Wadood Y. Hamad

Free-standing mesoporous silica films with tunable chiral nematic structures

Chirality at the molecular level is found in diverse biological structures, such as polysaccharides, proteins and DNA, and is responsible for many of their unique properties. Introducing chirality into porous inorganic solids may produce new types of materials that could be useful for chiral separation, stereospecific catalysis, chiral recognition (sensing) and photonic materials. Template synthesis of inorganic solids using the self-assembly of lyotropic liquid crystals offers access to materials with well-defined porous structures, but only recently has chirality been introduced into hexagonal mesostructures through the use of a chiral surfactant. Efforts to impart chirality at a larger length scale using self-assembly are almost unknown. Here we describe the development of a photonic mesoporous inorganic solid that is a cast of a chiral nematic liquid crystal formed from nanocrystalline cellulose. These materials may be obtained as free-standing films with high surface area. The peak reflected wavelength of the films can be varied across the entire visible spectrum and into the near-infrared through simple changes in the synthetic conditions. To the best of our knowledge these are the first materials to combine mesoporosity with long-range chiral ordering that produces photonic properties. Our findings could lead to the development of new materials for applications in, for example, tuneable reflective filters and sensors. In addition, this type of material could be used as a hard template to generate other new materials with chiral nematic structures.

Cellulose reinforced polymer composites and nanocomposites: a critical review

AbstractThis review provides a critical assessment of the use of cellulosic materials for reinforcement in polymer composites. The review focuses on structure–property interrelationships and the compatibilization of cellulosic materials for optimal performance of the resulting composite materials. Optimal material and physical properties are characterized on the basis of the reinforcement’s physical dimension and the nature of the interface between reinforcement and matrix. We explore how very different cellulosic materials—bacterial, microcrystalline, microfibrillated or nanocrystalline—can cause distinctly different reinforcment.

Chiral Nematic Mesoporous Carbon Derived From Nanocrystalline Cellulose

Template synthesis based on the self-assembly of lyotropic liquid crystals offers access to mesoporous solids with high specific surface areas and periodic structures. Incorporating mesopores (i.e., pores ranging from 2 to 50 nm in diameter) into carbonaceous materials may be advantageous for certain applications, including the adsorption of large molecules, electrochemical double-layer capacitors, lithium ion batteries, catalyst supports, and field-effect transistors. Ordered mesoporous carbon materials were first synthesized by using ordered mesoporous silica as a hard template. 7] In the hardtemplating (also referred to as nanocasting) approach, mesoporous silica is repeatedly infiltrated with a suitable carbon precursor (e.g., sucrose) that is carbonized within the pores of the silica at elevated temperature. After sufficient pore loading and etching of the silica, mesoporous carbon with a structure that is the inverse of the original silica template is obtained. Despite the potential benefits of using mesoporous carbon over traditional activated carbon, the cost of making these materials may be prohibitive. Finding more economical synthetic routes, both in terms of the number of steps involved and precursors used, is important if mesoporous carbon is to be implemented in new technologies. Direct surfactant-templating approaches (soft templating) have also been developed for the synthesis of mesoporous carbon by condensing polymerizable carbon precursors (e.g., phenolic resins) around block copolymer templates. Soft templating requires fewer synthetic steps than hard templating and offers improved control over the morphology of the mesoporous carbon products. For example, free-standing mesoporous carbon membranes have been synthesized through evaporation-induced self-assembly coupled with soft templating. The specific surface areas of these films, however, are considerably less than those of mesoporous carbons produced by hard templating. The use of both hardand soft-templating approaches has enabled mesoporous carbon to be synthesized with cubic and hexagonal pore systems that are ultimately derived from the self-assembly of surfactants into ordered mesophases. The synthesis of mesoporous carbon templated by other liquid-crystal phases, for example nematic and chiral nematic phases, has been virtually unexplored. In particular, the incorporation of chiral organization into mesoporous carbon could open the door for applications that involve enantioselective adsorption. Chiral nematic liquid crystals, which consist of mesogens organized into a long-range helical assembly, exhibit unique properties, such as the selective reflection of circularly polarized light. The incorporation of chiral nematic organization into solid-state materials could give rise to novel properties. Kyotani and co-workers have synthesized graphitic carbon with chiral nematic ordering by first polymerizing polyacetylene within a thermotropic chiral nematic liquid crystal followed by doping with iodine and pyrolysis. It is expected that these materials will display interesting electromagnetic properties. As the major constituent of plant cell walls, cellulose is the most abundant biological material on the planet. Recently, there has been significant interest in the study of cellulose fibrils with nanometer dimensions that have high surface area and can behave as lyotropic liquid crystals. Stable suspensions of nanocrystalline cellulose (NCC) can be obtained through hydrolysis of bulk cellulosic material with sulfuric acid. In water, suspensions of NCC organize into a chiral nematic phase that can be preserved upon slow evaporation, thereby resulting in chiral nematic films. The unique physical properties and natural abundance of NCC make it attractive as a potential template for porous materials. Although bulk cellulosic materials are commonly used to generate activated carbon, to date there have been no studies on the use of NCC as a template for mesoporous carbon. Recently, our research group reported that evaporationinduced self-assembly of NCC with different silica precursors can result in composite films with chiral nematic structures, and that the removal of NCC from these films generates chiral nematic mesoporous silica. Herein we report that NCC– silica composite films may also be used to generate mesoporous carbon with a high specific surface area and excellent retention of the chiral nematic organization. This provides the first example of using nanocrystalline cellulose as a template for mesoporous carbon as well as the first demonstration of a mesoporous carbon with chiral nematic ordering. We demonstrate that the use of silica is necessary for both the introduction of mesoporosity and the preservation of the [*] K. E. Shopsowitz, Prof. Dr. M. J. MacLachlan Department of Chemistry, University of British Columbia 2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada) E-mail: mmaclach@chem.ubc.ca

The Development of Chiral Nematic Mesoporous Materials

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 of host-guest chemistry, and identify future directions that exploit the unique combination of properties present in these materials. The examples covered in this Account demonstrate that there is a rich diversity of composite materials accessible using CNC templating.

Chiral nematic assemblies of silver nanoparticles in mesoporous silica thin films.

Silver nanoparticles (NPs) have been synthesized inside mesoporous silica films with chiral nematic structure. Circular dichroism measurements of the silver NP-loaded silica films show NP-based optical activity in the vicinity of the surface plasmon resonance. These materials, with an optical response associated with the chiral assembly of metal NPs, may be useful for developing new sensors.

Flexible and iridescent chiral nematic mesoporous organosilica films.

Nanocrystalline cellulose (NCC) has been used to template ethylene-bridged mesoporous organosilica films with long-range chirality and photonic properties. The structural color of the organosilica films results from their chiral nematic ordering, can be varied across the entire visible spectrum, and responds to the presence of chemicals within the mesopores. To synthesize these materials, acid hydrolysis was used to remove the NCC template without disrupting the organosilica framework. The resulting mesoporous organosilica films are much more flexible than brittle mesoporous silica films templated by NCC. These materials are the first of a novel family of chiral mesoporous organosilicas with photonic properties.

Rheology of nanocrystalline cellulose aqueous suspensions.

The rheological properties and microstructure of nanocrystalline cellulose (NCC) aqueous suspensions have been investigated at different concentrations. The suspension is isotropic up to 3 wt %, and phase separates to liquid crystalline and isotropic domains at higher concentrations where the samples exhibit a fingerprint texture and the viscosity profile shows a three-region behavior, typical of liquid crystals. The suspension behaves as a rheological gel at even higher concentrations where the viscosity profile shows a single shear thinning behavior over the whole range of shear rates investigated. The effects of ultrasound energy and temperature on the rheological properties and structure of these suspensions were studied using polarized optical microscopy and rheometry. Our results indicate that the amount of applied ultrasound energy affects the microstructure of the suspensions and the pitch of the chiral nematic domains. The viscosity profile is changed significantly at low shear rates, whereas the viscosity of biphasic suspensions at intermediate and high shear rates decreased with increasing temperature. This suggests that, between 30 and 40 °C, structural rearrangement takes place. At higher concentrations of about 10 wt %, the temperature has no significant effect on viscosity; however, a marked increase in viscosity has been observed at around 50 °C. Finally, the Cox-Merz rule was found to fail after a critical concentration, thereby implying significant structural formation. This critical concentration is much higher for sonicated compared to unsonicated suspensions.

Hard Templating of Nanocrystalline Titanium Dioxide with Chiral Nematic Ordering

Anatase TiO(2) nanocrystals have been organized into high-surface-area (150-230 m(2) g(-1)) mesoporous films with long-range chiral nematic ordering. The chiral structure of the anatase films causes them to selectively reflect circularly polarized light and appear iridescent. These materials show replication of structural features found in the silica template on nanometer to millimeter length scales.

Responsive mesoporous photonic cellulose films by supramolecular cotemplating.

Cellulose-based materials have been and continue to be exceptionally important for humankind. Considering the bioavailability and societal relevance of cellulose, turning this renewable resource into an active material is a vital step towards sustainability. Herein we report a new form of cellulose-derived material that combines tunable photonic properties with a unique mesoporous structure resulting from a new supramolecular cotemplating method. A composite of cellulose nanocrystals and a urea-formaldehyde resin organizes into a chiral nematic assembly, which yields a chiral nematic mesoporous continuum of desulfated cellulose nanocrystals after alkaline treatment. The mesoporous photonic cellulose (MPC) films undergo rapid and reversible changes in color upon swelling, and can be used for pressure sensing. These new active mesoporous cellulosic materials have potential applications in biosensing, optics, functional membranes, chiral separation, and tissue engineering.

Chiral Nematic Stained Glass: Controlling the Optical Properties of Nanocrystalline Cellulose-Templated Materials

Chiral nematic mesoporous materials decorated with metal nanoparticles have been prepared using the templated self-assembly of nanocrystalline cellulose (NCC). By adding small quantities of ionic compounds to aqueous dispersions of NCC and tetramethoxysilane (TMOS), the helical pitch of the chiral nematic structure could be manipulated in a manner complementary to the ratio of NCC/TMOS previously demonstrated by our group. We have studied the transformation of these ion-loaded composites into high surface area mesoporous silica and carbon films decorated with metal nanoparticles through calcination and carbonization, respectively. This general and straightforward approach to prepare chiral nematic metal nanoparticle assemblies may be useful in a variety of applications, particularly for their chiral optical properties.