Chemical approaches towards monolithic hydrophobic aerogels and xerogels

Abstract

The control of wettability is of paramount interest in Materials Science. Phenolics represent an important class of thermosets materials that are intrinsically hydrophilic due to free phenolic hydroxy groups. Yet, their conversion into hydrophobic materials may provide higher stability towards ambient conditions to explore hitherto impossible applications in wet environments. This is particularly important for hydrophilic organic aerogels and xerogels as their porous structure is prone to destruction by exposure to water. In that context, we wish to disclose strategies towards hydrophobic aerogels and xerogels based on phenolics. These approaches include the post-synthetic silylation of monolithic resorcinol-formaldehyde xerogels leading to highly hydrophobic material bearing silyl ethers functionalities. Beside such post-synthetic modifications of gels, a de-novo-synthesis of hydrophobic organic gels is also presented. Most syntheses reported in literature rely on the base-promoted addition of formaldehyde to the phenol followed and accompanied by condensation to polymers, limiting the syntheses to free phenolics ultimately producing hydrophilic polymeric gels. In turn, few examples are reported for the use acids of as promotors for addition and condensation, but these are restricted to free phenolics. We wish to report on a process using phenolic ether as starting material yielding a promising new subclass of organic aerogels and xerogels. With respect to both approaches, the scope of influencing structure and properties by means of chemical synthesis is discussed.