of dianhydrides [benzophenonetetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), and 4,4-bis(hexafluoroisopropylidene)phthalic dianhydride (6FDA)] and di-isocyanates [isophorone di-isocyanate (IPDI), toluene di-isocyanate (TDI), hexamethylene di-isocyanate (HDI) and 4,4′-diphenylmethane di-isocyanate (MDI)]. These intermediates could readily be converted to the corresponding polyimides. Films were cast from the resulting mixtures and the solvent was gradually evaporated at 130 °C to result in the formation of clear, transparent, pale yellow or amber-colored hybrid films in which the salts were dispersed at the molecular level. Pyrolysis of polyimide–silica gel hybrids at 600 °C gave mesoporous silica. Silica gel obtained from hybrids HPI-8 (containing no salt) and HPI-11 (containing ZnCl₂) had a pore radius (BJH method) of 2.9 nm, while that from hybrid HPI-9 (containing LiCl) had a pore radius of 11.4 nm. The surface areas (BET method) obtained were 203 m² g⁻¹, 19 m² g⁻¹ and 285 m² g⁻¹, while the pore volumes were 0.373 cm³ g⁻¹, 0.158 cm³ g⁻¹ and 0.387 cm³ g⁻¹, respectively, for samples obtained from hybrids HPI-8, HPI-9 and HPI-11. © 1997 by John Wiley & Sons, Ltd.