Gelation Kinetics and Network Structure of Cellulose Nanocrystals in

Sep 1, 2016 - aqueous solution when electrostatic repulsion is screened by addition of salt. ... structure of the CNC network, which we will address h...
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Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution Karthik R. Peddireddy, Isabelle Capron, Taco Nicolai, and Lazhar Benyahia Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01061 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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Biomacromolecules

Gelation Kinetics and Network Structure of Cellulose Nanocrystals in Aqueous Solution

Karthik R. Peddireddy†, Isabelle Capron‡, Taco Nicolai*,†, Lazhar Benyahia†,



LUNAM Université du Maine, IMMM UMR-CNRS 6283, 72085 Le Mans cedex 9, France



UR1268 Biopolymères, Interactions et Assemblages, INRA, 44316 Nantes, France

Abstract

Cellulose nanocrystals (CNC) are rod-like bio-sourced nano-particles that are widely used in a range of applications. Charged CNC was obtained by acid extraction from cotton and dispersed in aqueous solution using ultrasound and characterized by light scattering. Aggregation and gelation of CNC induced by addition of NaCl was investigated by light scattering as a function of the NaCl concentration (30–70 mM), the CNC concentration (0.5–5 g/L) and the temperature (10–60°C). Formation of fractal aggregates was observed that grow with time until they percolate and form a weak system spanning network. The aggregation rate and gel time were found to decrease very steeply with increasing NaCl concentration and more weakly with increasing CNC concentration. A decrease of the gel time was also observed with increasing temperature for T>20°C. The structure of the CNC networks was studied using confocal laser scanning microscopy and light scattering. The local structure of the networks was fractal and reflected that of the constituting aggregates. The gels were homogeneous on length scales larger than the correlation length, which decreased with increasing CNC concentration. The CNC gels flowed when tilted for C12 g/L gels formed that did not flow when titled. The effect of

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the ionic strength was tested by varying the NaCl concentrations between 0 and 250 mM for

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suspensions at C= 1 g/L. Sedimentation was observed for C≥30 mM NaCl, but the mechanism was

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strikingly different for [NaCl]≥50 mM and [NaCl]≤40 mM, see fig. 6. At higher ionic strengths, a distinct

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layer was formed within a few days with a relative height (h) that slowly decreased with time until it

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reached a constant value. For [NaCl]≤40 mM a dense layer was formed at the bottom after more than a

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month that slowly increased with time until it reached a constant value. We suggest that the different

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behavior at low ionic strength can be explained by the extremely slow aggregation process that allows

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sedimentation of the aggregates before they percolated. The bottom layer is in this case not the

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increasingly dense collapsing network, but an increasing layer of sedimented aggregates.

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h

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Figure 6. CNC in suspensions at C= 1 g/L at 40 mM NaCl (left) and 50 mM NaCl (right) at different times

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after preparation between two weeks and 6 months. The numbers on the images represent the age of

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the samples in months.

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The rate at which h decreased for [NaCl]≥50 mM, depended little on the ionic strength, but it

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was faster if the CNC concentration was lower. The time dependence of h at [NaCl]=70 mM is shown in

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fig. 7a. As expected, the steady state values of h increased with increasing CNC concentration, see fig.

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7b. We checked by light scattering that the amount of CNC in the clear top layer was negligible, which

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means that the CNC concentration in the sedimented layer (Cn) can be calculated as Cn=C.h. The CNC

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concentration in the sedimented layer after standing for two months increased with increasing CNC

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concentration from Cn=1.5 g/L at C=0.25 g/L to Cn=3.5 g/L at C=3.0 g/L.

1.0 a 0.8

0.25 g/L 0.5 g/L 1 g/L 2 g/L 3 g/L

0.6

h

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0.4

0.2

0.0 1

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10

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t (day)

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Figure 7a Evolution with time of the relative height of the collapsing network layer for CNC suspensions

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at [NaCl]=70 mM and different concentrations indicated in the figure.

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Figure 7b Photographs of the samples 1 month after preparation.

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Gel structure

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The mesoscopic structure of the gels was studied by CLSM for CNC suspensions at different

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concentrations at 70 mM NaCl that were aged at room temperature. CLSM images of the suspensions

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taken 15 h after preparation, show that the CNC networks were increasingly dense with increasing

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concentration, see fig. 8. No movement was observed on length scales probed by CLSM. The structure of

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the gels was analyzed quantitatively by calculating the pair correlation function (g(r)) of the fluorescent

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intensity fluctuations as described in detail by Ako et al. ref.17, see fig. 9a. Since the intensity of Nile blue

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bound to CNC is proportional to the CNC concentration, g(r) represents the pair correlation function of

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CNC concentration fluctuations. The decay of g(r) for r>0.2µm was approximately exponential: (g(r)-

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1)=B.exp(-r/ξ), and the results obtained at different concentrations superimposed after normalization of

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g(r) by B and r by ξ, see fig. 9b. The correlation length ξ decreased approximately linearly with

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increasing concentration from ξ=2.5 µm at C=1 g/L to ξ=0.3 µm at C=9 g/L, see inset of fig. 9b. Notice

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that not all Nile blue was bound to the CNC, which explains why the fluorescence intensity in the pores

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was not zero. This has no effect on the dependence of g(r) on r, but it reduces the contrast and

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therefore the prefactor B.

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160 µm

40 µm

160 µm

40 µm

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Figure 8 CLSM images at two different scales of CNC gels at [NaCl]= 70 mM and different concentrations

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between 0.5 g/L and 9 g/L as indicated in the figure. The images were taken 15 h after preparation, i.e.

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when steady state was reached, but before sedimentation was significant. For each concentration

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images at two different lengths scales are shown

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1.0

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(g (r)-1)/B

1 g/L 2 g/L 3 g/L 5 g/L 9 g/L

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ξ (µm)

a

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0.6 10-1 1

2

3

5

7

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C (g/L)

0.4

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1.00 0.1

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b

1.04

g(r)

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0.0 0.1

10

1

10

r/ξ

r (µm)

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Figure 9a Pair correlation functions of the concentration fluctuations of CNC networks at [NaCl]=70 mM

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and different CNC concentrations indicated in the figure.

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Figure 9b Master curve of the pair correlation function after normalization of g(r) by B and r by ξ. The

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inset shows the dependence of the correlation length on the CNC concentration. The solid lines

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represent fit results, see text.

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The structure on length scales smaller than ξ was characterized by light scattering at C=1 g/L,

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where the effects of turbidity and multiple scattering on Ir were negligible. The measurements were

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done 15 h after preparation when steady state was reached, but sedimentation was still negligible.

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When a network is formed the suspension is no longer ergodic, because concentration fluctuations are

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partially frozen-in. This means that the temporal average no longer corresponds to the spatial average.

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Therefore the intensity was spatially averaged by slowly rotating the sample. Fig.10 shows that Ir had a

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power law dependence on q over the whole accessible q-range: Ir∝q-1.6, which means that local

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structure was self similar on length scales down to about 10 nm and was characterized by a fractal

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dimension df=1.6. This value of df is significantly smaller than that obtained at [NaCl]= 200 mM and

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C=7.8 g/L by SANS measurements (df=2.1). One possibility is that the fractal dimension of the CNC

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network increases with increasing ionic strength. A weaker q-dependence was observed with the SANS

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measurements at [NaCl]=50 mM, but the results were not interpreted in terms of a fractal dimension.

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We also note that the lowest q-value in the SANS measurements was 10-2 nm, whereas with light

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scattering the q-range extended down to 3.10-3nm.

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Ir/KC (g/mol)

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108

107 -1

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q (m )

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Figure 10 Dependence of Ir/KC on q for a CNC suspensions at C= 1 g/L and [NaCl]= 70 mM. The

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measurement was done 15 h after sample preparation, i.e. when the intensity had reached a steady

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state value, but before sedimentation was significant. The straight line has slope -1.6.

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Redispersion of the CNC gels

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The bonds that were formed between the CNC particles were strong enough to resist dilution in

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pure water. However, the gels could be redispersed using a vortex mixer (Fisher) or by applying ultra

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sound. The efficacy of these two methods was tested for suspensions at C= 5 g/L and [NaCl]= 50 mM

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that had been left overnight to form gels. The evolution with time of the redispersed suspensions was

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characterized with light scattering and the results were compared with those obtained on freshly

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prepared suspensions. The effect of stirring time is shown in Fig. 11a. The value of Ir just after

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redispersion decreased with mixing time at 3000 rmp, but remained significantly larger than that of

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freshly prepared solutions even after mixing during 8 min. The gel time increased with mixing time

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showing that gels are formed quicker if the CNC is not fully dispersed. These measurements show that it

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is possible to break-up the gel into small aggregates by stirring, but not down to the original CNC

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trimers. Fig. 11b shows that the latter can be done very quickly using ultrasound. Sonication during 10 s

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at 0.3 J/s was enough to fully redisperse the gels into trimers. Sonication was found to be effective also

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at higher NaCl and CNC concentrations.

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11 10

10

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Ir

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a

Ir tg

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tg(h)

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Ir

tg(h)

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mixing time (s)

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sonication time (s)

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Figure 11 Initial value of Ir and gel time after mixing with a vortex mixer at 3000 rpm or sonication time

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at 0.3 J/s (b) for gelled CNC solutions at C=5 g/L and [NaCl]=50 mM.

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The results reported here for charged CNC obtained by acid extraction from cotton are

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qualitatively similar to those reported for other aggregating rod-like colloidal particles in the literature,

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see for a review ref. 12. Quantitatively, the results will depend on the charge density and the shape of

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the CNC particles that varies depending on the source and the extraction method. Lower charge density

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reduces the energy barrier that needs to be overcome to form bonds and therefore gelation will be

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faster. Indeed, preliminary experiments on cotton CNC with very low charge density showed rapid

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gelation even in the absence of added salt. The rate of gelation and the strength of the gels will also

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depend on the length of the CNC. For a given concentration we expect faster gelation and stronger gels

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if CNC with the same thickness is longer. However, in all cases the mechanism of gelation will be the

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same and the local structure of gels prepared at low CNC concentrations will reflect the fractal structure

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of the aggregates.

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Conclusions

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Aggregation of charged CNC in aqueous suspensions can be induced by addition of salt and leads

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to the formation of self-similar clusters that grow in size until they percolate into a system spanning

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network at a critical gel time. Networks can be formed even at very low CNC concentration down to at

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least 0.5 g/L. However, for C≤ 12 g/L the CNC gels are very weak and flow when the vials are tilted. For

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C< 4 g/L the network collapses under gravity leading to slow sedimentation. The sedimentation rate

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increases with decreasing CNC concentration, but does not depend much on the ionic strength. The gel

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time decreases very strongly with increasing ionic strength, e.g. at C= 5 g/L it varied between 2 months

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at [NaCl]=30 mM and less than 30 min at [NaCl]=70 mM. tg also decreases increasing CNC concentration,

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but the effect of the CNC concentration is less strong. The dependence of tg on C and [NaCl] can be

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described by a power law in the ranges investigated: (tg∝[NaCl]-10, tg∝C-1.7). The aggregation process can

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be accelerated by increasing temperature above 20°C indicating that hydrophobic interactions play a

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role in the crosslinking process. The local structure of the gels is insensitive to the NaCl concentration

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between 30mM and 70 mM and reflects that of the aggregates. CLSM shows frozen heterogeneous

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networks. The correlation length above which the gels are homogeneous decreases with increasing CNC

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concentration from 2.5 µm at C=1 g/L to 0.3 µm at C=9 g/L. Gels are broken-up in small aggregates after

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vigorous stirring and can be fully redispersed by sonication.

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ACKNOWLEDGMENT

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The research was financial supported by the MATIERES project and the “Région Pays de la Loire”.

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