Polymers, Colloids & Interfaces, Short talk
PI-017

Patience is a Virtue: Self-Assembly and Physico-Chemical Properties of Cellulose Nanocrystal Allomorphs

G. Delepierre1,3, W. Thielemans2, S. Eyley2, C. Weder1, E. Cranston3*, J. Zoppe4*
1University of Fribourg, Adolphe Merkle Institute, 2KU Leuven, 3University of British Columbia, 4Omya

Cellulose nanocrystals (CNCs) are bio-based rod-like nanoparticles with a quickly expanding market. Depending on their origin and chemical treatment, a variety of cellulose crystal allomorphs can be prepared. Industrially produced CNCs are isolated from the native crystalline allomorph cellulose I, giving cellulose I nanocrystals (CNC-I). An interesting feature of CNC-I is that they self-assemble into chiral nematic liquid crystalline phases upon sample concentration as a result of the increase in translational entropy due to the alignment of the CNC-rods. To date, the liquid crystalline properties have only been investigated for the native allomorph, CNC-I. To fill this void, we compare the physico-chemical properties and liquid crystalline behavior of CNC-I to those produced from a different crystal allomorph, known as cellulose II (CNC-II). First, cellulose II is obtained by treating macroscopic cellulose I fibers with a strong aqueous sodium hydroxide solution. Then, the allomorphs are isolated in a top-down manner from macroscopic cellulose I and cellulose II fibers, by sulfuric acid hydrolysis, resulting in colloidally stable sulfated CNC-I and CNC-II, respectively. The two allomorphs show similar surface charge densities and z-potential. Both CNC-types are determined to have a right-handed twist by induced circular dichroism. CNC-II are shorter in length and therefore have a slightly lower aspect ratio compared to CNC-I (Figure 1). Despite having very similar physico-chemical properties, the self-assembly of the two CNC allomorphs in water was distinctive. Whilst CNC-I equilibrate into a chiral nematic phase in under two weeks, CNC-II first phase separate into an upper isotropic and lower nematic phase, after which the CNC-II slowly reorganize into a long-pitch chiral nematic liquid crystal over a period of forty weeks. Here, we propose a number of hypotheses that could contribute to these observations. First, the different self-assembly behavior of the allomorphs may be caused by their faster diffusion, which is most likely related to their shorter lengths. The slow assembly could also be a result of the different crystal structures of the allomorphs, which presumably have distinct dipole moments. Finally, the CNC twist frequency and surface sulfate group distribution may likewise play a role.

Figure 1. AFM images of CNC-I (top) and CNC-II (down), and their self-assembly to a chiral nematic liquid crystalline phase.