Using Cellulose As Chiral Source For Constructing Chiral Functional Materials

Chirality,closely related to human life activities,is one of the mysteriously inherent properties in nature.Functional materials with chirality could be applied in enormous areas such as asymmetric catalysis,chiral resolution,and circularly polarized light materials.Recently,chiral functional materials demonstrate tremendous application potential in biomaterials,optoelectronic devices,and smart materials.The general pathway to prepare chiral materials is using small chiral molecules as chiral source.In such a way,the further progress of chiral functional materials may be obstructed due to high economic cost and limited resource of small chiral molecules.On the other hand,biomacromolecules(such as DNA,protein,and cellulose)demonstrate prominent and natural advantages as chiral source,due to abundant resource and hierarchical chirality in nature.Recently,chiral materials basing on DNA and protein have made significant progress,which benefits from the nanotechnology such as DNA origami and de novo assembly.Although cellulose is the most abundant biomacromolecules in nature,cellulose-based chiral materials are mainly used as chiral separation phase,and a wider range of chiral applications need to be developed.Derived from cellulose raw materials by acid hydrolysis,cellulose nanocrystals(CNCs)demonstrate three level chirality:(1)glucose unit in cellulose molecules with chiral carbon atom(the first level);(2)helical twisted structure of the CNC nanorods(the second level);and(3)cholesteric liquid crystal structure assembled by CNCs(the third level).To promote the development of nano-cellulose,and broaden the application range of cellulose-based chiral materials,three level chirality of CNCs was explored and new methods for using chirality at different levels were established in this dissertation.Moreover,chiral functional materials with different morphology and structure were constructed by combining cellulose with achiral substances such as substituted polyacetylene and semiconductor CuO.The main research contents are as follows:(1)Combining the third level chirality of CNCs and fluorescence of polymer,stimuli-responsive circularly polarized luminescent(CPL)composite film with tunable emission was prepared.The composite film was co-assembled from negatively charged CNCs and positively charged fluorescent polyacetylene(PM1)bearing dansyl chloride side groups.SEM,CD,and UV characterizations demonstrated that cholesteric liquid crystal structure of CNCs was retained in solid PM1/CNCs film.By adsorbing HCl and NH3 gas,the fluorescence of PM1/CNCs film could be reversibly emitted and quenched,which was due to dansyl chloride groups responding to acid/base.Varying adsorption content of water caused the film to undergo a uniform color change from colorless to blue,orange,and red.The change in color was ascribed to the helical pitch change in chiral liquid crystal structure.Therefore,the strength and wavelength of CPL emission could be simultaneously adjusted by controlling acid/base and water content.(2)Optically active nanoflowers were prepared by using twisted CNCs nanorods(the second level chirality)as chiral template.Under alkaline and heating(40?)conditions,CuO was generated in situ from Cu(NH3)42+ions coordinated with-OH groups which was helically distributed on CNCs surface.Finally,CuO/CNCs composite nanoflowers were formed according to SEM,TEM,and AFM images.The composite nanoflowers were composed of right-handed rod nanopetas with radial distribution.Besides,helical nanopetals were formed by helically arranged nanofibers with smaller size.After removing CNCs by calcination,the nanoflower structure was retained and nanopetals still kept right-handed helicity.CD spectra proved that both CuO/CNCs and CuO nanoflowers were optically active.These two kinds of nanoflowers exhibited high specific surface area and can be used in chiral resolution,more specifically,enantioselective crystallization of threonine racemate.(3)Optically active polymer microspheres were constructed by using CNCs as chiral source(the first level chirality).Taking CNCs as stabilizing agent in suspension polymerization of achiral monomers,optically inactive microspheres were prepared.Then CNCs were modified with alkynyl groups by silane coupling agents.Alkynylated CNCs can copolymerize with the achiral alkynyl monomers,transferring its chirality to the copolymers according to CD and UV characterization.Using alkynylated CNCs as stabilizing agents,comonomer,and chiral source simultaneously in suspension polymerization,polymer microspheres were formed.SEM images demonstrated that the alkynylated CNCs were individually distributed both outside and inside of microspheres,and some CNCs were even completely coated by polymer chains,indicating that the alkynylated CNCs participated in copolymerization in forming the microspheres.Further characterizations of CD and UV-vis absorption spectra proved the optical activity of polymer microspheres,and the chirality of alkynylated CNCs was successfully transferred to the achiral polyacetylenes.(4)The strategy established in part(3)was extended to carboxymethyl cellulose(CMC).CMC was modified with alkynyl groups through amidation reaction.The viscosity of alkynylated CMC solution decreased with increasing shear rate,indicating that CMC macromolecules could be oriented under shear stress.Then the alkynylated CMCs were used as stabilizing agent and comonomer in suspension polymerization.Adjusting the stirring rate in suspension polymerization,microparticles with different morphology were prepared.For example,spherical particles were constructed at 200 rpm.Increasing the stirring rate to 350 rpm,spindle-like particles were formed,while spindle-like particles with smaller size were prepared by further increasing the stirring rate to 600 rpm.SEM images demonstrated that the three kinds of macroparticles were assembled by small spheres with dimension about 5-10 ?m.Along the adjacent small spheres,holes were generated.Besides,the small spheres were found to be composed of fibrous architectures,indicating that hierarchical structures were formed in microparticles.CD and UV-vis absorption spectra proved the optical activity of both spherical and spindle-like particles,showing that the chirality of alkynylated CMC was successfully transferred to achiral polyacetylene.The resulting optically active microparticles were applied in enantio-differentiating release of chiral drugs and demonstrated effective enantioselectivity.