1460 | Phys. Chem. Chem. Phys., 2016, 18, 1460--1469 This journal is© the Owner Societies 2016

Cite this:Phys.Chem.Chem.Phys.,

2016, 18, 1460

Physical state of cellulose in BmimCl: dependenceof molar mass on viscoelasticity and sol-geltransition†

Yongjun Ahn,‡a Seung-Yeop Kwak,‡a Younghan Song‡b and Hyungsup Kim*b

In this work, we investigated the correlation between the molar mass and the rheological properties of

cellulose/1-butyl-3-methylimidazolium chloride (BmimCl) solutions, and provided the depolymerization

kinetics of cellulose in BmimCl. Gel permeation chromatography was used to track the change in molar

mass and kinetics as a function of the dissolution time. The molar mass of cellulose in BmimCl

decreased significantly as the dissolution time increased, following a zeroth order rate law. The decrease

of inter-chain friction induced by depolymerization resulted in a lower viscosity, shorter relaxation time,

and lower activation energy. The activation energies for flow were distinctly different above and below

the critical molar mass, which indicates that the relaxation mechanisms were not identical above and

below the critical molar mass. The transition behavior of liquid crystalline phase also changed at the

critical molar mass, which strongly demonstrated the effect of chain length on the formation of cholesteric

phase. The exponents of Mark–Houwink–Sakurada and the radius of gyration showed that cellulose in

BmimCl existed as a Gaussian chain in a theta solvent.

1. Introduction

Global environmental concerns have triggered researchers’ interestin replacing energy consuming processes and eco-hazardousmaterials with sustainable energy processes and eco-friendlymaterials.1–4 Cellulose is a promising eco-friendly material forreplacing synthetic polymers. Cellulose is the most abundantnatural organic material with B1.5 trillion tons on Earth at anygiven time.5,6 For thousands of years, natural cellulose (e.g.,wood, cotton, and hemp) has been used in textiles, furniture,and housing. The applications of cellulose have now extended tohigh-technology areas such as biomedical products, bio-energysources, and engineered structural reinforcement materials.7,8

Dissolution of cellulose is of fundamental importance for itsprocessing and chemical derivatization. However, solvent sys-tems for cellulose dissolution are quite limited because of itshigh crystallinity associated with extensive hydrogen bondingnetworks. In addition, most of the solvent systems9 for cellulose,such as N-methylmorpholine-N-oxide monohydrate, LiCl/N,N-dimethylacetamide, ammonium fluoride/dimethylsulfoxide,

and NaOH/water with urea, have serious drawbacks, includinghigh toxicity, volatility, oxidative side reactions, thermal instability,and high energy consumption during the dissolution process.As an alternative to these solvents, ionic liquids have receivedconsiderable attention for cellulose processing in a homogeneoussolution.10 Since Swatloski et al.11 reported cellulose dissolutionusing ionic liquids, many ionic liquids have been synthesized,such as 1-butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-methylimidazolium acetate (EmimOAc) and 1-allyl-3-methyl-imidazolium chloride (AmimCl). Ionic liquids have manyadvantages compared with traditional organic solvents, namely,excellent dissolving capability, negligible vapor pressure, easeof recycling, and high thermal stability.12–14

To design or improve a dissolution process, it is critical tounderstand the impact of ionic liquids on the properties ofcellulose and its solutions. Of particular importance is our under-standing of the rheological behavior of cellulose in ionic liquids.Rheological behaviors of polymer solutions play a significantrole in processes for fibers, films, and nonwoven materials. Moststudies on the rheological properties of cellulose-ionic liquidsolutions have focused on the influences of source, concen-tration and temperature on the rheological behavior of cellulosesolutions.15–17 Although linear viscoelastic properties were com-prehensively investigated for the cellulose/ionic liquids, there arefew studies on the influence of dissolution conditions on thechanges in molecular properties and structure, such as molarmass and distribution. Recently, it has been reported that

a Department of Materials Science and Engineering, Seoul National University,

Seoul, Republic of Koreab Department of Organic and Nano System Engineering, Konkuk University, Seoul,

Republic of Korea. E-mail: iconclast@konkuk.ac.kr

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cp06616f‡ Equally contributing co-first author.

Received 30th October 2015,Accepted 27th November 2015

DOI: 10.1039/c5cp06616f

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cellulose dissolved in some ionic liquids was fairly depolymerizedalong the dissolution time.18 It is expected that there is somedegree of molar mass reduction during the dissolution process,which strongly influences the rheological properties.

This study focused on the change in cellulose molar massduring dissolution, and the effect of this change on the rheologicalbehaviors of the solution and the physical state of cellulose inBmimCl. We observed a reduction of the molar mass duringdissolution of cellulose in BmimCl. The depolymerization mecha-nism of cellulose in ionic liquid BmimCl was investigated usingreducing ends analysis. According to the change in molecularstructure, the molar mass dependence on viscoelasticity was inves-tigated using the time–temperature superposition. The relaxationbehaviors of the solutions for different molar masses were inter-preted using the generalized Maxwell model. Based on the physicalstate, Mark–Houwink–Sakurada exponents and observation of thephase transition behavior were employed to assess the relationshipbetween the molar mass and the rheological properties, and theconformation of the dissolved cellulose. An understanding of therelationship between depolymerization and rheological propertieswould provide important information to help industries to opti-mally use cellulose-ionic liquid processing.

2. Experimental2.1. Materials and preparation of solutions

Cellulose from poplar was kindly provided by Hyosung Co.(Korea). Its molar mass and composition were determined by GPCand composition analysis (see Table S1, ESI†). Cellulose was driedovernight at 60 1C in a vacuum oven before use. BmimCl (499%,water content: 0.22 wt%) and N,N-dimethylacetamide (DMAc,499%) were purchased from BASF and Daejung Chemical &Metal Co. Cellulose and BmimCl were mixed in a sealed reactionvessel and stirred at 85 1C for 24–240 h. The cellulose concen-tration used in this study was fixed at 7 wt%.

2.2. Rheological properties

Rheological measurements were performed on a stress-controlledrheometer (RS-1, ThermoFisher Scientific, Germany) equipped witha 35 mm parallel plate geometry. Steady state viscosity measure-ments were carried out at 30 1C within the shear rate range10�3–102 s�1. Dynamic viscoelasticity was measured in the tem-perature range 30–110 1C. The range of the oscillatory frequencywas 10�2–102 Hz at each temperature. The data were reduced to thereference temperature (70 1C) data using the time–temperaturesuperposition principle.19 The open edge of the specimen sand-wiched between the plates was covered with a thin layer of siliconeoil (Shin-Etsu Chemical Co.) to prevent water uptake by the sample.

The intrinsic viscosity was determined from the viscositiesof dilute cellulose/BmimCl solutions calculated using eqn (1)and (2) proposed by Huggins and Kraemer.6,20–22

Zspc¼ ½Z� þ Kh½Z�2c (1)

ln Zrc¼ ½Z� � Kk½Z�2c (2)

where Kh is the Huggins coefficient, Kk is the Kraemer coeffi-cient, and Zr = Z0/Zs (Z0 = zero shear rate viscosity; Zs = solventviscosity) is the relative viscosity.

The plots of Zsp/c and ln Zr/c versus c gave two straight lineswith an identical intercept with the y-axis (c = 0). The interceptcorresponds to the intrinsic viscosity, [Z]. The graphicallydetermined intrinsic viscosities are summarized in Table 1.

2.3. Molar mass measurement

The prepared solution was poured into deionized water at roomtemperature, for cellulose precipitation. The precipitate was washedseveral times with deionized water until all residual BmimCl wasremoved. The precipitated cellulose samples were dried in avacuum oven for 1 day. For size-exclusion chromatography, theprecipitated cellulose was dissolved in 9% LiCl/DMAc using themethod reported by McCormick et al.23 This dissolution methodinvolved the following solvent exchange process. First, each cellu-lose sample was suspended in deionized water for 24 h. Afterfiltration, the sample was immersed in anhydrous methanol for30 min and filtered again. This process was repeated three times.After solvent exchange with methanol, the same process wasrepeated five times with DMAc. These solvent exchanges were carriedout at room temperature. The cellulose samples were dissolved in9 wt% LiCl/DMAc after being completely dried in a vacuum below60 1C. GPC was performed on a YL9100 (Young Lin Instruments,Republic of Korea), equipped with a refractive index concentrationdetector (RI), and two angle light scattering detectors at 901 and 451.A mobile phase consisting of 0.5 wt% LiCl in DMAc was used at aflow rate of 1 mL min�1. The temperature was set at 40 1C. Lightscattering (LS) constants were calibrated using standard poly-saccharides with molar masses of 34.4, 19.4, 10.7, 4.7, 3.3 and1.1 � 104 g mol�1 (see Fig. S1, ESI†). The injection volume was100 mL, and the run time was 30 min. The refractive index incrementconstant, dn/dc, representing cellulose in 9 wt% LiCl/DMAc was0.055 cm3 g�1, based on a value reported in the literature.23

2.4. Reducing ends measurement

In order to determine the degree of cellulose depolymerization,the number of cellulose reducing ends was measured via acolorimetric assay using 2,20-bicinchoninate (BCA). The assaysolutions were prepared by mixing two solutions. For assay

Table 1 Molar mass, number of reducing ends, and intrinsic viscosity ofcellulose in BmimCl for varying dissolution times

t (h)Mn

(103 g mol�1)Mw

(103 g mol�1)No. ofreducing ends

Intrinsicviscosity (cm3 g�1)

Raw 229 283 1 23024 195 252 1.3 22848 161 210 1.5 22572 123 165 1.7 21996 106 145 1.9 179120 89 128 2.1 171144 73 108 2.6 157168 61 92 2.9 152192 42 71 3.3 151216 35 63 3.7 121240 29 55 3.9 110

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solution A, Na2CO3 (5.4230 g), NaHCO3 (2.4198 g), and bicincho-ninic acid disodium salt hydrate (Sigma-Aldrich, 498%, 0.1944 g)were dissolved in milli-Q-purified water, and the volume of solutionwas adjusted to 100 mL. In assay solution B, CuSO4�5H2O (Sigma-Aldrich, 498%, 0.1242 g) and L-serine (Sigma-Aldrich, 0.1260 g)were dissolved in milli-Q-purified water, and the volume wasadjusted to 100 mL. BCA reagent solution was prepared bycombining solutions A and B in a 1 : 1 v/v ratio just prior to use.The BCA reagent solution was added to an aqueous slurry ofcellulose solution at 80 1C for 1 h. Assay tubes were quenchedin an ice bath and the absorbance at 560 nm was recordedusing an Optizen 3220UV UV/Vis spectrophotometer (Mecasys).

2.5. Optical microscope observation

The phase transition of cellulose/BmimCl solutions with differ-ent molar masses and concentrations was observed from opticalmicrographs taken using a BX41 Olympus polarized opticalmicroscope. 1-Methylimidazole (5 wt%) was added to preventfurther hydrolysis during dissolution.18 A 530 nm sensitive tintplate (U-TP530, Olympus) was used as a test plate compensator,which resulted in a magenta background in the micrographs.

3. Results3.1 Molecular characterization of cellulose dissolved inBmimCl for different dissolution times

Fig. 1 shows the calculated molar mass distribution of cellulosewith various dissolution times. The molar mass distributionwas fitted to a Zimm–Schultz (Z–S) distribution24 using eqn (3).The solid lines represent the fits to the Z–S distribution. Theparameters ‘‘a’’ and ‘‘b’’ in the Z–S distribution are related tothe weight- and number-average molar masses according toeqn (4) and (5).

w Pið Þ ¼abþ1

b!Mb

i exp �aMið Þ (3)

a ¼ 1

Mw �Mn(4)

b ¼ Mw

Mw �Mn(5)

The number-average (Mn) and weight-average (Mw) molarmasses were calculated using eqn (4) and (5), respectively, andsummarized in Table 1. The polydispersity index, Q, is given byeqn (6).

Q ¼Mw

Mn(6)

As shown in Fig. 1, the peak of the GPC curves shifted to lowermolar masses as the dissolution time increased. The shiftto lower molar masses without peak splitting suggests thatcellulose was homogeneously depolymerized during dissolu-tion. Based on a model of homogeneous depolymerization forcellulose, the plot of molar mass as a function of time can befitted by the zeroth order rate law proposed by Matthes,25 asshown in eqn (8).

dS

dt¼ k (7)

S = S0 + kt (8)

where, S, k, and t are degrees of splitting (1/Mw), depolymeriza-tion rate, and depolymerization time, respectively.

Fig. 2 shows the degrees of splitting and the polydispersityindex (PDI) of the regenerated cellulose with various dissolutiontimes. The degree of splitting linearly increased as the dissolu-tion time increased. From a regression analysis using the zeroth-order rate law, the depolymerization rate k was obtained as0.057 mmol h�1, indicating that the degree of splitting can benicely described by eqn (7). This value is consistent with theresults obtained under the same conditions reported by Gazitand Katz.18 According to eqn (7), the molar mass of cellulosecan be estimated based on the dissolution time at 85 1C. On theother hand, the PDI increased continuously from 1.2 to 1.9 asthe dissolution time increased. This trend is often observed fordepolymerization of cellulosic polymers.26,27 The data seem toindicate that cellulose randomly depolymerized during dissolu-tion, which led to the wide range of molecular sizes. This point

Fig. 1 Changes in molar mass distribution during the dissolution ofcellulose in BmimCl at 85 1C. The solid lines are fits to the Zimm–Schultzdistribution function.

Fig. 2 Changes in reciprocal molar mass, 1/Mw, and PDI during the dis-solution of cellulose in BmimCl. The slope of the solid line (blue) representsthe hydrolysis rate calculated by the zeroth order rate law.

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is discussed later in this paper based on the rheological propertiesof the cellulose solutions.

3.2 Steady shear viscosity of cellulose/BmimCl solutions

Steady shear viscosities were recorded over a range of differentshear rates. Fig. 3(a) shows the shear rate dependence of the steadyshear viscosity for 7 wt% cellulose/BmimCl solutions on the dis-solution times at 30 1C. Shear thinning behavior was observed forthe solutions dissolved for 24–168 h, which indicates that entangle-ment regions exist. However, the onset of shear-thinning shiftedto higher shear rates as the dissolution time increased. For thesolutions dissolved for longer times (192–240 h), all the solutionsshow Newtonian behavior without shear-thinning behaviors.

To quantitatively evaluate this shear thinning behavior,non-Newtonian flow (Fig. 3(a)) under steady shear conditionswas studied using the quantitative Cross model (eqn (9)), whichis known to describe well the dependence of viscosity on theshear rate.28

Zð _gÞ ¼ Z01þ ðt _gÞn (9)

where Z0, t, n, and _g represent the zero shear viscosity, char-acteristic relaxation time, power-law exponent and shear rate,respectively.

The values of Z0 and t for each solution are listed in Table 2.For uncrosslinked polymer solutions, the changes in shear-thinning behavior and viscosity can be ascribed to intermolecularinteractions. Shear flow contorts the polymer chain structureand changes conformation with certain relaxation times. Toconfirm the change in relaxation mechanism, the steady shearviscosity was normalized by the zero-shear viscosity, Z0, (seeFig. 3(b)). This normalization yielded good superposition of theviscosity curves, which implies universality in the viscoelasticproperties of cellulose/BmimCl solutions at different dissolu-tion times.

In addition, the characteristic relaxation time t is closelyrelated to the zero-shear viscosity. The definition of zero shearviscosity for entangled polymer solutions is shown in eqn (10).29

Z0 = GN(f)t(f,z0) (10)

where GN and f are the plateau modulus and volume fractionof the polymer, respectively.

As shown in Fig. 3(c), zero-shear viscosity had a logarithmicallinear relationship with relaxation time and the slope was foundto be 1 without regard to the dissolution times. These plotsindicate that the plateau modulus is independent of zero-shearviscosity and suggests that the inter-chain frictional coefficientwas the only change. In the calculations, the weight fraction ofcellulose was fixed as 0.07. This result can be explained in termsof the relationship between friction and the molecular size.Based on the relationship z = ZsR (z = frictional coefficient, Zs =solvent viscosity, R = molecular size), the frictional coefficientvaries linearly with the molecular size. The decrease of molecularsize resulted in a lower frictional force between neighboringchains, which led to the viscosity decrease.

Fig. 3 For cellulose in BmimCl at 85 1C at different dissolution times,dependence of (a) viscosity on the shear rate, (b) reduced shear viscosityZ/Z0 on the reduced shear rate _gZ0 and (c) zero shear viscosity on thecharacteristic relaxation time.

Table 2 Viscoelastic characteristics of cellulose in BmimCl for varyingdissolution times

t (h) Z0 (102 Pa s) t (10�2 s) �t (10�2 s) G0N (103 Pa) Me (103 g mol�1)

24 22.72 22.57 27.75 21.04 8.9848 11.01 10.40 16.74 21.01 9.0072 5.20 4.94 10.65 20.98 9.0296 2.93 2.69 6.46 21.23 8.91120 1.92 1.68 4.93 20.93 9.04144 0.87 0.78 3.82 20.88 9.06168 0.56 0.49 2.48 20.14 9.39192a 0.24 0.21 — — —216a 0.21 0.19 — — —240a 0.19 0.17 — — —

a The generalized Maxwell model cannot be adopted for these data.

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3.3. Dynamic frequency sweeps test of cellulose/BmimClsolutions at varying temperatures

Fig. 4 shows the master curves of 7 wt% cellulose/BmimCl solutionswith various dissolution times, obtained by the time–temperaturesuperposition principle. All of the curves shifted upward by a factorB except for the curves at 144 h. Over the range of dissolution times(24–168 h), a plateau region between G0 and G00 was observed whenthe relationships of G0 p o2 and G00 p o1 were held. Two cross-over points can be clearly seen, which is a typical behavior ofentangled polymer solutions. The first cross-over corresponds tothe macromolecular reputation, and the second crossover corre-sponds to entanglement relaxation. With the increasing dissolutiontime, the first cross-over point shifts to a higher frequency, indicat-ing the faster relaxation of the cellulose chains by low interaction(entanglement or hydrogen bonding) with its neighboring chains.For dissolution times above 192 h, the curve of G00 was larger thanthat of G0 over the entire o-region where the measurements werecarried out. This behavior is usually observed for dilute solutions,for which the dynamic viscoelastic behavior is similar to that of theterminal region in the Rouse model.30,31 Accordingly, the develop-ment of the network structure by entanglement was immature whencellulose was dissolved for longer times.

The dependence of the viscoelastic behavior on temperatureprovides useful information on the qualitative aspects of therelaxation process in polymer chains. The dynamic moduli ofcellulose/BmimCl solutions were measured at a series of tem-peratures, taken at every 20 1C interval from 30 to 110 1C. Thetime–temperature superposition principle was used to shiftall the rheological data to a reference temperature of 70 1C.

As shown in Fig. 5(a), the time–temperature superpositionprinciple applied well to the viscoelastic curves of the cellulose/BmimCl solutions. These results suggest that the relaxationmechanism does not change over the temperature range30–110 1C. The apparent activation energy for viscoelasticrelaxation, DEa, can be evaluated from the temperature depen-dence of the shift factor, aT. The shift factor is defined asZ(T)Trrr/(Tr)Tr, where r is the density and the subscript r refersto the properties of a reference state.19 Since the change indensity versus temperature is negligible, the definition of aT canbe simplified to Z(T)/(Tr). Near the glass transition temperature,Tg, the polymer flow is restricted to the free volume requiredto allow the necessary rotation of chain segments. Thus, theviscosity can be described by the Williams–Landel–Ferry (WLF)equation shown in eqn (11).

log aT ¼�C1 T � Trð ÞC2 þ T � Trð Þ (11)

where C1 and C2 denote experimental constants and Tr is thereference temperature.

Then, activation energy at the reference temperature can beestimated from eqn (12).

DEa T ¼ Trð Þ ¼ Rd ln aT

dð1=TÞ

����T¼Tr

¼ 2:303R C1=C2ð ÞTr2 (12)

Fig. 4 Dependence of dynamic storage modulus, G0 (open symbols), andloss modulus, G00 (filled symbols), on reduced angular frequency, oaT, forcellulose/BmimCl solutions dissolved for different times. These curves areshifted vertically by a factor B except for the solution dissolved for 144 h.The solid lines represent the curves reproduced by calculations using ageneralized Maxwell model.

Fig. 5 Relationship between (a) log aT and 1/T and (b) activation energy,DEa, and dissolution time, where aT and T represent the horizontal shiftfactor and absolute temperature, respectively. The solid lines show theresults of curve fitting using the WLF equation.

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The apparent activation energy of flow DEa is plotted against thedissolution time as shown in Fig. 5(b). As the dissolution timeincreased, the activation energy decreased and then leveled offwhen the time was longer than 196 h. Similar to the reason forthe viscosity change, the decrease can be explained in termsof the reduction of the molecular size and its effect on flow.However, the molecular size did not contribute to the changeof the activation energy further more when the size reached acertain value, i.e. when the dissolution time exceeded 196 h. Thediscussion on this topic will be continued in Section 4.3.

4. Discussion4.1. Cellulose depolymerization in BmimCl

Table 1 gives the characteristic values of the number averagemolar mass, Mn, weight average molar mass, Mw, polydispersity,number of reducing ends and intrinsic viscosity for 7 wt%cellulose/BmimCl solution with different dissolution times. Themolar mass and the intrinsic viscosity of the cellulose/ BmimClsolution rapidly decreased with the increase of dissolution timedue to depolymerization of cellulose in BmimCl. The cellulosedepolymerization can be attributed to acid hydrolysis in theionic liquid. According to the previous study,32 chloride anionsare able to form ionic bonding with protons in the imidazoliumcations. This reversible reaction accelerates dissociation ofimidazolium cations and protons, which results in the forma-tion of H3O+ in water. Hence, acid hydrolysis occurred in theionic liquid during dissolution. The number of reducing ends incellulose was measured using a 2,20-bicinchoninate (BCA) colori-metric assay in order to verify the type of hydrolysis. The numberof reducing ends significantly increased with the dissolutiontime. It is well known that the number of reducing endsincreases in acid-hydrolysis, while the number does not changein base or oxidative hydrolysis. In base and oxidative hydrolysis,cellulose was depolymerized via endwise chain cleavage, whichcan consume reducing ends and do not form new reducingends.33 The increase of reducing ends in the solution stronglysuggests that the cellulose is depolymerized by the acid hydro-lysis mechanism.

4.2. Relaxation behavior of cellulose chain

The measured dynamic modulus data provided specific informa-tion about cellulose/BmimCl solutions. The actual relaxation of apolymer chain has a large spectrum of relaxation times, whichcan be described using the generalized Maxwell model repre-sented in eqn (13) and (14).

G0ðoÞ ¼XNi¼1

Gi otið Þ2

1þ otið Þ2h i (13)

G00ðoÞ ¼XNi¼1

Gi otið Þ1þ otið Þ2h i (14)

where Gi and ti are the initial modulus and relaxation timecorresponding to the ith Maxwell element in the Maxwell model.

The calculated Gi and ti allow the evaluation of overallviscoelastic characteristics of cellulose/BmimCl solutions. Theseparameters were calculated using eqn (15)–(17).

G0N ¼

XGi (15)

Me ¼rRTG0

N

(16)

�t ¼P

ti2GiPtiGi

(17)

where G0N, Me, r, R, T, and �t are the plateau modulus, entangle-

ment molar mass, density, ideal gas constant, absolute tempera-ture and mean relaxation time, respectively.

The calculated values of Z0, t, �t, G0N, and Me for cellulose/

BmimCl solutions with various dissolution times are listed inTable 2. The difference between t and �t originating from theCross model became larger for longer dissolution times. Thisdifference, a result of the polydispersity of cellulose molecules,means that the viscoelastic properties are sensitive to the molarmass distribution.34 The large polydispersity resulted from randomhydrolysis of cellulose. This tendency was also observed in thevan Gurp and Palmen plot of the phase angle (Fig. 6).35 The vGPplot is a useful method to study the molar mass distribution,tacticity, and chemical composition (i.e. existence of copolymers).36

As the dissolution time increased, the minimum value of thephase angle increased, and the curves became wider. It was pre-viously found that shifting the vertical position and broadness ofthe minimum phase angle correlated with the change in molarmass and polydispersity, respectively. Similarly, the molar mass ofcellulose was reduced as its distribution increased during dissolu-tion, which corresponds perfectly with the measured GPC data.

The solid lines in Fig. 4 represent the curves calculated usingthe generalized Maxwell model with an appropriate combinationof Gi and ti values for the solutions. These curves, represented byeqn (13) and (14), are in good agreement with the experimentaldata. The well-fitted curves indicate the similarity and the uni-versality in the shape of their viscoelastic curves.37,38 The storagemodulus Gi shows a frequency-independent plateau over a

Fig. 6 Reduced van Gurp–Palmen plots of cellulose/BmimCl solutionshaving different dissolution times.

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certain frequency range, which is the plateau modulus G0N of the

solution. The estimated G0N was insensitive to the molar mass,

indicating that the entangled molar mass was almost constantas calculated by eqn (16) and shown in Table 2.

4.3 Relationship between rheological behavior and molarmass

Fig. 7 shows the double logarithmic plot of viscosity versus themolar mass. The point at which the slope changes in the plot ofviscosity versus molar mass can be assigned to the critical pointfor the transition between unentangled and entangled chains.The molar mass dependence on viscosity (Z p Mw

1.1 andZ p Mw

3.7) can be found for cellulose/BmimCl solutions, whichmutually intersect at the point corresponding to the criticalmolar mass, Mcr0 = 76 000 g mol�1. For the solutions consistingof molar masses below the critical point, the cellulose chainswere isolated and interacted with their neighbor chains onlyduring short times of encounter, thus viscosity increased slowlywith an increase of the molar mass. Above the critical point,cellulose chains became more entangled with each other andviscosity increased rapidly as the molar mass increased with aslope of 3.7. The relationship of viscosity and molar mass inpoly(cis-isoprene) was fitted by two straight lines with slopes of1.0 and 3.7,39 in good agreement with previous studies.

Based on the critical molar mass, the activation energyindependence of molar mass can be explained by the changein the relaxation mechanism. Although the chain conformationdid not dramatically change as the molar mass changed, therelaxation behaviors were different above and below the criticalmolar mass. Above the critical molar mass, the change in themolar mass greatly influenced the polymer–polymer interactionsdue to significant differences in the degree of entanglement. As aresult, it required more time to be relaxed and more energy toflow. The entanglement in the network above the critical molarmass became dense as the molar mass increased. Thus, thevalue of the activation energy strongly depended on the molarmass. On the other hand, the cellulose molecules below thecritical point had a finite size and less chance to interact with

their neighbor chains, which was not entangled sufficiently. Theunentangled molecules are easy to flow with freely translationaland rotational motion. It makes no significant contribution tothe relaxation process as indicated by the fact that the relaxationtimes were almost constant below the critical molar mass. Themolar mass independence of the activation energy was similarlyfound in a polysaccharide, curdlan, dissolved in 0.1 N NaOHaqueous solution.40

The Mark–Houwink–Sakurada (MHS) equation is efficient todescribe the molecular properties of a polymer, which correlatewith the intrinsic viscosity and the molar mass. Fig. 8 shows adouble logarithmic plot of the calculated intrinsic viscosityagainst the molar mass for cellulose in BmimCl at 30 1C. Thefigure contains the data published by other researchers.15,23,42

From the experimental data, the parameters of the MHS equation(eqn (18)) were obtained using eqn (19)

[Z] = KMaW (18)

[Z] = (0.64 � 0.3)M(0.47 � 0.06)/cm3 g�1 (19)

Using eqn (19), the parameters K and a for the cellulosesamples were obtained. The exponent a and coefficient K weredetermined to be 0.47 and 0.64, respectively, suggesting thatBmimCl may be a theta solvent for cellulose.15 Compared toother results (cellulose/DMAc/LiCl solution), a in the study wasmuch lower.23 It indicates that the intermolecular interactionswere significantly lowered by the ionic liquid used in the study.The inter-/intra-molecular hydrogen bonds should be brokenbecause the chloride anions released from the ionic liquid wereassociated with the backbone.

From the obtained intrinsic viscosity and molar mass, theradius of gyration Rg was estimated using the Flory approachfor flexible polymer chains as shown in eqn (20).41

Rg2 ¼ 1

6

½Z�FM

� �2=3(20)

where F is Flory constant 2.8 � 1023 mol�1. The radius ofgyration ranged from 11.3 to 25.1 nm for the molecules withmolar masses from 55 to 283 kg mol�1 respectively. Fig. 9 showsthe change in the radius of gyration along the molar mass.

Fig. 7 Dependence of viscosity on the molar mass for cellulose/BmimClsolutions at 30 1C. For each solution, the dependence can be approxi-mately represented by two straight lines with a slope of 1.1 for theunentangled region and a slope of 3.7 for the entangled region.

Fig. 8 Dependence of intrinsic viscosity on the molar mass for cellulose/BmimCl solutions at 30 1C.

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Fig. 9 also includes the data reported previously by otherresearchers.15,17,43 All the data showed a good linear relation-ship. This relationship can be expressed based on the assump-

tion that Rg ¼ bNn� ffiffiffi

6p

. An exponent of n was calculated to be0.54, which suggests that the conformation of the cellulosemolecules in BmimCl can be considered as a Gaussian chain ina theta solvent.

4.4 Phase transition behavior of cellulose/BmimCl solutions

Cellulose solutions are, in principle, able to form a cholestericphase in suitable solvents at certain concentrations. It is alsoknown that the formation of the liquid crystalline phasestrongly depends on molecular conformation and topologicalconstraints. To further clarify the influence of different physicalstates, above and below the critical molar mass, on the phasetransition, the birefringence observation and the rheologicalbehavior were compared. Fig. 10 shows the polarized opticalmicrographs of the cellulose/BmimCl solutions at 30 1C. For55 kg mol�1 molar mass solution, the changes in visible colorand phase transition are clearly distinguished. Below 10 wt%concentration, the solution is completely isotropic, and thenthe anisotropic texture gradually became more obvious uponfurther increasing the concentration. At 13 wt% concentration,fingerprint-like textures of the cholesteric phase appeared. As thecellulose concentration was further increased, apparent opticaltextures appeared. For 252 kg mol�1 molar mass solution, theoverall tendency for change from isotropic to anisotropic phasesis similar to high molar mass solutions, but the cholestericphase could not be observed distinguishingly. The reason for theabsence of a fingerprint-like pattern and appearance of a non-aligned cholesteric phase in the higher molar mass solutionsmay lie in their large domain size which accelerates aggregationof liquid crystalline phase.

The distinguishable behaviors of phase transition also can beobserved in rheological behaviors as shown in Fig. 11. For thesolutions with low molar masses, the viscosities of low molarmass solutions steeply increased up to a maximum value at aconcentration by 10 wt%, a slightly decreased by 14 wt%, andincreased again. This trend indicates the typical behavior oflyotropic liquid crystalline solutions.44 Compared to the low molar

mass solutions, zero shear viscosity exponentially increasedwith increasing concentration, suggesting a sol–gel transitionof cellulose/BmimCl solution.45 The different behavior of phasetransition can be explained in terms of the absence of an inter-connected structure by reduction below the critical molar mass.As shown in Fig. 12, each anisotropic domain forms a self-organization network chain by entanglement above the criticalmolar mass. The solution of united clusters makes a gel statewith a drastic increase of viscosity. Below the critical molarmass, the cellulose chains can also have opportunities to formliquid crystalline domains but do not have an inter-connection

Fig. 9 Dependence of the radius of gyration, Rg, on the molar mass forcellulose/BmimCl solutions at 30 1C.

Fig. 10 Polarized optical micrographs of cellulose/BmimCl solutions as afunction of concentration with different molar masses for cellulose/BmimClsolutions at 30 1C. The polarized optical micrographs in the left column arefor (a) isotropic solution, (c) 13 wt% and (e) 15 wt% solutions having a molarmass of 55 kg mol�1. The images on the right are for (b) 9 wt%, (d) 10 wt%and (f) 11 wt% solutions consisting of 252 kg mol�1. The insets are themicrographs taken without the sensitive tint plate insertion. The polarizedimage of 11 wt% solution with a high molar mass indicates that solubility ofBmimCl for cellulose having 252 kg mol�1 is limited above 11 wt%.

Fig. 11 Dependence of the zero-shear viscosity, Z0, on the molar massand concentration for cellulose/BmimCl solutions at 30 1C.

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of each domain due to the insufficient chain length for entangle-ment. As a result, the cellulose/BmimCl solution passes throughviscosity drop (cholesteric phase) between sol–gel transitions.

On the basis of this speculation, it would be possible to explainthe formation of the cholesteric phase of cellulose above andbelow the critical molar mass. The quantitative analysis of thecharacteristic time provides the physical state of cellulose chains.This time constant t can be approximated as follows:

t ¼ 12Z0p2nkBT

(21)

where n, kB and T are the number of molecules per unit volume,Boltzmann constant and the absolute temperature, respectively.Based on values of Z0 and t obtained above, we estimated then values, above and below the critical molar mass, to be 4.85 �10�6 mol cm�3 and 5.39 � 10�6 mol cm�3, respectively. Itimplies that cellulose chains were more packed closely and thestates changed from entangled to unentangled regions. The densestructure has more chance to interact each chain, which may helpto transit from the isotropic state to the cholesteric phase.

5. Conclusions

The dissolution time significantly changed the molar mass andrheological behavior of cellulose dissolved in 1-butyl-3-methyl-imidazolium chloride. The GPC and van Gurp–Palman plotconfirmed that the dissolution time increased as the molar massdecreased, which occurred at a rate of 0.057 mmol h�1. Depolymer-ization decreased the inter-molecular entanglement and friction,and, in turn, showed a decrease in the viscosity and terminalregion in the dynamic test, as well. The activation energy decreasedas cellulose was depolymerized. However, chain isolation by severedepolymerization resulted in leveled-off activation energy. In thesame way, the increasing rate of viscosity versus molar masschanged at the critical point of 76 000 g mol�1. The dependenceof molar mass on phase transition suggested a correlation betweenthe physical state and the cholesteric phase, which showeddifferent behaviors above and below the critical molar mass. Theexponents for Mark–Houwink–Sakurada and the result of theradius of gyration suggest that cellulose in BmimCl existed as aGaussian chain in a theta solvent.

Acknowledgements

This work was supported by the Industrial strategic technologydevelopment program funded by the Ministry of Trade, Industry &Energy (MI, Korea) (10041237). This work was supported by theNational Research Foundation of Korea (NRF) grant funded by theKorea government (MSIP) (2015R1A2A2A01007933).

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