High Strength And Highly Oriented Polysaccharide Based Multifunctional Materials: Construction,Structure And Properties

Facing the serious pollution problems caused by non-biodegradable plastics and the exhaustive consumptions of non-renewable fossil resources such as petroleum and coal,the development and utilization of the renewable and environmentally friendly biomass have attracted much attention.Cellulose and chitin,as the most abundant natural polysaccharides,have great chance to act as the ideal substitutes of the nonbiodegradable polymers due to the low-cost,wide availability and biodegradability,etc.However,cellulose and chitin are difficult to be dissolved for the further process due to the strong intermolecular and intramolecular hydrogen bonding within their chains.They are also limited used by the poor mechanical properties.This article aims to using energy-saving,environmentally friendly,“green”and convenient ways to construct polysaccharide-based functional materials with excellent mechanical properties and superior performance,which can be applied in various fields such as textiles,thermal management,packaging materials of communication devices and energy storage dielectrics et al.The main innovations of this work are as follows:（1）the cheap and renewable citric acid/trisodium citrate/glycol（CA/TC/glycol）coagulator was applied to fabricate the robust and highly oriented cellulose filaments by inducing the nanofibril-structured formation,showing great potential in the textile field.（2）The ultrastrong,flexible and high thermally conductive cellulose/BN-OH film were constructed via chemically and physically double crosslinking and stretching strategies.The composite film without the signal transmitted interference exhibited the fast heat dissipating speed and superior oxygen barrier properties,which showed great application prospects in packaging materials of communication devices.（3）Biodegradable,transparent and high-strength chitosan/BN-OH film was regenerated from alkali/urea solvent via double crosslinking and stretching strategies.Due to the orientation of BN-OH and chitosan nanofiber networks in the film,the composite film exhibited superior mechanical properties,dielectric constant and breakdown strength,which created great opportunity for chitosan in the field of energy storage application.The main research contents and conclusions of this thesis are summarized as follows.Robust cellulose filaments were produced from cellulose solution in Na OH/urea/Zn O aqueous system with the relative high concentration（7.5 wt%）via wet spinning technique by coagulating in the cheap and renewable 15 wt%citric acid（CA）/5 wt%trisodium citrate（TC）/40 wt%glycol at low temperature,followed by drawing in 5%sulfuric acid bath.In our findings,the solubility and the solution stability were significantly improved by introducing 0.8 wt%Zn O,and the cellulose chains self-aggregated in parallel to form nanofibers through the hydrogen bonding,as a result of the relatively slow exchange ratio between the coagulator and solvent.Moreover,the nanofibers with the average diameter of 30-50 nm were aligned to the fiber direction by stretching orientation in the second sulfuric acid coagulator,leading to the further enhancement of the filament strength.The nanofibril-structured cellulose fibers exhibited the high crystallinity of 66%and orientation degree with Herman’s parameter of 0.88,and the excellent tensile strength of 2.92 c N dtex^-1.The wet spinning process only took 8h,and the production costs were lower than the viscose,which was an efficient,low cost and“green”process without the discharge of toxic substances.Furthermore,the 80-160 nm Zn O nanoparticles could be generated into the cellulose filaments for achieving the ultraviolet and static resistance.This work provided a“green”and economical strategy for the spinning robust cellulose filaments.Such technology is being industrially trialed by cooperating with Yibin Grace Co.Ltd in China,showing potential impact on the sustainability in the industry,economy and environment.The robust cellulose/edge-hydroxylated boron nitride（BN-OH）composite film with superior thermal conductivity was constructed via the physically and chemically double cross-linked and stretching orientation strategy,resulting from the highly ordered cellulose nanofibers and oriented BN-OH nanosheets.BN was edge-hydroxylated to improve its dispersibility and compatibility in biodegradable cellulose matrix without damaging its in-plane structure,the crosslinking and stretching strategy could induce to the compact interconnection of aligned cellulose nanofiber and BN-OH,which led to the 2.6-fold enhancement of tensile strength（226 MPa）and 4.5-fold enhancement of in-plane thermal conductivity(20.41 W m K^-1).Moreover,the flexible and robust composite film exhibited superior oxygen barrier properties and did not interfere the signal transmission,as well as the fast heat dissipation for cooling the mobile phone,which offered great potential in the packaging field of communication devices.It could be also anticipated that the preparation strategy could be used for the highly oriented and robust composites construction of the other 2D nanosheets such as graphene and hydroxyapatite,endowing the composites better mechanical properties and higher performance.Transparent,flexible and high strength chitosan/BN-OH dielectric film were fabricated via chemically and physically double cross-linked strategy.Benefiting from the excellent intrinsic properties of BN-OH,the composite film dispalyed the excellent dielectric performance such as the superior dielectric constant and breakdown strength.In addition,the mechanical and thermal properties of composite film were also improved as the introduction of BN-OH.Moreover,the stretching process resulted in the reinforcement of tensile strength from 126 to 240 MPa and the enhancement of energy storage density from 7.22 to 13.73 J cm^-1 due to the oriented chitosan nanofiber network and aligned BN-OH nanosheets.This strategy opens a new window for the establishment of next-generation environment-friendly transparent dielectrics with excellent mechanical properties and superior dielectric properties.In summary,the original work mentioned above successfully constructed a series of robust and highly oriented fibers and film,and clarified the relationship between their structure and performance.These novel materials derived from the sustainable natural polymers can be used in the various fields such as textiles,thermal management and dielectrics.These fundamental research concerning utilization of the renewable resources exhibit theoretical and practical significance,according well with the principles of sustainable development.