Abstract
The self-assembly of cellulose nanowhiskers (CNWs) in confined geometries provides a powerful method for the fabrication of novel structures. Herein, ordered honeycomb microporous films were first prepared with surface-acylated CNWs (CNWs-SU) through the breath figure method. Resulting films showed highly porous order over large regions and the iridescent color was only displayed by their rims, which is different from traditional dish-cast CNW films showing the iridescent color over the whole area. This is mainly due to the condensation of water droplets forming three-dimensional (3D) geometry, which forced CNWs-SU to self-assemble into cholesteric architectures in confined geometry and resulted in the iridescent color of the rims after drying. The mechanism was further studied by investigating the critical influencing factors, primarily the concentration of CNW-SU suspensions, the relative humidity of the atmosphere and the surface-attached moieties. In particular, CNW-SU suspensions with a concentration of 3 mg/mL at the relative humidity of 75% preferentially formed honeycomb films with uniform pores. Too low or too high concentrations of CNW-SU suspensions or relative humidity are not preferable for uniform porous films. CNWs-SU with further immobilized octadecane or fluoroalkyl groups on their surface strongly affected the formation of uniform porous films because of higher hydrophobicity and accompanying inhomogeneous condensation of water droplets. This work provides a novel method to study the interactions of CNWs beyond the planar geometry and the formation of uniform porous films solely with CNWs with structural colors open up interesting possibilities for broad application in photonic nanomaterials.Graphic abstract
Highlights
The self-assembly of cellulose nanowhiskers (CNWs) has attracted intensive interest in the nanoscience and materials science community, owing to their ability to form chiral nematic structures in aqueous suspensions (Bondeson et al 2006; Habibi et al 2010)
The results demonstrated that the anisotropic drying and unidirectional propagation of the anisotropic phase in large areas were mainly induced by the saturated water vapor in one end of the capillary, resulting in the formation of chiral CNW films with uniformly oriented layered structures
We investigated the self-assembly of surface-acylated hydrophobic CNWs (CNWs-SU) in confined spaces during the breath figure process, which led to highly ordered honeycomb films
Summary
The self-assembly of CNWs has attracted intensive interest in the nanoscience and materials science community, owing to their ability to form chiral nematic structures in aqueous suspensions (Bondeson et al 2006; Habibi et al 2010). CNW-SU suspensions with a concentration of 3 mg/mL at the relative humidity of 75% preferentially formed honeycomb films with uniform pores. To the best of our knowledge, the self-assembly of CNWs in a complicated confined geometry formed by the condensation of water droplets during the breath figure process has not been reported.
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