Spontaneously assembled cellulose nanocrystal structural color films with tunable properties

Abstract

Cellulose nanocrystals (CNC), which possessing unique cholesteric liquid crystal properties, can self-assemble and then stack into layered structures to produce structural colouration. By controlling the periodicity of these layered structures, the films with various colors can be fabricated, which could be used as a new generation of environmentally friendly pigments. In this study bamboo-based CNC were utilized as the raw material for constructing vibrantly colored CNC film via layer-by-layer self-assembly process by adding hydroxypropyl cellulose (HPC) to regulate the pitch. By controlling CNC/HPC ratios at 100/0, 92/8, 87/13 and 82/18, the color of the film was in blue, green, yellow and red, respectively. Furthermore, by extending the equilibrium time and adjusting the solvent composition being typically required to allow for the self-assembly, the directional arrangement of the crystalline aggregates was facilitated, resulting in CNC films with more uniform colors and optimized optical and mechanical properties of the films.

Keywords: Cellulose nanocrystals; Film; Liquid crystal; Optical properties; Self-assembly.

Fig. 1

: Photos of CNC suspension (a) initial state; (b) keep static for 1 month.

Fig. 2

SEM images of CNC particles being acid hydrolyzed for different times: (a) 60 min; (b) 90 min and (c) 120 min.

Fig. 3

Digital photos of the film with a CNC/HPC ratio of 100:0, 92:8, 87:13, 82:18 being kept for 0, 2, 4, and 6 days.

Fig. 4

POM image of the film with a CNC: HPC ratio of 100:0, 92:8, 87:13, 82:18 being kept for 0, 2, 4, and 6 days (50 μm scale bar).

Fig. 5

SEM images of CNC films non-equilibrated and 6-day equilibrated with different CNC/HPC ratios: (a) 100:0; (b) 92:8; (c) 87:13 and (d) 82:18.

Fig. 6

The UV-Vis reflectance spectra of the films equilibrated for different times: (a) CNC/HPC(100:0); (b) CNC/HPC(92:8); (c) CNC/HPC(87:13); (d) CNC/HPC(82:18).

Fig. 7

Mechanical properties of CNC/HPC films with varying CNC/HPC ratio for non-equilibrated and 6-day equilibrated.

Fig. 8

The digital photo and POM image of CNC/HPC/CaCl₂ films at different ratio CNC/HPC: 100:0; 92:8; 87:13 and 82:18 (from left to right).

Fig. 9

SEM images of the CNC/HPC/CaCl₂ film: (a) CNC/HPC(100:0); (b) CNC/HPC(92:8); (c) CNC/HPC(87:13) and (d) CNC/HPC(82:18) (e) UV-Vis reflectance spectra of CNC/HPC/CaCl₂ films with different CNC/HPC ratio.

Fig. 10

Mechanical properties of CNC/HPC/CaCl₂ films with different CNC/HPC ratios.