Dispersed cellulose nanocrystals self-assemble into solid films while preserving their chiral nematic liquid crystalline phase. These dried films display a characteristic iridescence that is caused by underlying photonic structure of the film.
During the self assembly process a helical arrangement of nanocrystals form a pattern that repeats itself. This period of repetition is called a pitch. The pitch determines what wavelength of light is reflected off from it (in a very photonic crystal like manner).
Smaller pitches reflect shorter wavelengths of light. Longer pitches reflect larger wavelengths of light. In the visible spectrum the pitch needs to be comparable to the color of light its reflecting. This behaviour has been observed from blue to red light but in other photonic crystals IR reflection has been obtained. So no reason that something similar won't happen in nanocellulose if pitch length can be appropriately tailored(incidentally there have been reports of nanocellulose reflecting light in IR range for radiative cooling).
The concept here is not much different from diffraction grating. The grooves are etched on a cd/dvd that causes lights of different wavelengths to reflect based upon the size of the grating or the length of the groove.
Can photonic band gaps also be used to explain colors of cellulosic films?
Absolutely and it has been used to do so. For example structural colors in Polia fruit have been caused due to periodic variations in the cellulose arrangement inside the cells of the fruit. While the exact value of pitch & the length of the patterns would need instrumentation its easy to comparatively determine smaller & longer pitches simply by visual examination. Red colors will be longer. Blue colors will be shorter. Transparent films will have the shortest pitch if they are created using the same self assembly process(because they reflect UV it is possible that feature size could get even smaller but characterization would be difficult without instruments).
In the paper titled "Cellulose photonic pigments" by Tianheng H. Zhao and others it has been reported that the smallest obtainable pitch was 141 nm reflecting light in the UV spectrum.
https://www.researchgate.net/publication/361272462_Cellulose_photonic_pigments
What would be the microstructure of the nanocellulose film that appears completely transparent?
If a film from nanocellulose is self assembled and its completely transparent then that would mean that the nanocellulose periodic structures are smaller than the wavelengths of visible light. All the light then simply passes through obeying the classical laws of refraction. There is no dispersion of light occuring. This is quite common as nanocellulose particles as small as 20 nm can be easily obtained and have shown to form transparent films. And if the pitches formed are in the range of UV <300 nm the films would appear completely transparent.
As a material cellulose is in fact UV transparent
These films might still show thin film interference which is different from solid colors that self-assembled films with photonic band gaps in the visible region display.
Structural coloring indicates that the length of the periodic structure is comparable to the wavelength of light being reflected.
How is the white color of cellulose explained?
Cellulose is transparent. Its arrangement determines its color.Like salt is transparent but salt grains appear white. Same for ground sugar & quartz sand
Pure white colors could mean one of the two things.
1. That the arrangement of nanocrystals is not uniform & different domains overlap resulting in a composite white light being reflected(individual domains still reflect light according to their band gap but since they overlap the light just interferes and appears white). This property is actually exploited in super white paints produced from nano cellulose for radiative cooling.
2.The microstructured fibers obtained(from plants or other sources) are porus & have air filled cores that reflect light giving the appearance of white color when cellulose is actually transparent. This is usually the case with cotton & paper.
Regardless of that if the method of production of nanocellulose film is known then from its optical properties the microstructure can be known.
Does this also apply to cellophane?
No.
Cellophane or regenerated cellulose is formed by dissolution & reassembly of cellulose fibers. The nanoscale periodic assembly does not exist. Hydrogen bonded macro chains break down, dissolve & they are assembled again. In this process the air gaps are completely filled up & the natural transparency of cellulose comes through.No patterns are formed when the film is reassembled after removal of solvent by exchange,evaporation or some other procedure.
It is very important to remember that liquid crystals are colloids & dispersed within a liquid. Dissolution can't create liquid crystals because by definition it destroys the crystal structure.
In case fibers are produced from regenerated cellulose, the diameter of these fibers is limited by spinneret diameter in the range of 10s of micrometers.
>Suitable spinnerets are used to give a diameter of 15–20 μm and the fibre is cut into lengths usually of 4.8 cm.
https://www.sciencedirect.com/topics/medicine-and-dentistry/viscose
Electrospun fibers on the other hand can give nm sized diameters. Even then there is no process that allows pattern formation during the electrospinning process.
http://electrospintech.com/diameterparameters.html
All this is great but why should we care?
Two reasons
1. With self assembly we can consistently create REPEATABLE patterns on a nm scale.
2. These patterns can be carbonized & can give us nano scaled patterns on a semiconducting substrate which can be extremely useful for optoelectronic applications and quantum integrated circuits.
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