Aging and rheology of TEMPO-oxidized cellulose nanofibril gels

Nanocellulose is a renewable, biocompatible and biodegradable material with excellent mechanical properties and hygroscopic character. Two main types of cellulose nanoparticles are nanocrystals (nanocrystalline cellulose, CNC) and nanofibrils (nanofibrillated cellulose, CNF). Aqueous suspensions of CNF show stable gel properties at low solids content, due to the hygroscopic character as well as the high aspect ratio and specific surface area of nanofibrils. CNF is commonly obtained by mechanical disintegration of wood in aqueous medium. To enhance the specific surface area of nanofibrils and to prevent their agglomeration different chemical pretreatment methods, such as carboxylation, have been developed. TEMPO (2,2,6,6- tetramethylpiperidine-1-oxyl) mediated oxidation pretreatment, which converts the primary alcohol groups of the cellulose Dglucose units into carboxylate groups, provides the necessary electrostatic repulsion between fibrils. Recently, TEMPO-CNF gels have been studied and designed to meet requirements for applications such as drug delivery, wound covering, tissue engineering, cell encapsulation, aerogels, rheology modifier, nanofiber-reinforced composites. The present study concerns the development of rheological properties of TEMPO-CNF hydrogels prepared from a commercially available TEMPO-CNF powder. Ageing of hydrogels with concentrations from 1 to 3 wt % of TEMPO-CNF was investigated by monitoring shear flow and linear viscoelastic properties after different days from sample preparation. For quantitative representation of the results time dependencies of zero shear viscosity, yield stress and storage modulus are given. By superposition of individual data sets master curves displaying normalized values of the most significant rheological parameters are constructed, showing the effect of concentration on ageing kinetics. Constant parameters' values are reached in about 90 days of storage. The zero shear viscosity and storage modulus values increase for 2 and 1 decade, respectively