Preparation Of Cellulose Nanofibers Based On Room Temperature Ionic Liquid Dissolution

The increasing environmental issue and the diminishing of fossil resources have driven the society exploiting for alternative resources that are renewable and readily available.Cellulose nanofiber(CNF)is a high value-added natural nanomaterials derived from cellulose.It has excellent properties including large surface area,low density,high mechanical strength and stiffness,and low extent of thermal expansion,which entitle it broad applications in many fields,such as gas barrier films,engineering tissues,reinforcement for nanocompsites,and biosensors.At present,cellulose nanofibers are mainly produced by mechanical and/or chemical methods,which have the limitations of high energy consumption,high equipment requirements and are difficult for large-scale production.In this study,an energy efficient method to prepare CNF with uniform diameter has been developed based on a one-pot swelling-esterification process.Firstly,cellulose nanofiber(CNF)was prepared from pulp fiber in a binary solvent of tetrabutylammonium acetate(TBAA)/dimethyl sulfoxide(DMSO)at room temperature.Through the simple one-pot swelling followed by esterification process,an energy efficient method has been developed to prepare 3-5 nm wide carboxyl-functionalized cellulose nanofiber(CNF)from pulp fiber.The mechanism was also studied.By investigating the influence of the concentration of ionic liquids(W_TBAA)on the size,dispersion and structural properties of cellulose nanofibers,the highly efficient defibrillation of pulp fiber to CNF is believed due to two factors:(1)swelling in TBAA/DMSO effectively loosens the structure of cellulose supermolecules by breaking the intra-and inter-hydrogen bonds between cellulose chains;and(2)the carboxyl groups grafted on the cellulose molecules by esterification prohibit the reformulation of hydrogen bonds between cellulose chains and therefore stabilize the disperse CNF with uniform diameter in the solution.Secondly,two cyclic anhydrides(maleic anhydride and succinic anhydride)were used to modify cellulose for the preparation of cellulose nanofibers.The optimum reaction conditions were determined by investigating the effects of reaction time,the concentration of ionic liquids and the quantity of anhydrides added to the solution.The CNF modified with succinic anhydride has higher degree of substitution than maleic anhydride,which should be attributed to the factors that(1)the existence of conjugated structure in maleic anhydride causes a lower degree of electron deficiency and a weaker electro-philicity of carbonyl carbon,and(2)the cis structure of maleic anhydride leads more steric hindrance during the esterification of CNF macromolecules and cyclic anhydride.At the same time,through the investigation of the surface charge and degree of substitution of cellulose nanofibers modified by two cyclic anhydrides,it was found that a large number of carboxyl groups were attached to the cellulose.There was a strong electrostatic repulsion between cellulose chains,which effectively hindered the formation of hydrogen bonds between cellulose chains.In addition,the modification promoted the defibrillation of pulp fiber.Finally,the effects of ionic liquids with different cations and anions on the preparation of cellulose nanofibers were studied.The results showed that:(1)quaternary ammonium ionic liquids/DMSO displayed significantly higher ability to swell pulp fiber than that of imidazole ionic liquids/DMSO;(2)imidazole ionic liquids prefer to swell the amorphous region of cellulose than crystalline region;(3)quaternary ammonium ionic liquids with F^-anion showed the best swelling ability while and the lowest solubility of pulp fiber;while the quaternary ammonium ionic liquids with OH^-anion displayed the highest solubility but moderate swelling ability toward pulp fiber.Secondly,the effect of ionic liquids consisting of different ions on the degree of substitution of cellulose modification was studied.The results showed that the pulp fiber that has higher extent of swelling effect had a higher degree of substitution.