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Self-Assembled, Iridescent, Crustacean-Mimetic Nanocomposites with Tailored Periodicity and Layered Cuticular Structure Baochun Wang and Andreas Walther* DWI Leibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, Germany
ABSTRACT
Natural high-performance materials inspire the pursuit of ordered hard/soft nanocomposite structures at high fractions of reinforcements and with balanced molecular interactions. Herein, we develop a facile, waterborne self-assembly pathway to mimic the multiscale cuticle structure of the crustacean armor by combining hard reinforcing cellulose nanocrystals (CNCs) with soft poly(vinyl alcohol) (PVA). We show iridescent CNC nanocomposites with cholesteric liquid-crystal structure, in which different helical pitches and photonic band gaps can be realized by varying the CNC/PVA ratio. We further show that multilayered crustacean-mimetic materials with tailored periodicity and layered cuticular structure can be obtained by sequential preparation pathways. The transition from a cholesteric to a disordered structure occurs for a critical polymer concentration. Correspondingly, we find a transition from stiff and strong mechanical behavior to materials with increasing ductility. Crack propagation studies using scanning electron microscopy visualize the different crack growth and toughening mechanisms inside cholesteric nanocomposites as a function of the interstitial polymer content for the first time. Different extents of crack deflection, layered delamination, ligament bridging, and constrained microcracking can be observed. Drawing of highly plasticized films sheds light on the mechanistic details of the transition from a cholesteric/chiral nematic to a nematic structure. The study demonstrates how self-assembly of biobased CNCs in combination with suitable polymers can be used to replicate a hierarchical biological structure and how future design of these ordered multifunctional nanocomposites can be optimized by understanding mechanistic details of deformation and fracture. KEYWORDS: cellulose nanocrystals . nanocellulose . biomimetic materials . liquid crystals . photonic materials . crack propagation
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ature provides paradigms for future lightweight mechanical highperformance materials.1,2 Nacre,3 spider silk,4 wood,5 bone,5 crustacean cuticles,6 and other biological materials uniquely combine high stiffness, strength, and toughness. Therein, the slow and selfregulated growth of organic components and stepwise mineralization allow synergetic mechanical properties due to hierarchical and precise structuring, allowing the WANG AND WALTHER
shortcomings of the individual building blocks to be overcome. Common to these biological nanocomposites are the high fractions of reinforcements, which are arranged in an orderly, alternating hard/soft composite architecture within an energydissipating soft matrix. These principles and properties largely contrast presentday engineering nanocomposites, which typically use small amounts of reinforcements, such as clay or carbon nanotubes, VOL. XXX
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* Address correspondence to
[email protected]. Received for review August 14, 2015 and accepted September 15, 2015. Published online 10.1021/acsnano.5b05074 C XXXX American Chemical Society
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lack of an energy-dissipating binder phase. Previous work on ordered nanocomposites has mostly focused on a combination with monomers, evaporation of solvent, and subsequent polymerization, hence often following multistep pathways.3347 Surprisingly, there are only very limited reports on the direct self-assembly of water-based polymer systems with CNCs into ordered phases. The most notable recent single-step approaches toward hybrids include the mixing with latex particles or gold nanoparticles.48,49 Small amounts of anionic polymer or natural polymer were used to improve the flexibility of the iridescent film, but no deep insights into mechanical behavior were offered.50 Indeed, a large range of these studies focused on structure formation and photonic properties rather than mechanical strength. Here, we will demonstrate a simple single-step, fully waterborne, self-assembly concept for the preparation of simplistic crustacean-mimetic materials based on the concentration-induced colloidal self-assembly of polymer-coated CNCs obtained by in situ adsorption of a specifically selected polymer on CNCs. As a polymeric component, we chose poly(vinyl alcohol) (PVA), which is a nonionic polymer that undergoes strong hydrogen bonding with the polar and sulfonated CNC surface.5153 We select nonionic PVA to prevent interference with the charge distribution of the CNCs,54 which is one of the driving forces for cholesteric selfassembly,3032 and for its ability to provide a good cohesion within its soft phase due to hydrogen bonding. By varying the ratio of PVA/CNC, we demonstrate the tuning of the pitch height of the cholesteric phase and, thus, of the photonic band gap and optical reflection. Additionally, sequential deposition of different PVA/CNC dispersions on top of each other furnishes hierarchical materials containing different periodicities and properties, reminiscent of the cuticular structure in crustaceans. We further describe crack propagation and toughening mechanisms occurring as a function of the reinforcement level and demonstrate details on the transitions from a cholesteric (chiral nematic) to a nematic alignment.
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within commodity polymers.79 Despite improvements in such classical nanocomposites, it has been an extraordinary challenge to obtain synergetic mechanical performance, and classical nanocomposites still fall short of the biological ones, or expensive, man-made high-performance fiber-reinforced plastics. These reasons call for fresh approaches toward nanocomposites and motivate discovery of bioinspired nanocomposites, in which the relevant structural motifs of biological materials are captured;albeit often on a more simplistic level. One of the main bottlenecks to mimic the complexity of natural structures is to find suitable preparation pathways that allow the formation of hierarchical ordering, while still acting on suitable time and energy scales in the preparation. Recent research has shown great success in rebuilding the structure of the nacreous layer in sea shells,10 enabling the preparation of mechanically superior bioinspired, layered nanocomposites with an alternating hard/soft composite structure.1118 Processes have evolved from rather tedious multistep preparation pathways to quicker self-assembly concepts.19 This work will focus on a simple single-step pathway toward a simplistic biomimetic nanocomposite inspired by the mechanical strength of crustacean cuticles as found in crabs or lobsters, with which we are familiar from maritime cuisine. Crustacean cuticles contain highly crystalline, reinforcing chitin nanofibrils, which are surrounded by a binding and energydissipating protein shell, and are organized in a plywood structure with layers of different periodicity at the inside (endocuticle) and outside (exocuticle) of the shell. Additionally, the structure is slightly mineralized depending on the species, and channels perpendicular to the cuticle can be found.2028 We focus in our approach for a crustacean mimetic on the cholesteric organization of the plywood structure, the realization of an alternating hard/soft composite structure, and the possibility to tune the periodicity of the helical plywood structure, which we exploit to prepare layered materials with different periodicities as similarly found in the endo- and exocuticles. To this end, we harness the self-assembly capabilities of cellulose nanocrystals (CNCs). CNCs are short and entirely crystalline cylindrical bionanoparticles (length = 50150 nm, diameter = ca. 7 nm), which can be isolated from sustainable cellulose resources (trees, farming waste) by hydrolytic cleavage.29,30 They exhibit remarkable mechanical properties with a stiffness (Young's modulus, E) around 145 GPa at low density, which places them at the top end of highperformance natural building blocks. When evaporated in water, these CNCs form cholesteric nematic phases with photonic band gaps in the visible region. The self-assembly is possibly due to a twisted nanoparticle shape and anisotropic charge distribution.31,32 Pristine CNC films are extraordinarily brittle due to the
RESULTS AND DISCUSSION The strategy is schematically depicted in Scheme 1 and starts with the preparation of the hydrocolloid dispersions by slow addition of a dilute CNC suspensions (0.5 wt %) into well-stirred PVA (0.25 wt %, Mw = 85126 kDa) solution to reach target ratios of CNC/ PVA = 100/040/60 w/w. The CNCs in this work have an average height of 9 ( 2 nm and a length of 165 ( 50 nm as determined by statistical image analysis from atomic force microscopy (AFM) images. We focus on high fractions of reinforcements as implied by bioinspired design principles. The formed dispersions are stable over time, indicating little to no aggregation and coagulation during the mixing and coating process. VOL. XXX
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Scheme 1. Preparation of cholesteric, crustacean-mimetic CNC/PVA nanocomposites via an aqueous self-assembly of intermediately formed PVA-coated CNCs with intrinsic hard/soft structure. Sulfonic acid groups are present on the surface of the CNCs due to the preparation using H2SO4.
Subsequently, we left the suspensions to evaporate to form solid films, whereupon the films at higher CNC content display strongly iridescent films. This is already a very good macroscopic indication for the formation of ordered cholesteric structures. Figure 1ac displays cross-sectional scanning electron microscopy (SEM) and polarized optical microscopy (POM) of a range of films obtained at different ratios of CNC/PVA. One can clearly observe the cholesteric nematic phase at higher fractions of CNC, which is reminiscent of the plywood structure found in crustacean shells. This structure is also responsible for the photonic properties of the films, displaying iridescence as a function of the pitch heights. The continuous and smooth shift of the stop band, λmax, from ca. 560 nm for
Figure 1. Cholesteric liquid-crystalline structure of CNC/PVA nanocomposites as a function of composition. (ac) SEM images of fractured cross sections of CNC/PVA = 90/10 (a), 80/20 (d), and 60/40 (c) w/w. Inset in (a) shows a polarized optical micrograph with the characteristic fingerprint pattern. (d) Cholesteric liquid-crystal structure and the helical pitch. (e) VIS spectra of the CNC/PVA nanocomposite with various compositions. (f) Maximum wavelength calculated from the helical pitch from SEM images, λmax,calc, and minimum wavelength from transmittance, λmin,trans, as a function of CNC content. WANG AND WALTHER
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CNC/PVA Nanocomposites CNC/PVA (w/w)
P/2 (nm)a
λmax,calc (nm)b
λmin,trans (nm)c
100/0 90/10 80/20 70/30 60/40
355 ( 30 370 ( 45 406 ( 25 440 ( 29 486 ( 50
538 569 626 675 748
560 590 641 688 726
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Determined by SEM image analysis. b Calculated based on eq 1. c Determined from transmittance.
pure CNC films to ca. 726 nm at CNC/PVA = 60/40 w/w confirms a homogeneous integration of the PVA into the helical structure and absence of macroscopic phase segregation (Figure 1e). A quantification of the helical pitch heights by SEM confirms that additional PVA is indeed intercalated into the structure. The half of the pitch heights can be adjusted from ca. 355 nm for the pure CNC to ca. 486 nm for CNC/PVA = 60/40 w/w. The cholesteric structure becomes unstable when doping an excess of PVA into the material. We paid close attention to this transition point and identify it at a PVA content greater than 40 wt %. We emphasize that even casting at very slow evaporation speeds did not allow the formation of ordered structures, thus stressing that kinetic bottlenecks are most likely not responsible for the loss of order and that we observe near equilibrium structure. The peak wavelength reflected by a chiral nematic structure (λmax,calc) for incident light normal to the surface can be expressed as λmax, calc ¼ P=2navg
(1)
where navg is the average refractive index and P/2 is one-half of the helical pitch (Figure 1d).55 CNC and PVA have refractive indices of 1.5434 and 1.521.55,56 respectively. Assuming that the average refractive index is 1.54, it is possible to calculate λmax,calc from the helical pitch. The results are shown in Table 1 and Figure 1f and display good agreement between structural and optical properties. Interestingly, our approach also allows for the formation of materials with stacked periodicity. This is interesting not only from the point of view of structural control but also because encoding different mechanical properties as a function of the cholesteric nematic pitch may allow gradient and mechanically heterogeneous materials, for example, with a stiffer, more abrasive, and wear-resistant exterior and tougher interior, to be able to dissipate more tensile or impact energy.57 Figure 2 displays two examples with different periodicities, which were obtained by sequential casting of different CNC/PVA dispersions. The layered materials are flexible and iridescent, and the SEM images confirm different helical pitches and structures. The interface between both structures is very defined (
