Studies On The Homogeneous Synthesis, Structure And Solution Properties Of Cyanoethyl Cellulose

Cellulose is the most common and inexhaustible raw material and can beconverted into various regenerated materials and derivatives. However, cellulose stillhas not reached its potential applications in many areas because it is difficult toprocess in general solutions or in the melting state on account of its strongintermolecular and intramolecular hydrogen bonding. The chemical modification ofcellulose is the dominant route to tailor the functions, modify the properties, andimprove the overall utilization of this naturally occurring biopolymer. Nowadays, allcommercial cellulose derivatives have been prepared with heterogeneous procedureswith the cellulose slurry in industry. In case of heterogeneous reactions, theaccessibility and reactivity of the OH groups are clearly determined by hydrogenbond-breaking activation steps through alkaline compounds and by interaction withthe reaction media, which prevents effective synthesis of cellulose products withdesired degree of reaction, reproducible substitution patterns, and targeted properties.Therefore, homogeneous modification of cellulose has been one focus of celluloseresearch for a long time. In this thesis, cyanoethyl celluloses (CECs) werehomogeneously synthesized in alkali/urea aqueous solutions for the first time, byusing acrylonitrile as etherification reagents. The structure, properties and applicationsof CEC samples were well investigated.The novel creations of this work are as follows.(1) CECs were homogeneouslysynthesized in alkali/urea aqueous solutions for the first time. The structure, solutionproperties, thermotropic liquid crystalline behavior of the CEC samples wasinvestigated.(2) The effect of molecular weight (Mw), temperature and degree ofsubstitution (DS) on the rheological properties of water-soluble CEC samples weredetailed investigated.(3) Dynamic viscoelastic measurements indicated that thesol-gel transition of the CEC aqueous solution occurred at elevated temperature, andthe gel point Tgelwas determined by using the Winter-Chambon method.(4) Thescaling laws of CEC samples with high DS values in DMAc were decribed usingrheometry.(5) The CEC films were prepared by dissolving CEC samples with highDS values in DMAc using the casting method, which displayed excellent thermalstability, water resistance and dielectric properties.The main content and conclusions in this thesis are divided into the followingparts. The CEC samples were homogeneously synthesized in NaOH/urea aqueoussolutions under moderate conditions. The total DS values of the CEC samples increased from0.26to1.93with increasing molar ratio of acrylonitrile to AGU from1:1to9:1. The relative reactivity of hydroxyl groups is in the order of C-6﹥C-2﹥C-3. The DS value for water-soluble CEC was as low as0.54, but exhibited goodsolubility in organic solvents as the DS value increased to1.37. CEC with high DSvalue displayed apparent thermotropic liquid crystalline behavior at the temperatureabove its melting point.The CEC samples have been homogeneously synthesized in LiOH/urea aqueoussolution by using acrylonitrile as an etherification agent. The structure and solutionproperties of CEC samples were investigated by13C NMR, FTIR, element analysisand DLS. The DS of CECs obtained here were in the range of0.26~1.81, which canbe controlled by typical reaction conditions. The relative DS value at C-6position washigher than those at C-2and C-3positions. The DS had a great influence on itssolubility. The DLS and LLS results suggested that the individual chains coexistedwith their aggregates in0.9wt%NaCl aqueous solution for water-soluble samples.While water-insoluble samples showed no aggregation and existed as extended stiffchains in0.5%LiCl-DMAc.Solution properties of water-soluble CECs with different molecular weights havebeen investigated. CECs formed large aggregates spontaneously in aqueous solutionbecause of the strong hydrogen bonds. The rising concentration of CEC in thesolutions transformed the rheological behavior from Newtonian to shear thinning. Asthe molecular weight increased, the intermolecular interaction enhanced. The lossmodulus G′′and storage modulus G′increased and η*exhibited shear-thinningbehavior. We found that Carreau model can describe the shear-thinning of CECsolution. The derivation of complex viscosity and shear viscosity from Cox–Merz rulein dilute regime was related to the co-existence of single chain and large aggregates inCEC solution. As the concentration increased, the CEC system was transformed into ahomogeneous entanglement structure. Meanwhile, the CEC solutions displayed goodstability during long time storage.The dynamic viscelastic measurements showed CEC aqueous solution underwenta sol-gel transition with an increase of temperature. The results revealed that theviscoelestic properties of CEC aqueous solutions were very sensitive to thetemperature, concentration and DS. The critical gelation temperature Tgelandrelaxation exponent n of CEC solutions were accurately determined by theWinter-Chambon method. The sol-gel transition of CEC samples in water was thermoirrevesible. Above the gel point, a physical gel formed and micro-phaseseparation appeared in the system. The experimental data of rheological behavior ofG′and G can be described by four Maxwell elements.The solution behavior of CECs with DS of1.49,1.65and1.81in DMAc wereinvestigated using rheometry. The results indicated that water-insoluble CEC samplesformed weak gels in DMAc with increasing temperature, and the sol-gel transitionwas thermally irreversible. The exponent (n) values at the gel point showed anindependence of molecular weight due to the similar structure. The gel strengthincreased with the increase of the DS. The CEC solutions were decribed with twoscaling laws at20~80°C. The gelation process of CEC solution was also related totime, and the tgeldecreased with an increase of DS.The CEC films were prepared by dissolving water-insoluble CEC samples withdifferent nitrogen contents in DMAc by the casting method. The properties werecharacterized by tensile testing, DMTA, TG, water uptake experiments, UV-visspectroscopy, and dielectric frequency spectra. The results indicated that the CECfilms exhibited good optical transparency and mechanical properties. The σband Evalues of the CEC films increased from42.05MPa to119.61MPa, and1005MPa to1458MPa with increasing DS values, respectively. Meanwhile, the CEC filmsdisplayed excellent thermal stability and water resistance. Moreover, the CEC withhigh DS values could be used as dielectric materials due to its unusual dielectricproperties, namely high dielectric constant and relatively low dielectric loss factor.The basic research results mentioned above proved that alkali/urea aqueoussolution was a suitable solvent and reaction medium for the homogeneous synthesis ofCEC, which provided a new pathway for the preparation of cellulose derivatives.Meanwhile, this work clarified the relationships among the structures, properties andapplications of CEC with different DS. This thesis provided important informationand scientific evidences for the comprehensive utilization of cellulose derivativesthrough green methods. Therefore, there were great scientific significance andprospects of applications.