Polybenzoxazines (PBOs) are phenolic resins bearing Mannich rather than methylene bridges between their phenolic moieties, and are pursued mainly as cost-effective alternatives to polyimides for high temperature applications. Low-density, highly porous PBO aerogels have been synthesized via heat-induced ring opening polymerization (typically at 130 oC) of benzoxazine monomers, such as the one obtained from condensation of bisphenol A, aniline and formaldehyde. Gelation takes a few days. Curiously, the carbonization yield of those aerogels is &gt;2 times that of non-porous PBO. By implementing a new time-efficient (a few hours) room-temperature HCl-catalyzed synthesis of PBO aerogels, we were able to deconvolute gelation from curing processes and we found that in addition to completing polymerization, high temperatures curing in air caused oxidation of the Mannich bridges along the polymeric backbone, followed by fusion of the phenol and aniline rings in every polymer repeat unit. That oxidative aromatization process rigidized the polymeric backbone, and increased carbonization yields up to 61% w/w. Carbon aerogels from aromatized PBOs were microscopically similar to their parent aerogels, however, they had greatly enhanced surface areas, up to 520 m2 g-1 (from ≤70 m2 g-1) with up to 83% of those attributed to newly created micropores. Upon reactive etching (CO2/1000 oC), the total BET surface area of PBO-derived carbon aerogels skyrocketed to almost 2,500 m2 g-1. Using the PBO chemistry as a point of departure, we have explored whether oxidative ring fusion aromatization can take place in other phenolic resins including phenol-formaldehyde, resorcinol-formaldehyde, phloroglucinol-formaldehyde, and phloroglucinol-terephthalaldehyde. A pathway similar to that of PBO is followed by the phloroglucinol-based resins, whereas aromatization yields pyrylium cations along the polymeric backbone. Unlike the case of PBO, however, the carbonization yield of those systems did not improve. Instead, early rigidization via aromatization leads to carbon aerogels with higher surface areas

Oxidative ring-fusion aromatization and its role to pyrolytic carbonization of polybenzoxazines and other phenolic resins

255th National Meeting (2018)

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