Stabilization of Atactic-Polyacrylonitrile under Nitrogen and Air As Studied by Solid-State NMR

Insights into the carbonization mechanism of PAN-derived carbon precursor fibers and establishment of a kinetics-driven accelerated reaction template for atomistic simulation

To better understand the chemistry behind the carbonization process of the polyacrylonitrile (PAN)-based precursor fibers and provide a more authentic virtual counterpart of the process-inherited model for process optimization and rational performance design, we develop arrow-pushing reaction routes for primary exhaust gas product (H₂O/H₂/HCN/N₂/tar vapor) formation and a pragmatic kinetics-driven accelerated reaction template for atomistic simulation of the carbonization process overcoming traditional challenges in time scale discrepancy of the reaction-diffusion system. The results of enthalpy barriers from hybrid first principles calculations validate the rationality and sequence of conjectured reactions during the two-stage carbonization process. Conversion rates of the rate-determining steps under 300 s carbonization are also estimated based on Eyring's transition state theory realizing kinetics equivalency of the reaction extent. Process-control measurements are further demonstrated corresponding to the proposed mechanism. The iterative densified crosslinking scheme specially designed for the surface layer is implanted into the topological reaction molecular dynamics template and a series of highly devisable structural models during the whole evolutionary process from the pre-oxidized fiber to the pristine carbon fiber surface are successfully predicted. The ultimate structure of the model presents excellent similarity in carbon yield and elemental composition with the type II high strength carbon fiber surface.

Thermal Transformation of Polyacrylonitrile Accelerated by the Formation of Ultrathin Nanosheets in a Metal-Organic Framework

In this study, polyacrylonitrile (PAN) nanosheets with unimolecular thickness were successfully synthesized by cross-linking polymerization in the 2D nanospaces of a metal-organic framework. In contrast to 1D and 3D analogues, crystallization could be inhibited by the topological constraint of the ultrathin 2D network structure, allowing for an efficient thermal transformation reaction of PAN. The amorphous nature of the PAN nanosheets led to an increase in the access of oxygen molecules to the polymer chains, facilitating the thermal dehydroaromatization reactions to yield a ladder polymer structure with a highly extended conjugated system. Notably, further carbonization of this ladder polymer afforded graphitic carbon with a highly ordered structure because of the well-defined precursor structure.

In Situ Nitrogen Retention of Carbon Anode for Enhancing the Electrochemical Performance for Sodium-Ion Battery

Retaining nitrogen for polyacrylonitrile (PAN) based carbon anode is a cost-effective way to make full use of the advantages of PAN for sodium-ion batteries (SIBs). Here, a simple strategy has been successfully adopted to retain N atoms in situ and increase production yield of a novel composite PAZ by mixing 3 wt % of zinc borate (ZB) with poly (acrylonitrile-co-itaconic acid) (PANIA). Among the prepared carbonised fibre (CF) samples, PAZ-CF-700 maintains the highest N content, retaining 90 % of the original N from PANIA. It represents the highest capacity storage contribution (80.55 %) and the lowest impedance R_ct (117 Ω). Consequently, the specific capacity increases from 60 mAh g^-1 of PANIA-CF-700 to 190 mAh g^-1 of PAZ-CF-700 at a current density of 100 mA g^-1 . At the same time, PAZ-CF-700 exhibits a good rate performance and excellent long-term cycling stability with a specific capacity of 94 mAh g^-1 after 4000 cycles at 1.6 A g^-1 .

Effects of oxygen on the structural evolution of polyacrylonitrile fibers during rapid thermal treatment

In this study, the mechanism of stabilizing polyacrylonitrile (PAN) fibers in a short period of time is investigated through probing the effects of oxygen on the structural evolution of PAN under different temperature regimes. It has been found that oxygen has a significant influence on both the chemical and physical structural evolution of PAN fibers, even in a short period of stabilization time, and the influences are dissimilar at different stabilization temperatures. At lower temperatures (below 140 °C), there is no noticeable change in the chemical and physical structures of the PAN fibers. In the mid-temperature range (140-200 °C), oxygen can slightly induce the cross-linking of PAN chains and result in a higher rate of decreasing crystallinity. When the main chemical reactions are initiated at higher temperatures (200-260 °C), oxygen is directly involved in the oxidation reaction of the PAN chains and facilitates cyclization and dehydrogenation. These reactions initiate in the amorphous regions of PAN fibers, and extend to the crystalline regions at elevated temperatures.

Porous Heat-Treated Polyacrylonitrile Scaffolds for Bone Tissue Engineering

Heat-treated polyacrylonitrile (HT-PAN), also referred to as black orlon (BO), is a promising carbon-based material used for applications in tissue engineering and regenerative medicine. To the best of our knowledge, no such complex bone morphology-mimicking three-dimensional (3D) BO structure has been reported to date. We report that BO can be easily made into 3D cryogel scaffolds with porous structures, using succinonitrile as a porogen. The cryogels possess a porous morphology, similar to bone tissue. The prepared scaffolds showed strong osteoconductive activity, providing excellent support for the adhesion, proliferation, and mitochondrial activity of human bone-derived cells. This effect was more apparent in scaffolds prepared from a matrix with a higher content of PAN (i.e., 10% rather than 5%). The scaffolds with 10% of PAN also showed enhanced mechanical properties, as revealed by higher compressive modulus and higher compressive strength. Therefore, these scaffolds have a robust potential for use in bone tissue engineering.

The formation of conjugated structure and its transformation to pseudo-graphite structure during thermal treatment of polyacrylonitrile

Three types of polyacrylonitrile (PAN) were considered in order to investigate the effect of molecular composition and configuration on the formation of conjugated structures during stabilization and the conversion of that to pseudo-graphite sheets after carbonization. The stabilization process was performed in an inert or oxidative atmosphere with a temperature ramp from 180°Cto 280°C. The thermal behavior was studied by differential scanning calorimetry, and the change of chemical groups and conjugated structures was detected by in situ measurement of infrared (Fourier transform infrared) and ultraviolet–visible spectroscopy, respectively. The carbonization process of the stabilized samples was performed using a thermogravimetric analyzer under nitrogen atmosphere in the temperature range of 150°C–1200°C, and Raman spectra were applied to study the pseudo-graphite sheets of the residuals. It is suggested that the introduction of comonomer or the improvement of the isotactic regularity of the polymer chain are helpful to promote the stabilization reactions and accelerate the formation of conjugated structures rather than the extent of conjugation during stabilization in nitrogen. Moreover, they are also beneficial to obtain higher degree of graphitization and larger size of the pseudo-graphite sheets with less structural defects after carbonization. While stabilization is performed in air, atactic PAN copolymer has the highest extent of stabilization among these three PAN samples, but they are extremely close. PAN samples with comonomer or higher isotacticity still show a little advantage in the formation speed of the conjugated structures. After carbonization, PAN with higher isotacticity has the highest carbon yield and graphitization degree and the largest size of pseudo-graphite sheets with least structural defects. In addition, the presence of oxygen during stabilization is contributory to increase the extent of stabilization and generate some bigger conjugated structures, which leads to obtain higher graphitization degree and larger size of pseudo-graphite sheets, but it also brings more structural defects.

Determining carbon-carbon connectivities in natural abundance organic powders using dipolar couplings

We present a solid-state NMR methodology capable of investigating the carbon skeleton of natural abundance organic powders. The methodology is based on the (13)C-(13)C dipolar coupling interaction and allows carbon-carbon connectivities to be unambiguously established for a wide range of organic solids. This methodology is particularly suitable for disordered solids, such as natural or synthetic macromolecules, which cannot be studied using conventional diffraction or NMR techniques.

New understanding on the reaction pathways of the polyacrylonitrile copolymer fiber pre-oxidation: online tracking by two-dimensional correlation FTIR spectroscopy

FTIR spectroscopy in combination with scaling-MW2D and 2D correlation analysis is used to study the reaction pathways of polyacrylonitrile copolymer fibers pre-oxidation.