A Self-healing Nano-chitin/Polyvinyl Alcohol Antibacterial Composite Hydrogel

Chitin,one of the important green chemical raw materials,is a kind of natural polymer biomaterial with rich content,wide sources,and is renewable.Chitin has a natural hierarchical structure.The preparation of nano-chitin is an effective way to improve the utilization rate of chitin.Nowadays,the research on the applications of nano-chitin has attracted increasing attention,due to the fact that nano-chitin has large specific surface area,high strength and many exposed groups.Polyvinyl alcohol(PVA)is a kind of non-toxic,biodegradable,environmentally friendly,water-soluble polymer with good application prospects in biomedical,green electronics and other fields.In this paper,PVA and nano-chitin are used as raw materials to prepare a series of nanochitin/PVA composite hydrogels with excellent properties through physical or chemical methods.A series of meaningful results have been obtained.(1)The short rod-shaped chitin nanowhiskers(Ch NW)were prepared by the combination of acid hydrolysis and ultrasound treatment.Then the Ch NW/PVA composite hydrogel was prepared by the freeze-thaw cycle method.This method did not involve chemical reaction and retained the crystal structure of Ch NW.The mechanical test results showed that the tensile strength of the composite hydrogel with5 wt% Ch NW was 820.17 k Pa,which was 51.37 % higher than that of the pure PVA hydrogel.The tensile fracture work of the Ch NW/PVA composite hydrogel with 3freeze-thaw cycles was higher than that of the pure PVA hydrogel with 5 freeze-thaw cycles,indicated that the addition of Ch NW not only significantly improved the mechanical strength of PVA hydrogel,but also shortened the gel time of PVA and increased production efficiency.The SEM results showed that the composite hydrogel presented a three-dimensional network structure with abundant pores,and the pore size first decrease and then increase with the increase of Ch NW content.FTIR results showed that there was hydrogen bond interaction between Ch NW and PVA.The water content,swelling property,water retention and gel fraction of Ch NW/PVA composite hydrogel improved with the addition of Ch NW.(2)The needle-shaped chitin nanowhiskers(Ch NW)were prepared by acid hydrolysis following high-pressure homogenization.The Ch NW/PVA-B composite hydrogel with excellent self-healing property,p H-responsiveness and plasticity was prepared by chemical method using borax(B)as crosslinking agent.The interaction between Ch NW,PVA and borax was studied.The effect of the content of Ch NW and borax on the structure and properties of the composite hydrogels was investigated.The results showed that strong interactions,including hydrogen bonding and electrostatic interactions,were formed between Ch NW and borax.The Ch NW/PVA-B composite hydrogel exhibited excellent mechanical properties,and its tensile strength could reach63.07 k Pa,which was 30.03 times that of the borax cross-linked PVA hydrogel.Compared with borax cross-linked PVA hydrogels,the elongation at break of Ch NW/PVA-B composite hydrogels showed a downward trend,but both were greater than 400 %,showing good toughness.The results of SEM showed that the internal structure of the composite hydrogel could be "dendritic" or "honeycomb",and the microstructure of the composite hydrogel could be easily adjusted by adjusting the content of Ch NW and the amount of borax.Ch NW/PVA-B composite hydrogel had good plasticity and could be molded into a variety of shapes.It had excellent selfhealing properties,showing a high self-healing efficiency of 99 %.It had good p H responsiveness.A sol-gel transition can occur at least 4 cycles in acidic and alkali environment.(3)The rod-shaped chitin nanowhiskers(Ch NW)were prepared by TEMPO oxidation followed by high-pressure homogenization.Then SWCNT/Ch NW/PVA-B composite hydrogels were obtained by using borax as crosslinking agent and singlewalled carbon nanotubes(SWCNT)as the antibacterial agent.The prepared composite hydrogels showed good self-healing ability with excellent antibacterial properties.The tensile test results showed that the addition of SWCNT significantly improved the mechanical strength of the composite hydrogel to 99.10 k Pa,which was 11.43 times that without SWCNT and 47.19 times that of the borax cross-linked PVA hydrogel.SEM results indicated that the introduction of SWCNT made the "dendritic" network structure of composite hydrogel more dense,thus improving the ability to withstand external loads.At the same time,the composite hydrogel had good self-healing properties,and the self-healing efficiency could reach 85.39 %.The introduction of SWCNT gave the composite hydrogel excellent antibacterial properties and could effectively inhibit the growth of E.coli.