Structure and Thermal Behavior of a Cellulose I− Ethylenediamine

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Structure and Thermal Behavior of a Cellulose I-Ethylenediamine Complex Masahisa Wada,*,† Gu Joong Kwon,† and Yoshiharu Nishiyama‡ Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan, and Centre de Recherches sur les Macromole´cules Ve´ge´tales, CNRS, Affiliated with the Joseph Fourier University of Grenoble, BP 53, 38041 Grenoble Cedex 9, France Received June 23, 2008; Revised Manuscript Received July 25, 2008

We prepared highly crystalline samples of a cellulose I-ethylenediamine (EDA) complex by immersing oriented films of algal (Cladophora) cellulose microcrystals in EDA at room temperature for a few days. The unit-cell parameters were determined to be a ) 0.455, b ) 1.133, and c ) 1.037 nm (fiber repeat) and γ ) 94.02°. The space group was P21. On the basis of unit cell, density, and thermogravimetry analyses, the asymmetric unit is composed of one anhydrous glucose residue and one EDA molecule. The chemical and thermal stabilities of the cellulose I-EDA complex were also investigated by the use of X-ray diffraction. When the cellulose I-EDA complex was immersed in methanol or water at room temperature, cellulose IIII or Iβ was obtained, respectively. However, immersion in a nonpolar solvent such as toluene did not affect the crystal structure of the complex. The cellulose I-EDA complex was stable up to a temperature of ∼130 °C, whereas the boiling point of EDA is 117 °C. This thermal stability of the complex is probably caused by intermolecular hydrogen bonds between EDA molecules and cellulose. When heated above 150 °C, the cellulose I-EDA complex decomposed into cellulose Iβ.

Introduction Cellulose is one of the most abundant biopolymers. It exists in nature as crystalline (cellulose I) fibrils called microfibrils that are a few nanometers across. Two crystalline forms exist in naturally occurring cellulose: cellulose IR and Iβ.1-4 A sheetlike structure that is stabilized by intermolecular hydrogen bonds that are parallel to the pyranose rings and the staggered stacking of the sheets by distances of half-glucose rings along the cellulose chain axis is common in both forms. The difference between the two allomorphs in native cellulose lies in the mode of staggering: progressive staggering occurs in cellulose IR, and alternating staggering occurs in cellulose Iβ.5,6 When immersed in ammonia or amines, cellulose I fibrils inflate as the ammonia or amine molecules penetrate into the cellulose crystals, which forms a crystalline complex with the cellulose chain. After the evaporation or washing away of the guest molecules, the fibrils deflate and convert into another crystalline form called cellulose IIII.7-15 The crystal structure of cellulose IIII has a one-chain monoclinic unit cell with an asymmetric unit of only one glucosyl residue, which indicates that parallel cellulose chains are stacked with no stagger in the chain direction.16,17 The treatment with ammonia or amines is a simple method that can be used not only to produce cellulose IIII but also to enhance the accessibility and chemical reactivity of crystalline cellulose. A number of industrial applications that use this treatment have been developed in the fields of pulp and paper, textile, and cellulosic biomass conversion.18-23 Therefore, the interaction between ammonia or amines and cellulose chains has attracted much attention for the development of an efficient chemical modification process. To help understand such interac* Corresponding author. E-mail: [email protected]. Tel: +81-3-5841-5247. Fax: +81-3-5841-2677. † University of Tokyo. ‡ Centre de Recherches sur les Macromole´cules Ve´ge´tales.

tion, we have recently analyzed the cellulose I-ammonia complex.24 One ammonia molecule and one anhydrous glucose residue were found to form the asymmetric unit in a one-chain monoclinic unit cell. Cellulose chains are thus stacked without stagger in the chain direction in this complex, as in the case of cellulose IIII, with ammonia molecules included between the cellulose chains. The cellulose I-ethylenediamine (EDA) complex is another example of a cellulose I amine complex that converts to cellulose IIII when the complex is immersed in methanol.14,15 It also converts to cellulose I when immersed in water, but it does not alter its complex structure when treated with nonpolar solvents.25 An early X-ray diffraction study by Lee et al. reported the crystal structure of the cellulose I-EDA complex prepared from ramie fibers; two almost identical but independent parallel chains pack in a monoclinic unit cell (a ) 1.287, b ) 0.952, and c ) 1.035 nm and γ ) 118.8°) with P21 symmetry.26 In this asymmetric unit, there are two EDA molecules and two glucosyl residues with a 1:1 ratio of EDA molecules to glucosyl residues. However, the solid-state 13C NMR spectrum is very simple because it is composed of six resonance peaks of glucose carbon and one resonance peak of EDA, which suggests a structure with only one symmetrically independent glucose residue.27,28 The stoichiometry of the cellulose I-EDA complex of the X-ray model has also been challenged by solid-state 13C NMR results in which the intensity of the resonance peak from EDA carbon corresponded to one EDA molecule per two glucose residues.28 The ambiguities in the X-ray diffraction analysis can be partially removed by the use of highly crystalline and uniaxially oriented samples that result in better data quality than conventional higher plant samples such as ramie or cotton.5,6,17 In the present study, we prepared highly crystalline cellulose I-EDA samples to improve the data quality. Here we report the chemical stability, thermal stability, and stoichiometry of the complex thus prepared. In addition, we investigated the thermal expansion

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of this complex by using X-ray diffraction and compared it with the previously reported behavior of crystalline celluloses.29-31

Experimental Section Cellulose Purification. Green algae (Cladophora sp.) that were collected at the sea of Chikura (Chiba, Japan) were used in this study. After calcite was removed by boiling in 0.1 N HCl, the cellulosic algal cell walls were further purified by repeating treatments with 5% KOH and 0.3% NaClO2 solutions.32 The purified samples were hydrolyzed into cellulose microcrystals by sulfuric acid treatment, followed by reconstitution into oriented films, as reported.33 Conversion into the Cellulose I-EDA Complex. The bundle of oriented films was immersed in anhydrous EDA in the dark at room temperature for several days. Excess EDA was gently removed with a pipet, and the treated samples were dried in vacuum over P2O5 for 12 h.26 Chemical Treatment. The oriented films of the complex were immersed overnight in excess amounts of water, ethanol, methanol, propanol, butanol, toluene, carbon tetrachloride, acetone, or dimethylacetamide at room temperature. Excess solvents were gently removed with a pipet, and the samples were dried in vacuum over P2O5 for 12 h. The hydrothermal treatment of the oriented films of the complex was carried out by treatment in water at 260 °C for 30 min, as previously described.33,34 X-ray and Synchrotron Fiber Diffraction Analysis. The fiber diffraction analysis of the oriented complex films was carried out by the use of a flat-plate vacuum camera mounted on a rotating anode X-ray generator (RU-200BH, Rigaku). The patterns were recorded at room temperature with Ni-filtered Cu KR radiation (λ ) 0.1542 nm) and Fuji imaging plates (IPs). The sample-to-IP distance was calibrated by the use of NaF powder (d ) 0.23166 nm). The oriented complex films were also analyzed at the beam line BL38B1 at SPring-8 (Hyogo, Japan). The samples, which were mounted on a goniometer head, were irradiated for 120 s with synchrotron radiation (λ ) 0.1 nm). The fiber patterns were recorded at room temperature by the use of a camera system equipped with a flat IP (R-Axis V, Rigaku). The sample-to-IP distance was calibrated by the use of Si powder (d ) 0.31355 nm). Reflection positions of the patterns were measured by the use of Rigaku R-Axis software. After the indexation for the reflections, the unit-cell parameters were refined by the use of a least-squares algorithm. X-ray Diffraction During Heating. The X-ray diffraction analysis of the oriented complex films during heating was carried out by the use of a medium-temperature fiber specimen attachment (Rigaku) and a 1D position-sensitive proportional counter (PSPC) that was mounted on a Rigaku RINT 2200 goniometer. The films clamped on the sample holder were heated stepwise under a helium atmosphere from 20 to 250 °C. The equatorial profiles at each temperature were recorded by the use of Ni-filtered Cu KR radiation (λ ) 0.15418 nm) and the PSPC with an accumulation time of 3 min.29-31 The peak positions were determined by the peak fitting of the X-ray diffraction profiles, as reported.29-31 We refined the unit-cell parameters a, b, and γ by using the least-squares method using 010, 020, 100, 11j0, and 110 reflections in the equator. The following equation was used to determine the thermal expansion coefficient (TEC) R

R)

( )( ) 1 ∆l lt)0 ∆t

(1)

where l is the d spacing or a unit-cell parameter, t is the temperature in degrees Celsius (°C), and ∆l and ∆t are the changes in l and t, respectively. Thermogravimetric Analysis. Samples of cellulose I-EDA complex films and oriented cellulose films were subjected to thermogravimetry by the use of a TGD-9600 device (Ulvac, Tokyo). Vacuum-dried samples (∼50 mg) in a platinum holder were weighed under a nitrogen flow rate of 100 mL/min with a heating rate of 5 °C/min from room temperature to 600 °C. Liquid EDA (∼50 mg) was also subjected to thermogravimetry in the same way.

Figure 1. (a) Cu KR X-ray and (b) synchrotron X-ray fiber diffraction patterns of the cellulose I-EDA complex prepared from oriented films of algal (Cladophora sp.) cellulose. Arrows indicate the positions that correspond to d spacings of 0.6 and 0.54 nm.

Results and Discussion Structure. Two X-ray fiber diffraction patterns of the cellulose I-EDA complex that were recorded by the use of conventional Ni-filtered Cu KR radiation and synchrotron radiation are shown in Figure 1a,b, respectively. Both patterns show fairly high resolution, but Figure 1a contains broad tails and contributions from short-wavelength X-rays as a result of the impurity of the beam. The absence of equatorial reflections at 0.6 and 0.54 nm is a clear indication that there was no residue of cellulose I in the samples. The analysis of the X-ray diffraction pattern of Figure 1a yielded 47 independent reflections within 4 layer lines. All of these reflections could be indexed according to a one-chain monoclinic unit cell with dimensions a ) 0.453, b ) 1.136, and fiber repeat c ) 1.036 nm and monoclinic angle γ ) 93.80°. The volume of this unit cell is half of that previously proposed.26 On the meridian of the pattern, the 001 reflection is absent, 002 is very strong, 003 is absent, and 004 is weak. The presence and absence of these reflections are consistent with space group P21 with the 21 axis along the fiber axis, as it is in any other allomorph of cellulose. The pair of strong 002 and weak 004 reflections is a characteristic feature that is opposed to the pair of weak 002 and strong 004 reflections and indicates that the cellulose I-EDA complex does not show staggering along the fiber axis and is a common feature in cellulose IIII17 and cellulose I-ammonia complexes.24 All of the spots in the synchrotron fiber diffraction pattern that is shown in Figure 1b could also be indexed with a onechain monoclinic unit cell with a P21 symmetry (a ) 0.455, b ) 1.133, and fiber repeat c ) 1.037 nm and monoclinic angle γ ) 94.02°). This is consistent with the previous results of solid-

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Table 1. Unit Cell Parameters of Cellulose I-EDA Complex Compared with Cellulose I-Ammonia Complex and Cellulose IIII cellulose I-EDA complex cellulose I-ammonia complex24 cellulose IIII17

a (nm)

b (nm)

c (nm)

γ (deg)

V (nm3)

0.455 0.447 0.445

1.133 0.881 0.785

1.037 1.034 1.031

94.02 92.7 105.1

0.533 0.407 0.348

state 13C NMR in which only one nonequivalent glucose residue was detected.27,28 The unit-cell parameters of the cellulose I-EDA complex in this study and those of the cellulose IIII and cellulose I-ammonia complexes for comparison, are listed in Table 1. These three structures are all a one-chain monoclinic type with no staggering along the chain direction. Although the lengths of the a and c axes were found to be almost the same, the b axis significantly varied in the order EDA complex > ammonia complex > cellulose IIII. The anisotropic change of the unit cell by complexation suggests that this EDA complex retains the hydrophobic stacking of the pyranose rings almost parallel to the a-c plane and that the cellulose sheets are separated by the guest molecules along the b axis (Figure 2). This structure is quite similar to those of the cellulose I-ammonia,7,8,24 cellulose-amine,9-12 chitin-alcohol,36 and chitin-amine complexes.37 By comparing the unit-cell volume per cellulose chain of cellulose Iβ (0.329 nm3)6 with those of the cellulose I-ammonia and cellulose I-EDA complexes (Table 1), we could estimate the volume that is left for EDA molecules. From the densities of the condensed phase at room temperature, 0.899 and 0.610 g/cm3, the volumes occupied by the EDA and ammonia molecules were estimated to be 0.620 and 0.236 nm3, respectively. From these volume and the liquid density values, the number of molecules in the unit cell was calculated to be 1.8 for the EDA complex and 1.7 for the ammonia complex. X-ray diffraction analysis of the ammonia complex revealed that two ammonia molecules were incorporated into the one-chain unit cell.24 The small difference of 0.3 between the X-ray structure and the estimation from the free volume can be explained by the denser packing of ammonia molecules. By analogy, two EDA molecules were probably incorporated in the one-chain monoclinic unit cell of the EDA complex. Chemical Stability and Stoichiometry. The chemical stability of the complex was examined by changes in X-ray diffraction patterns. Figure 3 shows the patterns after water (a), hydrothermal (b), methanol (c), and toluene (d) treatments. By the immersing of the cellulose I-EDA complex in water at room

temperature, it was decomposed into cellulose Iβ. The crystallinity was lower than that of initial algal cellulose but almost the same as that of ramie or cotton cellulose.38 The EDA complex was also decomposed into cellulose Iβ by hydrothermal treatment at 260 °C. The crystallinity was higher than that of the sample that was treated in water at room temperature because one can see diffraction spots up to a wider angular region and with a smaller angular peak width in Figure 3b. We converted the EDA complex into another crystalline form, cellulose IIII, by immersing it in excess amounts of methanol or ethanol at room temperature. When the complex was immersed in nonpolar solvents such as toluene and carbon tetrachloride, the EDA molecules were not taken out from the EDA complex, although these solvents are fully miscible with EDA. Polar aprotic solvents such as acetone or dimethylacetamide also did not modify the complex structure. Even polar protonic solvents of low polarity such as propanol or butanol did not affect the complex structures. Water, a high-polarity protonic solvent, caused conversion into cellulose Iβ, and moderate polar protonic solvents such as methanol or ethanol caused conversion into cellulose IIII. However, low-polarity protonic solvents, polar aprotic solvents, and nonpolar solvents did not cause any change in the structure. The stability of the EDA complex in nonpolar solvents allowed the determination of the host-to-guest ratio of the complex, which is based on the density measurement. The density of the EDA complex, which was measured with a pycnometer in toluene-carbon tetrachloride as a medium, was 1.38 g/cm3. From this density and the unit-cell volume (0.533 nm3), the molecular weight of the EDA complex in the unit cell was calculated to be 443. This value can be interpreted as an indication that two EDA molecules are included in the onechain monoclinic unit cell. This stoichiometry is in good agreement with that estimated from the density of liquid EDA and the unit-cell volume of the EDA complex. Thermal Stability and Stoichiometry. Figure 4 shows the equatorial X-ray diffraction profiles of the EDA complex at given temperatures in the heating process from room temperature to 250 °C. The five peaks, which were indexed as 010, 020,

Figure 2. Schematic drawing of the cellulose I-EDA complex projected in the fiber axis direction by the replacement of the ammonia molecule of the cellulose I-ammonia complex24 with EDA.

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Figure 3. Cu KR X-ray fiber diffraction patterns of the chemically treated cellulose I-EDA complex. (a) Water treatment at room temperature, (b) hydrothermal treatment in water at 260 °C, (c) methanol treatment at room temperature, and (d) toluene treatment at room temperature.

100, 11j0, and 110 respectively, were observed at room temperature before being heated. With stepwise heating up to 133 °C, all five peaks shifted to smaller angles because of thermal expansion. In particular, the 110 peak merged into the 11j0 peak at 133 °C. When the temperature reached 150 °C, three peaks (11j0, 110, and 200) that corresponded to cellulose Iβ appeared, and the intensity of peaks that were due to the EDA complex started to diminish. At 167 °C, the cellulose Iβ phase became dominant. The EDA complex was completely decomposed into cellulose Iβ at 180 °C. After being heated to 250 °C, the sample was cooled to room temperature, where the profile was identical to that of a pure Iβ pattern. These X-ray results indicate that the EDA complex is stable at the boiling point of EDA, about 117 °C, but decomposes into Iβ at 150-167 °C. This thermal behavior allowed us to determine the stoichiometry of the EDA molecules in the complex by thermal analysis. The thermogravimetry diagrams of the EDA complex together with initial oriented cellulose films and liquid EDA are shown in Figure 5. The pyrolytic weight loss of the EDA complex is a characteristic two-step process that apparently shows the removal of EDA molecules by vaporization. The EDA complex lost approximately 27% of its initial weight between 20 and 180 °C. The latter temperature agrees well with the complete disappearance of the EDA complex in the X-ray diffraction results (Figure 4). Because pyrolysis of cellulose is negligible at this temperature, this 27% weight loss is ascribed to the thermal release of EDA molecules and leads to two EDA molecules incorporated into the one-chain (cellobiose) unit cell. Thermal Expansion. To analyze the thermal expansion behavior, we carried out peak separation of the diffraction profiles (Figure 4) and determined the d spacings at each temperature. Changes thus determined are shown in Figure 6. All increased gradually with increasing temperature from 20

°C to the decomposition temperature at 150-167 °C. Above the decomposition into cellulose Iβ, these d spacings also increased with increasing temperature up to 250 °C. The thermal expansion of d010 and d020 apparently has a flexion point at 70 °C, whereas expansions in the other direction can be approximated as being linear over the entire studied range. The corresponding TEC of the EDA complex that was calculated by the use of eq 1 is indicated in Figure 6. Theoretically, the TEC of R[010] that was calculated from changes in d spacings of d010 and d020 is the same because the relation d010 ) 2d020 should hold. However, at room temperature d010 was ∼1% larger than 2d020. A possible explanation of this is that the EDA molecules were eliminated during the sample preparation from the surface layer of the microcrystals (composed of about 30 × 30 chains). In fact, when the X-ray diffraction was taken immediately after wetting with EDA, d010 was close to 2d020. The last layers probably contribute to the low-resolution diffraction shifting the peak position to a higher angle, but they contributes less to higher angle diffraction because of the larger phase difference. This hypothesis is also supported by the fact that the unit-cell parameters, which were determined from the peak positions of 100, 11j0, and 110, gave a d020 value that was close to the observed value, whereas the d010 value that was calculated from this cell parameters gave a larger d010 than that observed. Therefore, in the following, only the d020 value was used to estimate R[010]. The linear TECs were R[010] ) 3.7 × 10-5 (70 °C), R[100] ) 1.1 × 10-4, R[11j0] ) 3.7 × 10-5, and R[110] ) 2.3 × 10-4 °C1-. The linear TEC of the cellulose I-EDA complex was highest along the [110] direction. The unit cell parameters of the EDA complex, the a and b axes, the monoclinic angle (γ), and the lateral area (S) were calculated from the d spacings by the least-squares method.

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Figure 4. Equatorial X-ray diffraction profiles of the cellulose I-EDA complex during heating from 20 to 250 °C in a helium atmosphere.

Figure 5. Thermogravimetric curves of the cellulose I-EDA complex, initial oriented cellulose films, and liquid EDA.

Changes in these unit-cell parameters with temperature are shown in Figure 7. The a axis increased linearly with increasing temperature by 1.2% from 20 to 133 °C. The b axis also increased with increasing temperature by 0.80% at 133 °C. The gradient showed a distinct increase at a temperature of 70 °C, which indicates that a phase transition may occur at this temperature. From these changes in the a and b axes, the TECs were calculated to be Ra ) 1.1 × 10-4, Rb ) 3.0 × 10-5(70 °C). The monoclinic angle γ decreased with heating; the difference between 20 and 133 °C was ∼2°. This change in γ is more significant than those of the cellulose allomorphs cellulose Iβ and IIII29,30 because the principal axis of thermal expansion anisotropy is diagonal to the unit-cell axis. The unit cell of the cellulose I-EDA complex is easily deformed because mobile EDA molecules were

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Figure 6. Changes in the d spacings of cellulose I-EDA complex during heating from 20 to 250 °C. The gray zones between 140 and 180 °C are regions in which the cellulose I-EDA complex converts to cellulose Iβ. Thermal expansion coefficients of the complex calculated from d spacings are also shown.

incorporated into cellulose chains. Particularly large thermal expansions occur along the direction of hydrophobic stacking of the cellulose chains because of harmonic molecular oscillation and the greater heat sensitivity of van der Waals interactions.29,30 However, the expansion of the a-axis direction corresponding to the hydrophobic stacking direction (Figure 2) was not high. Intermolecular hydrogen bonds through amine groups of EDA molecules probably prevent the thermal expansion along this direction. Further detailed discussion is not possible here because of the scant information that is available on the crystal structure of the cellulose I-EDA complex. A detailed crystal structure analysis from the fiber diffraction pattern (Figure 1b) that was recorded by the use of a synchrotron radiation X-ray source is under way to establish the exact configuration of the interaction. The thermal expansion along the cellulose chain axis direction is known to be very small. Therefore, the TECs of longitudinal directions are only 1-10% of those of lateral directions.30,31 This small change could be negligible (within experimental error) in the calculation of changes in the unit-cell volume. By assuming that the c axis of the cellulose I-EDA complex is constant at 1.037 nm (Table 1), we calculated the unit-cell volumes at each temperature from the measured a and b axes and the monoclinic angle γ in Figure 7. The unit-cell volume increased linearly with increasing temperature, and the calculated volume TEC was β ) 2.1 × 10-4 °C-1. This value is higher

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EDA molecules at ambient temperature and release them when heated to >150 °C.

Conclusions Fiber diffraction data that were recorded on the oriented films of the highly crystalline cellulose I-EDA complex indicated that the crystals of the cellulose I-EDA complex could be fully described with a one-chain monoclinic unit cell (a ) 0.455, b ) 1.133, and c ) 1.037 nm and γ ) 94.02°) and space group P21. In this cell, one glucosyl residue and one EDA molecule form an asymmetric unit. The EDA complex was found to be stable by treatment in nonpolar solvents or polar aprotic solvents, although it was decomposed into cellulose Iβ or cellulose IIII by treatment in polar protonic solvents. The EDA complex was stable when heated to ∼130 °C (above the boiling point of EDA of 117 °C) and decomposed into cellulose Iβ at >180 °C. The thermal expansion behavior of the EDA complex was monitored by the changes in X-ray diffraction profiles during heating. The volume TEC was calculated to be β ) 2.1 × 10-4 °C-1, which is twice that of other cellulose polymorphs. Acknowledgment. We thank Professor K. Noguchi of Tokyo University of Agriculture and Technology for his help during the synchrotron radiation experiment. The synchrotron radiation experiments were performed at BL38B1 in SPring-8 with the approval of the Japan Synchrotron Research Institute (JASRI). This study was partially supported by a Grant-in-Aid for Scientific Research (No. 18780131) and the French Agence Nationale de la Recherche.

References and Notes Figure 7. Changes in lateral unit-cell parameters of the cellulose I-EDA complex during heating. Thermal expansion coefficients of the unit-cell parameters are also shown.

than other cellulose allomorphs: 1.1 × 10-4 and 7.1 × 10-5 °C-1 for cellulose Iβ and IIII, respectively.29,30 Larger TEC values can be ascribed to the higher mobility of molecules in the complex structure compared with pure cellulose crystals. The thermal behavior of the cellulose I-EDA complex gives important insight into the interaction of cellulose with amines. Although the coordination of amines to cellulose is favored, part of the EDA molecules in the complex evaporate even at ambient temperature. The EDA molecules are very mobile, especially at >70 °C, as can be seen by the high TECs and the rate of evaporation. In the thermogravimetry in our condition, there is an approximately 50 °C shift in the evaporation curve between the bulk EDA and the complexing EDA. This is probably due to the very small diffusion rate inside the crystal, which behaves as a gas barrier, because no excess hydroxyl groups are present in the environment. Small alcohol will extract EDA molecules from the cellulose lattice because the interaction of amines with cellulose is due to the abundant hydroxyl groups of cellulose that serve as hydrogen bonding acceptors from amine groups. The hydroxyl groups in the solvent would compete with cellulose hydroxyl groups to extract the EDA. Polar molecules also break the cellulose hydrogen bonds on the crystal surface to enable the release of EDA molecules. Water will further plastify cellulose and induce crystalline conversion from cellulose III to I. Therefore, cellulose has the potential to be a kind of molecular sieve in an aprotic or nonpolar environment that can capture

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