Shrinkage and springback behavior of methylsilsesquioxanes prepared by an acid/base two-step processing procedure

We report on the effect of water concentration on the shrinkage and springback behavior of methylsilsesquioxane (MSQ) materials prepared by an acid/base two-step procedure in ethanolic solutions. The morphologies of the MSQ gels were characterized by SEM, nitrogen sorption, and mercury porosimetry. The chemical structures of these gels were also determined by solid-state C-13 and Si-29 NMR. As expected, shrinkage decreases as the volume of macropores in the material increases. Some samples, however, display shrinkage followed by expansion (springback phenomenon) even in the absence of surface modification or solvent exchange. The springback effect is attributed to the intrinsic properties of MSQ monoliths, namely, hydrophobicity and a low concentration of reactive groups on the surface. The implications of these findings for the development of low-shrinkage materials are discussed.