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On cellulose dissolution and aggregation in aqueous tetrabutylammonium hydroxide Marta Gubitosi, Hugo Duarte, Luigi Gentile, Ulf Olsson, and Bruno F. Medronho Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b00696 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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On cellulose dissolution and aggregation in aqueous tetrabutylammonium hydroxide Marta Gubitosi‡, Hugo Duarte§, Luigi Gentile‡, Ulf Olsson‡, Bruno Medronho§*

‡

Division of Physical Chemistry Department of Chemistry, Center for Chemistry and

Chemical Engineering, Lund University, SE-221 00 Lund, Sweden

§

Faculty of Sciences and Technology (MeditBio), University of Algarve, Campus de

Gambelas, Ed. 8, 8005-139 Faro, Portugal

*Corresponding author: [email protected]

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ABSTRACT Aqueous tetrabutylammonium hydroxide, TBAH(aq), has been found to dissolve cellulose and to be a potential solvent for chemical processing or fiber spinning. In this paper, we have investigated the dissolution state of cellulose in 40 wt.% TBAH(aq) solvent, and present an extensive study of rheology, combined with static light and small angle X-ray scattering, to correlate cellulose aggregation with changes in the rheological parameters. Two cellulose molecular weights are compared. Microcrystalline cellulose (MCC), with a degree of polymerization of ca. 260, and a dissolving pulp with an approximately ten times higher molecular weight. Scattering data demonstrate that cellulose is molecularly dissolved at lower cellulose concentrations, while aggregates are present when the concentration exceeds a certain value. The onset of the aggregate formation is marked by a pronounced increase in the scattering intensity at low q, shear thinning behavior and violation of the empirical CoxMerz rule. Additionally, the SAXS data suggest the presence of a solvation shell enriched in TBA+ ions, compared to the bulk solvent. The results are consistent with the recent suggestion that while native cellulose I may still dissolve, solutions are, above a particular concentration, becoming supersaturated with respect to the more stable crystal form cellulose II.

KEYWORDS. Cellulose dissolution; aggregation; quaternary ammonium hydroxide; rheometry; small angle X-ray scattering


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INTRODUCTION As the major carbohydrate produced by plant biosynthesis, cellulose occupies a prominent place as a ‘green’ polymer for the production of innovative and sustainable materials. 1 Unlike most other polymers, cellulose is not meltable and therefore most of its applications rely on an efficient dissolution step followed by shaping processes where the properties of the regenerated material are strongly dependent on how well cellulose is dissolved and organized in solution. 2 Already in the wood cell wall, such organization of the different molecules (involving aggregates of cellulose in microfibrils together with other matrix components such as hemicelluloses and lignin) is determinant for the plant structural properties. As a consequence of the hierarchical organization, the dissolution process is typically not simple and may involve multi-step pretreatments and hazardous chemicals in order to rupture the complex network of inter and intra hydrogen bonds and hydrophobic interactions among cellulose molecules. 3 The solvent systems developed so far are very different regarding the nature of the components (e.g. organic, salt or aqueous based) and operating conditions (e.g. extreme pHs and temperatures) but most of them share limited lab-scale applications. 4-6 Recently, alkylammonium hydroxide solutions (also known as onion hydroxides) with or without additives have been suggested as novel solvents for cellulose. Some of the systems are capable to dissolve high amounts of cellulose with reasonably high molecular weight in mild conditions and are apparently stable throughout the dissolving process, strongly suggesting a viable solvent recyclability. 7-9 In particular, aqueous tetra-n-butylammonium hydroxide TBAH(aq) has been found to dissolve lignocellulosic materials such as wheat straw or even wood. 10-13 Understanding of the rheological properties of cellulose-solvent systems is important for the design of proper spin dopes for fiber spinning, as well as solvents for chemical processing. Aggregation


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in solution may considerably affect the rheology of the cellulose dopes, in particular if aggregation is irreversible. In this paper we investigate cellulose dissolved in 40 wt. % TBAH(aq). Static light scattering (SLS) and small angle X-ray scattering (SAXS) is combined with precise rheometry experiments, with the object to correlate possible cellulose aggregation with changes in the rheological parameters. Two cellulose molecular weights are compared; microcrystalline cellulose (MCC), with a degree of polymerization of ca. 260, and a dissolving pulp with an approximately ten times higher molecular weight.

MATERIALS AND METHODS Materials. Microcrystalline cellulose (MCC) was obtained from Sigma (Avicel PH-101, average particles size of 50 µm). Tetrabutylammonium hydroxide (TBAH) of chromatographic grade was acquired from Sigma Aldrich as a 40 wt % solution in water. The cellulose pulp (“Purple”) was kindly offered by Södra and is produced at Södra Cell Mörrum. This pulp is designed for viscose production and most of the raw material is derived from birch.

Sample preparation. The cellulose dissolution was obtained following the known procedures in literature. 7 Essentially, different samples of MCC and pulp were prepared by weighing the proper amounts of TBAH and cellulose. After the addition of cellulose, the vial was vigorously stirred. The dissolution time depended on cellulose concentration; in average from approximately 3-5 minutes (0.5 – 2 wt%) to 1-2 hours (7-10 wt%). For the higher pulp concentration moderate heating was used to increase the kinetics of dissolution.


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Rheometry. The rheological measurements were carried out on an Anton Paar Physica MCR 301 instrument equipped with a cone-plate geometry (50 mm, 2º) and on a HAAKE MARS III rheometer (Thermo Fisher Scientific, Germany) also set with a cone-plate geometry (35 mm, 1º). In both cases Peltier units were used to control the temperature which was fixed at 30.0 ± 0.1 ºC. To minimize evaporation, the sample-air interface was covered with a low viscosity inert silicon oil and the measuring geometry was surrounded by an appropriate solvent trap. Linear and non linear shear experiments were performed; the flow curves were registered from 0.01 to 100 s-1 for the MCC samples and from 0.1 to 100 s-1 for the pulp samples while dynamic tests within the viscoelastic regime were performed to access the complex viscosity. Unless specified, the error bars of the experimental points are of the size of symbols.

Static light scattering. The setup used for the static light scattering measurements is an ALV/DLS/SLS-5022F, CGF-8F-based compact goniometer system from ALV-GmbH, Langen, Germany. The light source is a 22 mW He-Ne laser (632.8 nm) and the laser intensity is varied using a software-controlled attenuator. A perfect vertical polarization is achieved using a Glan laser polarizer prism with a polarization ratio better than 105 in front of the high-temperature cell housing. The cylindrical clean scattering cells of borosilicate glass are immersed in a thermostated vat filled with a refractive index matched liquid (cis-decahydronaphtalene). In our experiments, samples in the concentration range 0.05 – 0.1 g/cm3 were analyzed, and were previously filtered with pores of 200 nm to remove dust. The measurements were performed at scattering angles ranging from 60° to 140° with steps of 2° and the temperature was kept at 30.0 ± 0.1 °C by a Julabo heating circulator. The background subtracted scattering intensity I(q) was converted to absolute scale according to 14


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=

∆

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(1)

where n is the refractive index of the solution, and Iref(q), nref and Rref are the scattered intensity, refractive index and Rayleigh ratio, respectively, of the reference, toluene. q is the scattering vector magnitude, defined as

=

sin

(2)

with θ being the scattering angle and λ0 the laser wavelength.

Small-angle X-ray scattering. Small-angle X-ray measurements were performed on the Ganesha SAXSlab instrument (JJ X-ray, Skovlunde, Denmark). The instrument is equipped with a 2D 300k Pilatus detector from Dectris (Dectris Ltd., Baden, Switzerland) and a Genix 3D X-ray Source (Xenocs SA, Sassenage, France). The X-ray wavelength λ is 1.54 Å. All the samples were measured within one week after preparation, and the scattering curves were recorded at 30.0 ± 0.1 °C within the range of 0.004 < q < 0.7 Å−1, where q is still given by eq. 2, with n = 1. The data was brought to absolute scale using water as a primary standard.

RESULTS AND DISCUSSION Cellulose is molecularly dissolved at lower concentrations. Static light scattering (SLS) experiments were performed on dilute solutions of pulp, in the concentration range of 0.005-0.01 g/cm3, and compared to recent data obtained from MCC in the same


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solvent. 15 Figure 1a shows the scattering intensity values I(q) as a function of the scattering vector q for all the concentrations. For qRg < 1, i.e. in the Guinier regime, the scattering intensity can be written as

= 0exp −

$ $ "#

%

(3)

where I(0) is the scattering intensity at q = 0 and Rg the radius of gyration. Fitting eq. (3) to the data in Figure 1a (solid lines) we obtain Rg = 22 ± 2 nm which strongly suggests that cellulose is molecularly dissolved. Note that true molecular solutions are rarely observed in most of the known solvents. Binary mixtures of common polar organic solvents with ionic liquids are among the few known systems reported. 16 At lower concentrations I(0) can be written as

+

0 = &'(' ) *, + 20 )1

2+

(4)

-

where Mw is the weight averaged molecular mass, c is the cellulose concentration, in mass per volume, and B2 is the second virial coefficient. KSLS is the optical constant given by

&'(' =

34 $ 1 35

$ $ *

6 78

(5)

where n0 = 1.4063 is the refractive index of the solvent, dn/dc = 0.108 cm3/g the refractive index increment and NA in Avogadro’s number. Figure 1b shows the plots of KSLS c/I(0) as a function of cellulose concentration for both pulp and MCC. According 7 ACS Paragon Plus Environment

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to eq. (4), KSLS c/I(0) scales linearly with concentration, with the intercept being 1/Mw and the slope 2B2. A linear fitting allows an estimation of the second virial coefficient B2 and Mw. The calculated value of B2 for pulp is 1.2 x10-4 cm3/g2, slightly positive, indicating slight repulsive cellulose-cellulose interactions, as for MCC. 15 However, the value is smaller than the one of MCC (6.7 x10-4 cm3/g2), and this is consistent with the pulp having a molecular weight approximately 10 times higher than MCC, as obtained from SEC-MALS on both materials. 17 For excluded volume interactions, B2 can be written as 18

0 ≈ 4; %/ =>

$ 〉A/$ 〈"# $ ,-

B

(6)

where Ψ is the interpenetration factor, that approaches 0.24 for good solvents and is equal to 0 for θ-solvents. 19 From eq. 6, Ψ was estimated to be 0.01 for pulp cellulose and 0.02 for MCC, indicating that 40 wt.% TBAH(aq) is close to a θ-solvent for cellulose. For the pulp, the value of Mw estimated from the fit is 850 kg/mol, while for MCC the corresponding value is 93 kg/mol. 15 The light scattering led to an overestimation of the molecular weight also for the MCC system (60% higher). 15


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-3

10

3

0.01 g/cm 3 0.008 g/cm 3 0.005 g/cm 3 0.003 g/cm 3 0.002 g/cm 3 0.001 g/cm 3 0.0005 g/cm

-1

I(q) [cm ]

a)

-4

10

-5

10

0.014

0.016

0.018

0.02

0.022 0.024 0.026

-1

q [nm ] 3.0

2.0

-5

-1

MCC Pulp

b)

2.5

KSLSc/Ι(0) [10 g ]

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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1.5 1.0 0.5 0.0 0

2

4

6 -3

8

10

-3

c [10 g cm ]

Figure 1. Scattering intensity as a function of q (a) for 0.005-0.10 g/cm3 pulp solutions in 40 wt.% TBAH(aq) at 30 °C together with their respective fitting curves. KSLS c/I(0) as a function of cellulose concentration (b), compared with data for MCC. 15

Cellulose aggregation at higher concentrations. Small-angle X-ray scattering (SAXS) curves have been collected for both MCC 15 and pulp in the concentration range of 0.010.10 and 0.005-0.10 g/cm3, respectively. The concentration scaled curves are reported for both systems in Figure 2.


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Figure 2. SAXS patterns for 0.01-0.10 g/cm3 MCC (a) and 0.005-0.10 g/cm3 pulp (b) solutions in 40 wt.% TBAH(aq) at 30 °C. The curves are in absolute scale, normalized with cellulose concentrations. The data for MCC has been adapted from Behrens et al.15 except the lowest concentration 0.01 g/cm3 which was measured in this study.

The concentration trend for MCC reveals no structural differences from 0.01 to 0.04 g/cm3, i.e. the curves lay on top of each other. Above 0.04 g/cm3, however, a more and more pronounced upturn at low q-values is observed with the increasing of the cellulose concentration. This upturn indicates the presence of larger aggregates in solution. Profiles at high q-values are all comparable, and show a typical peak centred at 0.35 Å-1. 15

The presence of cellulose aggregates in solution with Rg ca. 150-250 nm has been

observed in other systems such as in the DMAc/LiCl 20, 21, viscose 22, NaOH/urea 23,


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NMMO 24 or even in one of the most efficient ionic liquids, 1-ethyl-3methylimidazolium acetate (EMIMAc). 25-27 Aggregation has also been observed with cellulose derivatives in aqueous solution. 28, 29 In Figure 3a we have plotted together the SAXS patterns obtained from 0.01 g/cm3 MCC and pulp, respectively. The scattering patterns related to pulp solutions exhibit overall profiles similar to MCC. For low concentrations, the extrapolated intensity at q = 0 is approximately 10 times higher than the one of MCC, in agreement with the higher molecular weight. These low concentration scattering patterns have been modelled using the form factor Pchain(q) of a semi-flexible chain with excluded volume interactions 30, 31, combined with a core-shell cross-section, Pcs(q), using the decoupling approximation (eq.1), as previously done by Behrens et al. 15

C = CDEFG ∙ CDI

(7)

The overall P(q) described in such a way is dependent on the contour length L, the persistence length λp, and the inner and outer cross section radii of the core and shell of the rod. The core-shell structure has been introduced to describe the feature at high qvalues, and is consistent with the presence of a first solvation shell around the cellulose chain, with a different composition and therefore a different scattering length density (SLD) with respect to the bulk. The radial SLD profile depicts a 5 Å thick shell around a core of ca. 5 Å in radius, with a scattering length density of ca. 90% of the bulk one. The TBA+ and water SLD values suggest the presence of a solvation shell richer in TBA+ cation compared to the bulk solvent which highlights the important role of TBA+ on cellulose dissolution. 32 In a recent NMR self-diffusion study it was concluded that ca. 1.2 TBA+ ions per glucose unit are reversibly bound to the cellulose. 33 This


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particular association of TBA+ to cellulose (1.2 TBA+ ions per glucose unit), is likely of electrostatic origin. Glucose and other similar sugars have typically pKa values between 12 and 13 (glucose pKa=12.3) 34, and hence we expect cellulose to be partly deprotonated and negatively charged in the present solvent, 1.3 M (40 wt.%) of the strong base TBAH(aq). This means that TBA+ acts as counterion, and that it is this electrostatic attraction that gives rise to the observed core shell structure. This form factor model has been combined with a structure factor S(q) that considers random phase approximation 35, 36 as a description of repulsive interactions among the single polymer chains, as suggested by the light scattering results on the dilute solutions. This structure factor is given by

J = K1 + M ∙ CN

where M =

+

'O

2+

(8)

− 1 is the interaction parameter that typically increases with increasing

concentration. MCC Pulp

0

10

-1

10

-2

10

-3

-1

I(q) [cm ]

-1

I(q) [cm ]

10

10

2

10

1

10

0

10

-1

10

-2

10

-3

a) MCC Pulp

b) 10

-3

10

-2

-1

10

] 1 Å

[ q

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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Figure 3. SAXS patterns on absolute scale (symbols) together with model calculations for (a) 0.01 g/cm3 and (b) 0.07 g/cm3 MCC (circles) and pulp (squares) solutions in 40 wt.% TBAH(aq) at 30 °C.

Finally, we calculate the scattered intensity using

= )&'>P' QR CJ

(9)

where, KSAXS is the optical constant given by

&'>P' =

∆S $

(10)

T5$ 78

Here, ∆ρ = 4.0 x1010 cm-2 is the SLD difference between cellulose and solvent and dc = 1.5 g/cm3 is the cellulose density. The model parameters used in these calculations were for MCC: L = 1.81 x103 Å, λp = 20 Å and ν = 5. For pulp, the corresponding parameters were: L = 1.32 x104 Å, λp = 15 Å and ν = 1. The persistence length λp = 20 Å corresponds to ca. 4 glucose units. Figure 3b shows the SAXS curves from MCC and pulp, respectively, obtained at 0.07 g/cm3, where the increased scattering at lower q-values suggest the presence of aggregates and hence effectively attractive interactions between the cellulose chains. Here in the semi-dilute regime, it is no longer possible to distinguish individual polymer chains and the intensity cannot be factorized into a product of the (effective) form factor and structure factor. 36 Instead, we focus on the intense scattering at lower q-values and assume that in this region the scattering is dominated by the aggregates. The aggregate


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scattering was modelled using the corrected Beacuage model describing fractal clusters, where the normalized form factor can be written as 37

CD = VWXY− Z,D /3] +

^

3

%T

KV_`YZ,D /√6]N

(11)

with

T

dT $

c = "3 eTeTT/ fg/2 #,5

(12)

Here, Rg,c is the radius of gyration of the fractal cluster and d is the fractal dimension. In Figure 3b, together with the experimental results, the calculated model scattering curves are shown. We have assumed that the cluster scattering, Ic(q), can be written as D =

)D &'>P' QR,D CD , where cc and Mw,c refer to the cellulose concentration in the cluster and the effective cluster molecular weight. cc = βc, where β ≤ 1 is the fraction of cellulose molecules participating in clusters. Mw,c can be written as Mw,c=naggMw, where nagg is the aggregation number, i.e. the average number of cellulose chains in the clusters. In the calculated curve for MCC we have used βnagg = 15, Rg,c = 280 Å and d = 1.9. For pulp, the corresponding values are βnagg = 22, Rg,c = 250 Å and d = 1.6.


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1

10

-1

I(0) [cm ]

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

10

Pulp - SLS MCC - SLS Pulp - SAXS MCC - SAXS

-1

10

-2

10

0.0

-2

2.0x10

4.0x10

-2

6.0x10

-2

-2

8.0x10

-1

1.0x10

-3

c [g cm ]

Figure 4. I(0) vs. concentration for 0.0025-0.10 g/cm3 MCC and 0.0005-0.10 g/cm3 pulp solutions in 40 wt.% TBAH(aq) at 30 °C. The grey lines represent I(0) calculated from the conformation space renormalization group theory for both systems. The empty symbols represent the I(0) data coming from SAXS analysis, while the filled symbols represent the SLS I(0) data, shifted using a multiplicative factor KSAXS/KSLS, where KSLS is given by eq. (5).

Using the models, we have estimated I(0)=cKSAXSMwS(0) for all the scattering curves for both MCC and pulp. The dependence of I(0) vs. concentration for both systems is reported in Figure 4. Here we also show the corresponding I(0) obtained from light scattering, where the data have shifted to X-ray contrast by multiplying with KSAXS/KSLS, where KSLS is given by eq. (5). In very dilute solutions, where no interactions between cellulose chains are assumed, S(0) = 1 and I(0) should increase linearly with the concentration of cellulose. For concentrations higher than the crossover concentration, the screening length ξ is related to the mesh-size of the entangled network, which decreases strongly with increasing concentration, leading to a decrease of the scattered


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intensity. For the high concentration analysis, an evaluation of I(0) has to involve virial coefficients of higher order. Instead, for polydisperse polymer chains, the dependence of S(0) as a function of concentration can be described using conformation space renormalization group theory 38, 39, which has its basis in the connection between the scattered intensity and the osmotic compressibility (∂Π/∂c)-1,

ij 2+

QR J0 = h * 1

(13)

iD

where R is the gas constant and T the absolute temperature. It calculates an explicit form of S(0) as a function of reduced concentration X ~ cB2Mw, where B2, the second virial coefficient, was estimated in the KSLSc/I(0) plot (Figure 1b).

+

J02+ = 1 + k *9m − 2 +

no +eP P

+ +

+

1 exp * pP + *1 − P $ 1 ln 1 + mr1

(14)

For polydisperse polymers, X is given by

m=

s$ D,-

yt wx y4 2 uv z

(15)

Figure 4 shows the calculated dependence of I(0) vs. concentration, and experimental data from both SLS and SAXS for the two systems. SLS data, obtained for low concentrations, are described with a good agreement by the calculated curves, while, for SAXS data, the calculated curves slightly overestimate the I(0) values below a concentration of 0.04 g/cm3, but the same trend is still described. This is not surprising if we take into account the q-ranges in which SLS and SAXS curves are collected. The 16 ACS Paragon Plus Environment

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SLS q-range (0.0014-0.0026 Å-1) is lower than the one of SAXS, and gives a more accurate estimate of the scattered intensity extrapolated at q = 0. In the high concentration regime, above 0.04 g/cm3, experimental data for both MCC and pulp do not follow the predicted trend by the conformation space renormalization group theory, confirming that the interactions among the cellulose chains are not repulsive, as assumed by the model (B2 > 0), but have become effectively attractive nature. An explanation for this transition from repulsive to effectively attractive interactions has recently been proposed by Behrens et al.15 in terms of the relative stability of the two crystal polymorphs, the native cellulose I, with parallel cellulose chains, and cellulose II where the chains are anti-parallel. Normally it is cellulose II that spontaneously forms when cellulose is regenerated, i.e. precipitated, from solution and it is considered to be the more stable form of the two. Being more stable means that chemical potential, and hence the solubility in the solvent is lower. We denote the solubilities of cellulose I and II by SI and SII, respectively, with SI > SII. For c < SII we have molecularly dissolved cellulose. However, when SII < c < SI the MCC being predominantly cellulose I will dissolve creating a supersaturation with respect to cellulose II that will aggregate and slowly precipitate. Finally, for c > SI, not all MCC will dissolve and it is thus SI which is normally reported as the cellulose solubility. From the scattering results we obtain the estimates SII(MCC) ≈ 0.04 g/cm3 15 and SII(pulp) ≈ 0.02 g/cm3.

Viscosity reflects the solution structure. In Figure 5 the steady state viscosity as a function of shear rate is represented for MCC and pulp dissolved in a 40 wt. % TBAH(aq.) solution. A Newtonian behavior is observed over more than 2 decades of shear rate for MCC concentrations up to 0.06 g/cm3. This suggests that the cellulose is dissolved and remains well mixed even under steady shear. Above 0.06 g/cm3, a shear


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thinning behavior appears at very low shear rates which suggests the presence of cellulose aggregates in good agreement with the SAXS data presented in Figure 2a. Such aggregates break down with increasing shear and/or reorient in the flow direction. On the other hand, for the pulp samples, the Newtonian behavior is observed until 0.01 g/cm3, while the shear thinning behavior appears at relatively high shear rates above ca. 0.005 g/cm3. The onset of the shear thinning behavior shifts to lower shear rates with increasing cellulose concentrations; the Newtonian plateau disappears and the shear viscosity shows an almost linear decrease with increasing shear rate when the pulp concentration exceeds ca. 0.02 g/cm3 which suggests the existence of a network of cellulose aggregates. Again, one note that the shear thinning behavior above at 0.005 g/cm3 follows the pronounced upturn at low q-values observed with the increasing cellulose concentration shown in Figure 2b.


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Figure 5. Flow curves for MCC (top) and pulp (bottom) in 40 wt % TBAH aqueous solution at 30 °C.

Small amplitude dynamic tests were also performed in order to provide information on the linear viscoelastic behavior of the cellulose dopes through the determination of the complex viscosity, η*. In Figure 6 the shear viscosity profiles (non-linear rheology) and the complex viscosity (linear rheology) are compared for some selected MCC and pulp concentrations. The empirical Cox-Merz relation 40 (which states that ||∗ ~| →O ≅

| ̇̇ →O ) is followed for the lower concentrations of cellulose where the shear and complex viscosity profiles match. For higher cellulose concentrations, a deviation from the shear and complex viscosities is striking. More specifically, the Cox-Merz rule is perfectly verified until ca. 0.06 g/cm3 for MCC and ca. 0.02 g/cm3 for the pulp. Since the empirical Cox−Merz rule provides a practical way to infer on the presence of any shear-sensitive microstructure in polymer solutions we believe the data evidences cellulose aggregation at high concentration in agreement with the SAXS and shear thinning behavior previously discussed.

shear & complex visc [Pa.s]

10

1

0.1

0.01 0.01

0.1 1 10 -1 shear rate [s ]

100

0.1

1 10 frequency [Hz]

100

0.1

1 10 frequency [Hz]

100

1000 shear & complex visc [Pa.s]

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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100

10

1

0.1 1

10 -1 shear rate [s ]

100

ACS Paragon Plus Environment

19

Biomacromolecules

Figure 6. Shear viscosity (left column, empty symbols) and complex viscosity (right column, full symbols) for MCC (top row) and pulp (bottom row) obtained at 30 °C. The selected MCC concentrations are 0.02, 0.04, 0.06 and 0.08 g/cm3.

The zero-shear viscosity, η0, was estimated from fitting the data in Figure 5 with the Carreau-Yasuda model 41 and in Figure 7 the relative viscosity, ηrel, (where ηrel = η0/ηsolvent) is plotted as a function of cellulose concentration. This plot allows the exclusion of the solvent contribution to the viscosity of the solutions.

η=

η0 − η∞

(

1 + (λ ⋅ω )

a

)

(1− n)

a

+ η∞

(16)

In the Carreau-Yasuda model, η0 is the zero-shear viscosity, η∞ is the infinite-shear-rate viscosity, λ is a time constant where 1/λ is the critical shear rate at which viscosity begins to decrease, (1-n) is the power law exponent and a is a dimensionless parameter that describes the width of transition between the zero-shear rate region and the powerlaw region.

1000

rel

100

η

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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10

c*Pulp ≈ 0.008 g cm-3 c*MCC ≈ 0.025 g cm-3

1 0.0001

0.001

-3

0.01

0.1

c [g cm ] 20 ACS Paragon Plus Environment

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Figure 7. The relative viscosity ηrel is plotted as a function of cellulose concentration for MMC (filled squares) and Purple pulp (filled circles) at 30 ºC. The lines represent the best power fittings for the dilute and concentrated regions being the critical concentration, c*, estimated from their crossover.

The critical concentration, c*, can be estimated from the crossover of the power fittings in Figure 7. A pronounced increase in the gradient is observed above c* with a power law scaling with an exponent of ca. 2.2 and 2.6 for pulp and MCC, respectively, in agreement with related systems. 42 Others have reported higher exponents but care must be taken when performing comparative analysis due to the differences in the raw materials and solvents. 43, 44 However, the general trend is well known for other systems such as synthetic polymer solutions and this is has been attributed to the transition from dilute solution conditions, where individual polymer molecules are present as isolated coils, to a more concentrated regime where the total hydrodynamic volume of the individual chains exceeds the volume of the solution. Therefore, c* marks the onset of significant coil overlap and interpenetration. This critical chain overlap concentration can also be regarded as the point where the concentration inside a single macromolecular chain equals the solution concentration, i.e. in the dilute solution limit:

c* ≈

N R

2 32

≈

1 [η ]

(17)

Where 1/2 is the root-mean-squared end-to-end distance of the linear polymer chain that has N monomers by the Fox-Flory relationship. 45 The intrinsic viscosity [η]


Biomacromolecules

or limiting viscosity number is a convenient index of the size (hydrodynamic volume) of isolated polymer coils. It is considered one of the most important indexes of dissolved polymer solutions not only because it is directly correlated to the size of the macromolecules but because it also reflects the thermodynamic quality of the solvent. It can be obtained by fitting the specific zero-shear viscosity data using Kulicke and Kniewske model 46: 2

η sp = c [η ] + K H ( c [η ]) + A ( c [η ])

n

(18)

where KH is the Huggins coefficient and A and n are fitting parameters. For a θ-solvent KH = 0.6 47, while reasonable values of n are between 1 and 6.8 46. The best fittings to equation 18 are displayed in Figure 8.

100

sp

/ mL/g

150

η

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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50

0

0

0.02

0.04

0.06

0.08

[cellulose] / g/ml Figure 8. Plot of the specific viscosity as a function of MCC (squares) and Pulp (circles) concentration calculated from the non-linear zero shear viscosity. The solid lines show the best fittings using equation 18.


Page 23 of 42

From the Kulicke and Kniewske model, we estimate for MCC [η]MCC = (46 ± 5) mL/g; c*MCC ≈ (0.022± 0.002) g/cm3, while for pulp [η]pulp = (210± 20) mL/g; c*pulp ≈ (0.005± 0.001) g/cm3. The c* values are in good agreement with the ones extracted from Figure 7. On the other hand the [η] for MCC and pulp obtained from equation 18 are in reasonable agreement with the intrinsic viscosity determined from the reduced viscosity as a function of cellulose concentration, c (the reduced viscosity, ηred, = (ηrel - 1)/c) plots (Figure 9). Note that the y-intercept, [η], is the intrinsic viscosity which was estimated to be 45 mL/g and 413 mL/g for MCC and pulp, respectively. 6

10

5

10 -1

η red [[mL g ]

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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4

10

3

10

2

10

1

10

0

0.02

0.04

0.06

0.08

0.1

-3

c [g cm ] Figure 9. The reduced viscosity as a function of cellulose concentration for MCC (squares) and Pulp (circles) at 30 ºC. The intrinsic viscosity [η] is estimated from the yintersect of the fittings.

The [η]pulp and [η]MCC are comparable with the values previous reported for similar cellulose samples dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate (EMIAc). 42


Biomacromolecules

In Figure 10 the effect of the cellulose hydrodynamic volume on the viscosity of the polymer solution is analyzed. According to the Flory-Huggins approach, the slope of ηrel versus c[η] should be 1. The best fit shows that the slope is ca. 1.7. Similar results were obtained for other polysaccharides, such as dextran, alginate and carboxymethylamylose, and are rationalized in terms of macromolecule aggregation in solution. 48 In addition to the physical entanglements of the overlapping coils, the data clearly suggests the interaction among cellulose chains. This aggregation is expected to increase with increasing concentration and molecular weight thus increasing the viscosity of the solutions. 6 5

η rel

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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4 3 2 1 0

0.5

1

1.5

2

2.5

[η]∗c [η]∗

Figure 10. Plot of the ηrel as a function of c[η] at 30 ºC. The black line represents the best linear fitting for both MCC (circles) and pulp (squares) data.

A rough estimation of the radius of gyration Rg using the Flory approach for flexible +

polymer chains can be obtained through the formula Z = * 1 . d

, /% ,

where M is

the cellulose molecular weight (M ≈ 57 kg/mol and 415 kg/mol for MCC and pulp, respectively, as determined by SEC-MALS) and Φ = 2.8 1023 mol is the Flory constant. 43, 49, 50

We obtain Rg(MCC) ≈ 8.6 nm and Rg(pulp) ≈ 34.6 nm are in good agreement with 24 ACS Paragon Plus Environment

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the values extracted from the SLS measurements (Rg(MCC) ≈ 10 nm15 and Rg(pulp) ≈ 22.2 nm). Cellulose aggregation can be further confirmed from consecutive up-and-down shear rate ramps as illustrated in Figure 11. As can be observed, no full relaxation is observed within the measurement time for both MCC and pulp at different concentrations (compare the full and empty squares). The dopes present a clear thixotropic behavior where the viscosity profiles while consecutively ramping up and down the shear rate do not match. Apart from the consecutive tests, we have performed an increasing shear rate ramp after a certain resting period from the last test. It can be observed that the initial state can be fully recovered after 15min for the lower cellulose concentrations (Figures 11b and 11d) while the hysteresis prevail with minimal recovery for the higher concentrations. The aggregates are destroyed when increasing the shear rate and do not re-form immediately after ramping down the shear rate. However, it seems that depending on the concentration, aggregates can re-form given an appropriate relaxation time. The kinetics of their reformation will be further addressed in a forthcoming work.


Biomacromolecules

100

10

b shear visc [Pa.s]

shear visc [Pa.s]

a 10

1

1

0.1 0.01

0.1 1 10 -1 shear rate [s ]

100

0.01

100

100

shear visc [Pa.s]

d

10

1

0.1 1 10 -1 shear rate [s ]

100

c shear visc [Pa.s]

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

Page 26 of 42

1


100

10

1 1


100

Figure 11. Flow curves performed on a) 0.09 g/cm3 and b) 0.07 g/cm3 MCC samples and c) 0.08 g/cm3 and 0.06 g/cm3 pulp samples. The full squares represent an increasing shear rate ramp direction (typically from 0.01 to 100 s-1 and 1 to 100 s-1 for MCC and pulp, respectively) while the empty squares represent the immediately after decreasing shear rate ramp. The grey circles were acquired while increasing shear rate after 15 min resting (same test as done for the full squares).

CONCLUSION We have performed an extensive study on the scattering and rheological properties of cellulose (MCC and pulp) dissolved in 40 wt% TBAH (aq.). This solvent dissolves cellulose molecularly at lower concentrations, with effectively repulsive cellulosecellulose interactions, while cellulose aggregation is observed at higher concentrations. 26 ACS Paragon Plus Environment

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Following Behrens et al.15 we understand this by considering the relative stability of the two crystalline polymorphs, the native cellulose I and cellulose II which normally forms upon recrystallization from solution. Cellulose II has a lower solubility compared to cellulose I. Hence, upon dissolving cellulose, the solution can become supersaturated with respect to cellulose II resulting in aggregation and slow precipitation of cellulose II. At lower concentrations, the shear viscosity is found to agree well with the complex viscosity determined from linear viscoelasticity measurements, and thus satisfying the empirical Cox - Merz rule. However, this rule is violated at higher cellulose concentrations where aggregates are formed in solution. An abrupt crossover from an essentially Newtonian to an overall shear thinning behavior is observed, indicating gel formation. Here, the percolated network can be destroyed by steady shear, but reforms again at rest.

ACKNOWLEDGMENTS This work was supported by Nils and Dorthi Troëdssons Foundation, The Swedish Research Council, the Swedish Research Council Formas, and the Portuguese Foundation for Science and Technology (FCT) through project PTDC/AGRTEC/4814/2014 and researcher grant IF/01005/2014.

AUTHOR INFORMATION Corresponding author Bruno F. Medronho; [email protected]

Notes The authors declare no competing financial interest.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

References 1. Klemm, D.; Heublein, B.; Fink, H. P.; Bohn, A., Cellulose: Fascinating biopolymer and sustainable raw material. Angew Chem Int Edit 2005, 44, (22), 3358-3393. 2. O'Sullivan, A. C., Cellulose: the structure slowly unravels. Cellulose 1997, 4, 173-208. 3. Singh, P.; Duarte, H.; Alves, L.; Antunes, F.; Le Moigne, N.; Dormanns, J.; Duchemin, B.; Staiger, M. P.; Medronho, M., "From Cellulose Dissolution and Regeneration to Added Value Applications - Synergism Between Molecular Understanding and Material Development, Cellulose - Fundamental Aspects and Current Trends", Dr. Matheus Poletto (Ed.), InTech, DOI: 10.5772/61402, 2015. 4. Medronho, B.; Lindman, B., Competing forces during cellulose dissolution: From solvents to mechanisms. Curr. Opin. Colloid Interface Sci. 2014, 19, 32-40. 5. Liebert, T., Cellulose Solvents: For Analysis, Shaping and Chemical Modification. In Acs Sym Ser, 2009; Vol. 1033, pp 3-54. 6. Heinze, T.; Koschella, A., Solvents Applied in the Field of Cellulose Chemistry - A Mini Review. Polímeros: Ciência e Tecnologia 2005, 15, 84-90. 7. Abe, M.; Fukaya, Y.; Ohno, H., Fast and facile dissolution of cellulose with tetrabutylphosphonium hydroxide containing 40 wt% water. Chem Commun 2012, 48, (12), 1808-1810. 8. Abe, M.; Kuroda, K.; Ohno, H., Maintenance-Free Cellulose Solvents Based on Onium Hydroxides. Acs Sustain Chem Eng 2015, 3, (8), 1771-1776. 9. Ema, T.; Komiyama, T.; Sunami, S.; Sakai, T., Synergistic effect of quaternary ammonium hydroxide and crown ether on the rapid and clear dissolution of cellulose at room temperature. Rsc Adv 2014, 4, (5), 2523-2525. 10. Zhong, C.; Wang, C. M.; Huang, F.; Jia, H. H.; Wei, P., Wheat straw cellulose dissolution and isolation by tetra-n-butylammonium hydroxide. Carbohyd Polym 2013, 94, (1), 38-45. 11. Abe, M.; Yamada, T.; Ohno, H., Dissolution of wet wood biomass without heating. Rsc Adv 2014, 4, (33), 17136-17140. 12. Hyvakko, U.; King, A. W. T.; Kilpelainen, I., Extraction of Wheat Straw with Aqueous Tetra-n-Butylphosphonium Hydroxide. Bioresources 2014, 9, (1), 1565-1577. 13. Abe, M.; Yamanaka, S.; Yamada, H.; Yamada, T.; Ohno, H., Almost complete dissolution of woody biomass with tetra-n-butylphosphonium hydroxide aqueous solution at 60 degrees C. Green Chem 2015, 17, (8), 4432-4438. 14. Neutrons, X-Rays and Light: Scattering Methods Applied to Soft Condensed Matter. Linder, P.; Zemb, Th. Ed, North Holland, Amsterdam, 2002. 15. Behrens, M. A.; Holdaway, J. A.; Nosrati, P.; Olsson, U., On the dissolution state of cellulose in aqueous tetrabutylammonium hydroxide solutions. Rsc Adv 2016, 6, (36), 3019930204. 16. Rein, D. M.; Khalfin, R.; Szekely, N.; Cohen, Y., True molecular solutions of natural cellulose in the binary ionic liquid-containing solvent mixtures. Carbohyd Polym 2014, 112, 125-133. 17. C. Löfgren, private communication, 2015. 18. Yamakawa, H., On the Theory of the 2nd Virial-Coefficient for Polymer-Chains. Macromolecules 1992, 25, (7), 1912-1916. 28 ACS Paragon Plus Environment

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19. Barrett, A. J., 2nd Osmotic Virial-Coefficient for Linear Excluded Volume Polymers in the Domb-Joyce Model. Macromolecules 1985, 18, (2), 196-200. 20. Terbojevich, M.; Cosani, A.; Conio, G.; Ciferri, A.; Bianchi, E., Mesophase Formation and Chain Rigidity in Cellulose and Derivatives .3. Aggregation of Cellulose in N,NDimethylacetamide Lithium-Chloride. Macromolecules 1985, 18, (4), 640-646. 21. Sjoholm, E.; Gustafsson, K.; Eriksson, B.; Brown, W.; Colmsjo, A., Aggregation of cellulose in lithium chloride/N,N-dimethylacetamide. Carbohyd Polym 2000, 41, (2), 153-161. 22. Fischer, K.; Schmidt, I.; Hintze, H., Investigations of Cellulose Xanthogenates in Respect to the Distribution of Its Substituents. Papier 1994, 48, (12), 769-774. 23. Chen, X. M.; Burger, C.; Wan, F.; Zhang, J.; Rong, L. X.; Hsiao, B. S.; Chu, B.; Cai, J.; Zhang, L., Structure study of cellulose fibers wet-spun from environmentally friendly NaOH/urea aqueous solutions. Biomacromolecules 2007, 8, (6), 1918-1926. 24. Morgenstern, B.; Roder, T., Investigations on structures in the system cellulose/Nmethylmorpholine-N-oxide monohydrate by means of light scattering measurements. Papier 1998, 52, (12), 713-717. 25. Cheng, G.; Varanasi, P.; Arora, R.; Stavila, V.; Simmons, B. A.; Kent, M. S.; Singh, S., Impact of Ionic Liquid Pretreatment Conditions on Cellulose Crystalline Structure Using 1-Ethyl3-methylimidazolium Acetate. J Phys Chem B 2012, 116, (33), 10049-10054. 26. Kyllonen, L.; Parviainen, A.; Deb, S.; Lawoko, M.; Gorlov, M.; Kilpelainen, I.; King, A. W. T., On the solubility of wood in non-derivatising ionic liquids. Green Chem 2013, 15, (9), 23742378. 27. Rinaldi, R., Instantaneous dissolution of cellulose in organic electrolyte solutions. Chem Commun 2011, 47, (1), 511-513. 28. Bodvik, R.; Karlson, L.; Edwards, K.; Eriksson, J.; Thormann, E.; Claesson, P. M., Aggregation of Modified Celluloses in Aqueous Solution: Transition from Methylcellulose to Hydroxypropylmethylcellulose Solution Properties Induced by a Low-Molecular-Weight Oxyethylene Additive. Langmuir 2012, 28, (38), 13562-13569. 29. Bodvik, R.; Dedinaite, A.; Karlson, L.; Bergstrom, M.; Baverback, P.; Pedersen, J. S.; Edwards, K.; Karlsson, G.; Varga, I.; Claesson, P. M., Aggregation and network formation of aqueous methylcellulose and hydroxypropylmethylcellulose solutions. Colloid Surface A 2010, 354, (1-3), 162-171. 30. Pedersen, J. S.; Schurtenberger, P., Scattering functions of semiflexible polymers with and without excluded volume effects. Macromolecules 1996, 29, (23), 7602-7612. 31. Chen, W. R.; Butler, P. D.; Magid, L. J., Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles. Langmuir 2006, 22, (15), 6539-6548. 32. Medronho, B.; Duarte, H.; Alves, L.; Antunes, F. E.; Romano, A.; Valente, A. J. M., The role of cyclodextrin-tetrabutylammonium complexation on the cellulose dissolution. Carbohyd Polym 2016, 140, 136-143. 33. Gentile, L.; Olsson, U., Cellulose-solvent interactions from self-diffusion NMR. Cellulose 2016, 23, (4), 2753-2758. 34. Applications of Ion Chromatography for Pharmaceutical and Biological Products, Bhattacharyya, Lokesh and Rohrer, Jeffrey S. (eds), John Wiley & Sons, Hoboken New Jersey, 2012. 35. Benoit, H.; Benmouna, M., Scattering from a Polymer-Solution at an Arbitrary Concentration. Polymer 1984, 25, (8), 1059-1067. 36. Higgins, J. S.; Benoit, H. C. Polymers and Neutron Scattering, Oxford University Press. Oxford, 1994. 37. Hammouda, B., Analysis of the Beaucage model. J Appl Crystallogr 2010, 43, 1474-1478. 38. Ohta, T.; Oono, Y., Conformation Space Renormalization Theory of Semi-Dilute Polymer-Solutions. Phys Lett A 1982, 89, (9), 460-464. 39. Schurtenberger, P.; Cavaco, C.; Tiberg, F.; Regev, O., Enormous concentration-induced growth of polymer-like micelles. Langmuir 1996, 12, (12), 2894-2899. 29 ACS Paragon Plus Environment

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40. Cox, W. P.; Merz, E. H., Correlation of Dynamic and Steady Flow Viscosities. J Polym Sci 1958, 28, (118), 619-622. 41. Dynamics of Polymeric Liquids, 2 Volume Set, 2nd Edition by R. Byron Bird, Charles F. Curtiss, Robert C. Armstrong, Ole Hassager, 1987. 42. Haward, S. J.; Sharma, V.; Butts, C. P.; McKinley, G. H.; Rahatekar, S. S., Shear and Extensional Rheology of Cellulose/Ionic Liquid Solutions. Biomacromolecules 2012, 13, (5), 1688-1699. 43. Gericke, M.; Schlufter, K.; Liebert, T.; Heinze, T.; Budtova, T., Rheological Properties of Cellulose/Ionic Liquid Solutions: From Dilute to Concentrated States. Biomacromolecules 2009, 10, (5), 1188-1194. 44. Sescousse, R.; Le, K. A.; Ries, M. E.; Budtova, T., Viscosity of Cellulose-ImidazoliumBased Ionic Liquid Solutions. J Phys Chem B 2010, 114, (21), 7222-7228. 45. Flory, P. J. Principles of Polymer Chemistry, 1953. 46. Kulicke, W. M.; Kniewske, R., The Shear Viscosity Dependence on Concentration, Molecular-Weight, and Shear Rate of Polystyrene Solutions. Rheol Acta 1984, 23, (1), 75-83. 47. Sakai, T., Huggins Constant K' for Flexible Chain Polymers. J Polym Sci A2 1968, 6, (8pa2), 1535-&. 48. Morris, E. R.; Cutler, A. N.; Ross-Murphy, S. B.; Rees, D. A.; Price, J., Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions. Carbohyd Polym 1981, 1, (1), 5-21. 49. Flory, P. J.; Fox, T. G., Treatment of Intrinsic Viscosities. J Am Chem Soc 1951, 73, (5), 1904-1908. 50. Flory, P. J.; Spurr, O. K.; Carpenter, D. K., Intrinsic Viscosities of Cellulose Derivatives. J Polym Sci 1958, 27, (115), 231-240.


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TOC graphic


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-3

10

3

0.01 g/cm 3 0.008 g/cm 3 0.005 g/cm 3 0.003 g/cm 3 0.002 g/cm 3 0.001 g/cm 3 0.0005 g/cm

-1

I(q) [cm ]

a)

-4

10

-5

10

0.014

0.016

0.018

0.02

0.022 0.024 0.026

-1

q [nm ] 3.0

2.0

-5

-1

MCC Pulp

b)

2.5

KSLSc/10 g ]

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

1.5 1.0 0.5 0.0

0

2


4

-3

6

-3

c [10 g cm ]

8

10

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M C C P u lp 0

1 0

-1

1 0

-2

1 0

-3

]

1 0

I(q ) [c m

-1

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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a ) 2

1 0

1

1 0

0

M C C P u lp

I(q ) [c m

-1

]

1 0

1 0

-1

1 0

-2

b ) 1 0

-3

1 0

-3

1 0

-2

q [Å ] -1


1 0

-1

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0

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Pulp - SLS MCC - SLS Pulp - SAXS MCC - SAXS

-1

10

-2

10

0.0

-2

2.0x10

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4.0x10

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6.0x10

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10

1

0.1

0.01 0.01

0.1 1 10 -1 shear rate [s ]

100

0.1

1 10 frequency [Hz]

100

0.1

1 10 frequency [Hz]

100

1000


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


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1

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100


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rel

100



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c*Pulp ≈ 0.008 g cm-3 c*MCC ≈ 0.025 g cm-3

1 0.0001

0.001

0.01

c [g cm-3]


0.1

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sp

 mL/g

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c [g cm ]


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rel

5 4



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

Biomacromolecules

3 2 1 0

0.5

1

1.5

c


2

2.5

Biomacromolecules

100

10

b shear visc [Pa.s]

shear visc [Pa.s]

a 10

1

1

0.1

0.01

0.1 1 10 -1 shear rate [s ]

100

100

0.01

100

shear visc [Pa.s]

d

10

1

0.1 1 10 -1 shear rate [s ]

100

c shear visc [Pa.s]

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

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1


100

10

1 1



100

On cellulose dissolution and aggregation in ... - ACS Publications

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