Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions

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Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents Johanna R. Bruckner, Anja Kuhnhold, Camila Honorato-Rios, Tanja Schilling, and Jan P. F. Lagerwall Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02647 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions Using High-Permittivity Solvents Johanna R. Brucknera, Anja Kuhnholdb, Camila Honorato-Riosa, Tanja Schillingb, Jan P. F. Lagerwalla* a

Experimental Soft Matter Physics Group, Physics and Materials Science Research Unit,

University of Luxembourg, 162a, Avenue de la Faïncerie, L-1511 Luxembourg, Luxembourg b

Theoretical Soft Matter Group, Physics and Materials Science Research Unit, University of Luxembourg, 162a, Avenue de la Faïncerie, L-1511 Luxembourg, Luxembourg

CORRESPONDING AUTHOR: *[email protected]

ACS Paragon Plus Environment

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ABSTRACT The helical liquid crystal self-assembly in suspensions of cellulose nanocrystals (CNCs), bioderived nanorods exhibiting excellent mechanical and optical properties, opens for attractive routes towards sustainable production of advanced functional materials. By convenience, the CNCs were in most studies until now suspended in water, leaving a knowledge gap concerning the influence of the solvent. Using a novel approach for aggregation-free solvent exchange in CNC suspensions we here show that protic solvents with high dielectric permittivity εr significantly speed up self-assembly (from days to hours) at high CNC mass fraction and reduce the concentration-dependence of the helix period (variation reducing from more than 30 µm to less than 1 µm). Moreover, our computer simulations indicate that the degree of order at constant CNC content rises with increasing εr, leading to shorter pitch and a reduced threshold for liquid crystallinity. In low-εr solvents the onset of long-range orientational order is coupled to kinetic arrest, preventing the formation of a helical superstructure. Our results show that the choice of solvent is a powerful parameter for tuning the behavior of CNC suspensions, enhancing our ability to control the self-assembly and thereby harvest valuable novel cellulose-based materials.

KEYWORDS: cellulose nanocrystals; chiral nematic; liquid crystal; non-aqueous solvents; solvent exchange.


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Introduction Cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), produced from one of nature’s most abundant resources, represent some of the most promising and intriguing nanoparticles to date.1–7 The stiff and rod-like CNCs, which are produced by acidic hydrolysis of cellulose,8 are known to form a liquid crystalline phase in aqueous suspension in a certain concentration range.9 Due to the anisotropic shape10 and the inherent chirality of the CNCs, the phase formed is chiral nematic (N*). Like a regular nematic, the N* phase exhibits no long-range positional order of individual CNC rods, but the liquid crystal orientational field, the director, is modulated helicoidally with a pitch p that is typically in the order of tens of micrometers.11–15 Thanks to the negative diamagnetic anisotropy of CNCs, the helix axis can be uniformly aligned throughout a macroscopic sample by applying a magnetic field of moderate strength.16, 17 By drying droplets of N* phase, films can be made in which p is reduced to the submicron range. Because the director orientation determines the local optical properties, this effectively turns the film into a photonic bandgap material with periodic modulation of refractive index, resulting in selective reflection of circularly polarized light within a narrow band of wavelengths.18–22 The photonic performance can be enhanced by incorporation of plasmonic nanoparticles,23, 24 and combined with a suitable polymer the helical CNC self-assembly may be used to produce light-weight high-performance composite materials that mimic the strongest biological composites, found in crustaceans.4,

25

By complexation of CNFs with boric acid or

borate, anisotropic foams can be developed that are of great interest as flame-retardant insulation materials.26,

27

Thus, the investigation of CNCs and CNFs is most interesting both from a

scientific point of view, e.g. concerning the mechanisms of chirality transfer,9, 28 as well as from


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an applied technological position, searching for new, sustainably produced advanced materials that could find use in diverse contexts, from modern buildings27 to security features.29 Essential for both is a profound and systematic investigation of CNC suspensions to obtain a deeper understanding of their properties, such as the pitch of the helix or the phase stability, and the factors that control these properties. Several parameters have been studied: the cellulose source,14, 30, 31 the acid used for hydrolysis,32 the sonication time18, 33 or the addition of salts.11, 34, 35 Surprisingly, a factor that has not yet been investigated systematically is the choice of solvent. Up to now, all systematic studies of the liquid crystal behavior of suspensions of pristine CNCs were restricted to water as solvent. In this paper we show that this is an important neglect: the solvent properties strongly influence the dynamics as well as the final state of the liquid crystalline self-assembly. While some non-aqueous CNC suspensions were studied with the purpose of dissolving polymer precursors,36–38 these studies were not concerned with the influence of the solvent on the properties of the CNC suspensions. There are also reports in which the CNC surface was functionalized39–42 or surfactants were added43–45 to enable the dispersion of the originally polar CNCs in non-polar solvents. However, these systems are difficult to compare with conventional aqueous CNC suspensions, since a strong influence can be expected from the surface modification and since the mass fraction of these modified CNCs for liquid crystal formation is typically an order of magnitude greater than for pristine CNCs.44 There are very few reports on pristine CNCs dispersed in non-aqueous solvents without further modifications or additives. These publications deal primarily with the problem of whether or not the CNCs can be suspended in certain solvents, leaving questions related to liquid crystal formation largely uninvestigated.


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Viet et al.46 studied the dispersion of unmodified freeze-dried CNCs in DMSO and DMF, observing a strong shear birefringence of the samples. They suggested that a certain amount of water within the suspensions might be crucial for their stability. However, to test this assumption they added molecular sieve, which strongly increases the salt content and this can greatly influence the behavior. This may have been the reason for the observed kinetic arrest of the systems.11 Blachechen et al.36 dispersed unmodified CNCs in N,N-dimethylformamide (DMF), tetrahydrofuran and ethyl acetate, finding maximum dispersibility in solvents with high relative dielectric permittivity εr as well as high viscosity. Okura et al.47 later confirmed this assumption, investigating 21 different organic solvents for suspending pristine CNCs. Furthermore, they found a correlation with the Lewis acidity and basicity of the solvents, which can be described by the Gutmann acceptor and donor numbers, respectively: if both values were high, as in strongly hydrogen bonding solvents, CNCs can be well dispersed. Thus, next to electrostatic interactions, hydrogen bonding might play a role in stabilizing CNC suspensions. Concerning the question of which CNC surface conditions favor suspension in non-aqueous media, contradicting results were found. While Espinosa et al.32 stated that highly charged CNCs would be easier to disperse in polar organic solvents, Cheung et al.48 found that neutralized CNCs formed more stable suspensions. Neither the phase diagram of pristine CNCs suspended in non-aqueous solvents nor effects on the N* helical pitch have been investigated so far. The main reason for this is most likely that the exchange of the solvent is challenging. Due to the process of extracting CNCs from the original cellulose source they are suspended in water to begin with. Simple evaporation of the water leads to irreversible agglomeration of the CNCs, making it impossible to re-suspend them in other solvents. In literature two protocols can be found for the solvent exchange, neither of which is


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satisfactory. The first consists of repeated steps of concentration by centrifugation, removal of the supernatant liquid and resuspension in the desired solvent.49 The second is to isolate the CNCs by freeze-drying followed by resuspension in the organic solvent.50 Both methods lead to some unavoidable CNC aggregation. In a comparative study, Tian et al.51 showed that freezedrying leads to more agglomeration than centrifugation, and in our preliminary studies we found that also the centrifugation method causes non-negligible agglomeration. In this work, we present an alternative method to exchange the solvent, based on the evaporation of water under reduced pressure. This does not cause agglomeration of the CNCs and thereby yields stable suspensions without any additives or modifications of the CNCs. The procedure can easily be applied also to aqueous CNF suspensions, expanding its usefulness. The new method allows us to report, for the first time for pristine CNCs, how the solvent influences the phase diagram, the magnitude of the chiral nematic pitch and the time needed for the formation of the helical superstructure. We find a dramatically enhanced self-assembly for high εr solvents, taking much less time at a given viscosity, being substantially less dependent of CNC concentration, and resulting in tighter helix pitch. The experimental results are corroborated by data from computer simulations for helically charged rods, a simple model for CNCs in solvent. These additionally reveal that also the dependence of the orientational order parameter on CNC concentration is affected by εr. As representative polar but non-aqueous solvents we choose formamide, N-methylformamide (NMF) and N,N-dimethylformamide (DMF), together representing a large span in dielectric permittivity and hydrogen bonding characteristics as shown in Table 1.


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Table 1. Chemical structure and selected physical properties of the solvents used. Chemical name

Chemical structure

Water

H

O

O

Formamide H

N-Methylformamide (NMF) N,NDimethylformamide (DMF)

H

Viscosity η / mPa s 25°C52

Hydrogen bond at characteristics

80.20

54.8 (18)

0.890

3D network

111.0

39.8 (24)

3.343

3D network

189.0

32.1 (27)

1.678

1D chains

38.25

16 (26.6)

0.794

No solventsolvent hydrogen bonds

NH2 O

H

NH

O H

Dielectric Acceptor permittvity εr at (donor) number53 20°C52

N


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Experimental Section Preparation of Cellulose Nanocrystals. Cellulose nanocrystals were prepared by sulfuric acid hydrolysis of commercial cotton. 30 g of cotton were dispersed in 600 mL of 64 wt% of sulfuric acid (aqueous) at 45°C and mixed using a mechanical stirrer for 120 min. The reaction was stopped by the addition of deionized water. The resulting suspension was concentrated by centrifugation three times with a large excess of fresh deionized water and subsequently purified using dialysis membranes in deionized water for a minimum of 4 days with daily exchange of the water. The CNC mass fraction in this initial suspension was between 2 and 3 wt%. Solvent Exchange. As solvents formamide (puriss. p.a., ACS reagent, ≥99.5% (GC/T)) and Nmethylformamide (NMF, 99%) from Sigma-Aldrich as well as N,N-dimethylformamide (DMF, ≥99.8%, p.a., ACS) from Carl Roth were used. To remove traces of water and ions, formamide and NMF were stirred over molecular sieve (3 Å, Typ 562, Carl Roth) at 40°C for 15 h, followed by distillation at 40°C under reduced pressure of approximately 0.1 mbar. Thereby the conductivity of formamide was reduced from 171 to 31 µS cm-1 and of NMF from 843 to 64 µS cm-1. As the conductivity of DMF was already at a very low value of 0.73 µS cm-1 this purification step was omitted for DMF. The aqueous CNC suspensions were diluted with about twice the volume of purified solvent and shaken until the solvents were well mixed. The water was then removed by distillation at 40°C under reduced pressure of approximately 50 mbar. At the end the pressure was briefly reduced further to ~0.1 mbar until also the non-aqueous solvent started to evaporate. This removed the last traces of water in the solvent (we cannot rule out that some CNC-adsorbed water might remain). The mass fraction of each CNC suspension was determined three times by drying several ml of the suspensions accompanied by weighing before and after drying.


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The CNC suspensions were now diluted with the respective purified solvent or deionized water to about 1.5 wt% of CNCs and sonicated in aliquots of 45 mL with an ultrasonic probe (Dr. Hielscher UP200St, 2 mm diameter titanium tip, effective power density 0.75 W cm-1). To avoid excessive heating, the samples were placed in an ice bath and sonicated four times 2.5 min, with rest phases in between to allow the samples to cool down. The CNC suspensions were then recombined and the solvent was removed by distillation until the suspensions became rather viscous but did not yet deposit films on the glassware. Finally, the mass fraction of the CNC suspensions was measured again. All further experimental details are provided in the Supporting Information.

Results and Discussion Quality of the suspensions after solvent exchange Our method for solvent exchange is based on mixing the aqueous CNC suspensions with the organic solvent, followed by evaporation of the water under reduced pressure and subsequent sonication (see Methods & Materials). The procedure ensures that the CNCs are suspended in a solvent at all times and therefore do not agglomerate. Furthermore, by working at reduced pressure, the temperature can be kept as low as 40°C, which prevents hydrolysis of the sulfate groups attached to the CNC surface.54 The method is only applicable for solvents that possess higher boiling points than water and do not form azeotropes with it. As confirmed in literature55, 56 the three solvents formamide, NMF and DMF are thus suitable. Considering that the formation of azeotropes also depends on the pressure, we confirmed that the solvents do not form an azeotrope with water under the given working conditions by measuring the refractive indices of the organic solvent after evaporation


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of the water. The obtained values (Supporting Information) are very close to the literature values, proving that no water is left in the investigated liquid. Already a simple optical evaluation revealed that no serious agglomeration of the CNCs took place during the solvent exchange. In addition, AFM measurements (cf. Supporting Information) were performed to confirm this observation also on the nanoscale and to compare the dimensions of the CNCs in the different solvents. The length distribution determined from the AFM measurements are shown in Figure 1. For all four solvents, the histograms are similar. The average

lengths

of

the

CNCs

are

lw = (0.18±0.09) µm

for

suspensions

in

water,

lf = (0.18±0.10) µm in formamide, lNMF = (0.16±0.07) µm in NMF and lDMF = (0.16±0.08) µm in DMF. The CNCs suspended in the latter solvent appear to have a somewhat larger average effective rod thickness, suggesting slightly stronger aggregation in DMF. All CNCs investigated thus possess almost the same properties, with a slight deviation for DMF-suspended CNCs. Any major differences found in the further experiments can thus readily be attributed to the varying properties of the solvents.


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Figure 1. Length distribution of the CNCs, as determined by AFM measurements on dried samples from sonicated suspensions in the different investigated solvents. For all four systems the CNCs exhibit comparable rod dimensions.

Phase diagrams of CNC suspensions in the different solvents The phase diagrams were established for CNC suspensions with all four solvents up to the highest accessible mass fraction of CNCs, varying in dependence of the solvent. To ensure a good separation of the isotropic and the anistropic phase, samples were centrifuged for 30 min at 4000 rpm and left standing for about 1 hour before pictures were taken. A comparison with


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samples that were left untouched for equilibration for 3 months showed that there was no significant difference in the volume fractions for the two methods (cf. Supporting Information). For the three solvents water, formamide and NMF, which all possess rather high εr, a phase behavior typically known for CNC suspensions was found (Figure 2). Depending on the solvent the suspensions were isotropic up to roughly 2 to 3 wt% of CNCs. After reaching this mass fraction threshold, w0, the N* liquid crystal phase forms in equilibrium with the isotropic phase. The volume fraction of the liquid crystal phase increases with increasing CNC content. The N* phase is strongly birefringent (Figure 3 and Supporting Information) and it sediments to the bottom of the vial in this two-phase regime, due to its higher density compared to the isotropic phase. By further increasing the CNC mass fraction to more than 7 wt%, the final isotropic component disappears and the sample is completely liquid crystalline beyond a second threshold w1. It is notable that the phase diagram is shifted to lower mass fraction if going from water to formamide and finally NMF, thus increasing εr. This difference might be explained by considering that the Debye screening length κ-1 is proportional to the square root of the relative permittivity, 𝜅 !! ∝ 𝜀! . In consequence, the effective volume of the CNC rods increases by increasing εr. The effective volume fraction for a given mass fraction of CNC thus increases, pushing the transitions into the two-phase region as well as into the 100% N* state to smaller mass fractions. The effect is further enhanced by the differences in Lewis acidity: the decreasing ability to donate protons from water to NMF (decreasing acceptor number, see table 1) means that the sulfate groups on the CNC surface are less neutralized in NMF than in water, yielding higher surface charge, consequently greater Debye screening length and a larger effective volume fraction.


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1 0.8

φ / vol%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 CNC/water CNC/form. CNC/NMF CNC/DMF

0.2 0 0

2

4 6 w(CNC) / wt%

8

10

Figure 2. Phase diagrams of the four investigated CNC/solvent systems, parametrized by the volume fraction 𝜑 of the anisotropic to isotropic phase. A second striking effect caused by the increased εr can be noted by looking at the CNC suspension between crossed polarizers, as shown in Figure 3 for suspensions with 6 wt% of nanoparticles. In CNC suspensions with water, the upper isotropic phase scatters light rather intensely. Furthermore, the phase boundary to the N* phase is blurry. By increasing the relative permittivity of the suspension medium, in case of formamide and even more so in case of NMF, the upper isotropic phase becomes increasingly clear and the phase boundary becomes increasingly sharp. This hints at a decrease of critical fluctuations, suggesting that the nature of the phase transition shifts to stronger 1st order. We noted the opposite trend as a function of increasing ionic strength in a recent study of aqueous CNC suspensions.11 Our computer simulations, described below, hint at an increase in the degree of orientational order, at given CNC mass fraction in the N* phase, with increasing εr. This may be the explanation to this behavior around the isotropic-N* transition.


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Figure 3. CNC suspensions with a mass fraction 6 wt% between crossed polarizers after centrifugation. The solvent used is noted at the top of each vial. The N* phase appears bright, at the bottom of all vials except the one with DMF as solvent. The latter is in a kinetically arrested state which is illustrated by the fact that the suspension does not flow down after centrifugation. For CNC suspensions in DMF a completely different behavior was found. At 1 wt% the suspension is completely isotropic and fluid, while the sample at 2 wt% appears as if it were completely liquid crystalline, but it is also dramatically more viscous, to the point of stopped flow. The latter effect of diverging viscosity also occurs in the other systems investigated but at much higher mass fractions and will be referred to as ‘kinetic arrest’, as the CNCs do not seem to be capable of moving freely within the suspensions. The deviating behavior of the CNC/DMF system may possibly be explained by considering the comparably low value εr = 38 of DMF, which is not even half the value of water. With such low permittivity of the suspension medium, the counter ions are solvated rather poorly and condense.57 As a result, the electrostatic stabilization of the CNCs loses its efficiency, causing partial agglomeration of the CNCs with interesting consequences for the macroscopic phase behavior. An additional contribution may come from the limited hydrogen bonding capability of DMF, which is solely able to accept but not to donate hydrogen bonds: two adjacent cellulose chains form strong hydrogen bond


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networks with each other, hence a close approach of CNC rods in a poor hydrogen bonding solvent is likely to cause irreversible aggregation, with the rods aligned parallel (maximizing the number of bonds in the network). We note that the development of long-range orientational order and the kinetic arrest take place simultaneously in the sample with DMF-suspended CNC. This suggests that the process driving liquid crystal formation also promotes percolation of the rods throughout the 3D phase. As the DMF-suspended CNCs, with their inability to form a fluid equilibrium liquid crystal phase, constitute a special case in this study, a full investigation of the origin of this phenomenon is outside the article scope. In the interest of stimulating follow-up studies we nevertheless propose a possible mechanism, being aware that it is speculative at this stage. If the dispersion stability of the CNC rods is breaking down in DMF at a CNC mass fraction of about 2%, the rods may rapidly form aggregates that diverge in size, forming a continuous (percolating) network throughout the volume of the phase and thus triggering the kinetic arrest. However, in order to maximize the hydrogen bonding between adjacent cellulose chains, aggregation with adjacent CNC rods aligned with each other is favored. As the system is highly polydisperse in length, the resulting aggregate has no well-defined length and instead it may grow continuously in length as more and more CNC rods of different sizes are added in an aligned fashion. A linear supra-rod thus develops, which through its great aspect ratio triggers liquid crystal formation at much lower mass fraction than for individualized CNC rods. Since the same process leads to percolation and kinetic arrest the equilibrium helical structure of the chiral nematic phase never has time to develop, explaining the absence of any traces of helix formation in the anisotropic DMF-based CNC suspensions (Figure 4a).


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Figure 4. Textures as observed after two to three weeks after filling into flat capillaries, between crossed polarizers, for 6wt% of CNCs suspended in a) DMF, b) water, c) formamide and d) NMF. The capillary is in each case 400 µm thick, with rectangular cross section.

Development of helical superstructure in the different solvents In contrast to the CNC/DMF system, a helical superstructure, revealing itself in form of characteristic fingerprint textures, was clearly identified for sufficiently concentrated CNC suspensions with the three solvents, water, formamide and NMF (Figure 4b-d), which also exhibit a conventional phase behavior (cf. Figure 2). The value of the N* pitch was measured directly from the fingerprint texture and in case of CNCs suspended in water and formamide additionally by using laser light diffraction (procedures described in Supporting Information). The results are plotted in Figure 5 and show that the consistency of the two methods is high. For CNC/water suspensions we found a rather large pitch with a strong dependence on the mass fraction, ranging from 48 to 16 µm. In suspensions with formamide we note a significant decrease of the pitch as well as of its dependence on the CNC mass fraction, resulting in values


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between 16 and 12 µm. An even more drastic reduction of the chiral nematic pitch to approximately 3 µm with almost no dependence on the mass fraction occurred for the CNC/NMF system.

Figure 5. Chiral nematic pitch p in dependence of the mass fraction w(CNC) for CNC suspensions with water, formamide and NMF. The measurements were performed by determination from the fingerprint texture (filled symbols) and laser light diffraction experiments (hollow symbols).

There is a clear correlation between an increase in εr of the solvent and a decrease of the chiral nematic pitch, paired with a strongly reduced dependence on the mass fraction. This observation suggests that the chiral interaction of the CNCs, and thus the chirality transfer from individual CNC rods to the macroscopic liquid crystal phase, is stronger in solvents with a high εr. In this context, a recent study of chirality transfer in a liquid crystal from non-ionic amphiphilic molecules in protic solvents is most interesting.58 While in that case the key parameter turned out to be the hydrogen bonding character of the solvent, this may at most be a secondary effect in case of the ionically stabilized CNCs, considering the hydrogen bonding characteristics of NMF versus water (see Table 1). Instead, we believe that the enhanced chirality transfer and reduction


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of helix pitch is due primarily to a higher degree of orientational order with higher εr, as indicated by the computer simulations to be discussed below. This is also in line with the reduced scattering of the isotropic phase in the high εr solvents, indicating a greater discontinuity in orientational order parameter at the phase transition, as discussed above in connection to Figure 2. The fingerprint texture is never visible directly after sample preparation. Due to the filling process by suction into flat capillaries, all CNC suspensions initially show planar alignment with the director aligned along the capillary axis. Within a certain time after filling, the CNCs in nonarrested samples rearrange to reveal the helical superstructure in form of a fingerprint texture. We measured this time t in dependence of the mass fraction as shown in Figure 6. In doing so, we distinguished between the ‘first signs’ of pitch lines, when they were at the limit of detection, and ‘clearly developed’ pitch lines, when there was no more doubt about their existence (cf. Suporting Information).

Figure 6. Time t needed for the development of pitch lines in dependence of the mass fraction w(CNC). At low mass fractions, up to 6 wt%, the CNCs need less than 4 h to rearrange into the helical superstructure. This time rises with increasing CNC content, up to about 7 wt%, fairly


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independent of solvent. Beyond this concentration a strong solvent dependence is seen. In case of water, the formation of clearly developed pitch lines takes more than 12 h at 7 wt% and more than 3 days at 8 wt%. In contrast, in the 8 wt% samples with formamide and NMF, the formation of clearly developed pitch lines takes only 14 h or 18 h, respectively. By increasing the CNC content to 9 wt% none of the investigated systems show any signs of pitch lines within 4 days, indicating that they are all in a kinetically arrested state at this mass fraction. As the time needed for developing the helical superstructure increases with CNC content, the question arises if this time dependence is simply due to increasing viscosity in the systems, or if other processes are also at play. To clarify this issue, we measured the flow curves for all systems (cf. Supporting Information) and plotted the helix development time t versus the viscosity η at a representative shear rate of 10 s-1 (Figure 7). An almost linear dependence between the development time and the viscosity is observed, which confirms the dependence on viscosity. However, the data points do not fall on a single master curve, as would be the case if viscosity would be the only relevant parameter. The slope of the fitted lines decreases significantly by changing the solvent from water to formamide and even more to NMF. This variation may only be explained in terms of a second factor originating from the properties of the solvent and their interaction with the CNCs. Considering that the slope decreases with increasing relative permittivity of the solvent, the efficiency of electrostatic stabilization of the CNCs within the suspensions might be this factor. The better the CNCs are stabilized, the less likely is agglomeration, even at high mass fractions. Less aggregated particles move more freely within the phase than large agglomerates, independent of the viscosity of the continuous phase, because they are not sterically hindered by other aggregated particles. This comparably higher mobility of


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less aggregated particles facilitates the development of the chiral nematic helix as long as the system does not enter a kinetically arrested state.

Figure 7. Time t needed for the development of pitch lines versus the viscosity η measured at a shear rate of 10 s-1. Empty symbols correspond to first signs of pitch lines, while filled symbols refer to clearly developed pitch lines.

An explanation for the phase and helix formation behavior in the different solvents is not obvious from the experimental result alone. The direct interactions between individual CNC rods in each solvent have to be considered. Therefore, we performed Monte Carlo simulations of the investigated systems to provide a deeper insight into the mechanism of liquid crystal formation and chirality transfer.

Monte Carlo simulations of helical self-assembly We model the CNC suspension as a system of helically charged, screened spherocylinders (rods terminated with spherical caps). This model was initially studied analytically by Wensink et al.,59 and has then been used by us in simulations of the phase behaviour of CNCs as a function of


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surface charge and salt concentration.11 As we found the model to reproduce the phase behaviour well, given its simplicity, we extend our simulations here to the case of different solvents with varying εr. The total surface charge Zs is split into npc discrete charges, which decorate the rod surface in a helical fashion with internal pitch pint. The charges interact via a screened Coulomb (Yukawa) potential, which depends, among others, on εr of the solvent, via the Bjerrum length λB: !!

𝑈! 𝑟 = 𝑘! 𝑇

!!"

!

𝜆! 𝑒

!!"

(1)

𝑟

𝜆! = 𝑒 ! /(4𝜋𝜀! 𝜀! 𝑘! 𝑇) 𝜅=

4𝜋𝜆!

(2)

!!!

(3)

!

Since λB increases the interaction strength (Eq. 1) but at the same time decreases the Debye screening length 𝜅 !! (Eq. 3), the solvent's influence on the phase behavior does not follow intuitively. Other constants and parameters are the elementary charge e, the vacuum permittivity ε0, the number of rods N in the volume V, the Boltzmann constant kB and the temperature T. The simulated system contains 1800 rods of aspect ratio 20 (length L over diameter D), with Zs=690, npc=17 and pint=20D. In water, the Bjerrum length is set to 0.07D, and for the other solvents it is multiplied with the ratio of the dielectric constants. The results of the simulations are shown in Figure 8, in terms of the nematic order parameter !

𝑆! = ! 3 cos ! 𝛽 − 1

(4)

where β is the angle of a rod with respect to the director and the brackets denote an ensemble average in a system with constant N, V, T, for different volumes V. The isotropic (S2 = 0) to nematic (S2 > 0) phase transition point shifts to lower mass fraction with increasing εr (water to


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formamide to NMF), in agreement with the experimental results. For individual DMF-suspended CNCs the transition takes place at a very high mass fraction, which we did not access by simulation. Considering that the opposite behavior was seen in experiments, this is a strong indication that the behavior in DMF involves multiple interacting phenomena, possibly with partial aggregation leading to an increase in effective rod size, as suggested above. As our simulations do not capture the hydrodynamics and represent a very limited sample size, we cannot reproduce such complex scenarios. 1 0.8 0.6

S2

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0.4 0.2 0 6

7

8 w(CNC) / wt%

9

10

Figure 8. Nematic order parameter S2 as a function of CNC concentration in water (circles), formamide (squares), NMF (diamonds) and DMF (triangles) as established by computer simulations.

For a fixed mass fraction the nematic order parameter (Eq. 4) increases with increasing εr. This effect is due to packing entropy: since the interaction strength becomes weaker with increasing εr—in particular at short distances—the rods can move closer and gain translational entropy. However, to maximize this entropy, they need to align more strongly at the expense of orientational entropy. Therefore the nematic order parameter increases with εr. As mentioned above, this can also contribute to the solvent dependence of the N* pitch: the stronger the local


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alignment of the actual CNC rods around the director, the more important the chirality of the interaction and the stronger is the rotation of the nematic director, resulting in decreasing pitch with increasing εr.

Rheological experiments To get a more quantitative and objective characterization of the kinetic arrest phenomenon and to estimate the mass fraction at which the transition into the arrested state occurs, we carried out oscillatory rheological experiments as a function of frequency for all four CNC/solvent systems (Figure 9). In case of the water-suspended CNCs, the sample with a mass fraction of 8 wt% exhibits an almost parallel slope of the storage modulus G´ and the loss modulus G´´, revealing that the CNC/water system is very close to the gel point (kinetic arrest) at the given concentration. By increasing the mass fraction to 9 wt% the storage modulus G´ takes on higher values than the loss modulus G´´, showing that the behavior of the CNC suspensions becomes elastic due to kinetic arrest. For the formamide-suspended CNCs, higher mass fraction than 9 wt% were unfortunately not experimentally accessible. Up to this mass fraction the suspension still behaved as a viscous fluid. For the CNCs suspended in NMF, the samples are kinetically arrested starting from a mass fraction of 9 wt% and for DMF the CNCs are in a kinetically arrested state already at 2 wt%. These results support our earlier observations, especially concerning the time dependence of the formation of the helical superstructure. For CNCs suspended in DMF the transition from the isotropic to a long-range ordered phase coincides with kinetic arrest, hence CNCs are not able to form a helical superstructure in DMF. In case of water-suspended CNCs the suspension with a mass fraction of 8 wt% still forms a helical superstructure but it needs more than 3 days


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development time, as the mass fraction is very close to kinetic arrest. At 9 wt% the CNCs suspended in water and NMF are in a kinetically arrested state, preventing the CNCs from rearranging into a helical superstructure. The helix development time at 8 wt% CNC was the smallest with formamide as solvent, as this suspension was still furthest away from the transition into the kinetically arrested state. Possibly, the combination of high εr and the ability to form a 3D hydrogen bonding network is beneficial in delaying kinetic arrest.


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Figure 9. Double-logarithmic plots of the storage modulus G´ (filled symbols) and loss modulus G´´ (hollow symbols) in dependence of the angular frequency ω. The investigated CNC/solvent system is indicated within the graphs.

Conclusions The chiral nematic liquid crystalline self-assembly of cellulose nanocrystal suspensions is strongly influenced by the choice of solvent. The key parameter that influences dynamics, equilibrium structural values as well as the onset of kinetic arrest is the dielectric permittivity, εr, hydrogen bonding characteristics probably playing an important additional role in preventing


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particle aggregation. Raising the permittivity 2.4 times compared to that of water by using Nmethyl formamide as solvent drastically reduces the viscosity-normalized self-assembly time, the dependence of the equilibrium helix pitch on CNC concentration, and also the final equilibrium value of the pitch. In contrast, a reduction of εr by a factor 2.1 by replacing water with N,Ndimethylformamide triggers immediate kinetic arrest upon liquid crystal formation, preventing helix formation. We speculate that this may be due to the poor CNC stabilization in a low-εr solvent with limited hydrogen bonding, leading to aligned and extended CNC aggregation with simultaneous development of long-range orientational order and percolation throughout the 3D volume. With our new method of exchanging the solvent via distillation, inducing no CNC aggregation since drying of the nanoparticles is avoided, an additional powerful parameter for tuning CNC self-assembly arises. This allows better control of the advanced materials that can be made from these fascinating nanorods. No modification of the CNCs is needed, only a shrewd choice of dispersion medium.

ACKNOWLEDGMENT We thank Rick Dannert for advice and guidance in performing the rheological experiments. Financial support by the Luxembourg National Research Fund (FNR; project MISONANCE, grant code C14/MS/8331546) and, for J. Bruckner, from the Dr. Leni Schöninger Stiftung are gratefully acknowledged.

SUPPORTING INFORMATION


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The following files are available free of charge: pdf file with further experimental details (verification of purity after solvent exchange; establishment of phase diagrams; pitch measurements; AFM characterization; rheological investigations) and additional data on phase sequence and rheological response. REFERENCES 1.

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TABLE OF CONTENTS GRAPHIC


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Table of Contents graphic 85x47mm (300 x 300 DPI)


Enhancing Self-Assembly in Cellulose Nanocrystal Suspensions

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