Structural and flow mechanisms of CNC suspensions under shear 

Breakdown and buildup mechanisms of cellulose nanocrystal (CNC) suspensions under shear or upon relaxation were elucidated through in situ small-angle X-ray scattering and small-angle light scattering (SAXS, SALS) coupled with rheometry.

Breakdown and buildup mechanisms of cellulose nanocrystal (CNC) suspensions under shear or upon relaxation were elucidated through in situ small-angle X-ray scattering and small-angle light scattering (SAXS, SALS) coupled with rheometry.

Objectives: The rheological behavior of cellulose nanocrystal (CNC) suspensions is known to follow a three-regime flow curve typical of liquid crystal systems, characterized by a shear-thinning region followed by a plateau region and then a second shear-thinning region.

The elementary particles (CNCs) have a parallelepiped shape with dimensions of (120 x 20 x 5) nm. The CNC suspensions are liquid crystal type and have a phase diagram that exhibits a coexistence domain of an isotropic phase and a cholesteric phase. Beyond a certain particle concentration, a phase separation occurs, containing randomly distributed CNCs (the isotropic phase) and CNCs that organize themselves to form a helical structure (the chiral nematic or cholesteric phase). The pitch of this helix is on the order of micrometers. This cholesteric phase is present within the isotropic phase as micrometric droplets called "tactoids." With increasing CNC concentration, these "tactoids" coalesce to form a single cholesteric phase throughout the entire sample volume.

Despite recent advances in monitoring the organization of CNC suspensions, until now, no in situ characterizations of the evolution of the cholesteric phase during its break-down under shear or its buildup upon cessation of flow had been performed.

To address this, structural observations were carried out at various length scales, from the nanometric scale of CNCs using SAXS to the micrometric scale of their cholesteric phase organization using SALS.

Results: Several advances have been made in understanding the flow properties of CNCs. The dynamics of structural organization changes on length scales ranging from nanometers to micrometers have been linked to the typical three-regime rheological behavior of liquid crystals (Fig. 1) and have allowed for updating interpretations previously proposed in the literature. Access to the presence or disappearance of the pitch during flow or restructuring, as well as the orientation of the helical axis of the cholesteric structure, obtained through in-situ SALS measurements, has led to significant progress in understanding CNC organization.

1) Disintegration Mechanism under Shear Flow:

First, at the lowest shear rates corresponding to regime I of shear thinning, a progressive breakup of large cholesteric domains into smaller, micrometer-sized "tactoids" has been observed. Furthermore, its cholesteric domains are oriented with their helical axis aligned perpendicularly to the flow direction. This specific mutual orientation of the cholesteric domains releases stresses within the suspension, which explains the decrease in viscosity in this shear-thinning regime I. This result clarifies what had been previously proposed in the literature, shifting the previously suggested interpretation of the CNC organization (Orts et al., 1998; Shafiei-Sabet et al., 2012; Haywood et al., 2017) towards lower shear gradient values.

Secondly, on the viscosity plateau of regime II, a progressive orientation of the cholesteric phase under flow was observed, with a vertical orientation of the helical axes. A progressive breakdown of the "tactoids" into smaller units (smaller than the helical organization pitch) is then observed until the helical organization pitch signature disappears. This direct observation of the "tactoid" rupture highlighted by SALS is reinforced by the quantitative analysis of rheo-SAXS measurements performed in radial and tangential observation modes.

Finally, it was shown that the second phase of shear thinning at the highest shear gradients (regime III) corresponds to the complete breakdown of the "tactoids," as demonstrated by SALS measurements where no significant diffusion intensity is detected. Nanoscale SAXS observations show a stabilization of the achieved orientation level, corresponding to a parallel flow of all nanocrystals along the velocity direction, resulting in a nematic organization.

Fig. 1: Flow curve and anisotropy parameter deduced from SAXS and SALS measurements, and corresponding 2D patterns. Schematic description of CNC suspensions from rest to increasing applied shear rates, showing the transition from an isotropic distribution of cholesteric tactoids at rest, to aligned cholesteric tactoids in regime I, then to the fragmentation of micrometer-sized tactoids in regime II, and finally to isolated oriented CNCs in regime III. Flow curve deduced from rheometric measurements, and anisotropy parameter deduced from SAXS and SALS measurements. Corresponding in situ SAXS and SALS profiles (C = 9 wt%, 5.82 vol%, [NaCl] = 0.01 mol·L⁻¹ at T = 24.5 °C). The blue circles or blue ellipses drawn in the diagram identify the cholesteric phase domains or "tactoids" in relation to the isotropic phase.

2) Buildup mechanism during relaxation after high shear:

The buildup mechanism during relaxation after high shear follows a three-step process: i) rapid reassembly of individual CNCs into a nematic organization established down to micrometer scales; ii) slower formation of large, oriented cholesteric domains whose helical axes are aligned perpendicular to the previous flow direction. This relaxation step is associated with a demixing process corresponding to the separation of the isotropic and cholesteric phases with an induction time of approximately 15 min. This suggests that the relaxation mechanism is probably primarily associated with a nucleation and growth process; iii) finally, a much slower reorganization of these large, oriented domains is observed, leading to an isotropic distribution of the cholesteric arrangements to build the isotropic equilibrium structure of the CNC suspensions at rest (Fig. 2).

Fig. 2: In situ SALS and SAXS characterization of the relaxation of CNC suspensions during relaxation after a steep shear rate of (1068 s⁻¹) belonging to regime III. Schematic description of the CNC organization and associated 2D light scattering profiles as a function of relaxation time. t = 0 s corresponds to stop of the flow. Time evolution of viscosity and the corresponding PCA anisotropy parameter, based on simultaneously recorded SAXS profiles of an CNC suspension at a shear gradient of 10⁻² s⁻¹ after pre-shearing at 1000 s⁻¹. C = 10 wt% (6.49 Vol%), [NaCl] = 0.01 mol·L⁻¹, T = 25 °C.

Références :

Pignon, F., Challamel, M., De Geyer, A., Elchamaa, M., Semeraro, E.F., Hengl, N., Jean, B., Putaux, J.L., Gicquel, E., Bras, J., Prevost, S., Sztucki, M., Narayanan, T., Djeridi, H., "Breakdown and buildup mechanisms of cellulose nanocrystal suspensions under shear and upon relaxation probed by SAXS and SALS", Carbohydrate Polymers, 260, 117751 (2021). doi.org/10.1016/j.carbpol.2021.117751

Orts, W.J., Godbout, L., Marchessault, R.H., Revol, J.-F. (1998). Enhanced ordering of liquid crystalline suspensions of cellulose microfibrils:  A small angle neutron scattering study. Macromolecules, 31, 5717–5725.

Shafiei-Sabet, S., Hamad, W.Y., Hatzikiriakos, S.G. (2012). Rheology of nanocrystalline cellulose aqueous suspensions. Langmuir, 28, 17124–17133.

Haywood, A.D., Weigandt, K.M., Saha, P., Noor, M., Green, M.J., Davis V.A. (2017). New insights into the flow and microstructural relaxation behavior of biphasic cellulose nanocrystal dispersions from RheoSANS. Soft Matter, 13, 8451–8462.