Solution properties of celluloses from different biological origins inLiCl · DMAc

Takayoshi Matsumoto*, Daisuke Tatsumi, Nobutake Tamai and Tatsuya TakakiDivision of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, 606-8502Kyoto, Japan; *Author for correspondence (e-mail: matsutk@kais.kyoto-u.ac.jp)

Received 5 February 2001; accepted in revised form 29 May 2001

Key words: Bacterial cellulose, Cellulose, Liquid crystal, Rheological property, Viscosity

Abstract

Differences in the solution properties of cellulose in 8% LiCl · DMAc (dimethyl acetamide) were investigatedusing celluloses from different origins. The latter included plants (dissolving pulp (DP), cotton linters (CC), andkraft pulp), bacteria (Acetobacter xylinum, BC), and marine animals (tunicin from Halocynthia). The cellulosesfrom plants and bacteria formed LiCl · DMAc solutions that were isotropic and anisotropic, respectively; and theanimal cellulose was insoluble. The weight average molecular weights, Mw, of DP, CC and BC were found to be98.2 × 104, 170 × 104 and 192 × 104, respectively. The solution viscosities were proportional to c� (c; polymerconcentration) in the dilute and semi-dilute regions, where the exponent � was 1 for all samples in the diluteregion; in the semi-dilute region, it was 4 for the DP and CC solutions and 3 for the BC solution. Molecularweight differences were compensated by plotting the viscosity against cMw or c[�] (where [�] is the limitingviscosity number). The difference in viscosity behavior at elevated solution concentration indicates that the cel-lulose molecules from DP and CC behave as flexible polymer chains and those of BC as rod-like ones. Theseresults suggest that differences in molecular structure and properties exist between celluloses from differentsources, and that these differences relate to the mechanism or the type of the intermolecular interaction betweenthe celluloses of plants (DP and CC) and those of bacteria (BC).

Introduction

Native cellulose is one of the most widely and abun-dantly produced polysaccharides on earth. Cellulosebiosynthesis is common in the Monera, Mycota, Plantand Animal kingdoms. The greatest amount of cellu-lose is generated by plants as part of photosynthesis.The genus Acetobacter in the Monera produces a bac-terial cellulose, and the class of Ascidians living inthe sea produces a cellulose called tunicin. It has con-ventionally been accepted that these celluloses are allidentical in molecular structure and physical proper-ties, irrespective of their origins and different biosyn-thetic pathways.

Recently Atalla and VanderHart (1984) andVanderHart and Atalla (1984), using solid state 13CNMR, have reported that the crystalline phase of thenative cellulose is composed of a mixture of two dif-

ferent phases, I� and I�. They also reported that themixing ratio of I� and I� is different in different cel-luloses. For instance, the cellulose of valonia and ofbacteria is rich in I�, and the cellulose of cotton,ramie and tunicin is I�-rich. Sugiyama et al. (1991)and Wada et al. (1997) reported that the crystallinestructures of I� and I� are characterized by the tri-clinic unit cell with one molecular chain and by themonoclinic unit cell with two molecular chains, re-spectively. These discussions focus on differences incrystalline structure, and they do not concern differ-ences in molecular structure between various cellulo-ses. The recent article by Horii (2000) is very helpfulto review these relating matters.

In our present article, we discuss some differencein the solution properties of cellulose from differentorigins in 8% LiCl · DMAc (dimethyl acetamide).

1Cellulose 00: 1–8, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

XPS 57193 (CELL) – product element 356226 – ICPC/Grafikon

Experimental

The cellulose samples employed included softwooddissolving pulp (DP) produced by Nippon Paper In-dustry Co., Ltd.; cotton linters cellulose (CC); bacte-rial cellulose (BC) from Acetobacter xylinum(BPR2001 strain), which was obtained from Biopoly-mer Research Co. (Yoshinaga et al. 1997); and tuni-cin cellulose (TN) from Halocynthia, which was pro-vided by Prof. Horii of Kyoto University. DP and CC,BC, and TN originated from the Plant, Monera andAnimal kingdom, respectively. Cellulose from hard-wood kraft pulp was also used. These cellulose sam-ples were conditioned by a pretreatment involving theimmersion of the samples in water (1×), acetone (2×)and DMAc (2×). These pretreatments were carriedout at room temperature. The cellulose samples weredissolved in the solvent, 8% LiCl · DMAc, after be-ing completely dried in vacuum below 60 °C. Thecellulose samples did not dissolve in this solventwithout being pretreated. This is in agreement withobservations by Turbak (1984).

Molecular weights were measured by means of aHe-Ne laser (wavelength = 633 nm) light scatteringphotometer (LALLS, Chromatix KMX6 andDLS5000, Otsuka Electronics). Viscosity and vis-coelastic properties were measured using Ubbelohdecapillary viscometers and a cone-plate type rheometer(Rheosol G-2000, UBM Co. Ltd., Kyoto). Wide angleX-ray (wavelength = 0.154 nm) scattering was mea-sured by means of a Rigaku X-ray diffractometer(RINT2200V, Rigaku Co. Ltd., Tokyo).

Results and discussion

The results of the dissolution tests of the samples atroom temperature are shown in Figure 1. DP dis-solved easily and produced isotropic solutions belowabout 15 wt%. Insoluble fiber particles were observedat higher concentrations. Hardwood kraft pulp andcotton linters (CC) were found to have similar solu-bility characteristics in this solvent, and they pro-duced isotropic solutions. BC also dissolved and pro-duces isotropic solutions at relatively low concentra-tion (below ca. 6 wt%), and anisotropic solutions (i.e.liquid crystalline solutions) at higher concentration.The solutions show birefringence in the restful con-dition, and this turns into clear iridescence by sheardeformation under cross polarizers. On the otherhand, TN failed to dissolve in this solvent. Generally,

there are several causes for polymer insolubility. Ifthe polymers consist of the same chemical compo-nents, they could have a crosslinking structure, and/orhigh crystallinity, and/or an extremely high molecu-lar weight. However, the possibility that TN has achemically crosslinked structure is extremely small,because we confirmed the dissolution of the TN inanother solvent, 1,3-dimethyl-2-imidazolidinone. Ithas been reported that the difference in solubility ofcellulose is not attributed to the degree of crystallin-ity (Kamide et al. 1992). We also made sure that analmost amorphous cellulose, which was preparedfrom DP without the pretreatment by freeze crushingusing a ball mill, did not dissolve in 8% LiCl · DMAc.Figure 2a shows a photograph under cross polarizersof DP and BC solutions at the concentration of ca. 14wt%. The DP solution is obviously isotropic, and theBC solution shows birefringence. Figure 2b shows aphotomicrograph of the BC solution under cross po-larizers. It should be noted that there is a significantdifference in solubility condition of different cellulo-ses. This suggests a difference in molecular structureor in molecular properties of different celluloses.

In Figure 3, the relationship between the scatter-ing intensity and the scattering angle 2� of the WAXSdata is shown for DP and BC at cellulose concentra-tions of ca. 14 wt%. Both systems produce virtuallyidentical data, despite the fact that the DP and the BCsolutions are isotropic and anisotropic, respectively. Ifthere were some crystalline structure in these systems,some sharp scattering peaks should be revealed in thescattering region. The data merely reflect a broadpeak around 2� = 18°, and this indicates that the so-lutions have an insignificant periodic ordering. Thismeans that the birefringence of the BC system cannotbe attributed to residual or insoluble crystalline partsof the original fibers. For the BC system, there must

Figure 1. Solubility of various celluloses in 8% LiCl/DMAc.

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be a certain ordered solution structure, probably dueto liquid crystallinity, which is detected as some pe-riodicity in the region of lower scattering angles.

Figure 4 shows the relationship between Kc/R0

and the cellulose concentration, c, for DP, CC and BCsolutions at 25 °C. Here, K is the optical constant andR0 is the excess Rayleigh ratio of the solutions whichis extrapolated to zero scattering angle. The weightaverage molecular weight, Mw, can be obtained as areciprocal value of the intercept of the ordinate. Thespecific refractive index increment, dn/dc, of 0.0575ml/g, which is obtained from work by McCormick etal. (1985), is used to calculate Mw for the cellulosesolution in 8% LiCl · DMAc. The refractive index ofthe solvent is 1.463.

McCormick et al. (1985) obtained relatively smallmolecular weights ranging from 12 × 104 to 70 × 104

for various celluloses. Kuga et al. (1989) measuredthe molecular weight (or degree of polymerization) ofcellulose using various derivatives by gel permeationchromatography (GPC) calibrated with polystyrene

standards. Recently Burchard et al. (1994); Seger etal. (1996) and Saalwachter et al. (2000) measured theweight average molecular weights, Mw, of the vari-ous celluloses representing CC and BC in water con-taining metal complexes by light scattering, and theyobtained values of ca. 40 × 104 for the cellulose fromsulfite pulp; 20 × 104 � 170 × 104 for CC; and60 × 104 � 180 × 104 for BC. The polymer concentra-tion range in these experiments is between 2 and5 × 10−4 g/ml.

Table 1 shows the Mw, the second virial coefficientA2, and the limiting viscosity number [�] (measured

Figure 2. (a) Photograph under crossed polarizers of the DP (left)and the BC (right) solutions. The DP system is isotropic and theBC system is anisotropic, showing birefringence. (b) Photomicro-graph of the BC solution under crossed polarizers.

Figure 3. Relationship between scattering intensity and scatteringangle of the WAXS data for the DP and BC solutions.

Figure 4. Relationship between Kc/R0 and cellulose concentrationat 25 °C for the DP, CC and BC systems.

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by Ubbelohde capillary viscometry) for our three cel-lulose samples. The Mw of DP is relatively high (i.e.,98.2 × 104). The Mw’s of CC and BC were found tobe 170 × 104 and 192 × 104, respectively, and this isvery high, as expected. The most noticeable featureof this table is that, although the value of Mw of theBC system is higher than that of the CC system, [�]of the BC system is much lower than that of the CCsolution.

The possibility that these cellulose molecules fromDP, CC and BC associate in solution is very small,because the polymer concentration in LiCl · DMAcsolutions (which are used for the light scattering mea-surement) is very low (4 × 10−4 g/ml [0.04%]), asshown in Figure 4. These concentrations are very sim-ilar to those used by Saalwachter et al. (2000) andmuch lower (about one tenth) than those used by Mc-Cormick et al. (1985). Irrespective of this fact, theMw’s obtained by McCormick et al. (1985) are rela-tively low as described above. And also, as shownlater, all solutions showed Newtonian flow behaviorbelow the concentrations of 0.1 or 0.2 wt%.

Figures 5 and 6 show the flow curves for the DPand CC solutions at various concentrations. In Fig-ure 6, some data points are displayed twice in orderto show the reproducibility of the measurements. Thesystems reveal Newtonian behavior in the relativelylow shear rate regions, and they indicate non-Newto-nian behavior at high concentrations and in the highershear rate regions. These flow behaviors are typicalfor isotropic polymer solutions.

Figure 7 shows the angular frequency, � depen-dence of storage, G�, and loss modulus, G�, for theBC solutions in which cellulose concentrationsranged between 0 and 13.7 wt%. Dynamic viscoelas-tic measurements were carried out at a strain ampli-tude of 0.1 where all samples show linear viscoelas-ticity. In the low concentration range, below ca. 3%,the solutions revealed flow characteristics typical ofsynthetic isotropic polymer systems; G� and G� wereproportional to �2 and �1, respectively. As the cellu-lose concentration increases, the frequency depen-dence of G� and G� changes from �2 and �1, respec-

tively. This means that the influence of the intermo-lecular interaction among cellulose molecules on flowbehavior becomes predominant, and this is related to

Table 1. Characteristic molecular data of the cellulose samples

Sample Mw/104 103A2/g−2 cm3mol [�]/cm3 g−1

DP 98.2 2.16 704

CC 170 1.33 1504

BC 192 1.12 935

Figure 5. Shear stress logarithmically plotted against shear rate atvarious concentrations for the DP solutions.

Figure 6. Shear stress logarithmically plotted against shear rate atvarious concentrations for the CC solutions.

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the formation of anisotropic structures at elevatedconcentration. At the concentration 8.47 wt%, the fre-quency dependence curves of G� and G� can be ap-proximated by straight lines with almost the sameslope. That is, G�(�) and G�(�) are proportional to�n, where n is the viscoelastic exponent, i.e.

G���� � G���� � �n (1)

The exponent n has the value of 0.67 in the case ofBC solutions (Figure 7). This means that the solutionwith 8.47% concentration is at the gel point (Winteret al. 1988). According to Scanlan and Winter (1991),the values of n change over a wide range, between0.14 and 0.92, for different systems. Actually, n-val-ues were found to range from 0.50 to 0.84 for algi-nate aqueous systems (Matsumoto et al. 1992a). Atthe gel point, tan �(G�/G�) becomes constant. Thevalue of n can be independently obtained from tan�by the equation (Scanlan and Winter 1991):

tan� � tan�n

2�� (2)

In Figure 7, the tan� for the solution of 8.47% con-centration is plotted also, and it has a value of ap-proximately 1.6. The n-value can be estimated as 0.65by Equation 2, which is almost equal to that obtainedby Equation 1.

Figure 8 shows the logarithmic plot of �0−�s,where �0 is the zero-shear viscosity of the solutionand �s is the viscosity of the solvent; and c (in g/g) isthe cellulose concentration, at 30 °C for the DP, CCand BC solutions. These relationships can be repre-sented by two straight lines representing low and highconcentration ranges, and this is similar to the case oftypical synthetic polymers (Berry et al. 1979). Theslope of the straight lines in the low concentrationrange (dilute region) is unity for all solutions. On theother hand, the data in the high concentration range(semi-dilute region) can be approximated by astraight line with a slope of 4 for the DP and CC sys-tems, and by a straight line with a slope of 3 for theBC system. According to the Doi–Edwards theory, �0

is proportional to c� where the exponent � is 1 andabout 4 in the dilute and the semi-dilute regions forflexible polymers, and � is 3 for rod-like molecularsystems in the semi-dilute region (Doi and Edwards1978). In application of this theory, it is presumed thatthe cellulose molecules from DP and CC behave asflexible polymer chains, and the BC molecules be-have as rod-like polymers. This presumption is ratherreasonable considering that the DP and CC solutionsare isotropic and the BC solution is anisotropic atrelatively high concentration. That is, the rod-likemolecules tend to arrange themselves easily into anordered structure. It should furthermore be noted thatthe viscosity in the dilute region of the BC solution is

Figure 7. Frequency dependence curves of storage modulus, G�, and loss modulus, G�, at various concentrations of the BC solution. Theslope of the dotted line is 0.67. The value of tan� is plotted also for the concentration of 8.47%.

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lower than that of the CC solution, irrespective of thehigher molecular weight of BC; and that the viscosityin the relatively high concentration region of the BCsolution is smaller than that of the DP solution, de-spite the fact that the molecular weight of BC is muchhigher than that of DP.

The concentration at the point of intersection ofthe two straight lines in the low and high concentra-tion ranges is called the critical concentration, c*, atwhich the polymer chains begin to contact each other.That is, at c < c*, the systems are in the dilute solu-tion state where the intermolecular interactions can beneglected; and at c > c*, the systems are in the semi-dilute solution state where the intermolecular interac-tions become predominant. At c*, the number of in-termolecular interaction points (IIP) per unit volumeis proportional to c*2. Since c/Mw is proportional tothe number of polymer molecules per unit of volume(c in g/g), the Bueche parameter cMw is proportionalto the number of IIP per polymer molecule (Graess-ley 1974). That is, the number of IIP at the criticalconcentration is proportional to c*Mw. According tothis consideration, it is expected that the difference inmolecular weight can be reduced or compensated byplotting the viscosity against cMw (Graessley 1974;

Berry et al. 1979; Matsumoto et al. 1992b). As shownin Figure 9, the relationship between the viscosity�0–�s and cMw can be actually superimposed for thesolutions of DP and CC but not for the BC system.This result suggests that the cellulose molecules ofDP and CC, which are derived from plant sources, aredifferent only by molecular weight but otherwiseidentical in terms of molecular properties. This resultalso suggests that the Mw’s of DP and CC are cor-rect. On the other hand, the difference in molecularweight between BC and the plant celluloses cannot bereduced by the same correction procedure.

Figure 10 presents the logarithmical plot betweenviscosity and the overlapping parameter, c[�] (Tamaiet al. 2000). It is reasonable to expect that the hydro-dynamic volume effect on viscosity can be reducedby this plot, since the limiting viscosity number [�]is a measure of the hydrodynamic volume of the poly-mer molecule in solution (Onogi et al. 1967; Graess-ley 1974; Morris et al. 1981; Ross-Murphy 1985).Actually all the data of the CC, DP and BC solutionscan be excellently superimposed in the dilute regionwhere the intermolecular interactions are essentiallynegligible. This also suggests that the limiting viscos-ity numbers of all the samples are measured correctly

Figure 8. Logarithmic plot of �o−�s and cellulose concentration,c, for the DP, CC and BC solutions.

Figure 9. Logarithmic plot of �o−�s and cMw for the DP, CC andBC solutions.

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and accurately. On the other hand, the data of the CCand DP solutions can be superimposed on each otherin the semi-dilute region (where the intermolecularinteractions are important), but the data of the BC so-lutions cannot. The c[�] value of the break point inthis plot is almost identical for the CC and DP solu-tions, but it is different for the BC solution. And thec[�] values of the break point, which range from ap-proximately 3 to 5, are almost identical to those byMorris et al. (1981) and Ross-Murphy (1985). Ac-cording to the plots of Figures 9 and 10, [�] is pro-portional to Mw

�, where � is nearly 1 as shown byMcCormick et al. (1985).

The results described in this paper suggest that theBC molecule has different molecular properties, ordifferent structural characteristics, from the DP andCC molecules, especially in regard to the mechanismor type of intermolecular interaction. This suggeststhat the molecular properties of celluloses from dif-

ferent sources, i.e. the Monera, Plant and Animalkingdoms, are different.

Conclusions

The celluloses derived from plant sources (DP andCC) were soluble in 8% LiCl·DMAc, and they pro-duced isotropic solutions. The bacterial cellulose(BC) from Acetobacter xylinum was also soluble, butthe solution formed a liquid crystalline solution atrelatively high concentration. The cellulose tunicinfrom Halocynthia did not dissolve in this solvent. Thecellulose molecules from DP and CC behave as flex-ible polymer chains whereas the BC molecules be-have as rod-like molecules in this solvent. The differ-ence in molecular weight between different cellulosescan be eliminated by plotting the viscosity againstcMw for the DP and CC solutions, but not for the BCsystem. Moreover, the hydrodynamic volume effectcan be eliminated by plotting the viscosity againstc[�] in the region of dilute solution concentration forall three systems, where [�] is the limiting viscositynumber. On the other hand, in the high concentrationregion, the effect can be eliminated for the DP andCC solutions, but not for that of the BC system.

That is, the cellulose molecules of DP and CChave virtually identical molecular properties as far assolution behavior is concerned, irrespective of differ-ences in molecular weight, but the BC molecules donot. These results suggest that differences in molecu-lar properties and/or molecular structure exist be-tween celluloses from different biological sources.

Acknowledgements

The authors thank Prof. F. Horii (Kyoto University)for a sample of tunicin, and Dr. K. Watanabe (Ajino-moto Co. Ltd.) for a sample of bacterial cellulose.

This work was supported by a Grant-in-Aid forScientific Research (No. 12460076) from the Minis-try of Education, Science, Sports and Culture of Ja-pan.

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Figure 10. Logarithmic plot of �o−�s and c[�] for the DP, CC andBC solutions.

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