Precursory reaction of thermal cyclization for polyacrylonitrile

• Published in: Polymer (Guilford) Vol. 226; p. 123780
• Main Authors: Furushima, Yoshitomo, Kumazawa, Ryoichi, Yamaguchi, Yoji, Hirota, Nobuhiro, Sawada, Keisuke, Nakada, Masaru, Murakami, Masataka
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 04-06-2021; Elsevier BV
• Subjects: Calorimetry; Differential scanning calorimetry; DSC; Electron paramagnetic resonance; Electron spin; Electron spin resonance; Endothermic reactions; Exothermic reactions; Glass transition temperature; Heating rate; Luminescence; NMR; Nuclear magnetic resonance; Polyacrylonitrile; Resonance; Spin resonance; Temperature; Thermal reaction; Thermal stability; Transition temperatures; X-ray diffraction; Polyacrylonitrile; Thermal reaction; DSC; Luminescence
• DOI: 10.1016/j.polymer.2021.123780
• EISSN: 1873-2291
• ISSN: 0032-3861

Simultaneous differential scanning calorimetry (DSC) and luminescence (LUM) measurements of polyacrylonitrile revealed that a precursory reaction progresses below the well-known endothermic cyclization reaction (i.e., peak temperature ca. 280 °C). The intensity of LUM increased above 90 °C, which is around the glass transition temperature. After the peak at 180–190 °C, the LUM intensity decreased with temperature up to 250 °C, i.e., below the onset temperature of cyclization reaction. Thermal signals cannot be seen on the DSC curve in the temperature range of the LUM peak. The activation energy (Ea) estimated from the heating rate dependence of the LUM peak temperatures was 122 kJ mol−1. However, the Ea obtained from the well-known exothermic peak temperatures of DSC measurements was 190 kJ mol−1. Electron spin resonance and nuclear magnetic resonance also support the chemical structure of the luminescence species. FT-IR and X-ray diffraction were also performed to confirm the change in chemical and physical structural. Finally, ab initio calculations suggest that thermally stable heterocyclic radical species with two to four fused six-membered rings are the main products of the precursory reaction.
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•Precursory reaction progresses below the cyclization reaction of polyacrylonitrile.•The intensity of luminescence increased above the glass transition temperature.•Thermal signals cannot be seen on the DSC curve for precursory reaction.•Radical species with two to four fused six-membered rings are the main products.