In biological architectures, material properties are optimized by the hierarchical structuring of components with a multiscaled order, from the nano‐ to the macroscales. Such designs enable, for instance, programmed yield points that maximize toughness. However, research efforts in biomimetic materials have focused on the assembly of nano‐ or macrostructures individually. In this study, high strength cellulose nanocrystals (CNCs), assembled into chiral‐nematically ordered structures, are tiled into a higher level, macro‐sized, architecture by topographical templating. As templates, two meshed architectures with distinct feature sizes are evaluated, and the optomechanical properties of the resulting films are compared to featureless, flat, CNC films. Controlling capillary stresses arising during CNC assembly is shown to enable control over the orientation of the chiral‐nematic director across the topography of the template. Tuning the specific reflections and multiscaled fracture propagation is demonstrated for the microtemplated CNC films. The latter phenomenon contributed to enhancing the toughness of the material through a high tortuosity of fracture propagation in all (x, y, z) directions. The presented findings are expected to pave the way towards the incorporation of current research in cellular metamaterials with the research focusing on the generation of nanoscaled biomimetic constructs.
Outstanding
optical and mechanical properties can be obtained from
hierarchical assemblies of nanoparticles. Herein, the formation of
helically ordered, chiral nematic films obtained from aqueous suspensions
of cellulose nanocrystals (CNCs) were studied as a function of the
initial suspension state. Specifically, nanoparticle organization
and the structural colors displayed by the resultant dry films were
investigated as a function of the anisotropic volume fraction (AVF),
which depended on the initial CNC concentration and equilibration
time. The development of structural color and the extent of macroscopic
stratification were studied by optical and scanning electron microscopy
as well as UV–vis spectroscopy. Overall, suspensions above
the critical threshold required for formation of liquid crystals resulted
in CNC films assembled with longer ranged order, more homogeneous
pitches along the cross sections, and narrower specific absorption
bands. This effect was more pronounced for the suspensions that were
closer to equilibrium prior to drying. Thus, we show that high AVF
and more extensive phase separation in CNC suspensions resulted in
large, long-range ordered chiral nematic domains in dried films. Additionally,
the average CNC aspect ratio and size distribution in the two separated
phases were measured and correlated to the formation of structured
domains in the dried assemblies.
The iridescence displayed by films made from cellulose nanocrystals (CNCs) has long been the subject of fundamental research. This has expanded our understanding of colloidal self-assembly towards the development of advanced materials. However, the application of such findings is less reported for visual designs that exploit structural color. Aesthetic outputs are already in reach, but requires input from trend setters in the design and art industries. In this realm, the CNCbased iridescence uniquely offers broadband, multicolored reflections through the ''coffee ring'' effect, which arises upon evaporation-induced self-assembly (EISA). Although this effect has been thoroughly studied in the context of axisymmetric patterns, complex geometries remain to be evaluated for large-scale implementation. This is central to the present efforts, where EISA of CNC suspensions occurred onto noncircular surfaces. We used orientation-dependent contact angle measurements, profilometry and fixed-light source photography to unveil the effect of asymmetric drying fluxes at sharp angles, between 30°and 90°, on CNC particle deposition and resulting color patterns. We also demonstrate the causality between increased capillary fluxes and deposition with the help of modelling via energy minimization of the suspension volume onto a given surface and using the diffusion equation to obtain the local concentration of water vapor during EISA. Lastly, we study the effect of background reflections as well as light and temperature resistance of CNC-based reflectors, both important for any deployment. The results from this multidisciplinary effort, involving applied design, art and colloid chemistry, point to the excellent prospects of CNC films for the development of structured and chromatic patterns.
Cellulose nanocrystals
(CNCs) can spontaneously self-assemble into
chiral nematic (cn) structures, similar to natural cholesteric organizations.
The latter display highly dissipative fracture propagation mechanisms
given their “brick” (particles) and “mortar”
(soft matrix) architecture. Unfortunately, CNCs in liquid media have
strong supramolecular interactions with most macromolecules, leading
to aggregated suspensions. Herein, we describe a method to prepare
nanocomposite materials from chiral nematic CNCs (cn-CNCs) with strongly
interacting secondary components. Films of cn-CNCs were infiltrated
at various loadings with strongly interacting silk proteins and bovine
serum albumin. For comparison and to determine the molecular weight range of macromolecules
that can infiltrate cn-CNC films, they were also infiltrated with
a range of poly(ethylene glycol) polymers that do not interact strongly
with CNCs. The extent and impact of infiltration were evaluated by
studying the optical reflection properties of the resulting hybrid
materials (UV–vis spectroscopy), while fracture dissipation
mechanisms were observed via electron microscopy. We propose that
infiltration of cn-CNCs enables the introduction of virtually any
secondary phase for nanocomposite formation that is otherwise not
possible using simple mixing or other conventional approaches.
The colorimetric and photovoltaic efficiency of different structural color coatings, including cellulose nanocrystals, were compared with spectrophotometry, photography, and power loss analysis in view of their application in solar architecture.
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