Preparation and Characterization of Low-Molecular-Weight Polyacrylonitrile

Polyacrylonitrile (PAN) is renowned for its excellent physical and chemical properties, making it a promising candidate for producing high-performance and energetic materials. However, traditional high-molecular-weight PAN suffers from poor solubility and low reactivity, which limits its application as a precursor for advanced materials. To overcome these issues, this study successfully synthesized low-molecular-weight PAN (M_η: 6.808 kDa) using an environmentally friendly aqueous precipitation polymerization method, utilizing ammonium persulfate (6 wt% relative to the monomer mass) as the initiator and isopropanol (400 wt%) as the chain transfer agent. The structures and properties of the synthesized low-molecular-weight PAN were analyzed in depth. The morphology and chain structure of PAN were characterized using field-emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance hydrogen spectroscopy (¹H NMR). The thermal properties were assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Additionally, the state changes during the heating process of PAN with different molecular weights were directly observed using a visual melting point analyzer for the first time. Furthermore, the influence of molecular weight on PAN's solubility was investigated in detail. Based on that, a linear regression between the viscosity average molecular weight (M_η) and the number average molecular weight (M_n) was established, providing simple and rapid access to the molecular weight of the synthesized PAN via viscosity measurements. Our study employed CTA-controlled aqueous precipitation polymerization to prepare low-molecular-weight PAN, which possesses significant potential in producing tetrazole-based energetic materials.