Rapid Dissolution Of Chitin In Potassium Hydroxide/Urea Aqueous Solution Under Low Temperature,and Preparation And Characterization Of Novel Materials

In the 21st century,human beings have entered into a new age in large-scale exploitations and utilizations of marine biomass resources.Chitin,mainly exists in marine sources,is the second abundant natural crystalline polysaccharide with excellent biocompatibility,biosafety,biodegradability,high accessibility and chemical reactivity,which is of potential values in applications of biomedical materials,optical and electrical functional materials,absorbent and catalyst carriers and packing materials,etc.However,natural chitin with abundant intra-and inter-molecular hydrogen bonding interactions,high crystallinity and complicated hierarchical structures neither dissolve in most of organic or aqueous solvents,nor undergo melting process.The common solvents for chitin are halogen organic solvent,ionic liquid and strong dipole/lithium salt,which are toxic,expensive and hard to recycle.Recently,the aqueous NaOH/urea solution is developed to dissolve chitin by freezing-thawing cycle.However,from the points of industrial production,this method with high energy consumption and low efficiency is not capable of sequential production.Therefore,the key point to promote chitin science and technology is seeking an efficient,cheap and "green" solvent for chitin.The research highlights of this work are as followed.(1)For the first time we developed KOH/urea aqueous solution for the rapid dissolution of chitin,and proposed the rapid dissolution mechanism;(2)Aqueous KOH/ureasolution was also used to dissolve ?-chitin rapidly and prepared ?-chitin-based hydrogels,films and aerogels with good mechanical properties;(3)Evaluated the self-assembly behaviors of chitin molecular chains,temperature-induced fast gelation and reversible sol-gel transition behaviors of chitin/KOH/urea aqueous solution;(4)Construction of extremely strong and transparent chitin films by using the efficient,energy-saving and"green" aqueous KOH/urea solution,and evaluation the effect of neutralization conditions on the structure of chitin hydrogels and films;(5)Fabrication of hierarchical porous structure cellulose/chitin and cellulose/chitosan hydrogels for heavy metal ions adsorption;(6)From the chitin dope,regenerated chitin multifilaments were successfully spun on a pilot plant scale,and the chitosan filaments with good mechanical strength and biocompatibility were prepared by heterogeneous deacetylation.The main contents and conclusions of this thesis are divided into the following parts.A non-derivative KOH/urea aqueous solution was developed to rapidly dissolve a-chitin at low temperature.Structural changes in the level of molecular chain,crystal and hierarchical structure were observed during a-chitin dissolving in aqueous KOH/urea solution.The results revealed that urea could preferentially bonded to chitin molecules by hydrophobic interactions and changed the conformations of chitin molecules,which allowed KOH hydrated quickly permeated into a-chitin crystal and formed strong ion-dipole interactions with carbonyl groups,leading to a total disruption of intra-and inter-molecular hydrogen bonding,and finally destroyed the crystal and hierarchical structure which made chitin rapidly dissolve into KOH/urea solution at molecular-level.After dissolution and neutralization,the a-chitin crystal form was remained.The high efficient,non-toxic and "green" aqueous KOH/urea solution had a great prospect in future application and production of chitin-based materials in a large scale.The rapid dissolution of ?-chitin in KOH/urea aqueous solution was also studied.Further,we prepared ?-chitin-based hydrogels,films and aerogels by neutralizing chitin solution in ethanol.It was revealed that a crystal transition from ?-to ?-chitin was occurred after dissolution and neutralization.The tensile stress,strain and Young's modulus of chitin hydrogel hydrogels were 3.24 MPa,163%and 4.6 MPa,respectively.After drying in air,the tensile stress,strain and Young's modulus of chitin films were90 MPa,19%and 3.3 GPa,respectively.After freeze drying,the tensile stress,strain and Young's modulus of chitin aerogels with BET surface area of 276 cm2/g were 24 MPa,21%and 427 MPa of aerogels,respectively.The rheological properties of a-chitin solution were investigated by employing dynamic viscoelastic measurement.The effects of temperature,chitin concentraion,molecular weight and degree of deacetylation on gelationprocess were studied.In thediluted chitin solution,chitin dispersed as single molecules with a diameter of 0.27 nm in the solution.But in the concentrated solution,a weak chitin gel structure was formed by chitin molecules self-assembly through hydrogen bonding interactions,which lead to a rapid gelation when temperature was close to gel point(Tgel).With an increasing chitin concentration from 5 wt%to 9 wt%,Tgel decreased from 22.9 ? to 11.5 ?.With decreasing Mw from 10.1 × 104 to 6.0 × 104 g/mol,Tgel of 6 wt%chitin solution increased from 18.8? to 28.5 ?.With increasing D.D.from 4.7%to 19.6%,Tgel of 6 wt%chitin solution increased from 18.8 ? to 36.0 ?.Notably,during sol-gel transition,gelation,phase separation and crystal transition were also occurred.By lowering the temperature,chitin gel re-dissolved into solution form.Interestingly,some anions such as Ac-,Cl-,I-,SO42-and SCN-had ability to reduce hydrophobility and promoted stability of chitin solution.The extremely strong and transparent chitin films were prepared through sol-gel transition of KOH/urea/a-chitin solution in aqueous ethanol solution neutralizer.The effects of neutralizer conditions,such as ethanol concentration,temperature of neutralizers and degree of deacetylation(D.D.)of chitin on hydrogels and films were evaluated by SEM,XRD,13C NMR and tensile tests.When altering the neutralizer conditions,the hydrogen bonding and hydrophobic interactions between chitin molecules would be differ and lead to great changes on crystalline hydrates and aggregation structures,which finally reflected by the significant variations on tensile stress.After undergoing uniaxial orientation,the 9 wt%chitin film neutralized from 0? ethanol had a best mechanical strength,which tensile stress,strain,Young's modulus and work of fracture were 226 MPa,13%,7.2GPa and 20.3 MJ/m3,respectively.This extremely strong and transparent chitin films had potential applications in enzyme and catalyst immobilizations,flexible electronic devices,packing materials and biomedical materials.Also,this simple and "green" method is a sustainable pathway for highly efficient utilization of marine biomass waste.The cellulose/chitin composite hydrogels with hierarchical porous structure were prepared by dissolving cellulose and chitin in LiOH/urea aqueous solution via freezing-thawing process and neutralizing in 5 wt%H2SO4 aqueous solution,ethanol and 5 wt%PEG1000/ethanol solution.After heterogeneous deacetylation,the cellulose/chitosan hydrogels were achieved and remained denser porous and hierarchical structure.From the results of spatial Raman imaging microscope,cellulose and chitin distributed uniformly in cellulose/chitin hydrogels while cellulose and chitosan distributed uniformly in cellulose/chitosan hydrogels.The tensile stress and strain of cellulose/chitosan hydrogels were 3.5 MPa and 155%,respectively.Furthermore,the hierarchical porous cellulose/chitosan hydrogels had remarkable heavy metal ions adsorption capacity,showing a great potential in water treatment applications.The a-chitin was dissolved in KOH/urea aqueous solution under-30 ? to prepare chitin dope.From the chitin dope,chitin multifilaments were wet-spun successfully on a pilot plant scale.The filaments had circular cross-sectional shape and smooth surface with draw ratio of 0.65.The chitosan filaments were also prepared by further heterogeneous deacetylation reaction.According to the results of 2D WAXD and SAXS,with an increasing D.D.,crystallinity and long period of the filaments decreased significantly and lead to a denser inner and surface morphology.The tensile stress and strain of chitin multifilaments were 2.4 cN/tex and 10%and increased to 4.1 cN/tex and 37%after deacetylation,respectively.Cell viability experiments showed that chitin and chitosan multifilaments had good biocompatibility.It should be noted that,the chitin filaments wet-spinning process is non-toxic and environmental friendly,which is very important and has great prospect in industrial production.This thesis developed a green,non-toxic and efficient aqueous KOH/urea solution to dissolve chitin at low temperature rapidly.Throughout numerous scientific researches from basic theory to pilot approach,we provided important scientific proofs to realize the efficient,non-toxic and low-cost process for chitin filaments industrially,promoting the utilizations of marine biomass resources in many applications.Therefore,there were of great scientific values and application potentiality for a sustainable development.