Abstract
Nanocelluloses are very attractive materials for creating structured films with unique optical properties using different preparation techniques. Evaporation-induced self-assembly of cellulose nanocrystals (CNC) aqueous suspensions produces iridescent films with selective circular Bragg reflection. Blade coating of sonicated CNC suspensions leads to birefringent CNC films. In this work, fabrication of both birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method is studied. Polarization optical microscopy and steady-state viscosity profiles show that non-sonicated CNC suspensions (concentration of 6.5 wt%) evolve with storage time from a gel-like shear-thinning fluid to a mixture of isotropic and chiral nematic liquid crystalline phases. Shear-coated films prepared from non-sonicated fresh CNC suspensions are birefringent, whereas films prepared from suspensions stored several weeks show reflection of left-handed polarized light. Quantification of linear and circular birefringence as well circular dichroism in the films is achieved by using a Mueller matrix formalism.
Highlights
Cellulose is the most abundant biopolymer on earth
The evolution of non-sonicated Cellulose nanocrystals (CNC) suspensions was imaged by polarization optical microscopy (POM) at different
For the the CNC concentration used (6.5 wt%) a suspension in the biphasic regime is expected [8], CNC concentration used (6.5 wt%) a suspension in the biphasic regime is expected [8], i.e., a coexistence of isotropic and anisotropic phases. The latter phase corresponds to a i.e., a coexistence of isotropic and anisotropic phases. The latter phase corresponds to a chiral nematic liquid crystal phase where the director n, which defines the preferential chiral nematic liquid crystal phase where the director n, which defines the preferential orientation of CNC in pseudo-planes, describes a helix
Summary
Cellulose is the most abundant biopolymer on earth. Polymer chains of cellulose conformed by β-1,4 glucopyranose units interact thorough hydrogen bonds leading to formation of semicrystalline fibrils [1]. Evaporation-induced self-assembly of CNC suspensions is often used to cast free-standing films [6,9]. In this type of CNC film, the chiral nematic order is retained with a microstructural organization, the so-called Bouligand structure, which is optically expressed as a selective reflection of left-handed circular polarization [10,11]. Among methods to control the spectral position of selective reflection, ultrasound treatment (sonication) of the CNC suspension has proved to produce an increase in pitch and thereby a redshift [12]. A mechanism for sonication-induced increase in pitch was proposed as result of changes in the interactions between CNC and the electrical double layer
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