The tensile strength of brittle fibers in liquids can be lower than that in air. Based on Griffith’s theory, this phenomenon can be explained by a change in the surface crack state owing to chemical or physical interactions. Therefore, this phenomenon can be used to analyze the surface crack state. In this study, single fiber tensile tests of glass fibers (GFs), polyacrylonitrile-based carbon fibers (PAN-based CFs) and pitch-based carbon fiber (pitch-based CFs), and silicon carbide (SiC) fibers were performed in air and liquids with different surface-free energies. The strength reduction mode was different for each fiber: GF and PAN-based CF had strength reduction at low strength, pitch-based CF had no strength reduction, and SiC fiber had strength reduction at full strength. The ratio of surface to internal defects in each failure probability region was estimated based on the test results. It was found that the strengths of the PAN-based CFs and SiC fibers were controlled by the surface cracks, and the strengths of the GFs and pitch-based CFs were controlled by the internal cracks. If surface defects are difficult to observe for their size, this method has the potential to analyze the location and proportion of defects that cause brittle fiber fractures.

To optimize the performance of low-molecular-weight poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] as an electron-transporting layer material, the electron-transporting behavior of P(NDI2OD-T2) in blends with polystyrene (PSt) was investigated. The results demonstrated that blends of P(NDI2OD-T2) with 10 wt% PSt after annealing exhibited significantly higher electron-transporting performance compared to unblended P(NDI2OD-T2). The electron mobility of the annealed blends reached 3.26 × 10^-4 cm²/V·s, approximately 30 times higher than that of annealed unblended P(NDI2OD-T2) (1.08 × 10^-5 cm²/V·s) and about 100 times higher than that of unannealed unblended P(NDI2OD-T2) (2.95 × 10^-6 cm²/V·s). This enhanced performance is attributed to changes in the aggregate state and phase separation, particularly influenced by the unique properties of low-molecular-weight P(NDI2OD-T2) in facilitating uniform film formation and phase stability.