Study On Graft Copolymers Of Cellulose Nanocrystals And Their Liquid Crystalline Behaviors

Cellulose is the most abundant,nontoxicity and renewable biopolymer provided by nature. The cellulose naonocrystals(CNC) can be prepared by acid hydrolysis of the biomass. Interesting behavior of CNC is mainly linked to their big surface ratios,high axis ratio, good mechanical properties,and percolation effect.A prospective development in nanocomposite materials is belong to CNC.In spite of their interesting properties,the formation of weak interfacial interactions between the CNC and the polymer matrix can lead to composite failure.The most efficient way to improve their compatibility and adhesion to other composite material components is to modify the cellulose surface by controlled grafting with hydrophobic or hydrophilic monomers.In addition,the CNC are rigid rodlike particles,which have a strong tendency to align along a vector director.The excellent reinforced nanocomposite materials are required to be highly ordered.The anisotropy is one of the most important factors to effect the performance of reinforced nanocomposite materials.The study of liquid crystalline phases behavior of CNC is helpful to design high performance inexpensive biodegradable nanocomposite materials.The liquid crystal-polymer composites have potential application in displays,smart windows,optical films,security papers and the optical actuators or devices.In this study,a series of graft copolymers of CNC were synthesized by atom transfer radical polymerization(ATRP):(1) Graft copolymers of rodlike cellulose nanocrystals(CNC) with hydrophobic poly(styrene)(PSt) were synthesized by ATRP.The liquid crystalline properties of the graft copolymers were investigated.The graft copolymers exhibit fingerprint texture in both thermotropic and lyotropic states.In thermotropic state,the PSt-grafted CNC orient spontaneously in isotropic melt(PSt side chains acting as a solvent).The thermotropic liquid crystal phase behavior is similar to the lyotropic phase behavior.The origins of chiral ordering may be the cause of geometric twists in the CNC.(2) The temperature-induced copolymers of poly(N,N-dimethylaminoethyl methacrylate) -grafted cellulose nanocrystals(PDMAEMA-grafted CNC) were synthesized through ATRP. The graft copolymers exhibited fingerprint texture in lyotropic state.The temperature-induced fingerprint texture changes of PDMAEMA-grafted CNC aqueous suspensions were investigated in variour temperature.With the increment of temperature, the spacing of fingerprint lines decreases.As temperature decreased,the spacing of fingerprint lines increases.Temperature-induced changes of PDMAEMA polymer chains result in changes of fingerprint texture. (3) The Poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate}(PMMAZO), which was a liquid-crystalline polymers(LCP),was successfully to graft from CNC via ATRP.The experimental results showed that the PMMAZO-grafted CNC exhibited both types of liquid crystal formation,thermotropic and lyotropic.The PMMAZO-grafted CNC shows smectic-to-nematic transition at 95℃and nematic-to-isotropic transition at 135℃, and exhibit analogous lyotropic liquid-crystalline phase behavior above the clearing point of PMMAZO.It shows a lyotropic nematic phase in chlorobenzene.The stiff properties of PMMAZO lead to the increment of CNC diameter,and the axis ratio of CNC is reduced. The rigidity of CNC increases because of PMMAZO.The rigidity of PMMAZO prevents the geometric twists in the CNC.Therefor,the PMMAZO-grafted CNC shows a nematic phase above the clearing point of PMMAZO and in chlorobenzene.The application of nanocomposite materials basing on grafted-CNC copolymer will be studied in future.