| Recently,the rapid development of carbon nanomaterials has attracted a wide range of research interests.Due to their special physical and chemical properties,carbon nanotubes(CNTs)have become the most promising nanofillers in polymer materials.Among them,single walled carbon nanotubes(SWNTs)composite reinforcement is currently a hot topic in the preparation of high-performance fibers.The research on fibrous polymer crystals induced by flow provides new ideas and approaches for the formation and preparation of fiber materials.In this paper,syndiotactic poly(vinyl alcohol)(s-PVA)fibers containing high amounts of SWNTs were formed via high-speed shear flow of s-PVA/SWNTs composite dispersions,and the forming process and mechanism were studied.The specific research content and conclusions are as follows:(1)Atactic poly(vinyl alcohol)(a-PVA)and s-PVA dilute solution were subjected to shear flow respectively,the effect of shear rate and solution concentration on fiber formation were explored.Due to poor regularity and crystallinity of a-PVA,there almost no precipitates formed from its solution even at a high shear rate(3000 rpm).In contrast,s-PVA that has high degree of syndiotactic,began to precipitate under low shear rate(500 rpm),and with the increase of shear rate,s-PVA precipitates were presented as fibrous form and wound around stirrer due to orientation of the molecular chain.Results indicated that the regular molecular structure,high shear rate and the appropriate concentration promote the formation of fibrous crystals.(2)Surface modification of SWNTs was performed by tea polyphenols(TP),the benzene ring structure on TP can form ?-?conjugate structure with SWNTs,and the rich hydroxyl group on its surface can form hydrogen bonds with s-PVA additionally.Thereby the compatibility of SWNTs with s-PVA was effectively improved,and the SWNTs can be uniformly dispersed in s-PVA matrix.(3)The study of shear property of s-PVA/SWNTs composite dispersions showed that with the increase of shear rate,the viscosity of the system decreased.Particularly,the viscosity of s-PVA/SWNTs composite dispersion decreases more than pure s-PVA solution,confirming the induced effect of SWNTs on polymer orientation.In addition,A simple simulation of the shear velocity under the coaxial flow field was performed,showing that as the stirring speed increases,the shearing effect of the rotating shaft on liquid gradually increased and the molecular chains were obviously stretched and oriented.(4)s-PVA composites fibers containing high amounts of SWNTs were formed by shearing the s-PVA/SWNTs composite dispersion at high stirring speed,and the fiber formation mechanism and fiber structure were studied.The results showed that in the high-speed shear process,s-PVA chain was oriented induced by SWNTs in the flow field,and with the increase of shear time,s-PVA gradually coated SWNTs and precipitated from the dispersion around the stirrer to form a fiber with three-level structure,that is,microfibrils,fibrils and fibers.TGA results indiacted that the s-PVA fibers containing high amounts of SWNTs(such as 20.7 wt%)can be obtained from dispersions with relatively low SWNTs loadings(10.0 wt%)through high-speed shear flow. |
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