Formation Mechanism,Structure,Properties And Application Of RRSF/PVA Blended Fibers

In the context of population explosion and rapid economic development,the global annual output of various textiles has been consistently rising.Among numerous textile fibers,silk fibers,known as fiber queen,own the lowest production and highest price.The silk fibroin in fibers is composed of abundant amino acids.The excellent biocompatibility and low immunogenicity make it promising for applications in tissue engineering,drug delivery,biological optics,and other biomedical fields.Nevertheless,after several years of use,silk products present declining mechanical properties,aging,and yellowing.There silk products are directly discarded because they cannot preserve their usability,resulting in serious resource waste and environmental pressure.At the present stage,the treatment methods of waste silk textile remain several problems to be solved.The common treatment means do not conform to the concept of green development.The recycling rate and added value of reused products are pretty low.The processing technology cannot be mass-produced,and the mechanical properties of reused products cannot meet the requirements for subsequent applications.Therefore,there are still major challenges in making sufficient use of waste silk products,forming a sustainable production system,and realizing the high-value recycling and utilization of waste silk fibers.To solve the problems mentioned above,we recycled discarded silk quilts used for one decade as raw material and extracted waste silk fibroin(WSF)fibers to make silk fibroin aqueous solution.Polyvinyl alcohol(PVA)was selected as a blending material due to its water solubility and excellent biocompatibility.Ethanol was chosen as a coagulation bath for its non-toxic and harmless properties.The recycled silk fibroin/polyvinyl alcohol(RRSF/PVA)blended fibers was prepared by dry-wet spinning.The recyclability of waste silk fiber was systematically studied.The rheological behavior of RRSF/PVA blended solution was explored adequately.The microstructure transformation of blended fiber in different spinning stages was investigated.The forming mechanism of RRSF/PVA blended fibers was constructed.The dry-wet spinning technology of RRSF/PVA blended fiber was formed.On the above basis,nano-TiO2 was added to prepare antibacterial blended fibers.The effect of the added amount of functional material on the spinning stability,structure,and properties of RRSF/PVA/TiO2 blended fibers was further studied.An optimal technical scheme of high-value recycling and utilization of waste silk fiber was carried out.The application performance of RRSF/PVA/TiO2 blended yarns as surgical sutures was preliminarily explored.The main results were obtained as the following aspects:(1)Feasibility study of recycling and reusing waste silk fiber.Compared with ordinary silk fibroin(OSF)fibers,WSF fibers have the phenomenon of aging and yellowing.What else,the diameter uniformity of WSF fibers was poor.The breaking elongation and strength of WSF fibers decreased by 65%and 31%,respectively.The decomposition temperature and thermal stability of WSF fibers declined.However,there was no significant difference in the crystal structure,secondary structure,and other microstructure between the two fibers.Both of OSF and WSF fibers were dominated by β-sheets crystal structure,which provided a feasible basis for the subsequent recycling of WSF fibers.WSF fibers were used to prepare RRSF fibers by dry-wet spinning.It was found that increasing the concentration of RRSF spinning solution could increase the viscosity and improve its spinnability,but the effect was not obvious.The RRSF fibers prepared by high concentration(28 wt.%)solution had poor spinning continuity and stability.Therefore,it is necessary to adjust the composition of the spinning solution to obtain a better spinning effect.(2)The study of rheological behavior and spinnability of RRSF/PVA blended solutions.Pure RRSF solution had low viscosity,high fluidity,and poor spinnability.With the increasing PVA contents,co-solutions’ apparent viscosity,storage,and loss modulus would rise.The addition of PVA solution can effectively improve the spinnability of the blended solution.When the ratio of PVA reached half or more,the blended solution owned preferable fiberizability.Still,too high content would lead to the excessive dominant effect of the elastic characteristics,which caused the phenomenon of extrusion swelling in the spinning process.Based on the purpose of fully recycling waste silk fibers,the optimal spinning solution parameters were determined as 20 wt.%RRSF/PVA blended aqueous solution with blending masee ratio of 4:6.When the temperature was higher than 30℃,silk fibroin protein denaturation would occur,resulting in abnormal growth of viscosity and modulus.So the spinning temperature should be lower then 30℃.(3)Stable and continuous preparation of RRSF/PVA blended fibers.The treatment time of RRSF/PVA blended fibers in the coagulation bath had the most significant effect on the continuous spinning stability.The optimal spinning process for continuous stability of RRSF/PVA blended fibers was as follows:needle diameter was 0.21 mm,treatment time in coagulation bath was 60 s,the drawing speed ratio was 1.2.The fabricated blended fiber possessed a breaking strength of 0.29±0.04 cN/dtex,an initial modulus of 12.22±2.55 cN/dtex,and an elongation breaking of 439±129%.The improvement of the drawing speed ratio during the spinning process effectually reduced fiber diameter and cross-section porosity.At the same time,the increase of the treatment time in the coagulation bath improved the β-sheets structure content and molecular chain orientation in RRSF.When the time exceeded 60 s,the molecular structure of silk fibroin protein remained to be stable.The forming mechanism of RRSF/PVA blended fibers was explained as follows.The spinning dope experienced shear action in the spinning tube and then promptly concreted a nearly cylindrical shape in the ethanol coagulation bath.During the coagulation process,the skin structure of the fiber became increasingly tight and impermeable,and the core part formed the gel structure gradually.The moisture inside fibers was probably locked up in this case,thus finally creating the pore structure.Under the winding action,the fibers were drawn out of the coagulation bath and further drafted.Finally,the fibers were wound to the collection device to form blended fibers.As the spinning process progressed,the surface of fibers underwent ragged variation.The cross-section showed a more numerous and intensive pore structure,and the distribution of voids gradually diffused from the center to the edge.The βsheet structure content witnessed a gradual rise from 30.8%to 48.9%,and the orientation of the molecular chain also increased.(4)The quasistatic and dynamic mechanical properties of RRSF/PVA blended fibers.Under uniform stretching,the diameter of fibers turned into decreasing.The axial vertical cracks gradually appeared on the surface and aggravated with the further action of tensile.When stretched to 200%,the fiber witnessed a striking dehiscence phenomenon in the skin layer.RRSF/PVA blended fibers revealed the typical features of ductile fracture.The main component of the core layer at the fracture was PVA.RRSF component presented axial fibrillike dispersion and split fragmentary areas gradually with further stretching.At the microlevel,the β-sheets content reached the highest value of 54.7%when stretched by 50%and then decreased.In Raman spectroscopy,the value of I⊥/I∥ witnessed a growth trend followed by a drop and achieved a peak of 1.25 at 100%elongation.The stretching process was divided into low,negative,and high viscoelasticity deformation stages.A nonlinear spring or dashpot element was introduced,respectively,according to the different microstructure of fibers.A parallel-connected mathematical model was established.The equation of the stress-strain curve at the different tensile process was simulated as follow:σ(ε)=-k0εn0+1+E0+η0s,σ(ε)=-k1sεn1+E1ε+η1s,σ(ε)=E2ε+η2s.The fitting degree was above 0.993,and the fitting effect was preferably.(5)Preparation and application properties of antibacterial RRSF/PVA/TiO2 blended fibers.Adding a small amount of TiO2 nanoparticles had no significant effect on the rheological properties of the blended solution.RRSF/PVA/TiO2 blended fibers were prepared by the dry-wet spinning method.TiO2 nanoparticles of fibers were evenly distributed.When TiO2 content was 1.50 wt.%,the maximum values of breaking strength and elongation were 0.41 ±0.083 cN/dtex and 421.34±106.46%,respectively.When the addition amount was excessive(2 wt.%),the mechanical properties of the blended fiber generally declined due to the aggregation of TiO2.RRSF/PVA/TiO2 blended fibers had an inhibition effect of more than 80%on the growth of Escherichia coli,and the blended fibers with 1.75 wt.%TiO2 had the best antibacterial rate,up to 97.41%.RRSF/PVA/TiO2 blended fibers had excellent cytocompatibility,no cytotoxicity and no variation in cell structure.Considering the fiber breaking strength and antibacterial effect,the optimal addition amount of TiO2 nanoparticles was 0.75 wt.%and 1.25 wt.%.The TiO2 content and post-treatment conditions of the prepared surgical sutures were as follows:0.75 wt.%TiO2-3 times after drafting-6 plying-1000 twists,1.25 wt.%TiO2-3 times after drafting-6 plying-500 twists,the knot strength was 1.99 N and 2.4 N,respectively.According to immature rats’ skin wound closure situation,laboratory-made sutures can effectively close wounds.By H&E staining tissue slice observation,sutures with more TiO2 were more effective in alleviating inflammation and promoting tissue regeneration.