Nanofibrillated cellulose rheology: Effects of morphology, ethanol/acetone addition, and high NaCl concentration
The effects of ethanol or acetone addition (2.5% to 40% w/w) and high ionic strength (50 mM to 1000 mM NaCl) on the rheology of carboxymethylated (NFC-carb) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized (NFC-TEMPO) nanofibrillated cellulose (NFC) suspensions were studied. Morphological characterization and centrifugation showed that NFC-TEMPO had a much finer overall morphology than NFC-carb. Rheological measurements were taken at 1.3 wt% using a stress-controlled rheometer equipped with cone and plate measurement tools with rough surfaces. The dynamic moduli were investigated through oscillatory stress sweeps. The results showed that as little as 2.5% (w/w) of either ethanol or acetone decreased the viscosity and the dynamic moduli, while 40% (w/w) increased the viscosity to values higher than those of the aqueous suspensions, doubled the storage modulus, and extended the gel-like behavior. Increasing the NaCl concentration from 50 mM to 100 mM drastically increased viscosity; moreover, the storage modulus in the elastic region linearly increased with increasing NaCl concentrations in the range of 100 mM to 1000 mM, suggesting the increased content of interparticle bonds with NaCl addition. The elastic domain was also extended from 10 Pa to 50 Pa and above 500 Pa with acetone addition (40%) and NaCl addition, respectively.