TEMPO-oxidised cellulose nanofibrils from bacterial cellulose: how the cellulose source influences the gelation mechanisms.

TEMPO-oxidation of cellulose is used to form water-dispersible, negatively-charged nanofibrils. It is known that the type of cellulose source influences the yield of TEMPO-oxidation, the number of carboxylate groups and the crystallinity index. In this paper, we aim to demonstrate its influence on water-based formulations. Dispersions of TEMPO-oxidised bacterial cellulose nanofibrils (OBCNFs) in aqueous suspensions were studied using both small-angle X-ray scattering (SAXS) and rheology as a function of nanofibril concentration and addition of NaCl or sodium dodecyl sulphate (SDS), and the results were compared with those obtained previously for oxidised cellulose nanofibrils (OCNFs) from wood pulp. With the same oxidation procedure, OBCNFs exhibit a slightly higher oxidation degree than their OCNF counterparts (29% and ζ-potential = -56 ± 4 mV versus 25% and -48 ± 2 mV). They also exhibit a much larger length, as seen in TEM, with an almost ribbon-like architecture, whereas OCNFs form short stiff nanorods. As with OCNFs, OBCNF suspensions can form gels by increasing nanofibril concentration or upon addition of salt or SDS. Yet, the gels formed upon increased concentration have frequency dependent storage and loss moduli associated with a low yield strain (ca. 1%), consistent with the formation of a weak entangled network, while no interactions are observed in SAXS, whereas OCNF gels were formed due to an increase in repulsive interactions between nanofibrils. Upon salt/SDS addition, the gelation mechanisms consist of a gradual charge screening of the carboxyl groups and an increase of attractive interactions between OBCNFs with salt, and depletion attraction due to the presence of negatively-charged micelles for SDS, which are the same mechanisms observed for OCNFs. Yet, the rheological properties are strongly dependent on the type of nanofibril used. OBCNF gels exhibit, in most cases, higher viscosity and storage moduli than their OCNF counterparts under similar formulation conditions and a higher tolerance to salt before syneresis or to SDS before gelation, which allows for finer tuning of their rheological properties, relevant for the design of health-care products.