Novel Value-added Applications for Cellulose Nanomaterials : Towards Optics and Electronics Applications

Anthropogenic climate change is one of the biggest global challenges of the 21st century and a green transition is an imminent mitigation measure. One facet of the green transition is a shift away from petroleum-derived materials towards sustainable lignocellulosic biomass-derived materials, while maintaining economic output. However, the available biomass must be utilized wisely and efficiently in order to preserve biodiversity and the ecological balance. Therefore, it is critical to explore application areas for lignocellulosic biomass where low production volumes can lead to high derived value.
This work focuses on the utilization of cellulose from biomass in novel highvalue applications in optics and electronics. The materials of interest in this thesis were cellulose nanomaterials and cellulose derivatives. Cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) were used to fabricate optical films with tunable transparency and haze. Novel thermochromic films from CNF and CNC were produced with the addition of thermochromic (TC) particles to the films. TC particle-doped films exhibited a pronounced reversible black-to-colorless transition upon heating above a transition temperature. The optical properties of CNF-TC and CNC-TC hybrid films could be significantly altered by controlling the particle doping and the film temperature. This feature was harnessed in an all-optical light modulator device where IR light was modulated using visible laser light. Such films hold potential in various thermally-stimulated sensing systems such as temperature monitoring, energy saving, logistics, and smart labels.
Cellulose is a strong emitter in the Earth’s mid-infrared atmospheric transmission window of 8-13 μm, making it a suitable material for passive radiative cooling. Following this, CNF-TC films were explored for passive radiative cooling application taking advantage of both the intrinsic properties of cellulose and the thermochromic functionality provided by the TC particles. Thin silver coatings on CNF-TC films were also explored. Here, a change in film transmission and reflectivity upon thermal stimulus enabled weather adaptive space cooling. A cooling potential of 1-10 °C was measured with the films in laboratory conditions and up to 12 °C in outdoor field tests. The demonstrated films could be applied in cooling roofs and windows in living spaces and greenhouses.
Further, nanocomposite films of CNF and hydroxyethyl cellulose (HEC) were fabricated for application as printed electronic substrates. The CNF-HEC films offered beneficial features such as strength, ductility, conformability, highresolution printability, and biodegradability. The CNF-HEC films were successfully processed in a typical printed electronics fabrication flow, as used for conventional polymer substrates, to fabricate a prototype wearable electrocardiograph (ECG) device. End-of-life scenarios for the devices fabricated on the CNF-HEC films were studied to enable material recovery and avoid incineration or landfilling.
Finally, carboxymethyl cellulose (CMC) optical fibers were fabricated via wet-spinning and crosslinking of CMC hydrogels with aluminum ions. The novel biopolymer optical fibers were produced in a core-only architecture to facilitate environmental sensing capabilities. The CMC optical fibers acted as waveguides in the visible and near-infrared regions and were successfully demonstrated for touch and respiratory rate sensing. Additionally, short-range high-speed optical signal transmission was demonstrated in both air and water media. Such biobased and biocompatible CMC fibers can be applied in environmental sensing, health monitoring, and medical diagnostics.
This work opens new avenues for the utilization of cellulose-based materials in a variety of use cases spanning the optics, electronics, and construction sectors. Future work could focus on studying the durability of the cooling films, improving the reliability of electronic devices, and reducing the attenuation of CMC optical fibers. However, the most important aspect would be the upscaling and commercialization of the demonstrated materials and concepts.