Producing crack-free colloid-polymer hybrid gels by tailoring porosity

Several mixtures obtained by ultrasonic agitation of colloidal silica with a sol solution containing tetraethoxysilane (TEOS) were used to form crack-free monoliths. The dry gels that were submitted to mercury porosimetry showed differences in behavior according to the proportion of silica colloid in the gel. For low proportions, the mercury isostatic pressure exclusively induced an irreversible compaction of material, but intrusion occurred in gels with a high proportion of colloid. This permitted the evaluation of the textural properties of intruded gels, whereas the pressurization-depressurization paths, where intrusion was prevented, provided information about their compression behavior. Specimens exhibited elastic strain, followed by yield and plastic hardening. Bulk modulus was significantly reduced as the content of silica particles in the gel was increased. This was related to the increase in free volume of the network. The yield point, which characterizes the limit of the elastic region, was taken as a reference for estimating the maximum capillary pressure of drying; the inclusion of colloidal silica progressively reduced the maximum capillary pressure occurring during the drying for all samples except those containing the highest proportion of colloid (82%). This anomalous behavior was associated with low TEOS content, which complicates the formation of siloxane bonds. Textural properties determined by nitrogen adsorption confirmed the results obtained by porosimetry. Both porous volume and pore size were gradually increased in line with the colloid content. Therefore, we offer an effective means of tailoring the derived sol-gel porosity by increasing the proportion of colloidal silica, starting from a dense microporous material to achieve a mesoporous gel with a high pore volume and reduced capillary pressure.