Cellulose Nanocrystal Liquid Crystal Phases: Progress and Challenges in Characterization Using Rheology Coupled to Optics, Scattering, and Spectroscopy

Abstract

Cellulose nanocrystals (CNCs) self-assemble and can be flow-assembled to liquid crystalline orders in a water suspension. The orders range from nano- to macroscale with the contributions of individual crystals, their micron clusters, and macroscopic assemblies. The resulting hierarchies are optically active materials that exhibit iridescence, reflectance, and light transmission. Although these assemblies have the potential for future renewable materials, details about structures on different hierarchical levels that span from the nano- to the macroscale are still not unraveled. Rheological characterization is essential for investigating flow properties; however, bulk material properties make it difficult to capture the various length-scales during assembly of the suspensions, for example, in simple shear flow. Rheometry is combined with other characterization methods to allow direct analysis of the structure development in the individual hierarchical levels. While optical techniques, scattering, and spectroscopy are often used to complement rheological observations, coupling them in situ to allow simultaneous observation is paramount to fully understand the details of CNC assembly from liquid to solid. This Review provides an overview of achievements in the coupled analytics, as well as our current opinion about opportunities to unravel the structural distinctiveness of cellulose nanomaterials.

Green Topochemical Esterification Effects on the Supramolecular Structure of Chitin Nanocrystals: Implications for Highly Stable Pickering Emulsions

“…Rheology can be a valuable tool to assess the self-assembly of nanostructures and their dynamic behavior in suspension. 39 First, we briefly discuss the ChNC phase behavior and the influence of ChNC size thereon, whereafter we focus on the influence of topochemical modifications on their nematic structuring. Similar to other liquid crystalline systems, with increasing concentration, ChNC suspension develops from an isotropic phase to a biphasic phase consisting of both isotropic domains and chiral nematic/nematic domains.…”

“…Nematic structures of ChNC water dispersions were observed under shear with the rheo-PLI technique (rheology coupled with polarize light imaging), directly correlating optical and rheological properties. 39 This technique allowed the drafting of a phase diagram of nanocrystal supramolecular interactions as a function of the topochemical moieties and their colloidal concentration.…”

Phase transitions of cellulose nanocrystal suspensions from nonlinear oscillatory shear

“…Such viscosity functions are characteristic of isotropic phase suspensions, see 2 wt% CNC. The second group corresponds to 3-9 wt% CNC, where the suspensions showed evidence of a three-region steady shear viscosity function, which is characteristic of liquid crystal systems (Kádár et al 2021). Onogi and Asada (1980) were the first to assign the three-region behavior in polymer liquid crystals, however, there is overall quite a large discrepancy among subsequent studies on the estimation of CNC phases based on the three-region viscosity curve.…”

“…The formation of ordered domains with increasing concentration has been described theoretically by Onsager (1949) who elaborated how suspensions of nanorods can spontaneously develop long-range orientational order as a result of the gain in translational entropy overcompensating the loss in rotational entropy. An additional increase in concentration will lead to a complete liquid crystalline phase (Kádár et al 2021). The liquid crystalline (LC) domains comprise chiral nematic and/or nematic CNC structures (Revol et al 1992;Abitbol and Cranston 2014).…”

“…Optical visualization techniques combined with rheological characterization can help understand the relationship between microstructure, phase transitions, and rheological properties (Kádár et al 2021). One of the commonly used rheological hyphenated methods for optically active suspensions is polarized light imaging (PLI).…”