Nanocellulose has emerged as a natural alternative to plastics for many applications because of its physical properties, such as tensile strength. However, nanocellulose also has an arsenal of other beneficial properties, one of which is a series of unique optical properties and a high optical transparency; as such, it has emerged as an effective material in the creation of optical films. In this article, we look at nanocellulose-based optical films and the applications they have been used in.
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Cellulose and Nanocellulose
Cellulose is a naturally occurring polysaccharide that can contain up to thousands of linked D-glucose monomer units. It is a natural polymeric type (pseudo-plastic) material that is the main structural component in the cell wall in plant cells.
By comparison, nanocellulose is nano-structured form of cellulose that can take multiple forms. There are three common types of nanocellulose. These are cellulose nanocrystals, micro/nano-fibrillated cellulose and bacterial nanocellulose. Of these, cellulose nanocrystals and fibrillated nano cellulose structures are the most widely used across all applications, and especially in the production of optical films.
These two forms are used more widely because they have a wide range of beneficial properties compared to bacterial nanocellulose, and they are a lot easier to produce in large quantities. The properties exhibited by nanocellulose structures range from a high chemical stability, to exhibiting thixotropic characteristics, efficient electronic and optical properties, a high tensile strength (up to 500 MPa), a high stiffness and strain, excellent barrier properties, no toxicity, and the ability to self-assemble when deposited (i.e. through coating methods).
Applications as Optical Films
Films created using nanocellulose are known to possess an optical transparency (i.e. a low light scattering) as well as many unique optical properties, such are iridescence, selective reflection of left circularly polarized (LCP) light, and transmission of right circularly polarized (RCP) light. These properties, coupled with their high strength and large surface area, make them an ideal material as optical films, and have been implemented into various optical applications. Additionally, when nanocellulose is self-assembled into multi-layer, multi-component, films, it is known (in certain conformations) to exhibit a birefringence through an optical anisotropy brought about by the orientation of the nanocellulose structures in the self-assembly process.
Here, we look at a few specific examples of where they have been used.
Liquid-Crystal Phase Optical Filters
Cellulose nanocrystals can be self-assembled into three-layer sandwiched films as optical filters, where the layers composed of nanocellulose are present in different liquid-crystal phases. These fabricated films are composed of a nematic-phase retardant layer which is sandwiched between helicoidal cholesteric reflector materials. The given layers and the way the nanocellulose reflects light in the different conformations mimics the polarization-insensitive reflectors found within the wings of Plusiotis resplendens beetles. These types of optical films are known to possess a high reflectivity which is tunable for both the visible and near-infrared regions of the electromagnetic spectrum. As such, these optical filters could be used for a range of applications including as color filters, smart cloths and in solar-gain-regulating building technologies.
Chromic Technologies
Cellulose nanocrystals can also be used to create flexible, iridescently controlled, and multistimuli-responsive optical films. Acting as a composite material, these nanocellulose films have a high mechanical toughness that enables them to be used as free-standing iridescent photonic films with tunable structural colours. They can be used to detect changes in an environment, such as humidity and mechanical compression. In response to an external stimulus, they change colour to recognize that there has been a change within the given environment. The colour change is also reversible, so they can be used many times in different environments. It is thought that these optical films could find use in colorimetric sensors, anticounterfeiting technology, and decorative coatings.
Security Paper
There is another potential application of cellulose in using ZnS-sodium carboxymethyl cellulose composite films as a type of security paper, for use with optical signatures. The optical properties, and in particular the optical transparency, of the composite are highly tunable and this is influenced by the number of ZnS-blende nanoparticles within the composite. The optical transparency of such films can be between 50% and 90% depending on how many nanoparticles are loaded into the cellulose-based film. However, this is not a well-established area compared to many other optical applications and could take some work before they are realized.
Sources:
- CelluForce: http://www.celluforce.com
- “Review of Bionanocomposite Coating Films and Their Applications” Haniffa M. A. C. M. et al, Polymers, 2016, DOI: 10.3390/polym8070246
- “Mind the Microgap in Iridescent Cellulose Nanocrystal Films”- Fernandes S. N. et al, Advanced Materials¸ 2017, DOI: 10.1002/adma.201603560
- “Birefringence in spin-coated films containing cellulose nanocrystals”- Cranston E. D. and Gray D. G., Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2008, DOI: 10.1016/j.colsurfa.2008.04.042
- “Optical and mechanical properties of nanofibrillated cellulose: Toward a robust platform for next-generation green technologies”- Simao C. D. et al, Carbohydrate Polymers¸ 2015, DOI: 10.1016/j.carbpol.2015.03.032
- “Cellulose-Based Reflective Liquid Crystal Films as Optical Filters and Solar Gain Regulators”- De La Cruz J. A. et al, ACS Photonics, 2018, DOI: 10.1021/acsphotonics.8b00289
- “Biomimetic Optical Cellulose Nanocrystal Films with Controllable Iridescent Color and Environmental Stimuli-Responsive Chromism”- He Y-D. et al, ACS Applied Materials & Interfaces, 2018, DOI: 10.1021/acsami.7b18440
- “Synthesis and optical characterization of ZnS–sodium carboxymethyl cellulose nanocomposite films”- Luna-Martinez J. F. et al, Carbohydrate Polymers, 2011, DOI: 10.1016/j.carbpol.2010.12.021
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