Sol-gel process: influence of the temperature on the textural properties of organosilsesquioxane materials

The hydrolytic sol-gel polymerization of a variety of molecular precursors, (RO)(3)Si-X-Si(OR)(3) with different structural features, has been investi gated varying the temperature from -20 to +110 degrees C. "Rigid" precursor s (R = Me, X = 1,4-C6H4, ferrocene-1,1'-diyl), "semi-rigid" precursors (R = Me, X = CH2CH2-1,4-C6H4CH2CH2; R = Et, X = CH2CH2CH=CHCH=CHCH2CH2) and alk ylene-bridged precursors [R = Me, X = (CH2)(6), (CH2)(10); R = Et, X = (CH2 )(2), (CH2)(14)] were studied. A molecular precursor containing three Si(OM e)(3) groups, 1,3,5-(MeO)(3)SiC6H3, was also considered. The reactions were performed using ionic and non-ionic catalysts. The ionic ones were a nucle ophile [tetrabutylammonium fluoride (TBAF)], an acid (HCl) and a base (NaOH ), and the non-ionic ones were nucleophiles [dimethylaminopyridine (DMAP), N-methylimidazole (NMI)]. The solvent employed was THF. Reactions were stud ied in MeOH only with TBAF as catalyst. In all cases the temperature appear ed to be a fundamental parameter and had a drastic influence on the specifi c surface area and the porosity of the resulting solids. Particularly at lo w temperatures (-20 to 0 degrees C), the solids were mainly microporous wit h a weak mesoporous contribution without narrow pore size distribution. At higher temperature (20 to +110 degrees C), the solids exhibited very high s pecific surface areas and were mesoporous with a narrow pore size distribut ion. The specific surface area, the porosity and the pore size were highly dependent on the temperature. This work came in our study on the kinetic co ntrol of the texture.