Absorbent fibers manufactured from polyacrylonitrile and silk

Abstract

Nowadays, textile materials are used as clothing as well as other advanced applications. For an example, polyacrylonitrile (PAN) is the main component of acrylic fibers and the main precursor for carbon fibers. Inspired by this concept, various preparation methods were explored to improve the water absorbency of textile materials and hence widen their applications. PAN, which is the main component of acrylic fibers, was selected to prepare the superabsorbent hydrogel. Facile and versatile electrospinning was employed to fabricate the PAN nanofibers as a nonwoven web. Further oxidative heat treatment was applied to stabilize the structure of PAN nanofibers. This stabilized structure was studied in depth by several characterization methods. The relative rate of three main reactions - dehydrogenation, cyclization and carbonylationwas investigated and revealed by Fourier transform infrared spectroscopy. The resultant PAN nanofibers were crosslinked that was confirmed by the gel fraction obtained from soxhlet extraction in DMF and the fiber integrity retained after alkaline hydrolysis. The heated PAN electrospun web was subsequently hydrolyzed in an alkaline solution to produce hydrogel. Oxidative stabilization plays a crucial role in maintaining the integrity of PAN nanofibers under harsh alkaline hydrolysis condition. The resultant hydrogel with highly interconnected three-dimensional pore network was obtained. Due to the formation of solubilizing groups and porous structure, the obtained hydrogel achieved both remarkable water absorption ratio and rate. The optimized hydrogels absorbed over 100 g/g within 1 min, making them to be useful in drug delivery systems and personal care products. Silk, which is another common natural textile fiber, was modified by in situ polymerization of two monomers acrylamide and sodium acrylate after swelling in 4.65 M lithium bromide solution. Because of its high swelling degree in LiBr as observed under optical microscope, these two synthetic monomers were able to penetrate into silk yarn. X-ray diffraction suggested the conformational changes of silk in 4.65 M LiBr, while dissolution test and Fourier transform infrared spectroscopy confirmed the formation of interpenetrating synthetic polymer network. An interesting ladder structure was observed in between the adjacent filaments of modified silk yarns. These modified silk yarns showed improvement in water absorption and mechanical property in both dry and wet states. The water absorption ratio of silk yarn modified by sodium acrylate achieved 7.6 g/g that was 9 times higher than that of raw silk yarn. This novel modified silk also showed low cytotoxicity towards skin keratinocytes, being promising in biomedical textiles such as wound dressings and sutures. Moreover, another effective approach to modify silk fiber via environmentally friendly photoredox catalysis was introduced. In this study, the photocatalyst tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate was employed. This green chemical reaction was able to induce the crosslinking of the tyrosyl in silk protein under visible light at room temperature; meanwhile, it was found to be useful in initiating the polymerization of sodium acrylate. This photocatalyst system is beneficial to silk fiber modification through crosslinking and polymerization. The modified silk fibers displayed enhancement in water absorption that achieved at least 4 times higher than that of the control silk fiber. These modified silks were ready to be woven or knitted into fabrics, having potentials in biomedical textiles like wound dressings and dermal sealants. To conclude, the water absorbency of two common textile materials PAN and silk has been successfully improved via various routes demonstrated in this study. These modification methods can be fully utilized in the textile materials which their applications are hence broadened.