Abstract:

Objective Graphene has the highest mechanical strength, electrical conductivity and thermal conductivity among all known materials, with unique characteristics in optics, acoustics, electromagnetism, catalysis and so on. Therefore, the combination of conventional materials and graphene would lead to novel composite materials with high performance and multi-functions. Among them, graphene composite fibers have been extensively explored in the last decade. Compared with conventional fibers, graphene composite fibers have demonstrated obvious advantages in mechanical strength, thermal conductivity, electrical conductivity, flame retardancy, antibacterial property, far infrared emission, UV protection, corrosion resistance and other properties. However, blending and surface treatment methods lead to defects in mechanical properties, durability, color diversity and weaving of composite fibers, which needs to be further improved.
Method An in-situ unfolding polymerization strategy was introduced to massively prepare multifunctional polyamide-6/graphene fibers, in which graphene sheets grafted with polyamide-6 (PA6) distribute uniformly. Flower-shaped graphene oxide (fGO), which was obtained by spray-drying of graphene oxide aqueous dispersion, was added into melted caprolactam under stirring. PA6/fGO chips were prepared by polymerization process. The whole reaction took place in a 3 L autoclave, using water as catalyst. The PA6/fGO chips were then melt spun into continuous fibers for characterization.
Results After a process of swelling, expansion and dissociation, fGO was tranformed into GO sheets in melted caprolactam, where strong interactions took place between caprolactam molecules and GO sheets via hydrogen bound (Fig.1, Fig.2). The homogeneous dispersion of GO sheets in melted caprolactam was obtained by dispersing fGO powder for 1.5 h. During the polymerization process, the surface of GO was grafted by PA6 molecules, which improved the interface compatibility between GO and PA6 (Fig.3). PA6/fGO was dispersed steadily in 64% formic acid, and such mechanism was further proved in AFM detection. The PA6/fGO copolymer was shown in the form of flakes with a height of 4-5 nm, higher than that GO (0.8 nm) and that of graphene (0.34 nm). The chips of PA6/fGO show excellent spinnability, and the composite fiber can be woven into fabircs (Fig.4). The polymerization process was not sufficient to fully reduce graphene oxide, while the addition of fGO led to the crystal transformation of PA6, conducive for obtaining higher strength (Fig.5). Low amount of fGO was able to improve the crystallinity of polymer and higher crystallization temperature, but the effect on the relative viscosity and thermal weight loss was not obvious (Fig.6, Fig.7 and Tab.1). The tensile strength of PA6 single fiber was increased by 25.4% and the tensile modulus 49.5% with the addition of 0.1% fGO, whereas increasing the fGO content by 0.6% resulted in decrease in the mechanical properties of the composite fiber (Fig.8). With outstanding functions in antibacterial performance, antiviral behavior, far infrared emission, negative ion generation, and UV protection, the PA6/fGO composite fabric shows broad application prospects (Tab.2).
Conclusion Flower-shaped graphene oxide and PA6 are compounded by in-situ unfolding polymerization. It was found that the flower-shaped graphene oxide microspheres gradually swell, expand and dissociate in melted caprolactam, and that the unfolded graphene oxide sheets are covalently grafted with PA6 molecules, forming a polymer brush structure, which improves the interface compatibility. The chain growth of PA6 will not be affected by low dosage addition of fGO, which would however induce transformation of PA6 crystal into a more stable form. The crystallinity of PA6 demonstrates a peak with the addition of fGO. The low dosage addition of graphene oxide has little effect on the relative viscosity and thermal weight loss of PA6, and the tensile strength and tensile modulus of the fiber increases by 25.4% and 49.5%, respectively. The composite fiber demonstrates multifunctionality in effective antibacterial performance, anti-virus behavior, far infrared emission, ultraviolet protection, and negative ion generation. The applications of PA6/fGO multifunctional fabric in different areas remain to be explored in the near future.

Key words: graphene, polyamide-6, composite fiber, multifunctional fiber, in-situ polymerization, anti-bacterial, high-speed spinning