Expanding the upper limits of robustness of cellulose nanocrystal aerogels: outstanding mechanical performance and associated pore compression response of chiral-nematic architectures

Abstract

Control over the nanoscale architecture of a material enables fine tuning of its physical characteristics and associated functions. Depending on the performance demands, properties such as active surface area, density, optical response, transport characteristics and mechanical resilience can be tailored by nanostructuring. Herein, we exploit the liquid crystalline phase transitions in aqueous dispersions of highly anisometric, nanoscaled and high strength (E_A > 150 GPa) cellulose nanocrystals (CNCs) to afford chiral-nematic ordered aerogels with controlled meso- and microstructures. Unprecedented levels of specific strength and toughness were achieved by controlling CNC assembly and derived architectures. We determined that the specific strength, and toughness, of CNC aerogels are improved by up to 137% and 60%, respectively, compared with the highest reported values for aerogels formed solely from cellulose nanofibrils or nanocrystals. Our results demonstrate that chiral-nematic ordered aerogels with controlled meso- and microstructures replicate the liquid crystalline phase transitions of CNCs in aqueous dispersions. The obtained architectures are evaluated systematically by varying the long-range order of the aqueous CNC dispersion from mostly isotropic to completely anisotropic. The resulting aerogels display a strong relationship between the mesopore fraction and selective light reflection (iridescence) as a function of mechanical load. Specifically, we find that the mechanical performance associated with pore compression under load is greatly enhanced by chiral-nematic ordering. The new limits in the mechanical properties of CNC-based aerogels point to new structural considerations for the synthesis of next generation porous constructs that exploit the inherent long-range order of such building blocks.