In Situ Study and Improvement of the Temperature Increase and Isothermal Retention Stages in the Polyacrylonitrile (PAN) Fiber Pre-Oxidation Process

The pre-oxidation process of Polyacrylonitrile (PAN) fibers is a complex procedure involving multiple stages of temperature increase and isothermal temperature retention. However, the impact of the temperature increase stage on PAN fiber has often been overlooked. To address this, samples were collected before and after the temperature increase and isothermal retention stages, treating them as separate influencing factors. Therefore, the pre-oxidation process can be divided into four distinct stages: (1) A temperature increase stage before the cyclization reactions: the PAN fiber's small-size crystals melt, and the crystal orientation changes under fixed tension, leading to shrinkage and increased orientation of the micropore. (2) An isothermal retention stage before the cyclization reactions: The crystal structure maintains well, resulting in minimal micropore evolution. The PAN fiber's crystal orientation and micropore orientation increased under fixed tension. (3) A temperature increase stage after the cyclization reactions: The PAN fiber's crystal melts again, reducing the average chord length and relative volume of the micropore. However, the PAN fiber can recrystallize under fixed tension. (4) An isothermal retention stage after the cyclization reactions: Significant crystal melting of the PAN fiber occurs, but the highly oriented crystals are maintained well. The average chord length and relative volume of the micropore increase. Recommendations for improving the pre-oxidation process are made according to these stages.

Fundamental Structural and Kinetic Principals of High Strength UHMWPE Fibers Production by Gel-Technology

One of the main research work in the field of polymeric materials was, is and always will be the improvement of their mechanical properties. Comprehensive structural studies of UHMWPE reactor powder, the features of its dissolution and the formation of a gel-state, as well as UHMWPE films oriented up to various draw ratios, were carried out using scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction. For comparison, decalin and vaseline oil were chosen as solvents. The mechanical properties of oriented UHMWPE films were also studied. In the process of orientation drawing, basing on the developed structural-kinetic principles of strengthening for highly oriented speciments gel-cast from UHMWPE powders, the average values of tensile strength of 4.7 GPa (about 6% of the samples had strength values up to 6.0 GPa) and an Young’s modulus of 170 GPa (about 6% of the samples had Young’s modulus values of 200 GPa). These values are among the highest according to the world scientific literature. A significant increase in the mechanical characteristics of highly oriented UHMWPE films was achieved using experimentally confirmed scientific approaches to revealing the structure-property relationship at each stage of the gel process.