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Carbohydrate Polymers 159 (2017) 86–93

Contents lists available at ScienceDirect

Carbohydrate Polymers

journa l homepage: www.e lsev ier .com/ locate /carbpol

ormation of cellulose-carbene complex via depolymerization in ILs:ependence of IL types on kinetics, conformation and dispersity

ongjun Ahn a, Younghan Song b, Hyungsup Kim b,∗, Seung-Yeop Kwak a,∗

Department of Material Science and Engineering, Seoul National University, Seoul 08826, Republic of KoreaDepartment of Organic and Nano System Engineering, Konkuk University, Seoul 05029, Republic of Korea

r t i c l e i n f o

rticle history:eceived 27 September 2016eceived in revised form 6 December 2016ccepted 7 December 2016vailable online 8 December 2016

eywords:

a b s t r a c t

This study focused on the influence of anion type on the depolymerization and its effect on the molecularstate, dynamics and dispersity of cellulose. GPC and the van Gurp-Palmen plot showed that molar masswas more significantly decreased by 1-butyl-3-methylimidazolium chloride ([C4C1Im][Cl]) comparing to1-butyl-3-methylimidazolium acetate ([C4C1Im][OAc]). Acid-catalyzed hydrolysis of cellulose in IL wasproved using base titration which was monitored by conductivity and pH value. On the contrary to thedepolymerization case, [C4C1Im][OAc] solution needed more base to be neutralized than [C4C1Im][Cl]

elluloseonic liquidcid-catalyzed hydrolysisarbeneispersity

solution. The generated carbene was combined with reducing ends of cellulose, which was facilitatedin low molar mass consisting of a large number of reducing ends. The formation of cellulose-carbenesubstitution caused steric hindrance of cellulose chain, thus resulting in increased segmental frictionwith high molecular density. The cellulose particle combined with carbene can be dispersed stably inaqueous media.

© 2016 Published by Elsevier Ltd.

. Introduction

Recently, ionic liquids (ILs) have attracted a great amount ofttention from many areas due to its eco-friendliness and vari-ty of valuable properties. The properties of IL can be modifiedasily by changing the combination of anions and cations. The con-rollable properties and the stability are broadening the potentialpplication areas of IL such as organic syntheses, electrochem-stry, separations and nano-fabrications (Plechkova & Seddon,008). Since the first report on the dissolubility for celluloseSwatloski, Spear, Holbrey & Rogers, 2002), ILs have been widelynd intensively studied in the areas of cellulose industries such asegeneration fiber, nanofiber, bio-fuel, delignification and chemi-al modification of cellulose (Ahn, Hu et al., 2012; Ahn, Lee et al.,012; Hong, Ku, Ahn, Kim & Kim, 2013; Sun, Rahman, Qin, Maxim,odríguez & Rogers, 2009). With the eco-friendly feature of the IL,he outstanding dissolubility for cellulose would promise the sus-ainable cellulose processes. Especially, ILs of dialkylimidazolium

ations combined with chloride or acetate anion show remark-ble solubility of cellulose up to 20 wt% under mild conditionWang, Gurau & Rogers, 2012). For those processes, it is crit-

∗ Corresponding authors.E-mail addresses: iconclast67@hotmail.com, iconclast@konkuk.ac.kr (H. Kim),

ykwak@snu.ac.kr (S.-Y. Kwak).

ttp://dx.doi.org/10.1016/j.carbpol.2016.12.022144-8617/© 2016 Published by Elsevier Ltd.

ical to understand the dissolution mechanism which has beenintensively investigated. However, a more complete understand-ing of the interactions and the possibility of reactions occurringbetween cellulose and ILs requires further investigation. For exam-ple, the possibility of depolymerization of cellulose by some ILswas reported either theoretically or experimentally (Cremer et al.,2010; Du & Qian, 2011; Ebner, Schiehser, Potthast & Rosenau,2008; Monti, Di Virgilio & Venturi, 2008; Zhao, Brown, Holladay& Zhang, 2012). The molar mass decreased by depolymerizationchanges significantly the physical and chemical properties of cel-lulosic products. Especially, the physical properties such as the glasstransition temperature, the rheological behavior and the mechan-ical performance are primarily determined by the molar mass(Bogoslovov, Hogan & Roland, 2010; Ding et al., 2004; Santangelo &Roland, 1998). Although the depolymerization gives huge impactson the performance of cellulose products, there were a few studiedand reported. It is needed consequently for a fundamental studyon the kinetics of the depolymerization and the conformationalchange of cellulose depolymerized in ILs.

In a recent study on cellulose depolymerziation in ILs,Rodríguez, Gurau, Holbrey, and Rogers (2011) showed exper-imentally that cellulose was significantly depolymerized by

alkylmethylimidazolium-based IL during dissolution. However, thestudy did not reveal the cause or the mechanism for experimen-tal results. Du and Qian (2011) took attention on the formations

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f carbene and proton by ionic interaction between anion andation. In the study, the free energy of carbene formation fromifferent types of ILs was calculated using quantum mechanicalalculation. They claimed that the resulted proton and the car-ene intermediate can hydrolyze the cellulose during dissolution.imilar reaction has been experimentally observed by Ebner et al.2008) using alkylimidazolium-based ILs. The theoretical approachas re-examined experimentally by Gazit and Katz (2012). Using

arious types of ILs and cellulose sources, they demonstrated theresence of free proton in cellulose/IL solution and claimed thatcid-catalyzed hydrolysis was unavoidable.

We purposefully focused cellulose-carbene reaction duringL-catalyzed depolymerization in typical condition for celluloseissolution and processing. The conformational change of the cel-

ulose by the reaction was studied using reheological approach.ase titration and FT-IR were applied to reveal the presence of car-ene and the difference in depolymerization behaviors of two ILs;-butyl-3-methylimidazolium chloride ([C4C1Im][Cl]) and 1-butyl--methylimidazolium acetate ([C4C1Im][OAc]). The study foundhat the types of anion in ILs gave significant effect on the aciditynd the depolymerization kinetics of the cellulose. We also inter-reted the relaxation behavior and the mobility in terms of theolar mass and the chemical structure of the cellulose. This studyill be beneficial for fabrication and modification of cellulose using

L design.

. Experimental

.1. Materials and solution preparation

Cellulose for the study was kindly provided as powder formrom Hyosung Co. (Republic of Korea). The cellulose sample has aumber average molar mass of 229 kg mol−1 and a weight aver-ge molar mass of 283 kg mol−1. 1-butyl-3-methylimidazoliumhloride ([C4C1Im][Cl]), 1-butyl-3-methylimidazolium acetate[C4C1Im][OAc]) and N-methylimidazole were purchased fromigma Aldrich. All chemicals were used as-received without furtherurification. Cellulose powder was washed with deionized waternd subsequently stored at 80 ◦C before dissolution.

For dissoluiton, the cellulose was placed in a sealed reactionessel, and IL was added to form a 7 wt% solution. Samples wereently stirred at 200 rpm, which was heated at 90 ◦C for desiredime (24–240 h).

.2. Characterization

Cellulose samples for gel permeation chromatography (GPC)ere prepared from the precipitation of the cellulose/IL solution

sing deionized water. The samples were repeatedly washed witheionized water and subsequently separated by centrifugation toemove IL and water. The obtained cellulose was dried for 48 h.

GPC was carried out using samples that were subjected to sol-ent exchange sequence to remove water and then was activated in-dimethylacetamide (DMAc) (McCormick, Callais & Hutchinson,985). The activated samples were dissolved in 9% LiCl/DMAc solu-ion and analyzed using Ultimate 3000 (ThermoFisher Scientific,ermany), equipped with a refractive index concentration detec-

or (RI), and two angle light scattering detectors at 90◦ and 45◦. mobile phase consisting of 0.5 wt% LiCl in DMAc was used atow rate of 1 mL min−1. The temperature was set at 40 ◦C. Lightcattering (LS) constants were calibrated using standard pullulan

22–343 kDa). The injection volume was 100 �L, and the run timeas 30 min. The refractive index increment constant, dn/dc, repre-

enting cellulose in 9 wt% LiCl/DMAc was 0.055 cm3 g−1, based onhe value reported in the literature (McCormick et al., 1985).

ymers 159 (2017) 86–93 87

The rheological characteristics of cellulose/IL solutions weredetermined using a stress-controlled rheometer, RS-1 (Ther-moFisher Scientific, Germany). A plate-to-plate geometry (35 mmdiameter) was used to measure the rheological properties at a con-stant temperature of 25 ◦C. Viscosity was measured over a shearrate range 0.01–100 S−1. Strain weep tests were performed at 1 Hzin order to determine the linear viscoelastic region for the solutions.The linear viscoelastic region was within 1% strain over angular fre-quency range of 1–50 rad s−1. The range of the oscillatory frequencywas from 10−2 to 102 Hz at each temperature.

The conductivity and the pH conductivity values were measuredat 298 K using Microprocessor pH meter (pH-290L, iSTEK instru-ments, Republic of Korea) equipped with a conductivity electrode.The calibration of the pH meter was carried out with two buffers(pH values of 4.00 and 7.00). Approximately 1 g of cellulose/IL solu-tion was diluted in 10 mL of deionized water for pH determinationand acetonitrile for conductivity measurement. Dilution of IL in ace-tonitrile is not expected to influence conductivity results due to itsvirtually negligible conductivity comparing with IL.

The samples were subjected to FT-IR spectroscopy (Nicolet iS5, ThermoFisher Scientific, Germany) with diamond attenuatedtotal reflectance attachment. Scanning was conducted from 4000to 650 cm−1 with 64 repetitious scans averaged for each spectrum.Resolution was 4 cm−1 and interval scanning was 2 cm−1.

The number of the reducing ends of cellulose was deter-mined using 2,2′-bicinchoninate (BCA) (McFeeters, 1980; Zhang& Lynd, 2005) methods: Solution A. 0.194 g of disodium 2,2′-bicinchoninate, 5.428 g of Na2CO3 and 2.420 g of NaHCO3 weredissolved in 100 mL of deionized water. Solution B. 0.125 g ofCuSO4·5H2O and 0.126 g of l-serine were dissolved in 100 mL ofdeionized water. BCA reagent solution was prepared by mixingsolutions of A and B in 1:1 of volume ratio. The prepared BACreagent was combined with cellulose aqueous dispersion in 1:1of volume ratio, and then, the mixture was incubated at 75 ◦C for0.5 h. The tube containing this mixture was cooled down to roomtemperature and the absorbance at 560 nm was recorded using anUV/Vis spectrophotometer (3220UV, Mecasys, Republic of Korea).

3. Results and discussion

Fig. 1(a) shows the cellulose molar mass distribution measuredby GPC for different dissolution times. The GPC curves of cellulosedissolved in [C4C1Im][Cl] shifted to lower molar masses with anincrease in dissolution time. The shift to lower molar mass withoutcurve splitting indicates that cellulose was homogeneously depoly-merized during dissolution. The depolymerization in [C4C1Im][Cl]is consequently accompanied by an approximate decrease in aver-age molar mass of one order magnitude. This trend is also observedfor cellulose dissolution using [C4C1Im][OAc]. The cellulose depoly-merization in ILs can be attributed to acid hydrolysis in IL. Accordingto the previous study (Cremer et al., 2010), anions are able to formionic bonding with proton in imidazolium cation. This reversiblereaction accelerates dissociation of imidazolium and proton, whichresults in formation of H3O+ in water. Due to its strong hydrophilic-ity, there remains the limiting amount of water needed for cellulosehydrolysis in IL, which is calculated below 40 ppm in case ofboth ILs (see Supporting information). On the other hand, thedepolymerization of cellulose less occurred in [C4C1Im][OAc] than[C4C1Im][Cl]. This trend in greater rate of decreasing molar massupon anion type of the IL used for cellulose dissolution is consistentwith Rodríguez et al. (2011), who remarkable differences of depoly-

merization was measured between 1-butyl-3-methylimidazoliumchloride and 1-ethyl-3-methylimidaozolium acetate.

In order to quantitatively compare the depolymerization kinet-ics, the plot of molar mass as a junction of time can be fitted

88 Y. Ahn et al. / Carbohydrate Polymers 159 (2017) 86–93

F dissolved in [C4C1Im][Cl], [C4C1Im][OAc] and ILs added with 5 wt% N-methylimidazolev

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ig. 1. (a) GPC curves and (b) change in reciprocal molar mass of 7 wt% cellulosearying different dissolution time.

y the zeroth order rate law (detail is expressed in Supportingnformation), as shown in Fig. 1(b), and the depolymerizationate was calculated. From these data, [C4C1Im][Cl] exhibits aigher depolymerization rate. The depolymerization rate mea-ured in [C4C1Im][Cl] was approximately 2 −fold faster than inC4C1Im][OAc] corresponding to the molar mass ratio betweenamples dissolved in [C4C1Im][Cl] and [C4C1Im][OAc] for 240 hs 1:2. This data demonstrates that the anion type of ILs has aignificant role in controlling the depolymerization rate. The capa-ility for acid formation between two ILs might be different asroposed from Du and Qian (Du & Qian, 2011). To test this dif-

erence, we added 5 wt% N-methylimidazole, which is alkalineeagent for synthesis of dialkylimidazolium ILs (Padmanabhan,im, Blanch & Prausnitz, 2011; Sun et al., 2011), to the cellulose/ILolutions sustained for different time. Fig. 1(a) shows almost nohange to the molar mass of cellulose in [C4C1Im][Cl] added with-methylimidazole. The prevention for the reduced molar massay be ascribed to N-methylimidazole acting as a proton accep-

or in acid-catalyzed hydrolysis reaction. However, the celluloseissolved in [C4C1Im][OAc] was still depolymerized despite theddition of N-methylimidazole. It means that more proton is pro-uced in [C4C1Im][OAc], indicating a lack of N-methylimidazole torevent the cellulose depolymerization. The discussion on the dif-

erent depolymerization behavior by the anion type is discussedater.

As shown in Fig. 1(a), the molar mass distribution shifted towardower molar mass as well as became broader as the dissolution timencreased. The changes of the distributions were confirmed usingan Gurp and Palmen plot (Trinkle & Friedrich, 2001) (i.e. vGP plot),s illustrated in Fig. 2. The loss tangent was plotted against theynamic modulus. As the dissolution time increased, the minimumalue of the phase angle increased and the curves became wider. Aseported previously (Trinkle & Friedrich, 2001), the shift of the ver-ical position and the broadness of the minimum phase angle wereorrelated to the changes of the molar mass and the polydispersity,espectively. The changes observed in the study demonstrate thathe dynamic data was well matched to the molar mass distributionf cellulose. Similarly to the molar mass distribution, the vGP plotor the cellulose/[C4C1Im][OAc] solution did not show significantifference by the addition of N-methylimidazole. However, the N-ethylimidazole addition to [C4C1Im][Cl] resulted in a similar plot

o the pristine solution even after 240 h dissolution. The similariscoelastic curve suggests that the addition of N-methylimidazolerevents the hydrolysis and the change of the chain conformation.

To understand the changes in the molecular conformation and

he rheological behavior, the viscosities of the solutions were mea-ured in dynamic mode and zero shear viscosities were obtainedrom the viscosity data (Fig. 3, calculation of zero shear viscosity

Fig. 2. van Gurp-Palmen plots of cellulose in (a) [C4C1Im][Cl] and (b) [C4C1Im][OAc]solutions with different dissolution times.

is expressed in Supporting Information). The viscosity is stronglyrelated to the molecular mobility, such as chain rigidity and chain tochain interaction. The change of viscosity can demonstrate the tran-sition of intermolecular interaction by depolymerization. When the

dissolution time was short, the solutions showed a conventionalpolymeric fluid behavior with Newtonian plateau and shear thin-ning. This behavior indicates that the celluloses in the solution havehigh degree of chain entanglements or more junctions between

Y. Ahn et al. / Carbohydrate Polymers 159 (2017) 86–93 89

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he molecules. As the dissolution time increased, the viscosity wasapidly decreased while the Newtonian plateau evidently becameroader and the shear-thinning shifted to higher shear rate. Theellulose/[C4C1Im][Cl] solution, in particular, exhibited significantiscosity drop for longer dissolution and finally showed Newtonianow when the dissolution was 240 h. The behavior was resulted

rom the reduction of molar mass, which in turns, significantlyecreased the intermolecular interaction.

To assess the rheological behavior more quantitatively, non-ewtonian flow behavior observed for cellulose/[C4C1Im][Cl]

olution was studied with the quantitative Cross model (Cross,979; Cross, 1969). Using the model, the complex viscosity cane represent as physical values: zero shear viscosity, characteristicelaxation time and power-law exponent (see Supporting Informa-ion). Viscoelastic characteristic parameters (zero shear viscositynd relaxation time) were obtained by fitting the dynamic sheariscosity curves as shown in Fig. 3. As expected, the zero sheariscosity was decreased linearly in log scale when the dissolutionime increased. As described in previous work (Tamai, Tatsumi

Matsumoto, 2004), the zero shear viscosity has proportionalelationship with molar mass. In turns, the decrease rate of the

olar mass in [C4C1Im][Cl] was higher than that in [C4C1Im][OAc].

hen N-methylimidazole was added to [C4C1Im][Cl], the viscosity

howed negligible decrease with dissolution time. As mentionedbove, it was resulted from the prevention of depolymerization by-methylimidazole. Similarly to the molar mass, the addition of

, (b) [C4C1Im][OAc] solutions and added with 5% N-methylimidazole for differentel.

N-methylimidazole did not give significant influence on the vis-cosities of cellulose/[C4C1Im][OAc] solutions. The two lines fromthe N-methylimidazole added solution and the N-methylimidazolefree solution were closely located for [C4C1Im][OAc] case, implyingthat there is still presence of acid to be enough to hydrolyze thecellulose.

Based on Figs. 1 and 2, the changing conformation of the cellu-lose can be observed by a plot of the zero shear viscosity againstmolar mass in Fig. 4. It is seen that the zero shear viscositiesof cellulose/[C4C1Im][Cl] solutions fall on a master curve, whereit follows the classical power low relationship, �0 ∝ Mw

�. Themolar mass dependence on viscosity (approximately �0 ∝ Mw

1.0

and �0 ∝ Mw3.5) can be found for cellulose dissolution in both ILs,

which appears in many polymeric system. The slope changes inthe viscosity plot can be assigned to the critical point for thetransition between unentangled and entangled chains. The depen-dence shows mutually intersect at the points corresponding tothe critical molar mass, 76,000 g mol−1 for cellulose/[C4C1][Cl]solution and 193,000 g mol−1 for cellulose/[C4C1Im][OAc] solu-tion. There are clearly traceable points of cellulose in eachILs corresponding to entanglement molar mass, 89,000 g mol−1

for cellulose/[C4C1Im][Cl] solution and 187,000 gnmol−1 for

cellulose/[C4C1Im][OAc] solution (calculation is detailed in Sup-porting Information). The different values of the critical molarmass suggest that the IL type acts significantly on conformationof cellulose chain. Compared to [C4C1Im][Cl], the higher solubil-

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ig. 4. Dependence of viscosity on molar mass for cellulose dissolved in [C4C1Im][Cl]nd [C4C1Im][OAc] solutions.

ty of [C4C1Im][OAc] (Wang et al., 2012) indicates strong repulsionoward a neighbor cellulose chain, suggesting weak intermolecularnteraction. Therefore, the molecules could be easily moved withonformation of freely motion, which makes requirement to higholar mass for entanglement.

We found that ILs have different kinetics of depolymerizationccording to the types of anion. The data implicates the capabilityf acid formation and its concentration are changed according tohe anion types of ILs. As studied previously (Gazit & Katz, 2012),he depolymerization was influenced mainly by the generated pro-ons in the solution. In order to study the proton generation duringissolution, every solution was titrated using N-methylimidazole.uring the titration, the conductivity and the pH value of the solu-

ions were monitored (Fig. 5). For the untitrated solutions, theonductivity became higher with increasing time because the longissolution time allowed more chance for IL to generate protons.he higher formation rate of proton in [C4C1Im][Cl] may act a majorole in extensively acidic depolymerization of cellulose. The trendn increasing conductivity upon the time is corresponded withH values, which the acidity of each solution was reached up topproximately 3 and 5 of pH value for cellulose/[C4C1Im][Cl] andC4C1Im][OAc] solutions, respectively.

The conductivity of the solutions consisting of [C4C1Im][Cl]howed rapid drops upon introducing N-methylimidazole. The con-uctivity drop continued until excess base was added to neutralizehe acid, at which the conductivity was slightly increased. Theeutralization point moved to higher base concentration as theissolution time was increased. It demonstrates the proton is con-inuously generated during the dissolution. It was also confirmedy the pH changes with titration (see the bottom Figure in Fig. 5(a)).

n similar way, the conductivity of the [C4C1Im][OAc] case wasecreased with lower rate, and then increased. Interestingly, theeutralization points were significantly shifted to higher base con-entration with the increase of the dissolution time relative toC4C1Im][Cl]. Although the conductivity of the [C4C1Im][OAc] caseas lower than the [C4C1Im][Cl], more N-methylimidazole was

onsumed to be neutralized. Comparing to the [C4C1Im][Cl] case,he pH change of the [C4C1Im][OAc] case was negligible duringitration after neutralization because more acidic products wereormed in [C4C1Im][OAc]. This result was quantified in terms ofhe volume of N-methylimidazole for neutralization point versus

he dissolution time as shown in Fig. 5(c). As shown in the fig-re, large difference between the two solutions was observed.he neutralization for cellulose/[C4C1Im][OAc] solution after 240 h

ymers 159 (2017) 86–93

required N-methylimidazole of 48,000 ppm while only 9000 ppmwas used for the cellulose/[C4C1Im][Cl] solution. (the calculationsare provided in Supporting information). Strong conjugate basesuch as acetate anion can easily accept a proton from imidazoliumcation and results in favorable formation of acid. On the contrast,weak conjugate base including chloride anion has less interactionbetween the anion and the proton in cation. In addition to the basic-ity, the size match of the cation and anions plays important role inthe formation of stable ion pair. Collin et al. (Collins, 1997, 2004,2006) reported that there was a greater chance to form an ion pairwhen the cation and anion have similar sizes. In that point of view,acetate anion has more chance to form acid comparing to chloride.

On the basis of the results, the formation of carbene interme-diate produced by cation-anion pair was studied using FT-IR ofthe precipitated cellulose as shown in Fig. 6(a) and (b). The unex-pected peak at 1562 cm−1 (C N stretching vibration attributedfrom imidazole-type cation) appeared even after extensive washfor removal of the IL from the precipitated cellulose, which is thesimilar with previous work. (Mahadeva & Kim, 2012; Ebner et al.,2008) The peak is considered as a result of carbene associatedwith the reducing end of cellulose. Theoretically, the dissociationof cation and anion in IL would result in carbene, highly reactivechemical intermediate. The carbene can further chemically com-bine to the reducing end of cellulose. For the precipitated cellulosefrom the [C4C1Im][Cl] solution, the peak was getting larger as thedissolution time increased (Fig. 6(a)). However, the peak size wasnot remarkably increased for the [C4C1Im][OAc] case. Based on theexplanation above, chloride-based IL cleaves more effectively theglucosidic bonds of cellulose chain, resulting in more reducing ends.The more reducing ends gave more chance for carbene to com-bine with cellulose. The combination of carbene with reducing endswas also confirmed based on the relation between the number ofthe remained reducing end and the degree of splitting (1/Mn). Inacid catalyzed hydrolysis of cellulose, the reducing end is linearlyincreased with the degree of splitting if the reducing ends are notchemically changed (Zhang & Lynd, 2005). Fig. 6(c), however, showsthat the number of reducing ends was leveled off. The result indi-cates strongly that the reducing ends were combined with carbeneat similar rate of its generation by depolymerization. It is an extraproof that the reducing ends generated by depolymerization wascombined with carbene.

In order to study the influence of the carbene combined withreducing ends on the molecular state of cellulose in the solution,the intermolecular frictional force was further evaluated based onthe rheological data obtained above. The intermolecular frictionalcoefficient can be expressed in terms of the zero shear viscos-ity and weight average molar mass, �0n̄−3.5 (Liao, Noda & Frank,2009) (calculation is detailed in Supporting Information), and dis-played in Fig. 7(a). The frictional coefficient for the cellulose in[C4C1Im][OAc] was lower than that for the cellulose in [C4C1Im][Cl]in the whole range of dissolution time. As mentioned above, it canbe attributed to more freedom of cellulose motion in higher sol-ubility of [C4C1Im][OAc]. As the dissolution time increased, thefriction was slightly decreased up to a certain point, and thensharply increased. The inflecting points were coincident to withthe critical molar masses observed in Fig. 4. On the contrary to theexpectation, the intermolecular friction increased although the cel-lulose became too short to be entangled each other. It was becausethe competition between the topological constraint and the car-bene combined with the reducing ends. In the entangled region,the effect of the carbene combined with cellulose chain may benegligible due to its low content and screening by strong entan-

depolymerization decreases the frictional coefficient until the crit-ical molar mass. However, in the unentangled region, the changeof end group becomes more critical to the chain-to-chain interac-

Y. Ahn et al. / Carbohydrate Polymers 159 (2017) 86–93 91

Fig. 5. Conductivity and pH value of cellulose/IL solutions dissolved for different dissolution time as a function of volume of N-methylimidazole. (a) cellulose/[C4C1Im][Cl]solution, (b) cellulose/[C4C1Im][OAc] solution and (c) their neutralization point.

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ig. 6. FT-IR spectra of cellulose precipitated from (a) [C4C1Im][Cl] and (b) [C4C1Imolar mass.

ion instead of intermolecular network. The carbene combined witheducing ends caused steric hindrance and resulted in severe inter-

olecular friction in the unentangled region. The similar behavioras already observed in copolymers having different side group

ontent (Liao, Noda & Frank, 2009).The carbene at the cellulose ends also influenced on the

olecule density in the solution as shown in Fig. 7(b). The num-er of molecules per unit volume, � (Bueche, 1954), was calculatedsing characteristic time constant which was obtained from theero shear viscosity (calculation is detailed in Supporting Informa-ion). The [C4C1Im][Cl] solution showed higher values of � in thehole range of dissolution time compared to the [C4C1Im][OAc]

olution. The dense state of cellulose in [C4C1Im][Cl] is related tohe stronger topological constraint than that of [C4C1Im][OAc]. As

he dissolution time increased, the molecule density of cellulosen both types of ILs increased. After the molar mass reached theritical value, the molecule density started to decrease. The trendxplains that the molecules are hard to pack each other due to their

], and (c) their number of reducing ends as function of reciprocal number average

own radius when the molar mass is larger than the critical value andthat small molecules are ready to pack in limited volume. However,the decrease of the molecule density was not dominant in spite ofsignificant decrease of molecular size. The carbene combined withthe reducing end causes steric and electrostatic hindrance, whichlead to interrupt closely packing of cellulose molecules. It is anotherproof that the carbene combined to the reducing ends prevents themolecular packing.

The results above suggest that carbene originating from cationcould react with the reducing ends of cellulose which continuouslygenerated by acidic hydrolysis of glucosidic bonding during disso-lution in IL. The association of carbene and the reducing ends wasstudied by observing the dispersity of cellulose in water. For thestudy, the celluloses dissolved for 240 h were precipitated in deion-

ized water. After drying and milling, the prepared cellulose particleswere dispersed in deionized water. After a week, the cellulose par-ticles from [C4C1Im][Cl] were still stably dispersed in water whilethe cellulose particles to from [C4C1Im][OAc] were aggregated and

92 Y. Ahn et al. / Carbohydrate Polymers 159 (2017) 86–93

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Fig. 8. (a) Photograph of aqueous dispersion of 0.1% cellulose particle. The disper-sions were stabilized at room temperature for 1 week after sonication. From left toright, the samples are cellulose precipitated from solutions dissolved in [C4C1Im][Cl]and [C4C1Im][OAc] for 240 h. (b) Schematic diagram illustrating mechanism for dis-

ig. 7. (a) Characteristic value for frictional coefficient and (b) number of moleculeser unit volume as function of dissolution time for cellulose dissolved in [C4C1Im][Cl]nd [C4C1Im][OAc].

eposited (Fig. 8(a)). According to previous work (Cremer et al.,010; Du & Qian, 2011), the cellulose combined with carbene hasositive charge at the end of the chain. Due to the positive charge onhe particle surface, the repulsive force acts between the particleshich leads more stable dispersion in water (Fig. 8(b)). As explained

bove, the carbene had more chance to combine with cellulosehen it was prepared from [C4C1Im][Cl] than from [C4C1Im][OAc].lthough many parameters, such as particle morphology, solvent

ype and pH value, have to be considered, it is still meaningful to anpproach towards a detailed understanding for preparation of cel-ulose particle without additional surface modification to enhancehe dispersity.

. Conclusion

The study revealed that the anion types of IL showed significantmpact on the degree and the kinetics of depolymerization of cel-

persion of cellulose particle reacted with carbene on surface. It demonstrates thatpositive charge derived from reacting carbene significantly influences on dispersityof precipitated cellulose particle in aqueous media.

lulose in IL. Comparing to acetate, chloride anion depolymerizedcellulose more effectively due to higher acidity which exhibits 2-fold higher depolymerization rate. In turns, the chain conformationwas greatly changed by chloride anion during depolymerization,confirmed by the rheological observation. The study also showedthe presence and the chemical reaction of carbene. The generatedcarbene from imidazolium cation in the solution was combinedwith the reducing ends of cellulose of which the number kept anequilibrium during the chain cleavage and carbine-reducing endsreaction. The combined carbene enabled cellulose particles to bestably dispersed in aqueous media. This suggests that cellulose inIL has the potential to be designed for variety of applications viagreen process.

Acknowledgements

This work was supported by the National Research Founda-tion of Korea (NRF) grant funded by the Korea government (MSIP)(2015R1A2A2A01007933).

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B

B

C

C

C

C

C

C

D

D

E

G

Y. Ahn et al. / Carbohydra

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.carbpol.2016.12.22.

eferences

hn, Y., Hu, D.-H., Hong, J. H., Lee, S. H., Kim, H. J., & Kim, H. (2012). Effect ofco-solvent on the spinnability and properties of electrospun cellulosenanofiber. Carbohydrate Polymers, 89(2), 340–345.

hn, Y., Lee, S. H., Kim, H. J., Yang, Y.-H., Hong, J. H., Kim, Y.-H., et al. (2012).Electrospinning of lignocellulosic biomass using ionic liquid. CarbohydratePolymers, 88(1), 395–398.

ogoslovov, R., Hogan, T., & Roland, C. (2010). Clarifying the molecular weightdependence of the segmental dynamics of polybutadiene. Macromolecules,43(6), 2904–2909.

ueche, F. (1954). Influence of rate of shear on the apparent viscosity of A—Dilutepolymer solutions, and B—Bulk polymers. The Journal of Chemical Physics, 22(9),1570–1576.

ollins, K. D. (1997). Charge density-dependent strength of hydration andbiological structure. Biophysical Journal, 72(1), 65.

ollins, K. D. (2004). Ions from the Hofmeister series and osmolytes: effects onproteins in solution and in the crystallization process. Methods, 34(3), 300–311.

ollins, K. D. (2006). Ion hydration: implications for cellular function,polyelectrolytes, and protein crystallization. Biophysical Chemistry, 119(3),271–281.

remer, T., Kolbeck, C., Lovelock, K. R., Paape, N., Wölfel, R., Schulz, P. S., et al.(2010). Towards a Molecular Understanding of Cation–AnionInteractionsöProbing the Electronic Structure of Imidazolium Ionic Liquids byNMR Spectroscopy, X—ray Photoelectron Spectroscopy and TheoreticalCalculations. Chemistry – A European Journal, 16(30), 9018–9033.

ross, M. M. (1969). Polymer rheology: influence of molecular weight andpolydispersity. Journal of Applied Polymer Science, 13(4), 765–774.

ross, M. (1979). Relation between viscoelasticity and shear-thinning behaviour inliquids. Rheologica Acta, 18(5), 609–614.

ing, Y., Novikov, V., Sokolov, A., Cailliaux, A., Dalle-Ferrier, C., Alba-Simionesco, C.,et al. (2004). Influence of molecular weight on fast dynamics and fragility ofpolymers. Macromolecules, 37(24), 9264–9272.

u, H., & Qian, X. (2011). The effects of acetate anion on cellulose dissolution andreaction in imidazolium ionic liquids. Carbohydrate Research, 346(13),1985–1990.

bner, G., Schiehser, S., Potthast, A., & Rosenau, T. (2008). Side reaction of cellulosewith common 1-alkyl-3-methylimidazolium-based ionic liquids. TetrahedronLetters, 49(51), 7322–7324.

azit, O. M., & Katz, A. (2012). Dialkylimidazolium ionic liquids hydrolyze celluloseunder mild conditions. ChemSusChem, 5(8), 1542–1548.

ymers 159 (2017) 86–93 93

Hong, J. H., Ku, M. K., Ahn, Y., Kim, H. J., & Kim, H. (2013). Air-gap spinning ofcellulose/ionic liquid solution and its characterization. Fibers and Polymers,14(12), 2015–2019.

Liao, Q., Noda, I., & Frank, C. W. (2009). Melt viscoelasticity of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers. Polymer, 50(25),6139–6148.

Mahadeva, S. K., & Kim, J. (2012). Influence of residual ionic liquid on the thermalstability and electromechanical behavior of cellulose regenerated from1-ethyl-3-methylimidazolium acetate. Fibers and Polymers, 13(3), 289–294.

McCormick, C. L., Callais, P. A., & Hutchinson, B. H., Jr. (1985). Solution studies ofcellulose in lithium chloride and N, N-dimethylacetamide. Macromolecules,18(12), 2394–2401.

McFeeters, R. (1980). A manual method for reducing sugar determinations with 2,2′-bicinchoninate reagent. Analytical Biochemistry, 103(2), 302–306.

Monti, A., Di Virgilio, N., & Venturi, G. (2008). Mineral composition and ash contentof six major energy crops. Biomass and Bioenergy, 32(3), 216–223.

Padmanabhan, S., Kim, M., Blanch, H. W., & Prausnitz, J. M. (2011). Solubility andrate of dissolution for Miscanthus in hydrophilic ionic liquids. Fluid PhaseEquilibria, 309(1), 89–96.

Plechkova, N. V., & Seddon, K. R. (2008). Applications of ionic liquids in thechemical industry. Chemical Society Reviews, 37(1), 123–150.

Rodríguez, H., Gurau, G., Holbrey, J. D., & Rogers, R. D. (2011). Reaction of elementalchalcogens with imidazolium acetates to yield imidazole-2-chalcogenones:direct evidence for ionic liquids as proto-carbenes. Chemical Communications(Camb), 47(11), 3222–3224.

Santangelo, P., & Roland, C. (1998). Molecular weight dependence of fragility inpolystyrene. Macromolecules, 31(14), 4581–4585.

Sun, N., Rahman, M., Qin, Y., Maxim, M. L., Rodríguez, H., & Rogers, R. D. (2009).Complete dissolution and partial delignification of wood in the ionic liquid1-ethyl-3-methylimidazolium acetate. Green Chemistry, 11(5), 646–655.

Sun, N., Rodríguez, H., Rahman, M., & Rogers, R. D. (2011). Where are ionic liquidstrategies most suited in the pursuit of chemicals and energy fromlignocellulosic biomass? Chemical Communications (Camb), 47(5), 1405–1421.

Swatloski, R. P., Spear, S. K., Holbrey, J. D., & Rogers, R. D. (2002). Dissolution ofcellose with ionic liquids. Journal of the American Chemical Society, 124(18),4974–4975.

Tamai, N., Tatsumi, D., & Matsumoto, T. (2004). Rheological properties andmolecular structure of tunicate cellulose in LiCl/1,3-dimethyl-2-imidazolidinone. Biomacromolecules, 5(2), 422–432.

Trinkle, S., & Friedrich, C. (2001). Van Gurp-Palmen-plot: a way to characterizepolydispersity of linear polymers. Rheologica Acta, 40(4), 322–328.

Wang, H., Gurau, G., & Rogers, R. D. (2012). Ionic liquid processing of cellulose.Chemical Society Reviews, 41(4), 1519–1537.

Zhang, Y.-H. P., & Lynd, L. R. (2005). Determination of the number-average degree

of polymerization of cellodextrins and cellulose with application to enzymatichydrolysis. Biomacromolecules, 6(3), 1510–1515.

Zhao, H., Brown, H. M., Holladay, J. E., & Zhang, Z. C. (2012). Prominent roles ofimpurities in ionic liquid for catalytic conversion of carbohydrates. Topics inCatalysis, 55(1-2), 33–37.