Polyacrylonitrile Fiber Has Mechanical Properties Comparable to Spider Silk

Polyacrylonitrile is a polymer compound obtained by free radical polymerization of the monomer acrylonitrile. Polyacrylonitrile fiber refers to fibers spun from polyacrylonitrile or copolymers with an acrylonitrile content of more than 85%.

Carbon fiber-reinforced polymer is a lightweight, high-strength material that can be used to make aircraft fuselages, for example. Currently, most commercial carbon fibers are obtained from polyacrylonitrile precursor fibers through high-temperature heating and stretching. In this process, polyacrylonitrile (PAN) cyclizes to form a ladder structure and then carbonizes to form the final disordered structure. The strength and stiffness of carbon fibers depend greatly on the composition of the precursor fiber and are restricted by defects including holes and impurity particles. Generally speaking, tensile strength and toughness cannot be balanced. However, researchers used polyethylene glycol diazide (PEG-BA) as a cross-linking molecule to give PAN fiber excellent tensile strength and toughness, with a tensile strength as high as 1236 ± 40 MPa and a toughness as high as 137 ± 21 J/g.

The preparation process is divided into three steps: first, a solution of commercial PAN and PEG-BA is electrospun to form yarn, and then the yarn is stretched in air at 160°C. Finally, the stretched yarn is further annealed at 120°C, 130°C and 140°C and under the action of tension for several hours to obtain the final yarn.

As the PEG-BA content increases, the maximum stress and modulus of the stretched yarn slightly decrease, but the toughness slightly increases. However, when the electrospinning solution contains PEG-BA, the annealing step has a significant impact on the toughness of the yarn. Annealing at 130°C for 4 hours can make the yarn have maximum strength, modulus and toughness. When the PEG-BA content is 4%, the draw ratio (length after stretching: length before stretching) is 8, and annealed at 130°C for 4 hours, the final tensile strength of the yarn is 1236 ± 40 MPa. The tensile modulus is 13.5 ± 1.1 GPa and the toughness is 137 ± 21 J/g, which is similar to spider silk. Although its mass is very small (0.008 mg), it can repeatedly bear a weight of 30 g without breaking and only stretches 2.5%. Even after 5000 cycles of tensile testing with a maximum tensile strength of 400 MPa, the final tensile strength only decreased by 5.3%, and slight plastic deformation occurred. Too much PEG-BA can have adverse effects. When PEG-BA is increased from 4% to 5% and 6%, the strength and toughness of the final yarn decreases.

The researchers found that the process of thermal drawing can significantly increase the crystallinity inside the yarn, while the annealing process does not significantly increase the crystallinity inside the yarn. Tension during annealing is very important for crystallinity. If no tension is applied during the annealing process, the high temperature during the annealing process will cause violent thermal motion inside the yarn, which will reduce the crystallinity and reduce the mechanical properties of the yarn. The researchers also confirmed a chemical reaction between PAN and PEG-BA. The cyano group of PAN is cross-linked with PEG-BA to form a ladder structure, which is very important for the mechanical properties of the yarn.

The results of this research open the door to new, forward-looking materials that are expected to have practical applications in industry in the future. In polymer science, their research results will be able to provide valuable services for further research and development of high-performance functional materials.