Abstract
Phthalonitrile (PN) resin is renowned for its absence of small molecule release during curing, high processing stability, and exceptional thermal stability, positioning it favorably in the realm of high-temperature resin applications. However, its narrow processing window and high melting point present challenges to its development and practical use. To address these limitations, this work synthesized PN prepolymers 4,4′-bis (sulfonylbiphenyl) thiophtalonitrile (BSPS) and 4,4′-bis(carbonylbiphenyl) thiophtalonitrile (BSPC) by introducing thioether bonds as a substitute for ether bonds, aiming to expand the prepolymers' processing window. Meanwhile, building upon the oxidation process of thioether to sulfone observed in pharmaceutical production, the potential for in-situ oxidation of thioether bonds during the curing of phthalonitrile prepolymers was investigated. A comprehensive analysis of the curing behavior, viscosity changes, and thermal stability was conducted using differential scanning calorimetry (DSC), rheological analysis, and thermogravimetric analysis (TGA). The DSC and rheological test results reveal that the experimental group prepolymers BSPS and BSPC exhibit lower melting points (BSPS 179 °C; BSPC 180 °C) and broader processing windows (BSPS 52 °C; BSPC 79 °C) than blank. Additionally, the thermal performance of resins show improvement over the blank, with BSPS achieving a decomposition temperature of up to 530 °C (Td5%). XPS analysis of the curing process confirmed that the enhanced thermal stability is attributable to the partial oxidation of thioether bonds into stable sulfone groups. This innovative structural approach allows for satisfactory processing performance in the initial stage, followed by a spontaneous transformation into a more robust and thermo-resistant structure post-shaping. This finding carries significant implications for the development of a balanced phthalonitrile system that harmonizes processing ease with superior thermal endurance.
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