Thermal Transformation of Polyacrylonitrile Accelerated by the Formation of Ultrathin Nanosheets in a Metal–Organic Framework

Abstract

In this study, polyacrylonitrile (PAN) nanosheets with unimolecular thickness were successfully synthesized by cross-linking polymerization in the 2D nanospaces of a metal–organic framework. In contrast to 1D and 3D analogues, crystallization could be inhibited by the topological constraint of the ultrathin 2D network structure, allowing for an efficient thermal transformation reaction of PAN. The amorphous nature of the PAN nanosheets led to an increase in the access of oxygen molecules to the polymer chains, facilitating the thermal dehydro­aromatization reactions to yield a ladder polymer structure with a highly extended conjugated system. Notably, further carbonization of this ladder polymer afforded graphitic carbon with a highly ordered structure because of the well-defined precursor structure.

Nanoconfinement Approaches to Two-Dimensional Polymeric Materials

“…As shown in Figure d, differential scanning calorimetry heating traces of 1D, 2D, and 3D PAN showed that the 1D and 3D PAN have clear melting peaks; in contrast, the 2D PAN showed a broad peak diagnostic of its amorphous nature. 2D PAN remains amorphous in the bulk state, facilitating thermal dehydroaromatization reactions to form a conjugated ladder structure …”

“…The dimensionality of the MOF nanopores is transcribed to the polymer structure. When monomers are polymerized in the 2D nanospace of a pillared-layer MOF are separated by coordination layers with the distance of the “pillar” molecules, 2DPMs were prepared on a gram-scale. ,, A similar synthetic trial of a 2DPM was performed in an earlier study using sodium montmorillonite clay; however, the obtained polymer was neither cross-linked nor isolated by removing the host . When we performed the cross-linking polymerization of styrene and ethylene glycol dimethacrylate (cross-linker) in a pillared-layer MOF, [Ni­(Hbtc)­(bpy)] n (btc = 1,3,5-benzenetricarboxylate, bpy = 4,4′-bipyridyl), we obtained 2D polystyrene (2D PS) after MOF removal (Figure a) .…”

“…The 1D and 3D PAN showed clear melting peaks; in contrast, the 2D PAN showed only a broad peak, indicating its amorphous nature. Reprinted with permission from ref . Copyright 2023 American Chemical Society.…”

“…Methods using pillared-layer MOF templates have enabled the synthesis of various 2DPMs from different monomers. For example, a 2D poly­(acrylonitrile) (2D PAN) and a 2D poly­(methyl methacrylate) (2D PMMA) were fabricated using the same method. , Both neutral and charged polymers are available. The hydrolysis of precursor 2D PMMA yields 2D poly­(methacrylic acid) (2D PMAA) in the form of a sheet-shaped polyelectrolyte .…”

“…In MOF Typical vinyl monomers and crosslinkers Variable degree of cross-linking, availability of 1D/3D counterparts, scalable, controllable thickness Random Flexible, obtained as monolayer, well-dispersed in solutions23,48,49 Confinement at Surface and Interface…”