Twisting of Fibers Balancing the Gel–Sol Transition in Cellulose Aqueous Suspensions

Abstract

Cellulose hydrogels and films are advantageous materials that are applied in modern industry and medicine. Cellulose hydrogels have a stable scaffold and never form films upon drying, while viscous cellulose hydrosols are liquids that could be used for film production. So, stabilizing either a gel or sol state in cellulose suspensions is a worthwhile challenge, significant for the practical applications. However, there is no theory describing the cellulose fibers’ behavior and processes underlying cellulose-gel-scaffold stabilizing. In this work, we provide a phenomenological mechanism explaining the transition between the stable-gel and shapeless-sol states in a cellulose suspension. We suppose that cellulose macromolecules and nanofibrils under strong dispersing treatment (such as sonication) partially untwist and dissociate, and then reassemble in a 3D scaffold having the individual elements twisted in the nodes. The latter leads to an exponential increase in friction forces between the fibers and to the corresponding fastening of the scaffold. We confirm our theory by the data on the circular dichroism of the cellulose suspensions, as well as by the direct scanning electron microscope (SEM) observations and theoretical assessments.

Zhurkov’s Stress-Driven Fracture as a Driving Force of the Microcrystalline Cellulose Formation

“…On the other hand, the nano- and microfibrils are twisted around each other and form a rather dense supercoiled structure that must be untwisted in the course of cellulose swelling in the acids [ 45 , 49 ]. The untwisting and subsequent twisting back and compaction of the cellulose fibrils allow explaining and managing the cellulose films’ properties [ 12 ], artificial cellulose fibers [ 44 ], and quality of nitrocellulose [ 45 ]. Following this hypothesis, we suppose the cellulose fibers are to be stressed in the twisting back after the acid removal.…”

“…To harvest the NC suspension, the obtained MCC was soaked again into the distilled water and treated on a RIM-150 share grinder (gap ∼ 500 m, shaft rotation speed 1250 rpm). The shear stress achieved by this machine was insufficient to convert MCC into NC [ 12 , 24 ], which allowed neglecting the generation of NC in the course of this treatment. The MCC particles were precipitated by centrifugation (10 min, 1300× g , Avanti J-15, Beckman Coulter, Brea, CA, USA), which resulted in the dense precipitate.…”

“…MCC is used as reinforcing material [ 8 ], in the food industry [ 9 ], and medicine [ 10 ]. NC is also used as reinforcing material, a base for films [ 11 , 12 ], and conductive materials’ production, as a rheological modifier (thickener), and for drug delivery [ 13 , 14 ].…”

“…More or less, all CNF production methods include the stage of the chemical pretreatment (TEMPO oxidation or enzymatic hydrolysis), followed by intense mechanical grinding: high-pressure homogenization, ultra-fine grinding, or microfluidization [ 24 ]. For the CNC production, lignin-free cellulose treated with relatively strong (40–80%) solutions of mineral acids at elevated temperature (40–80 °C) and then subjected to intensive mechanical grinding, such as ultrasonication or shear grinding [ 12 , 26 ]. The CNC production requires the rotor-stator gap to be about tens of microns, and the shaft rotation rate should be several thousand rpm, and dozens of processing cycles are required [ 12 , 24 , 25 ].…”

“…For the CNC production, lignin-free cellulose treated with relatively strong (40–80%) solutions of mineral acids at elevated temperature (40–80 °C) and then subjected to intensive mechanical grinding, such as ultrasonication or shear grinding [ 12 , 26 ]. The CNC production requires the rotor-stator gap to be about tens of microns, and the shaft rotation rate should be several thousand rpm, and dozens of processing cycles are required [ 12 , 24 , 25 ]. Power consumption for such a process could reach 30 kW/kg of the final product [ 25 ], which significantly increases its cost.…”