Enzyme-assisted Preparation Of Cellulose Nanofibers By TEMPO-mediated Oxidation

Due to their abundance, high strength and stiffness, low weight and biodegradability, cellulose nanofibers(CNFs) were considered as promising candidates to manufacture nanocomposites. In this paper, moso bamboo pulp fibers pretreated with cellulases were oxidized using TEMPO-mediated oxidation systems in water at p H 10. TEMPO-oxidized cellulose(TOCs) were converted into individual nanofibrils via ultrasonic treatments for 10 min. Two kinds of cellulases(Cellulases EG and Cellulases AC) were investigated in this research and the corresponding nanofibrils were named as EG-TOCNs and AC-TOCNs, respectively. Three main factors which affected TEMPO-mediated oxidation were researched in detail, including enzymatic pretreatment time, the amount of Na Cl O and cellulases dosage. EG-TOCNs and AC-TOCNs was characterized by atomic force microscopy(AFM), particle size analysis(PSA), Fourier transform infrared spectroscopy(FT-IR) and thermogravimetric analysis(TG).Enzyme pretreatment could loose cellulose structure, peel off microfibrils, promote TEMPO-mediated oxidation and facilitate disintegration of TOCs. As enzymatic pretreatment time increased, carboxyl contents of EG-TOCs and AC-TOCs increased at first and decreased subsequently illustrating that excess degradation of cellulose brought about the decrease of cellulose accessibility.A equilibrium shift model of cellulose accessibility was proposed to explain carboxyl contents variation triggered by enzymatic pretreatment. In terms of TEMPO-mediated oxidation, there may be a cellulose accessibility equilibrium during enzymatic pretreatment. Cellulose degradation including the destruction of hydrogen bond network and microfibrils crack will increase cellulose accessibility, however degradation of amorphous regions resulting in the decrease of C6 primary hydroxyl groups will decrease cellulose accessibility. These two effects function together to decided the accessibility. As cellulose accessibility increases, carboxyl content of TOCs tends to increase.The amount of Na Cl O added had a strong impact on TEMPO-mediated oxidation. As the amount increased, superficial oxidation moved towards internal amorphous regions which would improve the level of microfibrils exposure and facilitate the separation of microfibrils.The equilibrium shift model was applied to explain the effect of enzyme dosage on carboxyl contents. Within the adopted enzyme dosage from 10 IU to 50 IU, carboxyl contents of EG-TOCs and AC-TOCs were no obvious change. And EG-TOCNs yield increased gradually, but AC-TOCNs yield nearly kept constant. When enzyme dosage was 70 IU, EG-TOCNs yield decreased while AC-TOCNs yield increased.AFM images showed that the dispersions consisted of individualized cellulose nanofibrils 5-20 nm in width. Enzymatic pretreatment causes length decrease for EG-TOCNs and AC-TOCNs. Thermostability of TOCNs、EG-TOCNs and AC-TOCNs were inferior to moso bamboo pulp fibers. Meanwhile, enzymatic pretreatment had no obvious effect on nanofibrils thermostability.