Assembly of cellulose nanocrystals in a levitating drop probed by time-resolved small angle X-ray scattering

Abstract

Assembly of bio-based nano-sized particles into complex architectures and morphologies is an area of fundamental interest and technical importance. We have investigated the assembly of sulfonated cellulose nanocrystals (CNC) dispersed in a shrinking levitating aqueous drop using time-resolved small angle X-ray scattering (SAXS). Analysis of the scaling of the particle separation distance (d) with particle concentration (c) was used to follow the transition of CNC dispersions from an isotropic state at 1-2 vol% to a compressed nematic state at particle concentrations above 30 vol%. Comparison with SAXS measurements on CNC dispersions at near equilibrium conditions shows that evaporation-induced assembly of CNC in large levitating drops is comparable to bulk systems. Colloidal states with d vs. c scalings intermediate between isotropic dispersions and unidirectional compression of the nematic structure could be related to the biphasic region and gelation of CNC. Nanoscale structural information of CNC assembly up to very high particle concentrations can help to fabricate nanocellulose-based materials by evaporative methods.

Assembly, Gelation, and Helicoidal Consolidation of Nanocellulose Dispersions

“…45 We have also investigated a recently presented in situ SAXS investigation of the CNC assembly in a shrinking levitating drop that covered a very large concentration range. 46 The in situ SAXS study was performed on a single droplet without interference of a solid substrate or added surfactants, which may interfere with the phase transitions and assembly behavior. Figure 4a shows how a CNC aqueous drop (with an initial diameter of 1.5 mm) was levitating between the pressure nodes generated in the sonotrode.…”

Colloidal Behavior of Cellulose Nanocrystals Grafted with Poly(2-alkyl-2-oxazoline)s

“…This primary scattering peak is related to the center-to-center separation distance between neighboring particles ( d = 2π/ q peak ; q is the scattering vector). 44,45 Then, the addition of CNC- g -P(EtOx 95 - s -Ei 5 ) (in a ratio of 10:1 CNC/CNC- g -P(EtOx 95 - s -Ei 5 ), w/w) into a CNC dispersion imparts some tenuous structural changes, however, maintaining the nematic phase (Figure 5). The shift of the scattering Bragg peak toward higher q in the presence of polymer-grafted CNCs indicates a slight decrease in CNC interparticle distance to ∼57 nm.…”

“…Samples with the characteristic nematic organization did not flow with time, while isotropic dispersions behaved as a fluid of low viscosity (Figure 5). Investigation of the gravity-driven flowability of CNC dispersions was made at different concentrations by Liu et al., 45 whose work suggests that the gelation threshold of CNCs is around 6 wt %, corresponding to the concentration associated with structural organization dominated by the nematic phase. There is an undoubted correlation between gelation and the presence of a chiral nematic liquid crystalline phase.…”

Liquid Crystalline Tactoidal Microphases in Ferrofluids: Spatial Positioning and Orientation by Magnetic Field Gradients

“…20 Approaches to modifying the self-assembly and alignment of CNCs can bring control over and improvements to their optical properties. [21][22][23][24][25][26][27] Here, we demonstrate a new method to allow control over the position and orientation of chiral nematic liquid crystalline tactoids and their resulting macroscopic ordered phases in aqueous dispersions of CNCs ( Figure 1). Key to our approach is the discovery that isotropic phases can be endowed with significantly higher volume magnetic susceptibility (c m ) than liquid crystalline tactoids on the basis of the exclusion effects of tactoids on superparamagnetic magnetite doping nanoparticles (MNPs) (Figure 2A), 28 which is a result of the entropically driven microscopic phase separation of binary mixtures of rods and spheres.…”