‘Cellulose is a hierarchical structure. The cell wall in plants is composed of large [hemicellulose lignin] fibres inside which are smaller assemblies of cellulose fibres, twisted together. As you look at smaller length scales, you eventually get to cellulose nanocrystals, and below that the individual cellulose polymer molecules,’ explains Carson Meredith, a biomolecular engineer at the Renewable Bioproducts Institute, Georgia Tech, US.
Cellulose nanocrystals (CNCs) promise nanotechnology that is both recyclable and biodegradable. The single crystals, 5-70nm in diameter and 100-250nm in length, are stronger and stiffer than steel, yet are composed of a natural biodegradable carbohydrate.
CNCs can be obtained from wood, cotton, hemp, potato tuber, algae and some bacteria, though wood leads. They can be released from the natural lignocellulose fibres by use of strong acids such as sulfuric acid. Alberta Innovates – Technology Futures (AITF) in Canada is an example of a CNC producer, making about 25kg of research-grade CNCs each week. A wide variety of feedstocks is used, including hardwoods, softwoods and agricultural residues such as hemp and flax. ‘Any cellulosic material will do, but wood pulp is normally used,’ says AITF spokesperson, Scott Lundy.
Green composites and solar cells
Meredith’s team is looking to combine CNCs with polymer composites with improved strength and stiffness for aerospace and car components, and for sports equipment.
However, CNCs plus common polymers equals some serious challenges. ‘Most common polymers are hydrophobic thermoplastics. Yet cellulose likes water and chemical groups on it interact well with water, so putting CNCs into common polymers like polyethylene and polypropylene is a real challenge,’ says Meredith. One approach, which is attracting much attention, is to modify the cellulose so that it is more compatible with the polymers, although this so far has had limited success. An alternative approach is to add CNCs to thermoset polymers, such as epoxy resins and polyurethanes. These polymers tend to have reactive groups that can potentially bond covalently with CNCs, improving compatibility.
Meredith’s team has recently put reactive groups onto the CNC surface, so that they can covalently bind to epoxies and polyurethanes. The CNCs reinforce and boost the strength of these polymers; they could eventually take the place of fillers like calcium carbonate, silica or carbon black, which do the same job but have to be mined or are petroleum based. One complication for the greener alternative, however, is their thermal stability. Meredith’s CNCs are made via sulfuric acid hydrolysis, a commercially scalable process, but one which leaves sulfate groups on the cellulose. ‘Sugary molecules (ie cellulose) don’t perform well at high temperatures, and the sulfate groups make thermal performance even poorer,’ says Meredith. ‘If you cover the sulfate groups via surface modification, and “hide” them in a thermosetting polymer, it improves the thermal properties,’ he adds.
Meredith’ team put CNCs into waterborne epoxy coatings and examined their strength and stiffness, and how the CNCs distributed themselves. ‘We found that the order in which you put the CNCs turns out to matter a great deal,’ says Meredith. ’We obtained better results by first letting the CNCs mix and associate with the epoxy before curing (Polymer, 2015, 68, 111).
The team added up to 15% CNC and found a steady improvement in mechanical properties as they increased the amount of CNCs. These thermoset materials could prove useful for external coatings on aircraft or buildings or as a component of paint, for example.
Meredith is currently working on a collaborative project with five partners, including Australian company Futuris Automotive, to try to create interior car parts that incorporate CNCs. Their goal is to replace heavy steel structures, such as seat frames, with advanced reinforced polymers having CNCs as economical substitutes for carbon fibres. One of the partners, American Process, in Georgia, has built a demo plant for making the nanocellulose. Meredith is also working with a sports company to put CNCs into high-end sports equipment.
Meanwhile, physicist Bernard Kippelen and his team, also at Georgia Tech, are using CNCs in solar cells. In 2012, Kippelen discovered how to make the world’s first all plastic solar cell but the general feedback was that in trying to solve an environmental problem they were creating another (Science, doi: 10.1126/science.1218829). Plastic pollution in landfill and oceans has become a major concern. In the original design, the thin active layer, a few nanometres thick, was printed on plastic substrates a few hundreds of micrometres thick. Thus, most of the material goes into the substrate.
A year later, in 2013, Kippelen reported a renewable organic solar cell that was fabricated on a cellulose nanocrystal substrate (Scientific Reports, 2013, 3, 1536). ‘Using cellulose nanocrystals [as the substrate] you can replace 90% of the materials used in solar cells with natural products,’ explains Kippelen.
The main advantage of using CNCs in solar cells, he adds, is the environmental footprint, especially regards easier recycling. ‘PV is going to be the power generation of choice,’ says Kippelen. ‘There is no cleaner process for directly converting sunlight into electricity, with no byproducts.’ The solar cells he is making using CNCs can be dissolved in water or incinerated like paper, allowing the tiny amounts of other materials used to be recovered.
Kippelen’s lab has since demonstrated an organic light-emitting diode (OLED) on a CNC substrate, as efficient as the traditional devices on glass or plastic substrate (Applied Physics Letters, 2014, 105, 063305). One could make a cell phone display using woodchips in place of plastic, he says, and even a small amount of cellulose to start with will make recycling easier. He has also designed low voltage, field-effect transistors on cellulose nanocrystals (ACS Applied Materials & Interfaces, 2015, 7, 4804).
Reflective coatings and sensors
In his search for new hydrogen-storage materials, chemist Mark MacLachlan, at the University of British Columbia, Canada, added CNCs to glass. The idea was to attach CNCs to glass and then burn them off, leaving behind a porous glass. To his surprise, however, he discovered that the CNC-based glass materials were coloured and iridescent. ‘The cellulose nanocrystals organise into a helical structure on the nanoscale that allows them to reflect light, much the same way beetle shells are iridescent,’ says MacLachlan.
Each layer of CNCs rotates slightly and if the helical pitch matches a wavelength of light, the light is refracted. ‘We can transfer that organisation onto a piece of glass and when we remove the cellulose crystals we get glass with holes organised in a helical structure. The glass films are iridescent and coloured,’ MacLachlan explains. His team is now investigating the use of such glass as reflective coatings for windows. They are also looking to make glass films that reflect infrared or UV light selectively as well as materials that can separate chiral molecules for use the pharmaceutical industry.
The colour of the glass or film can be altered across the entire spectrum by changing the processing conditions slightly, since the pitch of the helical structure in a film or glass is sensitive to ionic strength and temperature. MacLachlan has no shortage of ideas. ‘We’ve been looking to make pressure sensors; materials, which when we press them change the helical pitch and their colour changes. We have also made hydrogels that swell and change colour and you can control the change by altering the analytes in the solution or by the concentration of something like alcohol,’ he explains, all of which is potentially useful for developing sensors. Other researchers interested in CNCs’ light reflective properties have been exploring their potential in colouring in cosmetics.
Membranes and medicines
While the charged surface of CNCs caused by processing in sulfuric acid can lead to poor thermal stability and thus be an issue for materials and electronic applications, it also confers on CNCs some useful properties.
‘You can functionalise various groups on the surface of these CNCs, so that you can build supra-molecular structures that can be utilised in water treatment,’ says Wadood Hamad, principal scientist at FPInnovations and assistant professor at the University of British Columbia, Canada. This is because the cellulose has chemical hooks by way of the acidic sulfate ester, hydroxyl and some carboxylic groups to hang other molecules onto for snagging specific pollutants, or for adding desired antioxidant or antimicrobial features.
Packaging is yet another promising sector for CNCs, because CNC films can block the transmission of oxygen and are almost entirely biodegradable. CNCs can be made transparent, so transparent semi-flexible windows are also on the cards, adds Hamad.
CNCs are also markedly cheaper, compared with some other nanomaterials, such as carbon nanotubes (CNTs). This is adding to their attraction as components of water-treatment membranes. ‘We have started to put CNCs into polymer matrices to make stronger membranes,’ explains Mark Wiesner, environmental scientist at Duke University in North Carolina, US (Environ. Sci. Technol, 2015, 49, 5277).
CNCs have a high surface area-to-volume ratio, low environmental impact, and high strength, as well as sustainability kudos, he explains. ‘You can make this stuff for about a dollar a gram, compared with perhaps hundreds of dollars for a gram of single-walled nanotubes, so that is a big advantage,’ says Wiesner.
‘CNCs can also be used to manipulate the viscosity and rheology of suspensions and gels, as in paints and coatings,’ says Hamad. He believes this is where we will see immediate benefits from CNCs, since emulsions and suspensions are used widely in various foods as well as in industrial applications.
CNCs may also have potential medical applications. ‘One thing that always excites me is that cellulose consists of repeat units of glucose, and that is something our body is very familiar with,’ says Maren Roman, a bio-based polymer researcher at Virginia Tech, US. She recently reviewed their toxicity in the body and says they are not completely in the clear in terms of inhalation, but ‘don’t seem to be harmful if they get on your skin or into our digestive system.’ The presence of OH groups makes them hydrophilic, a good thing in our watery bodies. Roman has combined CNCs with the natural polymer chitosan from lobster shells to make particles she hopes will prove useful for oral drug delivery.
Not surprising, then, that nanocellulose is attracting interest from all sorts of sectors, including paper and packaging, food additives, personal care products, biomedical, energy and composites for aerospace and automotive. ‘The limit to what one can do with these non-toxic, sustainable and highly versatile cellulose nanocrystals is one’s imagination. We have just begun to probe the applications suite,’ concludes Hamad.