Modified Methods To Prepare Nanofibers By TEMPO-mediated Oxidation

Cellulose is one of the main objects in the field of forest products. It’s a kind of organicpolymer which is inexhaustible, friendly to environment, and it comes from wide origins. At thesame time, nanotechnology becomes thehot research topic, because of its potential applications.TEMPO oxidation is selective, so many researchers study nanofibers prepared by TEMPOoxidation now.Nanofibers were prepared directly in TEMPO system with the assistant of ultrasonication.Redox titration, conductimetric titration and X-ray diffraction methods were used to derterminethe oxidation process and characteristics of products. SEM was used to observe themorphological changes of fibers, while TEM was used to observe the nanofibers.The results were as follows:The microstructures and crystal regions affected TEMPO oxidation, which was assisted byultrasonication. Material with lower crystallinity and more pores or pits produced longernanofibers, and the reaction was faster. From experiments, the crystallinities and crystal regionsizes of cotton linter pulp fibers (CPF), softwood pulp fibers (SPF) and bamboo pulp fibers (BPF)followed the sequence: CPF>SPF>BPF, and the surface area sequence is: CPF<SPF<BPF. Whenprepare nanofibers in the same ultrasonic condition, nanofibers’ carboxyl contents of CPF, SPFand BPF were1.66mmol/g,2.02mmol/g and2.10mmol/g, respectively. BPF nanofibershad thelength of400-800nm, SPF nanofibershad the length of400-800nm, and CPF nanofibershad thelength of200-400nm.Ultrasonic cleaning bath (UCB) and probe ultrasonicgenerator (PUG)had different effects onTEMPO oxidation, and PUG system is more efficient, and the nanofibers produced from PUGsystem were longer. When reacted in the same oxidizing condition, carboxyl content ofnanofibers produced by80min treatment in PUG system was1.85mmol/g; while carboxylcontent of nanofibers produced in UBC system for8h was2.02mmol/g.70min treatment in thePUG system led the nanofibers to be500-900nm long, and8h reaction in the UCB system madethe nanofibers400-800nm long.When using NaOH to swell fibers and using PFI mill to beat fibers, the microstructure couldbe broken and opened. These treatments increase the surface area of fibers, and facilitatedTEMPO oxidation. The carboxyl content of oxidized fibers was1.2mmol/g.high-speed mechanical dispersion or ultrasonic treatment of fibers after oxidation also produced nanofibers.These nanofibers are few millimeters long or even longer, while the width is few tensmillimeters.