Rheological Aspects of Cellulose Nanomaterials: Governing Factors and Emerging Applications

Abstract

of nanomaterials from green, low-cost, abundant, renewable, and biodegradable natural resources. One such attractive bioresource is cellulose, the most abundant biopolymer on earth, which is widely present in various forms of renewable biomass, such as trees, plants, tunicates, and bacteria.Structurally, cellulose is a linear, high molecular weight polysaccharide with repeating glucose units linked by 1-4 glycosidic bonds. Owing to the presence of hydroxyl groups, those linear chains of glucose units link together through van der Waals forces and intermolecular hydrogen bonding to form elementary fibrils, which are further packed into larger aggregates known as microfibrils. [3] Cellulose fibrils consist of disordered (amorphous) regions and highly ordered (crystalline) regions. In the crystalline regions, cellulose chains are closely packed together in highly ordered fashion (parallel to the direction of fibril length), dictated by the intricate intraand intermolecular hydrogen bonding; whereas in the amorphous regions, cellulose chain stacking is less ordered and not as closely packed. Therefore, the amorphous regions are more vulnerable to both physical disintegration and chemical hydrolysis in comparison with crystalline regions. When cellulose is prepared in nano-scale fibrillar or crystalline forms, it is broadly referred to as cellulose nanomaterials (CNMs).The isolation, characterization, modification, and application of CNMs are currently receiving much attention. Understanding Cellulose nanomaterials (CNMs), mainly including nanofibrillated cellulose (NFC) and cellulose nanocrystals (CNCs), have attained enormous interest due to their sustainability, biodegradability, biocompatibility, nanoscale dimensions, large surface area, facile modification of surface chemistry, as well as unique optical, mechanical, and rheological performance. One of the most fascinating properties of CNMs is their aqueous suspension rheology, i.e., CNMs helping create viscous suspensions with the formation of percolation networks and chemical interactions (e.g., van der Waals forces, hydrogen bonding, electrostatic attraction/repulsion, and hydrophobic attraction). Under continuous shearing, CNMs in an aqueous suspension can align along the flow direction, producing shear-thinning behavior. At rest, CNM suspensions regain some of their initial structure immediately, allowing rapid recovery of rheological properties. These unique flow features enable CNMs to serve as rheological modifiers in a wide range of fluid-based applications. Herein, the dependence of the rheology of CNM suspensions on test protocols, CNM inherent properties, suspension environments, and postprocessing is systematically described. A critical overview of the recent progress on fluid applications of CNMs as rheology modifiers in some emerging industrial sectors is presented as well. Future perspectives in the field are outlined to guide further research and development in using CNMs as the next generation rheological modifiers.

Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time

“…On the other hand, the viscosity profile of the non-sonicated CNC suspension stored for 120 days exhibits a plateau at shear rates below 1 s −1 and a shear thinning behavior at higher shear rates. These features in the viscosity profile are characteristic of a chiral nematic liquid crystal phase [17,18].…”

“…However, shear-thinning behavior of CNC suspensions does not ensure alignment of nanocrystals, but a breakup of CNC aggregates might appear [37]. Physicochemical properties of CNC suspensions are largely depending on many factors such as the source of cellulose and hydrolysis conditions, among others [17,18].…”

“…In contradistinction, the application of ultrasonic power decreases the steady-state viscosity [13]. Recent reviews of the rheology of cellulose nanomaterials account for the importance to process these materials [17,18]. However, to the knowledge of the authors, evidence of changes of viscosity of non-sonicated CNC suspensions over time have not been reported yet.…”

Preparation of Cellulose Nanoparticles from Foliage by Bio-Enzyme Methods

“…The FTIR signal scanned at around 3414 cm −1 (3500-3000 cm −1 ) was attributed to O-H stretching vibrations, and the absorption band at around 2918 cm −1 was attributed to C-H stretching vibrations [14]. The band at 1636 cm −1 was due to -OH groups of the absorbed water [15].…”

“…CNPs can be divided into cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) according to their morphology and scale [14]. The transmission electron microscopic images of CNPs prepared from five kinds of leaves are shown in Figure 3.…”