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MacLachlan Group


Sustainable Materials Research



Chiral Nematic Cellulose Nanocrystals Films


The chiral nematic structure of cellulose nanocrystals (CNCs) can be preserved in solid films of CNCs and beautiful helical structures are apparent in the films by scanning electron microscopy (SEM).1 The self-assembly process of CNC rods in suspension is complex, but our group has followed the formation and growth process of CNC agglomerates (called tactoids) for the first time by SEM.2 There has been considerable interest in controlling the helical pitch and color of CNC films over the last decade. In our lab, we have looked at many parameters that influence the helical pitch. Examples include casting CNC suspensions on different substrates3 and varying the evaporation time of water4, both of which produce colorful CNC films that span the visible range. We are currently investigating new ways to produce colorful CNC films!

Chiral Nematic CNC films. a) Photograph of a typical CNC film, prepared by completely evaporating a 4 wt% CNC suspension. b) Scanning electron micrograph of a CNC film taken at an oblique angle. Schematic representation (dashed box) of the chiral nematic organization of CNCs. Half the helical pitch (P/2) is shown.



CNC Composites


The self-assembly of CNCs into a chiral nematic structure is tolerant to additives, which has allowed us to prepare a range of interesting materials in our lab, including hydrogels,1 organosilicas,2 and thin-films.3,4 The use of additives gives us a way to tune the optical and mechanical properties of the resultant CNC materials. For example, pure CNC thin films are tough and lack flexibility, whereas the introduction of hydroxypropylcellulose (HPC) into the matrix can greatly improve the flexibility of the composite material.5 In other work from our lab, addition of inorganic materials, such as metallic nanoparticles,6 or addition of organic units via careful surface modification,3 has led to materials with unique chiroptical properties and that can change color when pressure is applied.4 We are constantly trying to find new ways to expand the library of CNC composites we can make.

CNC/composite thin films. a) CNC/HPC mixed in different weight ratios demonstrating the ability to tune the reflected color across the visible spectrum. Insets are of the chiral nematic structure at low and high amounts of CNC, scale bar = 3.5 cm. b) Photograph demonstrating the flexibility of a CNC/polymer composite. c) Pressure sensitive chiroptical properties of a melamine-urea-formaldehyde/CNC composite thin film.



Templating with CNCs


Cellulose nanocrystals (CNCs) form a chiral nematic structure in water, which can be used as a template to make inorganic materials with chiral nematic order. In our lab we have mixed aqueous suspensions of CNCs with inorganic precursors to obtain chiral nematic composite CNC films. Depending on how we treat these composite films, we can either obtain mesoporous carbon films1, or we can isolate free-standing mesoporous inorganic materials2; this is a process known as soft templating. Most notably, by using silica or organosilica precursors, we have made mesoporous silica with different colours2 and we can introduce functionality within the pores by changing the starting materials.3

Furthermore, the chiral nematic mesoporous silica can be used as a template for new inorganic precursors, and the silica can be etched away. By using this method, called hard templating, we have been able to synthesize chiral nematic mesoporous titania4 and germania5 materials. With their porous and periodic helical structures, these films have potential applications in photonics, chiral separation, catalysis, and energy-storage.1-5

Synthesis of chiral nematic mesoporous silica and mesoporous titania.



Responsive Photonic CNC Materials


CNCs can be mixed with polymers to embed the chiral nematic structure in composites. The arrangement of the CNC particles in the composite can be manipulated using a variety of stimuli to prepare responsive photonic CNC materials. Our group has developed elastomeric composites that change color upon stretching1-3 or compression,4 chemical-responsive photonic CNC materials that respond to ionic strength and solvent dielectric constant,5,6 and temperature-responsive photonic organosilica films.7 The photonic response of these materials is a result of the stimuli-driven change in the CNC helical pitch or chiral-to-nematic structural switching. We are now trying to find new responsive CNC composites for applications as mechanical/thermal/chemical sensors and high security materials.

Photonic responsive CNC materials. a) photonic response of CNC/acrylate elastomer to tension (transmitted light is imaged between cross-polarizers); b) photonic response of CNC/silicone aerogel to compression (reflected light is imaged); c) photonic response of CNC/acrylamide hydrogel to water and ethanol (in water the reflected color red-shifts while in ethanol the reflected color blue-shifts); d) photonic response of organosilica film prepared by CNC-templating to temperature (reflected light is imaged).



Stimuli-responsive CNC Gels


Gels are entangled networks that physically confine a fluid. Cellulose nanocrystals (CNCs) can spontaneously assemble into gels when mixed with organic or inorganic salts. The MacLachlan group relies on this ion-induced strategy to fabricate stimuli-responsive CNC gel materials that could be useful in wound dressings, water treatment, drug delivery, chiral separations, adsorbents, filters, sensors, and optical devices.

In the lab, we have made CNC gels using, e.g., (i) polyionic macrocycles, which trigger gelation and perform as molecular receptors (absorbing and recognizing guest molecules from solution),1 and (ii) pH-responsive species that can be switched between charged and uncharged states upon gas infiltration (CO2 vs. N2);2 this renders interesting materials with reversible sol-gel behavior.

CNC-based responsive gels. (a) Reversible gelation and decomposition of the CO2-switchable CNC gels containing imidazole in the presence and absence of CO2. (b) Schematic representation of the ion-induced gelation of CNCs.



CNC Aerogels


Aerogels are light-weight porous materials that generally have a disordered structure due to the random distribution of sol particles. In our lab we have exploited the ability of CNCs to form ordered structures to produce aerogels with interesting properties. We can prepare chiral nematic aerogels we use a solvent exchange method to prepare a chiral nematic gel that is then dried to yield an aerogel.1 These aerogels can be used as templates for silica and titania aerogels.1,2 If we use a hydrothermal gelation and freeze-drying technique, we obtain an aerogel with a layered-like structure but with chiral features inside. When we incorporate PDMS into the aerogel network we get an elastic composite that can be used as a pressure‐responsive chiral photonic material.3 In our lab we are also investigating ways to functionalize CNC aerogels for environmental remediation applications.4

CNC Aerogels. a) Chiral nematic CNC aerogel image (left) and SEM image (right). b) CNC/PDMS composite aerogels with and without compression is the dry and wet state.



Thermal Stability of CNCs


Individual CNCs have impressive mechanical properties,1 attracting enormous attention for the preparation of composites, where they reinforce polymers and inorganics.2 Cellulose itself is stable to above 340 °C. Unfortunately, the CNCs produced from the sulfuric acid hydrolysis are less stable than cellulose and begin degrading well below this temperature. This thermal instability is a significant drawback to widespread implementation of CNCs in composite materials, foams, and emulsions. In order to improve their stability, in our lab we are investigating the thermal degradation of CNCs, so that we can selectively prevent the degrading processes through mechanical stimuli, surface grafting, physical adsorption, and ion exchange.

Thermal degradation of CNCs. Photographs of birefringent CNC-H and CNC-Na powders at different stages of thermal degradation (10 °C/min, air atmosphere).



Bio-Inspired Materials


Bio-inspired materials take advantage of the complex multi-scale hierarchical self-assembly found in nature to emanate features such as chirality, porosity, photonics, structural color, hydrophobicity, and mechanical properties.1,2 Our research primarily focuses on bio-inspired materials based on naturally-sourced polysaccharides. Chitin is the second most abundant polysaccharide, after cellulose, present in the shells of crabs, shrimps, and insects.3 Despite the most popular application of chitin and chitosan in biomedical applications such as wound dressings and drug delivery,3 we are interested in how their self-assembly can be used to create materials for novel applications.

We have exploited the self-assembly of nanocrystalline chitin into nematic liquid crystalline phases as a template to create mesoporous silica and organosilica films with layered structures,4 which we can then modify to apply them as supercapacitor electrodes.5 We have also explored the use of chitin and chitosan for the development of tunable photonic nanomaterials including mesoporous membranes and hydrogels.7 Most recently, we have applied chitin bio-templating to create lightweight γ-alumina aerogels with potential as catalysts, catalyst supports, or thermal insulation.8 Ongoing research is aimed at chitin and chitosan based bio-inspired nanomaterials for a range of applications.

Chitosan-based bio-inspired materials. Structure of the biopolymer chitin and the derived materials made in our lab, including hydrogels, layered silica films, and aerogels.





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MacLachlan Group 2024.