The Development of Chiral Nematic Mesoporous Materials

Abstract

Cellulose nanocrystals (CNCs) are obtained from the sulfuric acid-catalyzed hydrolysis of bulk cellulose. The nanocrystals have diameters of ~5-15 nm and lengths of ~100-300 nm (depending on the cellulose source and hydrolysis conditions). This lightweight material has mostly been investigated to reinforce composites and polymers because it has remarkable strength that rivals carbon nanotubes. But CNCs have an additional, less explored property: they organize into a chiral nematic (historically referred to as cholesteric) liquid crystal in water. When dried into a thin solid film, the CNCs retain the helicoidal chiral nematic order and assemble into a layered structure where the CNCs have aligned orientation within each layer, and their orientation rotates through the stack with a characteristic pitch (repeating distance). The cholesteric ordering can act as a 1-D photonic structure, selectively reflecting circularly polarized light that has a wavelength nearly matching the pitch. During CNC self-assembly, it is possible to add sol-gel precursors, such as Si(OMe)4, that undergo hydrolysis and condensation as the solvent evaporates, leading to a chiral nematic silica/CNC composite material. Calcination of the material in air destroys the cellulose template, leaving a high surface area mesoporous silica film that has pore diameters of ~3-10 nm. Importantly, the silica is brilliantly iridescent because the pores in its interior replicate the chiral nematic structure. These films may be useful as optical filters, reflectors, and membranes. In this Account, we describe our recent research into mesoporous films with chiral nematic order. Taking advantage of the chiral nematic order and nanoscale of the CNC templates, new functional materials can be prepared. For example, heating the silica/CNC composites under an inert atmosphere followed by removal of the silica leaves highly ordered, mesoporous carbon films that can be used as supercapacitor electrodes. The composition of the mesoporous films can be varied by using assorted organosilica precursors. After removal of the cellulose by acid-catalyzed hydrolysis, highly porous, iridescent organosilica films are obtained. These materials are flexible and offer the ability to tune the chemical and mechanical properties through variation of the organic spacer. Chiral nematic mesoporous silica and organosilica materials, obtainable as centimeter-scale freestanding films, are interesting hosts for nanomaterials. When noble metal nanoparticles are incorporated into the pores, they show strong circular dichroism signals associated with their surface plasmon resonances that arise from dipolar coupling of the particles within the chiral nematic host. Fluorescent conjugated polymers show induced circular dichroism spectra when encapsulated in the chiral nematic host. The porosity, film structure, and optical properties of these materials could enable their use in sensors. We describe the development of chiral nematic mesoporous silica and organosilica, demonstrate different avenues...

Equilibrium Liquid Crystal Phase Diagrams and Detection of Kinetic Arrest in Cellulose Nanocrystal Suspensions

“…If the helix is allowed to reach the short sub-micron range, this can be seen by the naked eye as vivid iridescent colors being reflected from the sample. Typically, this happens when the water of an aqueous suspension of the rods is allowed to evaporate, leaving a thin paper-like film that displays striking colors across the whole visible spectrum (Beck et al, 2011(Beck et al, , 2013Tabor et al, 2012;Gray, 2013;Dumanli et al, 2014;Kelly et al, 2014;Lagerwall et al, 2014;Mu and Gray, 2014;Park et al, 2014). Moreover, due to the helical internal structure, the reflected light is circularly polarized with the same handedness as the helix (Dumanli et al, 2014;Lagerwall et al, 2014;Park et al, 2014).…”

Photonic Patterns Printed in Chiral Nematic Mesoporous Resins

“…[26][27][28][29] In these structures, the pores are aligned in layers, but their orientation within the layers rotates through the stack. This chiral nematic organization leads to the selective reflection of circularly polarized light with a wavelength (i.e., color) that depends both on the pitch of the helicoidal structure and the refractive index.…”

Anisotropic optical film embedded with cellulose nanowhisker

“…Considering the transmittance in Fig. 5a and the CD result that have the same maximum peak value (Kelly, Giese, Shopsowitz, Hamad, & MacLachlan, 2014), the CNW composite films prepared in this study may possess a pitch length smaller than 300 nm.…”

“…Rod-like CNWs spontaneously produce left-handed chiral nematic structure (or cholesteric structure) at a critical concentration according to Onsager (Eichhorn, 2011). The cholesteric structure encompasses stacked planes of nematic CNWs aligned along a director with the orientation twisted from one plane to the next in an anticlockwise fashion (Kelly, Giese, Shopsowitz, Hamad, & MacLachlan, 2014). Also, the chiral nematic structure, the pitch of which is of the order of the wavelength of visible light, is capable of reflecting left-handed circularly polarized light, in general, across the near-IR and visible spectrum (Kelly, Shopsowitz, Ahn, Hamad, & MacLachlan, 2012).…”