Tuning Chiral Nematic Pitch of Bioresourced Photonic Films via Coupling Organic Acid Hydrolysis

Abstract

DOI: 10.1002/admi.201802010 2–30 nm in width and hundreds of nanometers in length.[2] Typically, in the process of sulfuric acid hydrolysis, the amorphous regions of cellulose fibers are removed, and the crystalline regions of cellulose fibers are cleaved into nanocrystals.[3] Meanwhile, the negatively charged sulfate ester groups are introduced onto the CNCs surface.[4] This is an important step for the stabilization of aqueous colloidal suspensions of CNCs due to their electrostatic repulsion.[5] Interestingly, anisotropic whisker-like CNCs can spontaneously self-organize into chiral nematic (or cholesteric) liquid crystalline phases above a certain critical concentration.[6] Furthermore, the electrostatically charged CNCs organize along a direction, rotating slightly along the helicoidal axis layer by layer to form a left-handed helical structure.[7] This chiral nematic selfassembly behavior is usually influenced by many factors including pH, temperature, particle size, ionic strength, surface chemistry, etc.[8] Fundamentally speaking, the chiral structure of CNCs arises from the twist of cellulose nanocrystals.[9] One of the unique characteristics of CNCs is that the chiral nematic order in colloidal can be preserved in solid films after complete water evaporation. The CNC films exhibit the properties of a chiral photonic crystal with a strong circular dichroism (CD), behaving as a standard cholesteric Bragg reflector.[10] The intriguing capability of CNCs to self-assemble into chiral nematic iridescent films with a helical structure and their photonic properties and applications have attracted increasing attention.[11] Many research efforts have been put on the control of their optical properties.[12] And, it has been proved that the color of CNC films can be tuned in the drying process of crystalline nanorod assemblies, which is in response to certain stimuli.[13] Various methods have been explored to enhance the color of CNC films, including the addition of electrolytes or polymers to the CNC suspension, ultrasonication treatment prior to drying, and varying the drying temperature.[14] In addition, some researchers have used external electric or magnetic fields to control the CNC orientation in suspensions and to tune the photonic properties of the films.[15] Frka-Petesic et al. demonstrated that using electric fields is an effective method to dynamically control the iridescence properties of concentrated CNCs in a polar solvent.[16] Meanwhile, the use of magnets (NdFeB) has been reported to facilitate chiral nematic alignment Controlling the iridescent photonic film generated by self-assembly of colorless cellulose nanocrystal (CNC) from the nanoscale to macroscale is challenging. This study combines experimental and computational approaches to systematically investigate the correlation between electrostatic interactions and chiral nematic structures. The chiral nematic order of the CNC colloidal is preserved in solid films after the evaporation of water. The cross-sections of the iridescent film show a clear left-handed helical arrangement of nanocrystals. The helical structure with the aid of acrylic acid exhibits longer organized patterns. This work reveals that compared to CNC prepared by pure sulfuric acid (zeta potential −37.1 mV), the CNC prepared using coupled mineral sulfuric and organic acrylic acid has a higher zeta potential (−67.2 mV), which induces the increase of helical pitch of the cholesteric nematic phase from 312 to 409 nm and a red shift of the iridescent film. Consequently, the tuning of reflected light wavelength lies in the variation of chiral nematic pitch inside the layered structure, which gives rise to different iridescent colors. The bioresourced photonic film is appealing to both academia and industry where optical and photonic components are essential.