Studies On Functional Materials Based On Tunicate Cellulose Nanocrystals

With emerging the shortage of non-renewable natural resources and the increasingly serious pollution of plastic wastes and microplastics,the development and utilization of renewable resources have become an important direction for the sustainable development of human society.The ocean covering 70%of the earth surface is the most stable ecosystem with abundant biomass resources and runs little risk for valuable exploitation.It is reported that the tunicate cellulose with unique structure and properties is isolated from mantles of marine animals,the only animals that produce cellulose.The tunicate cellulose nanocrystals(TCNCs)were prepared from tunicate cellulose by the method of acid hydrolysis.Besides commonly known characters such as nano-size effects,biocompatibility,biodegradability and renewability of CNCs,TCNCs have a larger aspect ratio,a higher crystallinity,and a better modulus compared to CNCs from other cellulosic biomass.They are always used as reinforced fillers to fabricate composite materials.However,the construction and application of other TCNCs based functional materials are rarely reported.This study takes peculiar advantage of TCNCs to construct multiple functional materials including composite materials,separation and responsive optical materials,the relationship between their structure and properties was also studied.The experimental results demonstrated the potential application of TCNCs based functional materials in the fields of flexible solar cells,oil-water separation and optical devices.The innovations of this work was as follows:(1)For the first time,TCNC aqueous suspension,being self-assembled to cholesteric liquid crystals was used to prepare hydrophilic/hydrophobic nanoporous membrane for the highly effictive oil/water separation;(2)Alkylated TCNCs was attached on the surface of polyurethane(PU)sponge to fabricate oleophylic/hydrophobic sponge for rapid oil/water separation;(3)The TCNC based solvent-free liquid crystal fluids was fabricated by grafting abundant flexible polymers on the surface of TCNCs.It exhibited liquid crystal behavior at room temperature and could be responsive to temperature,deformation and alternating electric fields;(4)The nematic arrangement of TCNCs obtained from shearing was locked by the cross-linking network of poly(N-isopropylacrylamide)(PNIPAM)to construct anisotropic hydrogels with uniform interference color,which responded to the change of temperature and external force;(5)For the first time,transparent cellulose-based nanocomposite papers were prepared by introducing high-aspect ratio and high-modulus TCNCs to O-(2,3-dihydroxypropyl)cellulose(DHPC),which were used as the substrates of flexible solar cells.The primary content and conclusion of study is divided into five parts.The tunicate nanocrystals(TCNCs)was successfully obtained by the purification of tunicate cellulose from tunics and followed by sulfuric acid hydrolysis.It was proved that the TCNCs aqueous suspension was self-assembled to form liquid crystal.The novel nanoporous cellulose membranes were fabricated by facile vacuum-assisted filtration.The characterization of transmission electron microscopy(TEM)and scanning electron microscopy(SEM)were used to demonstrate that TCNC membranes composed of rod-like TCNCs had nanopore architecture but no pore walls,which facilitated water permeation.The thickness,pore size,water flux and oil rejection of the TCNC membranes could be controlled by the dosage of TCNCs.The membranes were superhydrophilic/underwater superoleophobic,which could be applied to separate various emulsions.The separation performance of the emulsions was characterized by optical microscope,laser particle size analyzer and gas chromatography(GC).For isooctane-in-water nanoemulsions,TCNC membranes exhibited excellent separation performance with reaching high separation efficiency of 99.99%and water flux of 1700L m^-2 h^-1 bar^-1.The tested membranes also presented excellent mechanical strength,the high flux of water,temperature-and p H-resistance and cycling capability.They were not only able to separate various oil-in-water nanoemulsions,but also applicable for the filtration oil-in-water microemulsions and water-in-oil emulsions.The results revealed that TCNC membranes with cholesteric liquid crystal structure contributed to improve the water flux and separation selectivity.Via the modification of substituting with n-hexadecyl bromide,and the wettability of TCNCs changed from superhydrophilic/underwater superoleophobic to oleophylic/hydrophobic.The oleophylic TCNCs(O-TCNCs)was still able to show fibrous morphology from TEM image.It could construct uniform oil-water separation interface as a surface modifier.This approach effectively improved the utilization rate of O-TCNCs while without changing the original structure of the substrate materials.For example,oil/water absorptive separation sponge could be constructed by the adhesion of O-TCNCs on the surface of polyurethane(PU)sponge.The experiment results showed that the oleophylic/hydrophobic wettability and the micro-nano structure of the pore wall determined the separation selectivity and high adsorption rate of the O-TCNC@PU sponge,whereas the porous structure offered the high adsorption capacity of the O-TCNC@PU sponge(34?77 g/g).The O-TCNC@PU sponge provided great cycling stability and environmental tolerance under harsh conditions.It could be a good candidate for complex treatment of oily wastewater.For the first time,the TCNC based solvent-free liquid crystal fluids were fabricated by covalently linking TCNCs with organosilane through condensation reaction and bind with an oppositely charged flexible polymer as a canopy.The structures and properties of the solvent-free fluids were studied by Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),small-angle X-ray scattering(SAXS),wide-angle X-ray scattering(WAXS),TEM,differential scanning calorimetry(DSC)and rheometer.The core-shell structure was proved by using molecular dynamics simulation(MD).The abundant flexible long side chains attached on the TCNC cores increased the fractional free volume,resulting in a significant mobility.The stiff TCNCs with soft canopy displayed the liquid-like fluidity at room temperature and bright shear birefringence.The self-suspended and solvent-free characteristics improved the stability of the system and capability to adapt to dry or even vacuum environment.The shear birefringence was responsive to the change of temperature,deformation and alternating electric fields.Moreover,the thermo responsiveness could be regulated by designing the shell.This work provided an important scientific basis for the preparation of liquid crystal devices from sustainable cellulose biomass.Due to the high aspect ratio of TCNCs,its cholesteric liquid crystals could easily transform to nematic liquid crystals through shearing along a certain direction.Stimulus responsive hydrogels were prepared by using the thermo-sensitive poly(N-isopropylacrylamide)(PNIPAM)cross-linking network to fix its nematic arrangement.Due to the ordered structure of TCNCs,the hydrogels showed uniform interference color between the crossed polarizer.The hydrogels did swell or shrink when it was stimulated by temperature and external force change,leading to the variation of TCNC arrangement,thus the interference color changed.Therefore,the TCNC-based optical anisotropic hydrogels could be used for the fabrication of responsive optical sensors.O-(2,3-Dihydroxypropyl)cellulose(DHPC)was synthesized by homogeneous etherification of cellulose in Na OH/urea aqueous system.DHPC exhibited a high level of transparency,outstanding ductility,and good adhesiveness but poor mechanical properties.Thus,stiff TCNCs with large aspect ratio and strong modulus were introduced to construct tough nanocomposite papers.The excellent interfacial compatibility between TCNCs and DHPC was characterized by the test of SEM?TEM?FTIR?atomic force microscope(AFM)mapping technology.The toughness,tensile strength,tensile modulus,and impact resistance of the nanocomposite papers were increased significantly by introducing the TCNCs.The dynamic mechanical analysis(DMA)results proved that the reinforcement was attributed to the formation of a percolating TCNC network which matched up with the output of percolating model.The nanocomposite papers gave smooth surface,high transparency,as well as satisfactory mechanical properties,which making it suitable for the construction of flexible polymer solar cells.Power conversion efficiency(PCE)of the flexible polymer solar cells on cellulose-based substrate reached 4.98%,comparable to that on glass,which showing enormous potential for creating a new types of flexible optoelectronic devices.Based on the unique structure,distinct self-assembly behavior,excellent mechanical properties of the tunicate cellulose nanocrystals,a series of functional materials were successfully constructed in this paper,including oil/water separation membranes,hydrophobic oil/water absorptive separation sponges,multiple responsive solvent-free liquid crystals,the thermal/mechanical responsive optical hydrogels,transparent and flexible cellulose-based nanocomposite papers.They are expected to be widely used in complex water treatment,liquid crystal display,sensors,energy transformation and other fields.These fundamental research concerning utilization of marine renewable resources exhibit theoretical and practical significance,according well with the principles of sustainable development.