Polybenzodiazine Aerogels: All-Nitrogen Analogues of Polybenzoxazines─Synthesis, Characterization, and High-Yield Conversion to Nanoporous Carbons

Abstract

Tetrahydroquinazoline (THQ) was designed as an all-nitrogen analogue of main-stream benzoxazine monomers. THQ solutions in DMF gelled at 100 °C via HCl-catalyzed ring-opening polymerization to polybenzodiazine (PBDAZ) wet gels, which were dried in an autoclave with supercritical fluid CO 2 to aerogels. These as-prepared PBDAZ-100 aerogels undergo ring-fusion aromatization at 240 °C under O 2 . This oxidized form is referred to as PBDAZ-240. Chemical identification of PBDAZ-100 and PBDAZ-240 relied on consideration of all nine possible polymerization pathways, in combination with elemental analysis, infrared and solid-state 13 C NMR spectroscopy, and 15 N NMR spectroscopy of aerogels from the selectively 15 N-enriched THQ monomer. Fully oxidized PBDAZ-240 aerogels were carbonized at 800 °C under Ar to carbon aerogels with 61% w/w yield and with retention of the nanomorphology of the parent PBDAZ-100 aerogels. Direct pyrolysis of PBDAZ-100 at 800 °C, i.e., without prior oxidation, resulted in only 40% w/w yield and complete loss of the fine nanostructure. The evolution of PBDAZ-240 aerogels along pyrolysis toward carbonization was monitored using progressively higher pyrolysis temperatures from 300 to 800 °C under Ar. Aerogels received at 600 and 800 °C (referred to as PBDAZ-600 and PBDAZ-800, respectively) had relatively high surface areas (432 and 346 m 2 g −1 , respectively), a significant portion of which (79% in both materials) was assigned to micropores. The new polymer aerogels, together with polybenzoxazine aerogels, comprise a suitable basis set for comparing N-rich versus O-rich porous carbons as adsorbers.

CO2 Uptake by Microporous Carbon Aerogels Derived from Polybenzoxazine and Analogous All-Nitrogen Polybenzodiazine Aerogels

“…Reflecting the common ring-opening polymerization chemistry, PBO and PBDAZ aerogels were structurally similar; however, the former contains both oxygen and nitrogen heteroatoms while PBDAZ contains only nitrogen (Scheme ). Reasoning by analogy to the pyrolytic chemistry of polyacrylonitrile, , it was discovered that for quantitative carbonization, both PBO and PBDAZ should be subjected to an oxidative ring-fusion aromatization step at relatively low temperatures under oxygen or air. , The resulting materials are termed PBDAZ-240 and PBO-200, where the numerical extensions refer to the oxidation/aromatization temperatures. The prevalent structures of the oxidized forms are included in Scheme .…”

“…The benzodiazine (BDAZ) monomer and the benzoxazine (BO) monomer were prepared as published previously. 34,45 Preparation of Polybenzodiazine and Polybenzoxazine Aerogels (PBDAZ and PBO). In a typical procedure, the BDAZ monomer (20% w/w) or the BO monomer (7 wt %) was dissolved in DMF.…”

“…In this work, we studied the role of oxygen versus nitrogen heteroatoms in the CO 2 uptake capacity of carbon aerogels by comparing the behavior of carbon aerogels coming from PBDAZ 45 and from PBO aerogels. 34 As shown in Scheme 1, PBDAZ contains only nitrogen while PBO contains both N and O. Owing to the structural similarity of PBDAZ and PBO aerogels, their pyrolytic chemistry is identical: for quantitative carbonization, both types of aerogels need to undergo complete oxidative ring-fusion aromatization, which is carried out under O 2 or air at relatively low temperatures (200−240 °C).…”

“…39,40 However, most carbon aerogels come from phenolic-resin type of polymeric aerogels like those from condensation of resorcinol formaldehyde (RF), 33,41−43 phenol formaldehyde, phloroglucinol formaldehyde, and phloroglucinol−terepthaldehyde 33 and like those from ring opening of suitable monomers like polybenzoxazine aerogels (PBOs) 34,44 and more recently polybenzodiazine (PBDAZ) aerogels. 45 Motivated by the capture of CO 2 by solutions of amines, 10 it has been proposed that a way to improve CO 2 absorption is by incorporating nitrogen into the carbon framework. 46,47 However, the role of nitrogen heteroatoms in CO 2 absorption by carbon aerogels has been debated, as among the best absorbers in this class of materials are carbon aerogels that do not contain any nitrogen at all.…”

Kinetic Modeling of Ring-Opening Polymerization of Benzoxazines Using MIL-53-Al as a Potent Catalyst

“…PBzs have recently gained considerable interest because of their outstanding potential of exhibiting the excellent properties of traditional phenolic resins, which make them attractive alternatives to a wide range of end-use applications. Aerospace and naval composites, − automotive frames and components, , composite pipes for gas or oil, , circuit boards, , aerogels, , self-healing materials, − coatings and corrosion protection, − and paints , are among these end-use applications. PBzs possess highly attractive features such as high glass transition temperature ( T g ), , low water absorption, near zero shrinkage upon polymerization, , high char yield, , fast evolution of mechanical properties with the conversion, , similar and even lower costs than epoxy and bismaleimides resins, and excellent dielectric properties. , These outstanding properties and intrinsically rich molecular design flexibility allowing good compatibility with other structures have rendered PBz an important contender to epoxy and traditional phenolic resins in the composite industry.…”