Mechanical Strong Nanofibril-Structured Polysaccharide Materials Based On Mild Regeneration

With the increasing attention on sustainable development,the comprehensive utilization of biomass resources became more and more important.Cellulose and chitin are the most abundant biomass resources on earth,clarified as carbohydrate polymers,generally known as the natural polysaccharides.They exhibit characteristics of safe,nontoxic,biocompatible and biodegradable.Hence,polysaccharide-based materials have attracted widespread attention,and showed promising potential applications in fields of wastewater treatment,food and agriculture industry,tissue engineering and textile engineering.However,the poor mechanical properties are serious impediments for their practical applications.Recently,several methods such as chemical cross-linking,nanofiller reinforcement and blending with other synthetic polymer have been used to enhance their mechanical strength,but these techniques result in only a moderate enhancement and sometimes even partly sacrifice the intrinsic properties of these polysaccharide.This work focus on new mothed for constructing mechanically strong polysaccharide materials,and exploring their applications in biomedical and textile engineering.In this work,based on the dissolution of chitosan,cellulose and chitin in alkali/urea aqueous solution at low temperature,we prepared mechanically strong materials(such as hydrogel,film and fibers)via mild regeneration method.The structure and properties of these materials were characterized by atomic force microscopy(AFM),scanning electron microscope(SEM),wide angle X-ray diffraction(WAXD),small angle X-ray diffraction(SAXS)and mechanical testing.The correlation between structure and properties was also studied.Meanwhile,the potential application of chitosan film in biomedical fields was evaluated by biological experiments,and the potential application of cellulose fibers and chitin fibers in textile engineering was evaluated by comparing with traditional viscose rayon.The innovativen of this thesis are as follows:(1)Based on the alkali/urea solvent system,mechanically strong nanofibril-structured chitosan films were fabricated via mild regeneration mothed,for the first time.The formation mechanism of nanofibril structure was studied.Meanwhile,the nanofibril structure is easy to orient and the oriented chitosan film can guide directional growth of the cells;(2)Based on the alkali/urea solvent system,mechanically strong nanofibril-structured cellulose fibers was fabricated via mild regeneration method,for the first time;(3)Based on the alkali/urea solvent system,mechanically strong nanofibril-structured chitin fibers was fabricated via mild regeneration method,for the first time.The chitin fibers exhibited good mechanical properties and dying properties.The main contents and conclusions in this thesis are divided into the following parts.Different from traditional dissolution of chitosan using dilute acid,chitosan were dissolved successfully in 4.6 wt%LiOH/7 wt%KOH/8 wt%urea aqueous solution via freezing/thawing method.Mechanically strong nanofibril-structured chitosan hydrogels and films were fabricated based on the formation of nanofibers,whose mechancism is described as follow:the alkali/urea complexes surrounding the chitosan chains was destroyed slowly in a mild regenerating environment in ethanol solution(?60 wt%)at low temperature(? 0 ?).Then,the naked chitosan chains could arrange in parallel to form nanofibers via hydrogen bonding.The nanofiber then form hydrogel with network structure by entanglement and hydrogen bonding.The as-prepared chitosan hydrogels and films exhibited high mechanical properties and could fold to various complex structures.The chitosan film reached a high tensile strength of 282 MPa by cold-drawing,which is the highest among the as-reported chitosan films.The chitosan films have good biocompatibility,the cell viability cultured on them were higher than 96%.Moreover,the oriented chitosan film could guid the directional growth of the cells,on the basis of the filopodia of the cells extend and adhere on the nanofibers.These nanofibril-structured chitosan hydrogels and films could be promising for tissue engineering applications.In order to improve the solubility and stability of cellulose solution,LiOH was added to NaOH/urea aqueous solvent.Cellulose was dissolved completely in 7 wt%NaOH/0.5 wt%LiOH/12 wt%urea aqueous solution to prepare concentrated cellulose solution with good stability.Highly strong multifilament fibers with nanofibril structure are spun successfully from the cellulose-alkali-urea solution on a small pilot wet-spinning machine by coagulating in 15 wt%phytic acid/5 wt%Na2SO4 aqueous solution at 5 ?.The naked chitosan chains could arrange in parallel to form nanofibers via hydrogen bonding under such a mild regenerating environment.The construction of nanofibril structure made the cellulose fiber mechanically strong.The fibers consisted with cellulose nanofibers with 25 nm in diameter had high crystallinity of 65%and Herman's parameter of 0.9.The tensile strength of the multifilament fibers achieved up to 3.5 cN/dtex,much higher than that of the commercial viscose rayon and similar to that of Lyocell.This new technology to fabricate mechanically strong cellulose fibers with the construction of nanofibers is a new breakthrough in the traditional spinning method.In addition,the fiber production are low costs,short cycle,environmentally friendly,and is expected to replace the traditional viscose fiber for industrial production.Imitating the multi-scale structure of shrimp and crab shell,we have designed a new strategy of the construction of nanofibril-structured chitin fibers.Chitin was dissolved completely in 11 wt%NaOH/4 wt%urea aqueous solution to prepare chitin solution,then mechanically strong fibers with nanofibril structure are spun successfully from the chitin solution on a small pilot wet-spinning machine by coagulating in 15 wt%phytic acid/5 wt%Na2SO4 aqueous solution at 5?,based on the fact that the extended chitin chains easily arranged in parallel to form nanofibers in mild regenerating environment.The chitin fibers have smooth surface,circle cross-section and homogeneous nanofibril structure,and exhibited good mechanical properties and dying properties.The fibers consisted of chitin nanofibers with 24 nm in diameter had high crystallinity of 74%and orientation degree of 0.78.The tensile strength of the fibers was 2.33 cN/dtex,much higher than that of the repored pure chitin fiber,indicating that the formation of nanofibril structure is useful for improving the mechanical strength.In addition,the fiber production are low costs,short cycle,environmentally friendly,and is expected for industrial production.This thesis developed a series of mechanically strong polysaccharide materials with nanofibril structure via mild regenerating method through alikali/urea aqueous solution.The formation and regulatory mechanism of nanofibril structure was studied.Therefore,we provided important scientific proofs to producing mechanically strong cellulose and chitin fibers,which would accelerate the upgrade and renovations of viscose industry.The topic of this thesis well accorded with the target of our country for sustainable development,showing great scientific significance and prospects of applications.