Anisotropic Diffusion and Phase Behavior of Cellulose Nanocrystal Suspensions

Abstract

In this paper, we use dynamic light scattering in polarized and depolarized modes to determine the translational and rotational diffusion coefficients of concentrated rodlike cellulose nanocrystals in aqueous suspension. Within the range of studied concentrations (1–5 wt %), the suspension starts a phase transition from an isotropic to an anisotropic state as shown by polarized light microscopy and viscosity measurements. Small-angle neutron scattering measurements also confirmed the start of cellulose nanocrystal alignment and a decreasing distance between the cellulose nanocrystals with increasing concentration. As expected, rotational and translational diffusion coefficients generally decreased with increasing concentration. However, the translational parallel diffusion coefficient was found to show a local maximum at the onset of the isotropic-to-nematic phase transition. This is attributed to the increased available space for rods to move along their longitudinal axis upon alignment. This increased parallel diffusion coefficient thus confirms the general idea that rodlike particles gain translational entropy upon alignment while paying the price for losing rotational degrees of freedom. Once the concentration increases further, diffusion becomes more hindered even in the aligned regions due to a reduction in the rod separation distance. This leads once again to a decrease in translational diffusion coefficients. Furthermore, the relaxation rate for fast mode translational diffusion (parallel to the long particle axis) exhibited two regimes of relaxation behavior at concentrations where significant alignment of the rods is measured. We attribute this unusual dispersive behavior to two length scales: one linked to the particle length (at large wavevector q) and the other to a twist fluctuation correlation length (at low wavevector q) along the cellulose nanocrystal rods that is of a larger length when compared to the actual length of rods and could be linked to the size of aligned domains.

SANS study of mixed cholesteric cellulose nanocrystal – gold nanorod suspensions

“…These results are in line with our earlier work which showed that structuring occurs in the isotropic phase before macroscopic phase separation commences. 22 They also agree with earlier observations in the literature, showing that the development of the cholesteric liquid crystalline phase of CNCs covers a wide concentration range, involving the initial formation of small anisotropic domains within the isotropic phase which slowly evolve towards a macroscopic liquid crystal as the CNC concentration increases. 20,22 The "isotropic" phase thus exists as a kinetic equilibrium between a large isotropic region with local anisotropic domains, the ratio of which depends on CNC concentration (concept illustrated in Figure 2c).…”

“…22 They also agree with earlier observations in the literature, showing that the development of the cholesteric liquid crystalline phase of CNCs covers a wide concentration range, involving the initial formation of small anisotropic domains within the isotropic phase which slowly evolve towards a macroscopic liquid crystal as the CNC concentration increases. 20,22 The "isotropic" phase thus exists as a kinetic equilibrium between a large isotropic region with local anisotropic domains, the ratio of which depends on CNC concentration (concept illustrated in Figure 2c). 20,22,20,26 The position of the structure peaks shifted when compared to the position of the CNC reference (in the absence of AuNRs, black curve in Figure 2a, and ESI Figure S4.1), indicating that AuNRs interact with CNCs, thereby influencing CNC structuring.…”

“…In general, the development of structured domains starts at concentrations that are highly dependent on the CNC characteristics and the ionic strength of the solution. 22,25,26 Here, for CNC concentrations of 5 wt% and above, distinct SANS peaks were observed for both the chiral phase (Figure 2a) and the isotropic phase (ESI Figure S4.1). These results are in line with our earlier work which showed that structuring occurs in the isotropic phase before macroscopic phase separation commences.…”

“…The position of the structure peaks shifts to higher q-values as the concentration increases and the CNC correlation distance decreases, in accordance with recent studies regarding the concentration effect of CNCs on the formation of the liquid crystalline phase. 19,20,22 The calculated correlation distances were found to decrease from 53 to 43 nm when the CNC concentration increased from 5 to 8 wt% (values used as reference values throughout the manuscript).…”

Arrested dynamics in a model peptide hydrogel system

“…A similar interpretation was also made recently in a study of cellulose nanocrystals. 47 Another possible source of slow mode could be the self-diffusion, as the particles are highly polydisperse. 48 Selfdiffusion modes have been observed clearly for example for polydisperse microemulsion droplets.…”