Thermoplastic resins have been widely used in fiber reinforced polymer composites because of its recyclability and short cycle times. However, the high viscosity after heating and melting restricts its infiltration on the surface of fiber. In this study, a series of thermoplastic epoxy resins were prepared via the chain extension reaction of epoxy groups with liquid aniline using triphenylphosphine (TPP) as catalyst. The relationship between polymer network structure and performance was comprehensively investigated. The solubility tests indicated that excessive aniline or TPP facilitated the crosslinking of resins. Besides, on the premise of thermoplasticity, appropriate TPP could increase the degree of chain extension, molecular weight, and glass transition temperature of resins. Furthermore, the in‐situ polymerization process facilitated infiltration between epoxy resin and the fibers before chain extension reaction. The bending test showed that the flexural performance of the sample with 2 phr of TPP was improved by 38.8%. Therefore, this work provides a feasible method to prepare the thermoplastic epoxy resins and its fiber‐reinforced composites with good mechanical properties.

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Large-tow carbon fibers have great potential in industrial and civil fields due to their high production capacity and processing efficiency. However, the dense packing of large-tow filaments exacerbates the heat accumulation within the bundle during the exothermic stabilization reaction, which significantly adds to the difficulty of process control and reduces product performance stability. These thermal behaviors are strongly affected by the configuration of the filament bundle, such as the tow size and the tow spreading width. Therefore, having a clear understanding of the effect of tow configuration on the stabilization process is crucial for producing large-tow carbon fiber with desired qualities. In this study, the effect of tow size and tow spread width on the thermal behavior and the structural transformation of polyacrylonitrile (PAN) fibers during stabilization was quantitatively investigated through dynamic monitoring of the temperature in the center of the bundle. The results indicated that the center temperature of large-tow bundles rises much slower due to the poor heat conductivity of PAN polymer fibers. When the exothermic reaction is initiated in the center of the bundle, the center temperature quickly rises as a result of poor heat dissipation within the densely packed large-tow fibers. The surface-center temperature differences result in significant structural heterogeneities within the bundle, as evidenced by the Fourier transform infrared spectroscopy, dynamic scanning calorimetry, and single-filament tensile tests. These phenomena are produced for a bundle size higher than 48 K. Finally, a processing model covering the tow size, the spread width, and the maximum differences in temperature between the bundle center and environments is proposed for product quality assurance.