Ion Modified Cholesteric Liquid Crystal Films Of Cellulose Nanocrystals For Gas Sensing

Cellulose is a biopolymer resource which continues to attract more and more interest globally due to its excellent biocompatibility and biodegradability.Cellulose have proven to be reliable substitutes to petroleum-based feedstock due to their high ability to biodegrade in nature.Furthermore,have attracted lots of interest both in the academic and industrial arena since has a soft spot to undergo easy modification due to its hydroxyl groups,low cost,among other excellent factors makes them very useful and powerful resource for scale-up industrial production.In this thesis work,the main aim was to characterize the structure and properties of cholesteric liquid crystal films produced via solution casting.Here,cellulose nanocrystals?CNC?,a derivative of cellulose,made up of twisted helical nanorods was employed to fabricate a color-sensitivity functionalized cholesteric liquid crystal?CLC?films.Through modification with organic or inorganic compound simple,low cost,highly sensitive and low-performance functional materials were prepared and the composite material was applied as a an effective sensor for gas monitoring activity of which the results could be observed by the naked eye.Salt additives of Cl-,NO₃^-,SO₄^2-,K⁺,Ca²⁺ and Fe³⁺ions were directly added into sulfuric hydrolyzed microcrystalline cellulose?MCC?suspension to obtain different kinds of ion modified CNC composite films whose color and morphology were studied and characterized by polarized optical microscopy?POM?and scanning electron microscopy?SEM?,respectively.The results showed an obvious chiral nematic fingerprint and lamellar structure in the composite films.Notably,CNC-SO₄^2-film had similar effect on the structural properties of the modified films because of its similarity to the sulfonic acid groups on the surface of the nanocrystals.On the other hand,NO₃^-and Cl-modified CNC films showed improved spectral reflectance with a blue shift.In the case of the cation modified CNCs,as the ion concentration K⁺,Ca²⁺ increased,the reflective wavelength initially increased and later decreased,displaying a blue shift.On the other hand,Fe³⁺showed a reduction in reflectivity of the modified films as the ion concentration increased,thereby leading to a redshift.The ion took effect on the self-assembly of cellulose nanocrystals which consequently changed the pitch in the modified film thereby leading to a change in the film reflectivity and the maximum reflection wavelength of the modified film.Modifying CNCs with Cu²⁺ ions modified film,the maximum reflection wavelength from 415nm to 490nm with the increase in Cu²⁺ content,causing the pitch to increase from 0.603um to 0.703um.Meanwhile,the interlayer spacing of the lamellar structure of the modified film also increased.However,further increasing the ion concentration to a certain degree such as in the case of CNC-Cu???175 and CNC-Cu???275,the lamellar structure became unclear and discontinuous.Particularly,the cholesteric liquid crystal structure of CNC-Cu???375 film with excess copper ions eventually disappeared as the structure was destroyed by the presence of copper chloride crystals.Interestingly,the maximum reflection wavelength in CNC-Cu???125 shifted from 444.63 nm to 500.97 nm,showing a total red shift of 56.34nm,however,the reflectivity decreased by 30%.It is important to mention that CNC-Cu???375 with high copper ions concentration loading showed no sensing effect on ammonia.