Study On The Preparation And Properties Of Silk Fibroin-based Aerogels

Silk is a type of important natural protein fiber,having been used in the textile field for several thousand years.As the main component of silk,silk fibroin(SF)with excellent biocompatibility,controlled biodegradability and exceptional mechanical properties has been widely used in the fields of biomedicine,flexible electronic devices and energy devices.Different from traditional inorganic and organic polymer aerogels,the aerogel materials prepared from SF also have biocompatible,chemically modifiable and structurally adjustable characteristics on the basis of the characteristics of traditional aerogels,showing promise for various applications,including thermal management,medical implant materials as well as smart wearables.However,SF aerogels generally suffer from the poor mechanical properties,which prevents its further applications.Therefore,we firstly compared the effects of alkaline and non-alkali degumming processes on the structure and properties of SF as well as prepared aerogels,and clarified the mechanism of non-alkali urea degumming to improve the mechanical properties of SF aerogels.Secondly,the cellulose acetate/polyacrylic acid@silk fibroin(CA/PAA@SF)aerogel fibers with core-shell structure were fabricated by the coaxial wet spinning.Meanwhile,we investigated the microstructure,mechanical properties and thermal insulating performance of aerogel fibers.Based on this,we further prepared CA/PAA@SF/GO aerogel fibers with both the infrared radiation heating and thermal insulation functions by doping graphene oxide(GO).The specific research contents and results are as follows:(1)Effects of degumming on the structure and properties of silk fibroin:We degummed the raw silk with the alkaline(sodium carbonate)and non-alkali(urea)degumming process,respectively,and comparatively studied morphology,molecular weight distribution and solution properties of SF.The results confirm that some fibroins are dissolved in the traditional sodium carbonate degumming,resulting in a higher degumming rate,while the whiteness of sodium carbonate degummed silk is lower.Simultaneously,compared with the alkaline sodium carbonate degumming,the non-alkali urea degumming has a lesser effect on silk crystallinity,and can reduce the damage to the SF peptide chain,leading to the SF solutions with a higher molecular weight,higher viscosity,and lower Zeta potential.As a consequent,the SF molecules in the aqueous solution are easier to self-assemble to form SF aggregates with a larger particle size,which is beneficial to form a stable skeleton structure of the subsequent prepared SF aerogels.(2)Preparation and characterization of silk fibroin aerogels:The SF aerogels with a low density of 13.43±0.77 mg/cm3 and a high porosity of 99.1%were fabricated from the urea degummed silk via freeze-drying.The morphology and mechanical properties of aerogels were systematically studied.We also quantitatively analyzed the secondary structures of aerogels based on Gaussian peaks of infrared spectroscopy,and compared with the SF aerogel material prepared by the sodium carbonate degumming process.The results indicate that the non-alkali urea process reduces the damage of degumming to SF,rendering the stable three-dimensional skeleton structure of the prepared aerogels.Furthermore,compared with the SF aerogel prepared by alkali degumming,the SF aerogel prepared by non-alkali degumming has higher crystallinity and content of β-sheet structure,resulting in the better mechanical properties and thermal stability.In addition,compared with conventional warm fabrics such as wool,air movement is blocked in the honeycomb porous structure of SF aerogel,limiting thermal convection.With the ultra-high porosity,the thermal conduction of aerogel is significantly reduced.Simultaneously,the pore walls of the aerogel increase the reflection of infrared light and suppress thermal radiation.Consequently,SF aerogels demonstrate excellent thermal insulation between 40 and 200℃,which is expected to be applied to the field of textiles thermal insulation.(3)Preparation and thermal insulation characteristics of continuous,strong silk fibroin aerogel fibers:CA/PAA hollow fiber was prepared by coaxial wet spinning to used as a template and protective layer for the SF aerogel fibers.Subsequently,the SF solution was injected into the hollow fiber to fabricate CA/PAA@SF aerogel fibers via freezing and freeze drying toward textile thermal insulation.The sheath of this aerogel fiber has multiscale porous structures,including microvoids(11.37 ± 4.01 μm),sub-micron pores(217.47±46.16 nm),as well as nanopores on the inner(44.00± 21.65 nm)and outer(36.43 ± 17.55 nm)surfaces,which are crucial to the formation of SF aerogel core.Furthermore,the porous CA/PAA@SF aerogel fibers has many advantages such as low density(0.21 g/cm3),high porosity(86%),high tensile strength(2.6± 0.4 MPa),as well as continuous and large-scale production potential.The unique incorporation of hierarchical porous CA/PAA hollow fiber and ultra-low density SF aerogel core significantly reduces the contributions of three heat transfer patterns,i.e.thermal convection,thermal conduction and thermal radiation.As a result,that the CA/PAA@SF aerogel fibers exhibits excellent thermal insulation performance in a wide working temperature ranging from-20 to 100℃.This work opens a new way for the development of SF aerogels in high-performance wearable thermal insulation materials and their application in the textile insulation.(4)Preparation of silk fibroin/graphene oxide aerogel fiber and its infrared radiation heating properties:Aiming at realizing the application of SF aerogel fibers in personal thermal management,we demonstrate a hierarchically porous and continuous CA/PAA@SF/GO aerogel fibers with excellent radiative heating performance and thermal insulation performance by doping GO on the basis of CA/PAA@SF aerogel fibers.A strong interaction can be formed between SF and GO,resulting in the two components being able to be blended uniformly.After freeze-drying,a complete aerogel skeleton structure can be formed in the hollow fiber.Meanwhile,the hierarchical porous hollow fiber is not only beneficial to the formation of the SF/GO aerogel core,but also improves the mechanical strength of the aerogel fiber.The maximum tensile strength of the CA/PAA@SF/GO aerogel fiber is 3.0 ± 0.2 MPa,which is equivalent to or even higher than previously reported aerogel fibers.Moreover,GO can dramatically.improve the infrared radiation heating properties of CA/PAA@SF/GO aerogel fiber,and its surface temperature is increased by 2.6℃ after exposure in the infrared radiation for 30 s,greatly higher than the hollow fiber and SF aerogel fibers.More importantly,the doping of GO can not reduce the thermal insulation performance of SF/GO aerogel fibers.The analysis of heat transfer mechanism of the mcroenvironment between the human body and textiles indicates that the integration of hierarchically porous hollow fiber and SF/GO aerogel prevents the thermal convection,decreases the thermal conduction,and suppresses the thermal radiation,rendering the CA/PAA@SF/GO aerogel fiber with excellent thermal insulation performances,which paves the way for the fabrication of high performance aerogel fibers used for the personal thermal management.