Zhongyu Fu

Pre‐oxidized PAN‐based fibers (OPFs) are prepared by continuous mimicking industrial line. A correlation is established between the skin‐core structure and mechanical properties of OPFs. SEM and OM reveal that the skin‐core structure of OPFs stems from radial oxygen content heterogeneity. EA and FTIR indicate that cyclization dominates in the early stage of pre‐oxidation, with the surface O/C ratio remaining essentially constant. At a residence time of 30 min, the bulk O/C ratio increases by 75%, and the degree of cyclization increases by about 28%. The abundant cyclic structures formed provide ample reactive sites for subsequent oxidation and crosslinking reactions. During pre‐oxidation, the crystalline damage in OPFs originates from the edges of crystallites and gradually extends to the whole crystal. Meanwhile, the radial heterogeneity of OPFs causes uneven stress distribution and microcrack formation at the skin‐core interface, ultimately deteriorating the mechanical properties of OPFs. A quantitative analysis of the relationship between the core area fraction and mechanical properties reveals that the elongation at break decreases when the core area fraction falls below 30%. This study provides a reference for understanding the correlation between the skin‐core structure and the mechanical properties of pre‐oxidized fibers.

This study investigates the microscale dual‐diffusion process of polyacrylonitrile (PAN) solutions in the coagulation bath and the process of PAN fiber formation. The interaction peaks assignment between solvent and water were identified by analyzing the Fourier transform infrared spectroscopy (FTIR) characteristic peaks of red or blue shift. The time‐resolved attenuated total reflection FTIR technique was employed to analyze the dual‐diffusion process. Enabling quantitative determination of diffusion coefficients based on Fick's second law. A correlation was established between diffusion coefficients, the microstructural characteristics of PAN membranes and the cross‐sectional morphology of PAN nascent fibers. Additionally, examination of the microstructure of triple times stretched PAN nascent fibers revealed the formation mechanism of fibers with “Fibrils Within a Filament”.

In the production process of carbon fibers (CF), the quality of PAN spinning solution (PANss) is one of the critical factors influencing the performance of CF. In this study, PANss with different mixed solvents were prepared, and its viscosity, linear and nonlinear rheological behavior were characterized by rheological measurement. The results demonstrated the PANss of this manuscript belong to non‐Newtonian fluids. The zero‐shear viscosity(η₀) decreased with the increase in lower‐viscosity solvent, while the index of flow (n) increased with the addition of the lower‐viscosity solvent, the variation in η₀ and n as a function of solvent content could be fitted using the equation . Furthermore, the slope of log G″ versus log G′ increased with the increase in dimethylformamide (DMF) content in both PAN/dimethyl sulfoxide (DMSO): DMF spinning solution and PAN/dimethylacetamide (DMAC): DMF spinning solution. Conversely, the slope decreased as raised DMAC content in PAN/DMSO: DMAC spinning solution. Additionally, the degree of nonlinearity of the three types of PANss gradually decreased as the content of DMAC or DMF increased.

Polyacrylonitrile precursor was pretreated under nitrogen to study the effect of the extent of cyclization of pretreated fibers on the chemical structure, crystal structure and skin‐core structure of thermal oxidative stabilization (TOS) fibers. Based on the FTIR spectra, the cyclization degree of pretreated fibers was quantitatively calculated by a combination of peak‐fitting and second‐derivative operations. Combining the results of wide‐angle X‐ray diffraction, Raman spectroscopy and scanning electron microscope analysis, pretreatment was beneficial to increase the cyclized structure of TOS fibers, but inhibited the oxidation reactions in the TOS process. As the increase of cyclization degree of pretreated fibers, the oxygen content and crystal size of TOS fibers decreased and the crystal interplanar spacing increased; in the skin layer of TOS fibers, the I_D/I_Gratio increased and brittle fracture area decreased. When the cyclization degree of pretreated fibers was in the range of 10%–35%, the skin‐core structure of TOS fibers was weakened, which demonstrated that proper pretreatment before TOS process can improve the skin‐core structure.

A new kinds of waterborne polyurethane acrylate (WPUA) dispersions were synthesized based on isophorone diisocyanate (IPDI), hydroxyl‐terminated polybutadiene (HTPB), 1, 4‐butanediol (1, 4‐BDO), polytetramethylene ether glycol (PTMG), dimethylolpropionic acid (DMPA), 2‐ethylhexyl acrylate (EHA), and methyl methacrylate (MMA) as main raw materials. The influence of the HTPB content on the particle size, storage stability of the WPUA dispersions and the damping property of cured films were studied by using dynamic light scattering (DLS), dynamic mechanical analysis (DMA), respectively. It can be found the WPUA dispersions have a good stability and the damping temperature range of loss tangent (tan δ) >0.3 reaches a maximum (−26.6–79.14°C) when the content of HTPB is 20%. The HTPB based WPUA could be applied to damping material.

The effects of chemical and physical structural evolution of polyacrylonitrile (PAN)‐based carbon fibers precursor during thermal oxidative stabilization (TOS) on the mechanical properties of stabilized fibers were systematically studied. The results of Fourier transform infrared spectroscopy, wide‐angle X‐ray diffraction, and density gradient column showed that the PAN fibers treated with high temperature and for long time have higher extent of cyclization, oxygen content, and crosslinking content. The crystallinity and crystallite size decreased with the increase of TOS time and temperature, whereas the bulk density of the stabilized fibers increased. The mechanical property results indicated that the decrease in tensile strength was inseparable from the formation of the cyclic structure and the amorphization transition of the crystal structure. The fibers have better structural stability when the extent of cyclization was 80–83%, the crystallinity was 34–45%, and the bulk density of stabilized fibers was 1.33–1.35 g/cm³, but exceeding these ranges, a serious skin‐core structure appeared.