Self-assembly of cellulose nanocrystals of different lengths

Abstract

HypothesisThe self-assembly (SA) of cellulose nanocrystals (CNC) in suspensions is important both from the fundamental and advanced technology development perspective. CNC of different lengths self-assemble differently in suspensions by balancing attractive and repulsive interactions which depends strongly on morphology, surface chemistry and concentrations.ExperimentsTwo different commercial CNC samples (CNC-M and CNC-C) of different lengths were dispersed in Milli-Q water at different concentrations (0.5–10 wt%). CNC-M is provided as a gel at a solid concentration of 10.3 wt% which was diluted in Milli-Q water. CNC-C is sold as a powder which was dispersed in Milli-Q water with a mixer to achieve the desired concentrations. TEM was used to determine morphology of CNC. Polarised optical microscopy is performed to get microscale visualisation of the chiral nematic self-assembly. High flux synchrotron SAXS is applied to evaluate and compare the nanoscale self-assembly mechanisms of CNC of different lengths.FindingsThe SA of two different types of CNC rods of similar diameter but different lengths is investigated. SAXS analysis shows the short rods in suspension form an isotropic phase (randomly oriented) at lower concentration (0–4 wt%); as concentration is increased, the rods become systematically aligned in a nematic phase. The interrod distance d varies as c−0.33 at the lower concentration, which changes to c−0.5 and even c−1 at the higher concentrations. In contrast, the long rods in suspension remain in the isotropic phase throughout the measured concentration range from 0.5 to 10 wt%. The interrod distance also follows the isotropic power law slope of c−0.33. Suspensions made of the short CNC rods show long range order and large interrod distance compared to those formed by the long rods. POM agrees with the SAXS results. A specific equilibrium between attractive and repulsive forces is required to maintain SA and ordering of the rods. DLVO calculations reveal that the long rods maintain van der Waal attractive force dominating over the electrostatic repulsion, which hinders rods alignment in an ordered manner. However, for the short rods, the weaker attractive interactions are well compensated by the repulsive force which aligns rods in an ordered assembly. This fundamental understanding of the SA of rods in suspensions facilitates the engineering of novel CNC composites of unique optical properties which enables novel applications such as in sensors and bio-diagnostics.