Synthesis of RF Aerogels

Resorcinol (R) is dissolved in a 37% aqueous formaldehyde solution in a molar ratio of 1 2. An aqueous solutitm of Na2C03 is added as catalyst [29, 53]. Water is added to the initial mixture to control the density of the final product The solution is mixed, poured into glass vials, and sealed. Stable RF wet gels are formed by heating for 1 day at 25 C, 1 day at 50°C, and 3 days at 90°C [27]. Aerogels are obtained by suitable drying processes, as described in detail below. 

The key parameters for controlling the stmctural properties of RF aerogels are the catalyst concentration, defined as the molar ratio of resorcinol to catalyst (R/C ratio) and the monomer concentration of reactants (R + F) in solution (RF wt%) [53, 54]. 

Resorcinol reacts with formaldehyde under alkaline conditions to form mixtures of addition and condensation products. The major reactions include (1) the formation of hydroxymethyl (-CH2OH) derivatives of resorcinol, (2) the condensation of the hydroxymethyl derivatives to form methylene (-CH2-) and methylene ether (-CH2OCH2-) bridged compounds, and (3) the disproportionation of methylene ether bridges to form methylene bridges plus formaldehyde as a by-product [29]. These reactions have been studied extensively by NMR [58]. The crosslinked polymer grows into discrete particles, which eventually form a three-dimensional network or a gel (Chap. 13). 

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Synthesis of RF-MOx interpenetrating networks is straightforward and highly versatile. Importantly, these materials can be used as precursors for the synthesis of other aerogels whose synthesis either has not been reported before or would be quite elaborate by other methods. The method can be modified easily to make films, thus comprising an economic alternative to chemical or atomic layer deposition methods CVD and ALD, respectively), which require expensive equipment. 

The resorcinol-formaldehyde polymer is classified as a phenolic resin. In fact, phenol-formaldehyde condensates are the first synthetic polymers introduced commercially (Bakelite) in the beginning of the twentieth century. Thus, the phenolic resin chemistry, on which there is a vast amotmt of information, can be used to understand the chemistry of the RF aerogel synthesis. In the first step, resorcinol reacts with formaldehyde to form hydro-xymethylated resorcinol in the second step, the hydroxymethyl groups condense with each other to form nanometer-sized clusters, which then crosslink by the same chemistry to produce a gel. The formation of clusters is influenced by typical sol-gel parameters such as the temperature, pH, and concentration of reactants. Though the first RF aerogel is synthesized by the aqueous polycondensation of resorcinol with formaldehyde with the use of sodium carbonate as catalyst [6, 12, 13], now several literature reports exist for aerogels made from phenol and formaldehyde [14—16]. 

Scheme 11.1. Mechanism of the base-catalyzed RF aerogel synthesis.

Figure 36.1. Reaction mechanism for the synthesis of resorcinol-formaldehyde (RF) wet gels, aerogels, and caitxm aerogels.

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