Supramolecular Engineering of Hierarchically Self-Assembled

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Supramolecular Engineering of Hierarchically Selfassembled, Bioinspired, Cholesteric Nanocomposites formed by Cellulose Nanocrystals and Polymers Baolei Zhu, Remi Merindol, Alejandro Benitez, Baochun Wang, and Andreas Walther ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00410 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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ACS Applied Materials & Interfaces

Supramolecular Engineering of Hierarchically Self-assembled, Bioinspired, Cholesteric Nanocomposites formed by Cellulose Nanocrystals and Polymers Baolei Zhu, Remi Merindol, Alejandro J. Benitez, Baochun Wang, Andreas Walther*

DWI – Leibniz-Institute for Interactive Materials, Forckenbeckstr. 50, 52056 Aachen, E-mail: [email protected]

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ABSTRACT Natural composites are hierarchically structured by combination of ordered colloidal and molecular length scales. They inspire future, biomimetic and lightweight nanocomposites, in which extraordinary mechanical properties are in reach by understanding and mastering hierarchical structure formation and deformation processes. Here we describe a hierarchically self-assembled, cholesteric nanocomposite with well-defined colloid-based helical structure and supramolecular hydrogen bonds engineered on the molecular level in the polymer matrix. We use reversible addition-fragmentation transfer polymerization to synthesize well-defined hydrophilic, non-ionic polymers with a varying functionalization density of 4-fold hydrogen-bonding ureidopyrimidinone motifs. We show that these copolymers can be co-assembled with cellulose nanocrystals (CNC), a sustainable, stiff, rod-like reinforcement, to give ordered cholesteric phases with characteristic photonic stop bands. The dimensions of the helical pitch are controlled by the ratio of polymer/CNC, confirming a smooth integration into the colloidal structure. Towards the effect of the supramolecular motifs, we demonstrate that those regulate the swelling when exposing the biomimetic hybrids to water, and allow engineering the photonic response. Moreover, the amount of hydrogen bonds and the polymer fraction are decisive in defining the mechanical properties. An Ashby plot comparing previous ordered CNC-based nanocomposites with our new hierarchical ones reveals that molecular engineering allows to span an unprecedented mechanical property range from highest inelastic deformation (strain up to ~ 13 %) to highest stiffness (E ~ 15 GPa) and combinations of both. We envisage that further rational design of the molecular interactions will provide efficient tools for enhancing the multifunctional property profiles of such bioinspired nanocomposites. KEYWORDS: biomimetic materials, nanocellulose, self-healing polymers, mechanical properties, crustacean-mimetic

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INTRODUCTION Bioinspired materials mimicking the mechanical high performance structures of native composites are considered next generation lightweight nanocomposites, which can integrate advanced mechanical properties and functionalities.1-2 Ideally, such hybrid materials, typically composed of crystalline reinforcing particles in a polymer matrix, display highly ordered structures and are hierarchically engineered on molecular, polymeric and colloidal length scales towards optimum mechanical response, delivering synergetic combinations of stiffness, strength and toughness.3-5 In recent years there has been much progress in two concurrent directions of bioinspired structural materials research. On the one hand, great success was reported to design bioinspired nanocomposites, such as mimicking the ordered, layered structure of natural nacre, using commodity polymers.6-12 On the other hand, rationally designed self-healing polymers and polymers incorporating sacrificial bonds, known from non-covalent interactions and structures found in proteins, are emerging as tools for designing molecular energy dissipation mechanisms.1316

However, there is still a profound lack in combining both worlds, which would lead to higher

level control, an improved mimicry of natural composites, and possibly, significantly improved molecular engineering of bioinspired nanocomposite properties. Towards such highly ordered bioinspired nanocomposites incorporating molecularly defined supramolecular energy dissipation mechanism, we recently demonstrated that tailored amounts of supramolecular units allow strong synergetic combinations of stiffness and toughness with stable crack propagation in a highly reinforced self-assembled polymer/clay nacre-mimetics.17 This approach highlights the importance of careful molecular design of the polymer phase in bioinspired nanocomposite settings. To promote sustainable approaches towards bioinspired nanocomposites, it is important to find renewable constituents that are globally abundant, and can be isolated with great ease at low costs. Plant-derived nanocellulose bionanoparticles are emerging as one of the key building blocks for such nature-based mechanical high performance materials, because they are based on one of the 3



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stiffest natural crystalline material (Ecellulose-I = 138 - 145 GPa).18-19 Nanocellulose can be divided into long and entangling cellulose nanofibrils (CNF) and short, highly crystalline cellulose nanocrystals (CNC). CNFs have been shown to be interesting building blocks for the preparation of transparent nanopapers20-23, nanocomposites24-27 and fibers28-29 with outstanding mechanical and functional properties30, as well as have shown promising results as biocompatible substrates and scaffolds for biomaterials.31-36 CNCs are remarkable in their ability to self-assemble into cholesteric phases due to an asymmetric charge distribution on the twisted crystal structure, leading to photonic bandgaps in the visible regime.37-38 In addition, their mechanical stiffness has been exploited to reinforce disordered nanocomposites or unidirectionally ordered fibers.39-42 The cholesteric structure is reminiscent of the plywood structure found in crustacean cuticles (such as lobster), in which highly crystalline, reinforcing chitin nanofibrils are surrounded by a binding energy-dissipating and slightly mineralized protein shell.43-47 Given the strength and toughness of the crustaceans, known from maritime cuisine, it would be highly desirable to pursue such cholesteric nanocomposite structures in advanced biomimetic nanocomposites and integrate them with molecular energy dissipation mechanisms. From previous work, it is know that appropriate coassembly conditions allow the integration of gold nanorods for chiral plasmonics,48-49 or the templation of mesoporous inorganic silica structures and organic resins via co-casting of monomers and subsequent polymerization.50-52 Interestingly enough, reports about direct co-assembly of watersoluble polymers and CNCs to make cholesteric bioinspired CNC/polymer nanocomposites are still scarce – albeit being the possibly most simple approach.53-54 In addition, photonic properties are more in focus compared to mechanical properties.50-56 One of the central challenges towards direct co-assembly is to identify polymers that do not interfere with the formation of the colloidal length scale.41 We recently demonstrated that non-charged polyvinylalcohol can be co-assembled with CNCs bearing sulfate groups at their surface into cholesteric structures, and lead to mechanically stiff and strong nanocomposites.54 However, the materials typically fracture at relatively small 4


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strain (< 1.5 %) in the ordered cholesteric phases, due to insufficient dynamics in the soft phase when set under tension. Here, we will overcome this problem and demonstrate a single-step, direct, hierarchical selfassembly approach of hydrophilic, non-charged copolymers equipped with supramolecular binding units in combination with CNCs to carefully engineer the mechanical properties of crustaceanmimetic polymer/CNC nanocomposites. We demonstrate that high stiffness, as well as higher ductility and toughness can be molecularly tailored by appropriate selection of (i) the molecular composition of the polymer, that is the fraction of supramolecular units and the glass transition temperature, Tg, and (ii) the ratio of polymer/CNC to tailor the helical pitch and inter-particle spacing of the CNCs. The molecular engineering of the polymer dynamics and the colloidally defined meso-structural dimensions allow frictional, dissipative movement of the reinforcements against each other under tensile stress, and substantially higher elongation. Additionally, we will demonstrate that the ability for swelling and the response of the photonic band gap can be modulated via the amount of supramolecular bonds. We believe this approach to be a starting point for careful hierarchical engineering of sustainable, crustacean-mimetic nanocomposites, displaying advanced and tunable mechanical properties, and providing access to robust, flexible, photonic films.

RESULTS AND DISCUSSION The concept for the preparation of cholesteric CNC/polymer nanocomposites with defined supramolecular interactions is based on mixing tailor-made copolymers bearing supramolecular hydrogen bonding moieties as side groups with cellulose nanocrystals (CNCs) and letting these mixtures evaporate via film casting. This leads to hierarchically self-assembled crustacean-mimetic nanocomposites with four folds hydrogen-bonding self-assembly on a molecular scale, and a cholesteric self-assembly of the CNCs into a helically arranged structure on the colloidal scale. 5



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Scheme 1. Self-Assembly of tailored mixtures of EGUPyX polymers modified with different fractions of 4-fold hydrogen-bonding 2-ureido-4-pyrimidinone (UPy) segments and cellulose nanocrystals (CNCs) into cholesteric phases during solution casting.

The self-assembly of the CNCs into cholesteric phases is due to a twisted nanoparticle shape and anisotropic charge distribution.37-38, 57 Recent work suggests that the formation of such ordered phases of the CNCs in presence of polymers requires non-charged, highly water-soluble polymers, so that the electrostatically driven self-assembly of the colloidal-scale CNCs is not disrupted by altering the CNC charge distribution or by excessive agglomeration of CNCs.53-54 Ordered phases are expected to occur at CNC weight fractions around 60 wt%.54 Following the hypothesis on the importance of non-ionic polymers, and bearing in mind that inelastic deformation in highly reinforced bioinspired nanocomposite settings requires polymers of sufficiently high dynamics6, we chose to target low Tg (glass transition temperature) fully water-soluble copolymers based on poly(oligoethylene glycol methacrylate) (OEGMA). Those can be synthesized using reversible addition-fragmentation transfer (RAFT) polymerization, which is suitable to prepare well-defined copolymers with high tolerance of functional groups (see Supporting Information, Figure SI1, SI2). To incorporate tailored levels of hydrogen bonding units, we copolymerized different amounts of UPy-MA (a methacrylate derivative with a UPy group; UPy = 2-ureido-4-pyrimidinone). The copolymers are abbreviated with EGUPyX, where X stands for the fraction (in mol%) of integrated UPy groups. UPy itself is a well-understood supramolecular motif. It dimerizes via four-fold 6


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hydrogen bonding, and can further form fibril-type assemblies via stacking of the dimers when assisted by flanking units.58-60 The developed synthesis protocol (see exp. section) allows for a straightforward incorporation of the UPy-MA units up to a high degree of loading (30 mol %) and appreciable molecular weights. Table 1 displays a summary of the three synthesized copolymers with overall similar molecular weights in the range of (Mn ≈ 60 kDa). The polymers are designed to be just in the range of the entanglement molecular weight,61-62 so that the supramolecular bonds are decisive in defining the internal cohesion of a supramolecularly-bonded polymer phase. Macroscopically, the effect of adding the UPy units manifests in turning the viscous EGUPy0 polymer into increasingly elastomeric solids, clearly indicating the presence of supramolecular linkages. All polymers display a desirably low Tg originating from the parent POEGMA backbone.

Table 1. Overview of EGUPyX copolymers prepared by RAFT polymerization. Sample codes

Mn [kDa] (Đ)a

UPy-MA units [mol%]b

Tg (°C)c

EGUPy0

55 (1.23)

0

-61

EGUPy13

65 (1.24)

13

-52

EGUPy30

57 (1.28)

30

-34

a

Mn and Đ obtained from DMF SEC, calibration vs. PMMA standards. b Molar ratio of the UPy-MA is calculated from 1H NMR of the copolymers in CDCl3, c Tg obtained from DSC.

The film preparation starts with the formulation of homogeneous and stable colloidal dispersions of sulfate-stabilized CNCs and the functional copolymers by addition of a 0.2 wt% CNC dispersion into a 0.5 wt% polymer solution until the desired mixing ratios are reached. The CNCs used in the work have an average length and diameter of 165 ± 51 nm and 9 ± 2 nm, respectively, as derived by statistical image analysis in earlier work.41 From previous work on PVA/CNC cholesteric nanocomposites, it can be derived that excess brittleness occurs at CNC above 80 wt%.54 This is due to the fact that rather small amounts of polymers intercalate into the voids of the nanoporous CNC films rather than being present as an interstitial phase between the CNCs. Furthermore, the

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self-assembled order is lost when decreasing the CNC concentration below 50 wt%. Therefore we fixed the ratios of EGUPyX/CNC nanocomposites to 40/60, 30/70 and 20/80 w/w. As hypothesized from the design of the copolymer (non-ionic), the presence of the non-charged EGUPyX copolymers is indeed not detrimental to the formation of an ordered colloidal-scale liquid crystalline, cholesteric CNC film in the investigated concentration regime. In this respect it is important to point out that our UPy motifs are designed without flanking units and hence only show strong association after water removal (absence of hydrogels), and hence, they do not interfere the formation of the cholesteric structure of the CNC composite.60,

63

This can already be seen

macroscopically after solution casting via the strongly visible, homogeneous iridescence of the dried films (inset of Figure 1a). This iridescence originates from diffraction of visible light at the helical, cholesteric texture of the EGUPyX/CNC crustacean-mimetics nanocomposites. Scanning electron microscopy (SEM) of the cross-section further reveals the mesostructure and displays cholesteric textures with long range order (Figure 1). The CNCs are organized into mesoscopic layers, where the CNCs are aligned parallel to each other, and rotate helically through the stack. We find similarly layered structures regardless of the polymer content in the films, thus confirming that the development of the cholesteric structure is unperturbed in the investigated regime of polymer content. Importantly, phase separation of polymer e.g. on top of the film or within domains cannot be observed, being a good indication of high compatibility, positive interactions between the polymers and the CNC, and successful intercalation of the polymer between the CNCs. While we did not study the absorption of the different polymers in detail, it is known that poly(ethylene oxide) chains, as found in the side chains of the EGUPyX copolymers, can undergo hydrogen bonding with hydroxyl and acid groups as present on the surface of the sulfate-stabilized CNCs, leading to polymer adsorption on the CNC.64-66 This adsorption may be reinforced by the addition of the UPy segments, which are good hydrogen bonding donors and acceptors, and by increasing the concentration during evaporation. Adsorption is also strongly suggested by the 8


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absence of depletion flocculation, which would occur in case of colloidal dispersion and nonadsorbing polymers.67 The copolymers adsorbtion onto the CNCs assists in controlling the definition of the overall structure.

Figure 1. Structural characterization of EGUPy13/CNC = 20/80 nanocomposites. (a-c) SEM images showing the long-range cholesteric order. The inset of (a) depicts the macroscopic photonic properties. Further SEM in Figure SI3.

The intercalation of the polymer can be traced by analyzing the pitch height of the cholesteric structure. Since the cholesteric structure results in photonic properties, the periodicity can be directly detected by UV-Vis spectroscopy. Figure 2a-c displays the reflection spectra with distinct reflection peaks that undergo red shifts for increasing polymer content. This can be understood by larger structural periodicities, originating from a successful intercalation and expansion of the cholesteric structure.

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The peak wavelength reflected by a chiral nematic structure (λmax) for incident light normal to the surface can be related to the half of the helical pitch height, P/2: 𝜆!"#, = 𝑃/2 ∙ 𝑛!"#

(I)

where navg is the average refractive index of the materials.68 CNCs have a refractive index of ca. 1.5451, while most standard organic polymers, such as aliphatic polymethacrylates range between 1.46 to 1.53. Assuming that the average refractive index is 1.5, it is possible to calculate the dimension of the periodicity of the structure, P/2. The results for all crustacean-mimetic nanocomposites are displayed in Figure 2d and show an almost linear correlation of the structural periodicity (P/2) with the polymer content, independent of the amount of supramolecular units present. This confirms that the structure formation is dominated by the colloidal-level objects. A direct measurement of the periodicity of the cholesteric structure can be done using SEM. These measurements are in excellent agreement and corroborate the optical reflection model with deviations usually below 20 nm. In summary, the developed co-assembly approach of supramolecularly modified polymers in combination with CNCs allows the formation of well-defined crustacean-mimetic nanocomposites with a cholesteric structure, which can be tuned via the ratio of both components. Importantly, contrary to other approaches to modulate the photonic properties in CNC-based materials,50-51,

53, 55, 69

our procedures are free from organic

solvents, crosslinking or silicification reactions, and are simply based on mediating the interactions in solution to allow for quick, single-step self-assembly in a waterborne process. Overall, this corresponds to a very rapid, facile, robust and also environmentally benign approach towards functional CNC-based nanocomposites in the future.

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Figure 2. Photonic properties of the films measured at 90° irradiation. (a-c) UV-Vis reflectance spectra for all EGUPyX/CNC nanocomposites at different weight fractions. (d) Comparison of the helical pitch (P/2) as a function of CNC content and in comparison to data derived from SEM.

Since we use a fully water-borne process and water-soluble constituents, one can expect that the films respond to water or other solvents able to swell the polymers. Interestingly enough, when exposed to water, the films do not fully disintegrate, but swell to quasi-equilibrium after immersion in water for 2 days. This swelling leads to a strong red shift of the photonic bandgap as exemplarily shown in Figure 3 for the films based on EGUPyX/CNC = 40/60. The most remarkable and unexpected observation is that the degree of swelling and the shift in the photonic bandgap is in fact controlled by the amount of UPy units within the film. Crustacean-mimetics without any UPy show a change in the photonic band gap from ca. 725 nm to ca. 1050 nm, while those with 30 mol% UPy only change from ca. 750 nm to ca. 900 nm. The reason for this behavior must be sought in the physical crosslinking and higher hydrophobicity of the incorporated UPy segments, which lead to 11



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an overall lower degree of swelling. Hence, the structures formed during drying are at least metastable to an extent that there is no further change during several days after initial hydration. This demonstrates that the photonic response of the films to water can be engineered through supramolecular interactions as encoded within the polymers. This offers new possibilities of manipulating the properties of CNC-based photonic devices, and in approaching e.g. sensor applications.

Figure 3. Swelling response of the photonic stop band of EGUPyX/CNC = 40/60 to water for different fractions of UPy.

Next we turn to the tensile properties of the crustacean-mimetics to elucidate to what extent the macroscale mechanical properties of the co-assembled films can be modulated by changing the molecular interactions through modification with supramolecular binding units. This can be done in a straightforward manner because the copolymers lead to similar mesoscale structures, independent of the amount of supramolecular units. Therefore, changes in the mechanical properties can be directly linked to the presence of the UPy segments. Figure 4a-d depicts the stress-strain curves of all films, on the one hand individually for the different polymers (Figure 4a-c), and on the other hand as a direct comparison for the different polymers at two weight fractions of the CNCs (Figure 12


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4d). Figure 4e, f and Table 2 summarize the important mechanical characteristics, such as the stiffness (Young’s modulus, E), tensile strength (σUTS), the strain-to-failure (εb), and the work-offracture (wof), obtained by integrating the area under the curve. All films were conditioned at 55 % relative humidity for at least 24 hours before testing under these conditions. A first comparison of the materials at different weight fractions of CNCs shows that an increase of the CNC content from 60 to 80 wt% leads to a stiffening and strengthening of the materials, typically leading to a 3 fold increase of Young’s modulus and tensile strength. Interestingly, in particular the materials at low content of CNCs (EGUPyX/CNC = 40/60) are very ductile considering the level of reinforcement and the tight, cholesteric structure, and display large strain-to-failure around 8 - 13 %. Such large strain-to-failures in cholesteric CNC/polymer nanocomposites are unusual as most other CNCbased nanocomposites, such as the ones with polyvinylalcohol (PVA) or phenol formaldehyde resin (PF), typically fail below 3 % strain.53-56, 70 Our films are also in sharp contrast to mesoporous, cholesteric photonic silica films, which are built by stepwise silica growth in the interstitial phases and subsequent thermal removal of CNC, leading to mostly brittle films prone to fracture.51 This large ductility and higher toughness is a clear result of the low Tg and suitable dynamics of the copolymers, which allow a dissipative frictional movement of the tightly packed reinforcements against each other. Figure 4d depicts the influence of the supramolecular binding motifs on the mechanical properties of the crustacean-mimetics at two different CNC fractions. From this data, it becomes obvious that the mechanical properties cannot only be tailored by the content of reinforcements, but moreover can be controlled on a molecular level using the amount of UPy segments. Compared to the crustacean-mimetics without UPy units (EGUPy0/CNC), the addition of UPy up to 30 mol% in (EGUPy30/CNC) leads to a significant stiffening and strengthening of the materials. Across all compositions, the Young’s modulus, E, approximately doubles and leads to maximum values of around 15 GPa in EGUPy30/CNC = 20/80, while the corresponding material without

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supramolecular units only has a stiffness of ca. 7 GPa. The reinforcing effect of the supramolecular UPy units is on the one hand provided by the formation of strong 4-fold hydrogen-bonding dimers leading to higher internal cohesion in the soft phase, and on the other hand associated to a better stress transfer from the soft low Tg polymer matrix to the reinforcing high stiffness CNCs via enhanced interfacial adhesion.

Figure 4. Tensile mechanical properties of EGUPyX/CNC crustacean-mimetic nanocomposites. (a-c) Direct comparison of tensile behavior of EGUPyX/CNC nanocomposites at different weight fractions of the CNC for the individual polymers. (d) Influence of the supramolecular units on the mechanical properties as demonstrated for two different fractions of CNC. (e) Comparison of stiffness (Young’s modulus, E), tensile strength (σUTS) and (f) comparison of strain-to-failure (εb), work of fracture (wof).

The more ductile films based on EGUPyX/CNC = 40/60 are of higher relevance in discussing strain-to-failure, yield points, and tensile strength, because their higher toughness limits the susceptibility to premature fracture by statistical defects typically encountered in more linear elastic materials, such as the highly reinforced EGUPyX/CNC = 20/80. In the less reinforced materials, we find that the UPy content controls the elastic and inelastic properties as well as the transition between both regimes. A consistent increase of stiffness, yield points, and tensile strength can be derived from Figure 4d (lower part). The increase in these properties goes along with a shortening of the maximum elongation. The latter can be understood considering the lower ability of 14


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deformation in the supramolecularly crosslinked materials under the applied deformation and nanoconfinement conditions. Overall, these detailed analyses demonstrate that the mechanical properties can be controlled and manipulated on a colloidal, as well as on a molecular scale using rational design principles of supramolecularly reinforced polymers. Furthermore, thinking about the combination with optical properties, an increase of the UPy content emerges as an efficient way of tailoring the mechanical properties in photonic films, without altering the structural dimensions of the colloidal mesoscale organization that govern the optical properties.

Table 2. Mechanical properties of EGUPyX/CNC composites. CNC content Polymer E (GPa)a σUTS (MPa)b (wt%) 6.8 ± 0.7 50 ± 11 80 3.4 ± 0.4 25 ± 5 EGUPy0 70 1.9 ± 0.4 19 ± 2 60 9.5 ± 1.2 58 ± 4 80 3.5 ± 0.3 25 ± 5 EGUPy13 70 2.4 ± 0.3 22 ± 4 60 14.9 ± 1.9 72 ± 11 80 6.9 ± 1.1 44 ± 5 EGUPy30 70 3.7 ± 0.5 27 ± 5 60 a

εb (%)c

wof (MJ⋅m-3)d

1.5 ± 0.2 5.9 ± 0.6 12.8 ± 2.2 1.6 ± 0.4 5.0 ± 1.1 10.1 ± 2.2 1.2 ± 0.2 1.7 ± 0.2 8.2 ± 1.8

0.5 ± 0.1 1.2 ± 0.2 1.8 ± 0.3 0.6 ± 0.1 1.1 ± 0.2 2.0 ± 0.3 0.5 ± 0.1 0.6 ± 0.1 1.6 ± 0.2

Young’s modulus; b Tensile strength; c Strain-to-failure; d Work of fracture.

It is interesting to compare the range of mechanical properties obtained by our molecular engineering approach to previous data on the mechanical properties of cholesteric polymer/CNC nanocomposites. Even though approaches towards understanding the mechanical properties in such crustacean-mimetic nanocomposites are still rare, some design criteria for the synthesis of suitable structures can be derived. On the one hand there are increasing reports about co-casting of monomer mixtures with CNCs and subsequent polymerization within the liquid crystalline phase.55-56, 70 On the other hand, there is our direct self-assembly approach, which reveals that non-ionic polymers, which do not interfere with the electrostatic stabilization pattern of the CNCs, can directly form 15



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cholesteric phases with CNCs above a critical weight fraction.53-54 Based on the literature values, we plot the Young’s modulus (E) vs. strain-to-failure (εb) in an Ashby plot to provide an instructive comparison between stiffness and ductility (as an indicator for toughness; most previous literature does not list the work of fracture Figure 5). The best materials, combining stiffness and ductility, are located on the top right in this plot. In general, all of the previous work produced either relatively brittle materials with εb < 3 %, or rather soft materials with E < 2 GPa. As common to nanocomposite materials, increased stiffness is usually obtained on the expense of ductility. An interesting comparison can be made regarding the PF/CNC and the mesoporous PF film, in which the CNCs were removed (PF = phenol formaldehyde resin).55 Here the mechanical properties can be changed from high stiffness to an interesting combination of stiffness (3 GPa) and strain-to-failure (4.8 %). However, the stiffness in the mesoporous material derives from the stiff resin and not from the CNCs. More flexible crosslinked films can be obtained by using a soft epoxy resin, leading to films with low stiffness (E < 0.7 GPa), yet with an elongation up to ca. 7 %.70 Our previous work on polyvinylalcohol/CNC (PVA/CNC) materials in the top left shows the relatively small property range (in this plot), which can be obtained by only varying the fraction of CNCs using classical commercial polymers without defined supramolecular interaction patterns.54 In very sharp contrast, our EGUPyX/CNC films reach into previously inaccessible areas of the property charts. EGUPy30/CNC = 20/80 films with highest amount of CNC reinforcements and highest amount of UPy segments are the stiffest crustacean-mimetics ever prepared with E = 15 GPa. Looking at the right hand side of the Ashby plot, EGUPyX/CNC = 40/60 films also explore new areas of highest strain-to-failure, reaching up to 13 %, while at the same time providing significantly enhanced stiffness (ca. 5 – 8 fold) compared to the most ductile, previous epoxy/CNC nanocomposites. More interestingly, all of the hierarchical crustacean-mimetic nanocomposites are located to the top right of the previous materials, and combine new levels of stiffness and ductility. We suggest that this beneficial property combination arises from some dynamic hydrogen bonding in the soft phase, as well as at the interface. It is furthermore evident that the simultaneous control over (i) the colloidal 16


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periodicity by changing the ratio of polymer/CNC and (ii) the extent of supramolecular interactions by varying the fraction of hydrogen bonding units enables to span a very large area in the property chart. This greatly assists in tailoring such hierarchical crustacean-mimetic nanocomposites to application needs, and allows tuning photonic properties and mechanical properties with some degree of independence simultaneously.

Figure 5. Comparison of Young’s modulus vs. strain-to-failure of polymer/CNC crustacean-mimetic nanocomposites in an Ashby plot. PF/CNC = 15/85, Mesoporous PF (PF: phenol formaldehyde resin; CNC removed with strong basic solution).55 PEG/CNC = 10/90, PEG/CNC = 5/95, PEG/CNC = 2.5/90 (PEG: polyethyleneglycol, Pure CNC.53 PVA/CNC = 10/90, PVA/CNC = 20/80, PVA/CNC = 30/70, PVA/CNC = 40/60 (PVA: polyvinylalcohol).54 PHEMA/CNCiso = 7/93, PHEMA/CNCani = 7/93 (PHEMA: poly(2-hydroxyethyl methacrylate).56 Epoxy/CNC =50/50, Epoxy/CNC = 44/56, Epoxy/CNC = 38/62, Epoxy/CNC = 28/72.70

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CONCLUSIONS In this work we demonstrated that highly functional hydrogen-bonded copolymers can be coassembled with CNCs to prepare hierarchical crustacean-mimetic nanocomposites with control over the colloidal and molecular self-assembly scales. The process is simple, water-borne and harnesses the self-assembly capabilities of both components. We find that the fraction of supramolecular units controls the degree of swelling and the photonic response of the cholesteric, iridescent films. Furthermore, we show that both the fraction of the polymer, which defines the helical pitch and separation distance between the CNCs, and the fraction of hydrogen bonds govern the mechanical behavior and allows reaching previously inaccessible areas in the mechanical properties. Interestingly enough, the tunability of the mechanical properties via supramolecular bonds, while keeping the photonic band gap stable at one fraction of CNCs, allows some independence in orthogonally designing multifunctional CNC-based photonic devices tailored with respect to mechanical and optical properties. We suggest that the clear design criteria and versatility and simplicity of the synthesis of the copolymers will in future allow broadening the concept towards rational design of other types of supramolecular interactions to further improve property space and target multifunctional biobased high-performance materials.

EXPERIMENTAL SECTION Materials 6-methylisocytosine (MIC, 98%), 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (>97%), 2,2′azobis(2-methylpropionitrile) (98%), poly(ethylene glycol) methyl ether methacrylate (OEGMA, Mn ≈ 475 g/mol, 8-9 repeating units of EG, 98%), 2-isocyanatoethyl methacrylate (98%), 1,4dioxane (99.8%), N,N-dimethylformamide (DMF, analytical grade) and dimethyl sulfoxide (DMSO,

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analytical grade) were purchased from Aldrich. OEGMA was purified by passing through basic alumina. Whatman filter paper (grade 1, Aldrich). Milli-Q water was used for all experiments.

Synthesis of UPy-containing monomer (2-(3-(6-Methyl-4-oxo-1,4-dihydropyrimidin2-yl)ureido)ethyl methacrylate) (UPy-MA) 6-methylisocytosine (MIC, 4.0 g, 32.0 mmol) was added to 50 mL DMSO and heated to 170 °C for 10 min. Once the solid dissolved, the oil bath was removed. 2-isocyanatoethyl methacrylate (5.5 g, 35.0 mmol) was added immediately to the flask under vigorous stirring. The mixture was quickly cooled using a water bath. A white solid precipitated upon cooling, collected by centrifugation, washed with excess acetone for at least 4 times and dried to obtain the pure product. 1H NMR can be found in Figure SI1.

RAFT polymerization of OEGMA and copolymerization of OEGMA and UPy-MA 5.0 g of OEGMA (10.50 mmol), 9.8 mg (0.035 mmol) 4-cyano-4-(phenylcarbonothioylthio) pentanoic acid was weighed and dissolved in 8 mL 1,4-dioxane in a vessel with a stirrer. Afterwards, 1.15 mg (0.007 mmol) 2, 2′-azobis (2-methylpropionitrile) was added, and the vessel was sealed with a rubber septum and bubbled with N2 for 20 minutes to remove oxygen. The vessel was placed into an oil bath at 65 °C. Samples were taken during the reaction to monitor conversion and molecular weight evolution. After 15 hours, the reaction vessel was cooled down in an ice bath. The polymer was dialyzed against water for over one week and freeze dried to yield the purified polymers. The product was a honey-like fluid. The RAFT copolymerization of OEGMA and UPyMA was conducted similarly, but using DMF as solvent. Different molar ratios of UPy-MA relative to OEGMA were used for the copolymerization (13 and 30 mol%), and the ratio of monomer/CTA was kept constant to the reaction conditions above. The resulting products turned rubbery compared to pure POEGMA.

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Preparation of cellulose nanocrystals (CNCs) CNCs were isolated from Whatman filter paper via acid hydrolysis with 64% H2SO4 at 70 °C for 30 min. The reaction was quenched by immersing the reaction flask into an ice bath. The suspension was washed three times by centrifugation at 8000 rpm for 30 min. The resultant slurry was dialyzed against deionized water for several days to remove excess acid, low molecular weight carbohydrates, and other water-soluble impurities. The final pH was constant around 4–5. The CNC suspension was then ultra-sonicated for 2 h. The sulfur content is ca. 0.75 wt% as derived from conductometric titrations.

Preparation of CNC/polymer films A 0.2 wt% dispersion of CNCs in water was added to a 0.5 wt% aqueous polymer solution using a syringe pump with a flow rate of 0.1 mL/min under vigorous stirring to ensure homogeneous dispersions. The CNC/polymer dispersion was stirred for further 24 hours, and then concentrated to ca. 1 wt% using rotary evaporation before being poured into a petri dish for solution casting to prepare films of ca. 20 µm thickness. The solution casting usually takes 6-7 days. The film can be peeled off from the bottom of the petri dish after drying.

Scanning electron microscopy SEM was performed on a Hitachi S4800 field emission microscope, operating at 1–1.5 kV. A thin Pd/Au coating was sputtered onto the films prior to imaging.

Mechanical tensile tests Mechanical tensile tests were carried out on a DEBEN mini-tester equipped with a 20 N load cell. All measurements were conducted at room temperature and specimens were conditioned at 55% relative humidity for a minimum of 24 hours. The specimen sizes used were in the range of 2 cm × 2.0 mm × 20 µm. At least 7 specimens were tested for each sample. A nominal strain rate of 0.5 mm/min was used. The slope of the linear region of the stress-strain curves was used to determine

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the Young’s modulus, E. The stress value at crack initiation is used as tensile strength. The work of fracture was determined by integrating the area of the stress-strain curve.

UV-Visible spectroscopy UV-Visible spectroscopy was performed on a Jasco V-630 UV/Vis Spectrophotometer. The films prior to UV-vis measurement are in the condition of 35 % relative humidity.

Size Exclusion Chromatography SEC with DMF (HPLC grade, VWR) as eluent was performed using an Agilent 1100 system equipped with a dual RI-/Visco detector (ETA-2020, WGE). The eluent contained 1 g/L LiBr (≥99%, Sigma-Aldrich). The sample solvent contained traces of water as internal standard. One precolumn (8 x 50 mm) and four GRAM gel columns (8 x 300 mm, Polymer Standards Service) with nominal pore widths of 30, 100, 1000 and 3000 Å were used at a flow rate of 1.0 mL/min at 40 °C. Narrowly distributed poly(methyl methacrylate) standards were used for calibration (Polymer Standards Service).

Differential scanning calorimetry The DSC (Differential scanning calorimetry) measurements were performed using a Netzsch DSC 204 unit. 5-10 mg of dried powder was enclosed in standard Netzsch 25 µL aluminum crucibles with a heating or cooling rate of 10 °C/min under a nitrogen atmosphere. Temperature range of 90 °C to 100 °C was used for each polymer samples.

Supporting Information NMR of UPy-MA, RAFT polymerization kinetics and further SEM images of the cross sections of the nanocomposite. The material is available free of charge via the internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS We thank the VW Foundation, and the BMBF for support in the Aquamat Research Group. This work was performed in part at the Center for Chemical Polymer Technology, supported by the EU and North Rhine-Westphalia (EFRE 30 00 883 02).

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67. Jenkins, P.; Snowden, M. Depletion Flocculation in Colloidal Dispersions. Adv. Colloid Interface Sci. 1996, 68, 57-96. 68. De Vries, H. Rotatory Power and Other Optical Properties of Certain Liquid Crystals. Acta Crystallogr. 1951, 4, 219-226. 69. Cheung, C. C. Y.; Giese, M.; Kelly, J. A.; Hamad, W. Y.; MacLachlan, M. J. Iridescent Chiral Nematic Cellulose Nanocrystal/Polymer Composites Assembled in Organic Solvents. ACS Macro Lett. 2013, 2, 1016-1020. 70. Zoppe, J. O.; Grosset, L.; Seppala, J. Liquid Crystalline Thermosets Based on Anisotropic Phases of Cellulose Nanocrystals. Cellulose 2013, 20, 2569-2582.

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Graphical Abstract for

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Supramolecular Engineering of Hierarchically Self-Assembled

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