Adjustment of the Chiral Nematic Phase Properties of Cellulose

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Adjustment Of The Chiral Nematic Phase Properties Of Cellulose Nanocrystals By Polymer Grafting Firas Azzam, Laurent Heux, and Bruno Jean Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00690 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 12, 2016

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Adjustment Of The Chiral Nematic Phase Properties Of Cellulose Nanocrystals By Polymer Grafting Firas Azzam,†,# Laurent Heux†,# and Bruno Jean†,#,*

†

Université Grenoble Alpes, Centre de Recherches sur les Macromolécules Végétales

(CERMAV), F-38000 Grenoble, France #

CNRS, CERMAV, F-38000 Grenoble, France

KEYWORDS. Cellulose nanocrystals, TEMPO oxidation, polymer grafting, chiral nematic phases

ABSTRACT The self-organization properties of sulfated cellulose nanocrystals, TEMPO-oxidized cellulose nanocrystals and polymer-decorated cellulose nanocrystals suspensions in water were investigated and compared. Polarized light optical microscopy observations showed that these three systems phase separated to form a lower anisotropic chiral-nematic phase and an upper isotropic phase following a nucleation and growth mechanism, proving that surfacegrafted polymer chains did not inhibit the self-organization properties of CNCs. The phase diagrams and pitch of the suspensions were shown to strongly depend on the surface

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chemistry of the nanoparticles and the nature of the interacting forces. Especially, the entropic repulsion contribution of the polymer chains to the overall interactions forces resulted in a decrease of the critical volume fractions due to an increase of the effective diameter of the rods. Additionally, above a cellulose volume fraction of 3.5 % v/v, the pitch was significantly smaller for of polymer-decorated CNC suspensions than for sulfated as-prepared CNC ones, revealing stronger chiral interactions with the surface-grafted chains. In all cases, the addition of small quantities of monovalent salt induced an increase of the critical concentrations but values for polymer-decorated CNCs were always the smallest ones the due to entropic repulsion forces. Overall, results show that polymer grafting provides more tunability to the chiral-nematic phase properties of CNCs, including an enhanced expression of the chirality. INTRODUCTION One of the most spectacular behaviors characterizing colloidal rodlike particles suspensions is their spontaneous assembly into ordered structures. Above a critical concentration, a separation occurs in the suspension and two phases appear: an isotropic phase where the particles are randomly distributed and an anisotropic one where the rods point toward a common direction, which is a characteristic of nematic lyotropic liquid crystals. This finding dates back to one century with vanadium pentoxide aqueous suspensions observed by Zocher.1 Later, this phenomenon was observed in numerous systems like metallic oxide nanoparticles,2, 3, 4 poly(tetrafluoroethylene) “whiskers”,5 tobacco mosaic virus (TMV),6 filamentous bacteriophage fd virus,7, 8 DNA fragments,9 collagen fibers10 chitin nanocrystals11, 12

etc. The origin of this self-assembly is based on the balance between the translational and

the orientational components of the entropy. Indeed, at high volume fraction, the parallel alignment of the rods results in a decrease of the excluded volume leading to a gain in translational entropy that overcomes the loss in orientational entropy. This phenomenon was analytically demonstrated by Onsager for very long rods that interact only through the


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excluded volume effect and the concentration range where the two phases coexist was calculated.13 Since then, the theory was extended in order to take into account rods with lower aspect ratios,14 polydispersity15 and electrostatic interactions.16 This phenomenon was observed as well for cellulose nanocrystals (CNCs),17 which consist of crystalline nanorods resulting from the acid hydrolysis of native cellulose microfibrils and stabilized by electrostatic repulsions thanks to their surface negative sulfate groups.18, 19 In this case, the nematic order presents a regular twist of the director leading to the so-called chiral nematic - also referred to as cholesteric - structure, which displays particular optical properties. When observed between crossed polars, the structure appears as an alternation of light and dark bands corresponding to a continuous rotation of the nematic director. This helical structure is characterized by the helical pitch, P, that corresponds to the distance needed for a rotation of 360° of the particles. The seminal work of Gray and coworkers who discovered the ability of CNCs to self-organize into a chiral nematic phase was followed by an important body of literature devoted to the study of this intriguing property. In fact, this topic is of fundamental importance since the very origin of the helical assembly of CNCs is still not clearly understood and because this property paves the way to the design of functional materials with advanced mechanical or optical properties. The self-organization of CNCs into a cholesteric liquid crystal phase indeed enabled the formation of dried thin films exhibiting photonic bandgap properties20, 21, 22, 23, 24 and was also used as a template to synthesize multifunctional inorganic mesoporous materials displaying photonic crystal properties.25, 26, 27, 28, 29, 30, 31 These two topics as well as the liquid crystal self-assembly in aqueous suspension were recently discussed in a critical review by Lagerwall and coworkers.32 It is this thus of prime importance to control and understand the cholesteric phase organization of CNCs and this issue can be addressed by investigating the influence of particle-particle interactions on cholesteric phase properties like the required critical


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concentrations for self-organization and the helical pitch. In water, electostatic repulsions originating from the surface sulfate ester groups are the dominant interactions. Accordingly, increase in the ionic strength has strong consequences on the chiral nematic phase.33, 34, 35, 36 First, an increase of the critical concentrations with the increase of the ionic strength is observed and attributed to the screening of the interactions between the objects.35 Second, the increase in ionic strength reduces the thickness of the double layer, which increases the chiral interactions between the crystals and consequently decreases the value of the cholesteric pitch.35 In organic solvents such as cyclohexane, where electrostatic interactions are minimized or suppressed and replaced by steric interactions between surfactant coated CNCs, significantly higher critical concentrations and lower pitches than in water were reported.37

In this paper, we investigate the role of the tuning of the interactions forces on the chiral nematic phase properties of CNCs, whose surface have been chemically modified. Namely, we report a comparison of the self-organization behavior of three CNC-based aqueous systems: (i) unmodified sulfated CNCs, (ii) TEMPO-oxidized CNCs (TO-CNCs) and (iii) Jeffamine® polyetheramine M2070-grafted CNCs (M2070-g-CNCs). Compared to initial sulfated CNCs, the TEMPO oxidation introduces new charges leading to increased electrostatic repulsions, while the surface grafting of polymer chains generates steric repulsion forces. The effect of such interaction forces modification on the texture, phase boundaries and the helical pitch of the cholesteric structure are investigated. The role of the ionic strength was evaluated as well and discussed. The comparison of the systems with different interacting forces brings new insight in the understanding of the isotropic-anisotropic transition occurring in these systems and demonstrate a possible adjustment of the chiral nematic phase properties that could be benefitial for the design of CNC-based functional materials.


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EXPERIMENTAL Materials. Cotton linters were provided by Buckeye Cellulose Corporation and used as the cellulose source without any further purification. Jeffamine® polyetheramine M2070, a monoamine terminated statistical copolymer of propylene oxide (PO) and ethylene oxide (EO) was donated by Huntsman Corporation. Its PO/EO composition is 10/31 and its molecular weight 2070 g mol-1. Sodium hypochlorite solution (NaClO, reagent grade), sodium bromide (>99 %), 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO, 98 %), N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDAC, commercial grade) and N-hydroxysuccimide (NHS, 98 %) chemicals were purchased from Sigma-Aldrich. Deionized water was used for all experiments. Preparation of Cellulose Nanocrystal Suspensions. To prepare CNC suspensions, cotton linters were hydrolyzed according to the method described by Revol et al.38 by treating the almost pure cellulosic substrate with 65 wt.% sulfuric acid during 30 min at 63 °C. The suspensions were washed by repeated centrifugations, dialyzed against distilled water until neutrality, and ultrasonicated for 4 min with a Branson sonifier model 450 at 30% intensity. After these treatments, the resulting suspensions were filtered through 8 μm and then 1 μm cellulose nitrate membranes (Sartorius). At the end of the process, 3 wt. % stock suspensions were obtained. Such sulfated CNCs will be referred to as S-CNCs. Chemical Modifications of Cellulose Nanocrystals. Cellulose nanocrystals resulting from sulfuric acid hydrolysis of cotton linters were subjected to TEMPO-mediated oxidation as previously reported.39 The carboxyl content (DO) was measured by conductimetry and 13C solid-state NMR and was 0.12 mol/mol. Such TEMPO-oxidized samples will be referred to as TO-CNCs.


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Grafting of amine-terminated Jeffamine® polyetheramine M2070 was achieved through peptidic coupling as reported elsewhere.40 The pure polymer was added (Np moles per carboxyl unit measured by conductometry) to a 1 wt. % TO-CNC suspension and stirred until dissolution. The reaction was performed at room temperature. The pH was adjusted to 7.5−8.0 before the addition of 2 mL of an aqueous solution containing N-(3-dimethylaminopropyl)-Nethylcarbodiimide hydrochloride (EDAC) and N-hydroxysuccimide (NHS; NEDAC and NNHS mol per carboxyl group, respectively). Samples were prepared with NP = NEDAC = NNHS = 4. The reaction lasted 24 h at room temperature under stirring while maintaining the pH of the mixture at 7.5−8.0 using 0.5 M NaOH or 0.5 M HCl. The pH was finally decreased to 1−2 by addition of 0.5 M HCl and the resulting suspension was dialyzed against distilled water to remove excess reagents including non-grafted polyetheramine. The degree of substitution (DS) was measured by conductimetry and 13C solid-state NMR as well and was equal to 0.06 mol polymer per mol anhydroglucose unit. The remaining unreacted carboxylated group was then 0.06 mol/mol. Jeffamine® polyetheramine M2070-grafted CNCs samples will be referred to as M2070-g-CNCs. A schematic representation of the chemical modifications is given in Supporting Information Figure S1. All samples were studied at pH ~7 where oxidized groups, when present, are predominantly in their carboxylate form. Phase Diagram. Suspensions at different concentrations were prepared by diluting with deionized water aliquots of a concentrated suspension prepared by ultracentrifugation. The resulting samples were homogenized by vortexing. The final concentrations were determined by weighting aliquots of the samples after evaporation of water in an oven, typically during 15 h at 60 °C. The different suspensions were introduced in 0.2 × 2.0 × 50 mm3 glass capillaries (Microslides, VitroCom Inc.) by capillarity or with the help of a vacuum membrane pump for


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the most viscous samples. The capillaries were sealed at both ends and then left to equilibrate, standing vertically at ambient temperature until a clear interface was formed. Unless otherwise mentioned in the text, the corresponding equilibration times were in the order of one month and slightly depended on the concentration. The proportion of anisotropic and isotropic phases was measured as the ratio between the volumes occupied by each phase in the capillaries. The critical concentrations were measured as the linear extrapolation of the volume fraction in the biphasic gap at 0 and 100 vol. % for the critical concentration φi and φa, respectively. The systems that were kept at room temperature for two years were found to stay unchanged. The relation between measured concentrations and cellulose volume fractions is given in Supplementary Information. Polarized Light Optical Microscopy (PLOM). The samples sealed in flat glass capillaries were observed between crossed polars using a Zeiss Axiophot 2 optical microscope. Images were recorded with a Colorview 12 CCD camera (SIS). The pitch, P, was measured directly from the micrographs on the so-called fingerprint patterns of the cholesteric texture in homeotropic conditions after alignment of the structure in a 9.8 T NMR magnet field overnight. This texture is characterized by an alternation of light and dark bands corresponding to a continuous rotation of the nematic director. Values of P/2 were determined as the distance separating two light (or dark) bands. For better precision, the measurements have been made on 10 successive alternating bands, and 20 different measurements were made in the most homogeneous regions.

RESULTS AND DISCUSSION Phase Separation and Texture For the three different types of CNCs, namely sulfated CNCs (S-CNCs), TEMPO-oxidized CNCs (TO-CNCs) and Jeffamine® Polyetheramine M2070-grafted CNC (M2070-g-CNCs),


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aqueous suspensions of increasing concentrations were prepared and introduced in glass capillaries. As already reported, sulfated cotton CNC suspensions exhibit a macroscopic phase separation above a critical concentration.17, 34, 35, 41, 42 Such a behavior was not only observed in our case for sulfated CNCs but also for TO-CNC and M2070-g-CNC suspensions. This observation shows that neither the presence of carboxylic acid groups and higher charge density in the case of TO-CNC (0.12 mol/mol) nor the grafting of polymer chains on the surface for M2070-g-CNC samples prevented the phase separation of the modified CNCs to take place. Polarized light optical microscopy (PLOM) was then used to visualize the phase separation in each case and to follow its evolution. Examples in the case of 7-8 wt. % suspensions are shown in Figure 1.

Figure 1. Evolution of three suspensions observed by PLOM: a 7 wt. % S-CNC suspension (cellulose volume fraction = 4.5 vol %) (a,d,g), a 8 wt. % TO-CNC suspension (cellulose


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volume fraction = 5.1 vol %) (b,e,h) and a 8 wt. % M2070-g-CNCs suspension (cellulose volume fraction = 3.05 vol %) (c,f,i) at t < 1h (a-c), 24 h (d-f) and 1 month (g-i). Observations a, c, d, e and f were performed with the use of a lambda plate. The three types of samples exhibited the same features over time. During the first hour after introduction in glass capillaries, small birefringent elliptical tactoids with a size ranging from one to a few tens of micrometers nucleated in the continuous isotropic phase (Figure 1.a-c). After 24 hours, these tactoids coalesced, leading to larger domains that sedimented to the bottom of the capillary (Figure 1.d-f). The separation was more pronounced over time and equilibrium, namely the macroscopic phase separation was reached after 4 to 8 weeks, depending on the sample concentration. These observations are consistent with a nucleation and growth phase separation mechanism for the three types of samples. At equilibrium, two different phases are observed: an upper isotropic phase, which appears dark and a lower anisotropic phase, which appears birefringent (Figure 1.g-i). Within the anisotropic phases of the three suspensions, two regions with different aspects coexisted (Figure 2.a-c): a black zone (white arrow) and a colored clear one (red arrow). When observed at higher magnification, the clear region exhibited a so-called “fingerprint” texture (Fig.2.d-f), which is characteristic of a chiral nematic (cholesteric) structure. On the basis of this finding, it can be concluded that similarly to S-CNCs, TO-CNCs and M2070-g-CNCs were able to selforganize into chiral nematic liquid crystalline phases.


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Figure 2. PLOM micrographs at t = 2 months of the anisotropic phases for the different suspensions: a) 7 wt. % S-CNC (cellulose volume fraction = 4.5 vol %), b) 8 wt. % TO-CNC (cellulose volume fraction = 5.1 vol %) and c) 8 wt. % M2070-g-CNC (cellulose volume fraction = 3.05 vol %). Micrographs d, e and f are magnified images of the regions with a fingerprint texture pointed out by the red arrows in a, b and c. White arrows point to regions with a planar texture. Both regions correspond to the chiral nematic structure but with two different orientations of the cholesteric axis towards the glass surface (planar or homeotropic anchoring for the black or colored regions, respectively). Effect of Surface Modifications of CNCs on the Phase Diagram. A systematic quantitative study of the phase separation for the three systems was performed. Two months after the introduction of suspensions at different concentrations in glass capillaries, the separation into distinct, isotropic and anisotropic phases was complete. The PLOM observation then allowed us to determine the volume fraction of the anisotropic phase that was plotted as a function of the cellulose volume fraction to yield the phase diagram. To evaluate the effect of surface chemical modification, the phase diagrams of the three systems are presented in Figure 3. It has to be noted that the x-axis for this plot is the cellulose volume fraction and not the total volume fraction. This representation was indeed more relevant since


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a given cellulose volume fraction corresponded to an equal number of particles (polymerdecorated or not) in the suspension, i.e. similar interparticle distances, which highlights the different interactions between the various types of nanoparticles. 100

Anisotropic phase volume fraction (%)

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80 60 40 20 0

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Cellulose volume fraction (vol. %)

Figure 3. Phase diagram for S-CNCs (black squares), TO-CNCs (blue open circles) and M2070-g-CNCs (red diamonds) suspensions. Dotted lines are guides for the eyes. Figure 3 shows that S-CNCs, TO-CNCs and M2070-g-CNCs suspensions all qualitatively follow Onsager’s theory.13 Indeed, three distinct domains can be seen: the first one corresponds to cellulose volume fractions lower than φi, a first critical volume fraction, where the suspensions are isotropic. The second domain, between φi and φa, is defined as the volume fraction region where there is coexistence of an isotropic upper phase and a chiral nematic lower phase. In this domain, the volume fraction of anisotropic phase increases with increasing cellulose volume fraction. The third domain corresponds to cellulose volume fractions higher than φa, a second critical volume fraction, where suspensions are totally anisotropic. The critical volume fractions for the different suspensions were measured as the linear extrapolation of the biphasic domain at 0 and 100% for φi and φa, respectively, and are shown in Table 1.


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For sulfated CNC suspensions, φi and φa are 2.8 and 7.5 vol.%, respectively. These values are very close to the one already reported in the literature for sulfated cotton-derived CNCs.34, 35 As shown by Dong and coworkers, a relatively good agreement between theoretical predictions of critical concentrations and experimental results can be obtained in the case of salt-free suspensions if the Stroobants, Lekkerkerker and Odijk’s theory is used.16 Indeed, this model takes into account electrostatic interactions that were not considered in Onsager theory. Compared to the phase diagram of S-CNCs suspensions, the one for TO-CNCs is shifted to the smaller concentrations (Figure 3), with φi and φa values, respectively equal to 2.4 and 6.8 vol. %. For charged rods, the physical diameter of the particles is not the relevant parameter and an effective diameter that takes into account the electrical double layer has to be considered. TO-CNCs have a higher charge density (0.12 mol/mol) than S-CNCs (0.05 mol/mol), which might affect the contribution of the charged particles to the ionic strength of the system. The magnitude of the double layer repulsion could also be higher for TO-CNCs than for S-CNCs. For M2070-g-CNCs, φi and φa are 1.7 and 5.0 vol. %, respectively. These values are significantly lower than the ones obtained for both sulfated and TEMPO-oxidized CNC suspensions. M2070-g-CNCs had a surface charge of 0.06 mol/mol (mainly originating from the remaining carboxylate groups that were not consumed during the peptidic coupling reaction), comparable to the one of sulfated CNCs (0.05 mol/mol) and significantly lower than the one of TO-CNCs. Thus, taking only electrostatic interactions into account, the critical concentrations should be the lowest for TO-CNC suspensions, followed by those for M2070g-CNC and then those for sulfated CNCs. This order is not respected here since polymerdecorated rods also display steric interaction forces. As already shown for M2070-g-CNCs, the presence of a polymer corona around the CNCs imparts long term colloidal stability even at high ionic strength (1 M NaCl) due to the contribution of entropic repulsion forces between the grafted chains.40 Interaction forces between polymer-decorated CNCs can therefore be


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described as a combination of steric interactions provided by the polymer grafted chains and electrostatic interaction that come from the remaining carboxylate groups (COO-). A contribution of the entropic repulsion forces must then be taken into account in the effective diameter, leading to lower critical concentrations. For S-CNCs and TO-CNCs, it is worth noting that the anisotropic phase volume fraction curve does not follow a perfectly linear dependence. For these samples, the points corresponding to high concentration deviate from the linear extrapolation. This phenomenon was already explained as an effect of intrinsic ionic strength of the CNCs that becomes important at high concentration and results in a varying effective shape and size of the rods with concentration.32, 35 The effect was shown to be weaker for CNCs with a lower surface charge and with a smaller onset concentration for liquid crystalline ordering (e.g. CNCs with a higher aspect ratio like bacterial cellulose).32, 33, 36 Accordingly, for M2070-g-CNCs suspensions the anisotropic volume fraction dependence shows a very linear dependence on the whole biphasic domain, and therefore a behavior closer to neutral rigid rods, in agreement with Onsager theory and due to a low initial critical concentration.

Effect of Polymer Grafting on the Cholesteric Pitch The second characteristic of the cholesteric phase is the pitch, P, which corresponds to the distance over which a full rotation (360°) of the director is completed. Any change in the nature and range of the interactions between the nanoparticles should lead to pitch variations. To get more insight into the influence of chemical modification onto the surface of CNCs, pitch values as a function of cellulose volume fraction for S-CNC and M2070-g-CNCs suspensions were compared (Figure 4). In both cases, a decrease of the pitch was observed with the increase of the concentration. This phenomenon was already observed for CNC issued from Whatman paper and for other rods systems like fd virus.8, 35, 37 Recently, Schütz


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and coworkers, working with aqueous cholesteric phase of CNC suspensions, used small angle X-ray scattering to measure the interparticle distance and laser diffraction to obtain the pitch of the organized phase. These measurement enabled the calculation of the twist angle, whose increase was related to the increase in the magnitude of the electrostatic repulsions.43 Most probably, the decrease of the pitch with increasing particle concentration is due to a compression of the helix because the decrease in the interparticle distance facilitates the twisting.

Figure 4. Variation of the chiral nematic pitch as a function of the cellulose volume fraction for S-CNCs (black squares) and M2070-g-CNCs (red diamonds) and corresponding Log-Log plot in the inset. Many theories have tried to predict the variation of the cholesteric pitch as a function of the concentration. Assuming chiral rods can be considered as threaded rods or screw-like cylinders, Straley predicted that the most favorable situation from the excluded volume point of view is when they approach at a given angle so that the grooves interpenetrate. Straley suggested a relation that could be written as P ∝ C –υ where P is the pitch, C is the rod concentration and υ is an exponent that depends on the objects flexibility.44 For chiral stiff rods, Straley found a coefficient υ equal to 1. With a similar calculation, Odijk has predicted a


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value of υ equal to 5/3 for semi-flexible objects.45 In our case, for the sulfated CNC and M2070-g-CNC suspensions, the variation of log P was plotted as a function of log C and the slopes of the obtained straight lines, corresponding to υ, were 1.1 and 1.5, respectively (inset in Figure 4). For S-CNCs, this result is in a good agreement with Straley's predictions for chiral stiff rods (υ equal to 1), and with the value reported by Elazzouzi et al. for cotton CNCs46. The grafting of polymer chains on the surface of CNCs does however seem to affect to a certain extent the stiffness and/or the chirality of the objects in the framework of Straley’s theory. In Figure 4, the comparison of the pitch values for both suspensions (sulfated and M2070-gCNC suspensions) shows that in the cellulose volume fraction range 3.5-5.2 vol. %, the pitch is lower for polymer-grafted CNCs than for sulfated CNCs. For example, at a cellulose volume fraction of 5.2 vol.%, the pitch for M2070-g-CNCs (6.1 µm) was significantly smaller than the one for S-CNCs (10 µm). This phenomemon had already been described by Araki et al. who found at 9.5% cellulose volume fraction a decrease of the pitch from 7 to 4 µm after the grafting of 1000 g mol-1 polyethylene glycol chains onto CNCs.47 Here this result is extended from a single concentration to a whole concentration range and to polymer chains with even larger molecular weight (Mw = 2000 g mol-1). This result first appears counter intuitive since a first guess would be to consider a screening of the shape of particles by the polymer corona, making the particles effective shape less chiral. Results seem however to show an opposite effect. In the case of M2070-g-CNCs, the chirality might be transmitted by the grafted polymer chains to a larger distance from the crystal core, leading to a better expression of the chirality and resulting in lower cholesteric pitches.

Effect of the Ionic Strength on the Phase Diagrams


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To probe the electrostatic interactions dependence of the phase diagram for the three systems, a small amount of NaCl was added to the suspensions in order to reach concentrations of 0.25, 0.5 and 1 mM. Figures 5 a-c show the evolution of phase diagrams of the three suspensions as function of the added salt concentrations. For S-CNC and TO-CNC suspensions, a shift of the diagram to the higher f values was observed. In the case of sulfated CNC suspensions, with an ionic strength increasing from 0 to 1 mM, fi increases from 2.75 to 3.5% (v/v) and fa increases from 7.5 to 8.6 % (v/v). In the case of TO-CNC, for the same cellulose volume fraction, the volume fraction of the anisotropic phase decreased when the ionic strength increases. Similar results were reported by Dong et al.35 and Araki et al.33 on sulfated CNC suspensions issued respectively from Whatman paper and bacterial cellulose. This variation is explained by the variation of the electrostatic interactions between the negatively charged nanoparticles. The addition of salt results in a decrease of the electrical double layer thickness, thus inducing a decrease in the effective diameter of the nanoparticles, leading to the increase of the critical volume fractions.


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Figure 5. Variation of the phase diagram as a function of salt concentration for: (a) S-CNCs, (b) TO-CNCs and (c) M2070-g-CNCs. (d): Variation of the critical volume fractions, fi (filled squares) and fa (open triangles), as a function NaCl salt concentration: S-CNCs (black), TOCNCs (red) and M2070-g-CNCs (blue).

The same increase of critical volume fractions appeared for M2070-g-CNC suspensions. For these samples, fi increased from 1.6 to 3 vol. % and fa increased from 5 to 6 vol. % when the ionic strength increased from 0 to 1 mM. This result illustrates the electrostatic contribution of the remaining carboxylate groups that is weakened by salt addition. Figure 5d shows the evolution of φi and φa as a function of added NaCl concentration for the three suspensions. Whatever the salt concentration, the critical volume fractions for M2070-g-CNC suspensions were always lower than the ones for sulfated and TO-CNC, which could be explained by the existence of steric repulsion between the CNCs. Zhang et al. grafted poly(Nisopropylacrylamide) chains on fd virus nanoparticles that are able to organize into chiral


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nematic phases.48 By increasing the ionic strength, they found an increase of the critical concentrations as well. Above a certain salt concentration, the critical concentrations were stable because the system was sterically stabilized and the effective diameter was equal to the geometrical one. In our case, there is a clear tendency towards leveling-off when the ionic strength increases towards 1 mM.

Effect of the Ionic Strength on the Pitch Figure 6 shows the variation of the cholesteric pitch as a function of the ionic strength for suspensions of sulfated and M2070-g-CNCs having a similar cellulose volume fraction (5.2 vol. %) and consequently corresponding to similar interparticle distances. For the unmodified CNC suspension, the pitch decreased when the ionic strength increased. This behavior was already observed for such suspensions35, 36 and for DNA chains as well.49 It was attributed to the decrease of the double layer thickness created by the surface charges. This makes the apparent form of the nanoparticles more twisted and leads to a decrease of the pitch.

10 9

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8 7 6 0

0.2

0.4

0.6

0.8

1

NaCl concentration (mM)

Figure 6. Variation of the chiral nematic pitch as a function of NaCl concentration for S-CNCs (black squares) and M2070-g-CNCs (red diamonds). The cellulose volume fraction is 5.2 vol. % in both cases.


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In the case of M2070-g-CNCs, unlike the S-CNCs, a slight increase of the cholesteric pitch was observed when the ionic strength increased. This increase was unexpected considering that the ionic strength should decrease the electrostatic contribution that first should blur the chiral shape of the particles, and consequently decrease the pitch. However, Belamie et al. showed a non-monotonic dependence of the pitch as a function of HCl concentration, for chitin nanocrystals suspensions.12 A similar increase of the pitch was indeed obtained, followed by a decrease above 1 mM HCl concentration. The origin of this behavior is still not elucidated. In a study on bacterial cellulose, Hirai et al. showed a decreasing pitch upon the addition of NaCl up to 0.75 mM, following the expected behavior. However, after 0.75 mM, surprisingly the pitch diverged and started to increase as more salt was added. Recently, Lagerwall et al. proposed an explanation for this behavior based on a supramolecular polymerization triggered by nanoparticles end-to-end aggregation resulting in a diverging of rod length.32 Grelet and Fraden who studied polyethylene glycol-grafted fd virus systems8 found that the cholesteric pitch of this system depended on ionic strength even after screening all remaining charges. Obviously, in our case the understanding of such a surprising behavior would require a dedicated study. This intriguing result might arise from a subtle interplay between the inherent properties of the nanoparticles (size, geometry, presence of a permanent electric dipole moment50) and the type and range of the different interactions forces involved.

CONCLUSION A systematic investigation of the self-organization properties of sulfated CNC, TEMPOoxidized CNC and polymer grafted-CNC aqueous suspensions was carried out. Even though the three CNC types exhibit different surface chemistry and topography, a macroscopic phase separation into a chiral nematic lower phase and an upper isotropic phase was qualitatively observed in all cases. However, a quantitative comparison of the onset for phase separation


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and pitch values clearly shows that there is a strong dependence of the chiral nematic phase properties with the type and strength of the interaction forces between the nanoparticles. Namely, additional electrostatic forces brought by the TEMPO oxidation increases the effective diameter of the rods and consequently decreases the critical concentrations. The shift towards smaller concentrations is even larger in the case of polymer-grafted samples and was attributed for this type of CNCs to the combination of steric and electrostatic repulsions. Pitch variations measurements revealed that above a concentration threshold of about 3 vol. %, polyetheramine-grafted samples exhibit smaller pitches than their as-prepared sulfated counterparts, showing that the presence of a polymer corona enhances the twisting factor. The effect of ionic strength increase was also evidenced with expected effects in the case of SCNCs and a more intriguing behavior in the case of the pitch variations of M2070-g-CNC suspensions. The tunability of the the chiral nematic features provided by polymer-grafting shown in this study is a significant asset towards the adjustment of the functional properties of materials derived from helical CNC-based architectures. Using more complex polymeric structures should indeed give accurate control of the self-assembly and final materiels characteristics.

Supporting Information available The Supporting Information is available free of charge on the ACS Publications website and contains a schematic description of the chemical modifications of CNCs and a description of the relationship between the measured dry weight and the cellulose volume fraction.

AUTHOR INFORMATION Corresponding Author *[email protected]


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Author Contributions The manuscript was written through contributions of all authors. Funding Sources The authors are grateful to French Ministry of Higher Education and Research for the PhD grant of F.A ACKNOWLEDGMENT The authors are grateful to H. Chanzy and J.-L. Putaux for valuable help during the editing of the manuscript. B.J. thanks Huntsman Corporation for the gift of Jeffamine® polyetheramines samples. REFERENCES 1. Zocher, H. Über freiwillige Strukturbildung in Solen. (Eine neue Art anisotrop flüssiger Medien.). Z. Anorg. Allg. Chem. 1925, 147 (1), 91-110. 2. Buining, P. A.; Philipse, A. P.; Lekkerkerker, H. N. W. Phase Behavior of Aqueous Dispersions of Colloidal Boehmite Rods. Langmuir 1994, 10 (7), 2106-2114. 3. Davidson, P.; Gabriel, J.-C. P. Mineral liquid crystals. Curr. Opin. Colloid Interface Sci. 2005, 9 (6), 377-383. 4. Davidson, P.; Garreau, A.; Livage, J. Nematic colloidal suspensions of V2O5 in water— or Zocher phases revisited. Liq. Cryst. 1994, 16 (5), 905-910. 5. Folda, T.; Hoffmann, H.; Chanzy, H.; Smith, P. Liquid crystalline suspensions of poly(tetrafluoroethylene) 'whiskers'. Nature 1988, 333, 55-6. 6. Bawden, F. C.; Pirie, N. W.; Bernal, J. D.; Fankuchen, I. Liquid crystalline substances from virus-infected plants. Nature 1935, 138, 1051-2. 7. Dogic, Z.; Fraden, S. Cholesteric Phase in Virus Suspensions. Langmuir 2000, 16 (20), 7820-7824. 8. Grelet, E.; Fraden, S. What Is the Origin of Chirality in the Cholesteric Phase of Virus Suspensions? Phys. Rev. Lett. 2003, 90 (19), 198302. 9. Strzelecka, T. E.; Davidson, M. W.; Rill, R. L. Multiple liquid crystal phases of DNA at high concentrations. Nature 1988, 331, 457-60. 10. Giraud-Guille, M. M.; Mosser, G.; Belamie, E. Liquid crystallinity in collagen systems in vitro and in vivo. Curr. Opin. Colloid Interface Sci. 2008, 13 (4), 303-313. 11. Revol, J. F.; Marchessault, R. H. In vitro chiral nematic ordering of chitin crystallites. Int. J. Biol. Macromol. 1993, 15 (6), 329-335. 12. Belamie, E.; Davidson, P.; Giraud-Guille, M. M. Structure and Chirality of the Nematic Phase in α-Chitin Suspensions. J. Phys. Chem. B 2004, 108 (39), 14991-15000. 13. Onsager, L. The effects of shapes on the interaction of colloidal particles. Ann. N. Y. Acad. Sci. 1949, 51, 627-59.


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Adjustment of the Chiral Nematic Phase Properties of Cellulose

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