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Abstract
Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration.
Highlights
Cellulose nanofibers (CNFs) with thicknesses of 3–15 nm are extracted from plant cell walls as dilute aqueous colloidal suspensions
The aim of this study is to reveal when and how colloidal CNFs three-dimensionally assemble from randomly dispersed CNF suspensions in the evaporation–condensation process to form nanopapers
The unfibrillated pulp particles had almost disappeared after passing through a Star Burst. Both the dispersions with the concentrations of 0.087 and 0.52 wt% showed high stability derived from the surface charges with the averaged zeta potentials of −54.9 and −45.6 mV, respectively. These charges are thought to be derived from the TEMPO oxidization treatment introducing carboxyl groups on the CNF surfaces [21,22]
Summary
Cellulose nanofibers (CNFs) with thicknesses of 3–15 nm are extracted from plant cell walls as dilute aqueous colloidal suspensions. These suspensions create flexible “nanopapers” consisting of 100% CNFs with high transparency and low haze of ~10% [1] These nanopapers exhibit lower thermal expansion coefficients [1,2], higher dielectric constants, higher thermal conductivity [2,3], and greater strength [4,5] than glass and other plastic films due to the highly crystalline, rigid CNFs [6,7]. Compared with traditional transparent substances, such as glass and plastic films, nanopaper is a unique transparent material because it has hierarchical structures in which cellulose molecule chains form crystalline CNFs, which aggregate or align to form macroscopic nanopaper This hierarchy, causes a problem—the optical properties of the nanopaper, including the transparency or haze, vary due to the various drying processes affecting CNF alignment [14,15,16]. CNF alignment during the evaporation of a uniform suspension still needs to be clarified to determine their assembling sequence
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