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Articles Effect of Sulfate Groups from Sulfuric Acid Hydrolysis on the Thermal Degradation Behavior of Bacterial Cellulose Maren Roman† and William T. Winter*,‡ Department of Chemistry, Pulp and Paper Research Centre, McGill University, Montre´ al, Quebec H3A 2A7, Canada, and Cellulose Research Institute and Department of Chemistry, SUNY College of Environmental Science and Forestry, Syracuse, New York 13210 Received December 10, 2003; Revised Manuscript Received April 16, 2004
When used as fillers in polymer composites, the thermostability of cellulose crystals is important. Sulfate groups, introduced during hydrolysis with sulfuric acid, are suspected to diminish the thermostability. To elucidate the relationship between the hydrolysis conditions, the number of sulfate groups introduced, and the thermal degradation behavior of cellulose crystals, bacterial cellulose was hydrolyzed with sulfuric acid under different hydrolysis conditions. The number of sulfate groups in the crystals was determined by potentiometric titration. The thermal degradation behavior was investigated by thermogravimetric analysis. The sulfate group content increased with acid concentration, acid-to-cellulose ratio, and hydrolysis time. Even at low levels, the sulfate groups caused a significant decrease in degradation temperatures and an increase in char fraction confirming that the sulfate groups act as flame retardants. Profile analysis of the derivative thermogravimetric curves indicated thermal separation of the degradation reactions by the sulfate groups into low- and high-temperature processes. The Broido method was used to determine activation energies for the degradation processes. The activation energies were lower at larger amounts of sulfate groups suggesting a catalytic effect on the degradation reactions. For high thermostability in the crystals, low acid concentrations, small acid-to-cellulose ratios, and short hydrolysis times should be used. Introduction Sulfuric acid hydrolysis of native cellulose fibers causes breakdown of the fibers into rodlike fragments. These highly crystalline cellulose needles form stable aqueous suspensions due to sulfate groups, which have been introduced during the hydrolysis through esterification of surface hydroxyl groups.1 The colloidal suspensions of cellulose crystals first gained attention as model systems for rigid rods2 and have then been extensively studied for their ability to form ordered, birefringent phases when the crystal content in the suspensions exceeds a critical value.3-8 In recent years, the mechanical properties of cellulose crystals have attracted attention for application as reinforcing fillers in polymer nanocomposites.9-11 With typical processing temperatures for thermoplastics often exceeding 200 °C, the thermostability of the crystals is important for this application. Inorganic salts and acids are known to act as flame retardants in the pyrolysis and combustion of cellulose; that is, they increase the char yields at the expense of flammable tars by promoting the dehydration reactions.12,13 In accordance, impregnation of cellulose with sulfuric acid has been shown to lower the onset of thermal degradation,14-16 * To whom correspondence should be addressed. E-mail: wtwinter@ syr.edu. Telephone: 315-470-6876. Fax: 315-470-6856. † McGill University. ‡ SUNY College of Environmental Science and Forestry.
decrease the yield of levoglucosan,14 and increase the yield of carbon.15 Consequently, it might be expected that the sulfate groups in the crystals prepared by sulfuric acid hydrolysis diminish the thermostability of the crystals, in which case hydrochloric acid might be a better choice for the hydrolysis. The present study was conducted to elucidate the relationship between the hydrolysis conditions, the amount of sulfate groups introduced, and the thermal degradation behavior of cellulose crystals prepared by sulfuric acid hydrolysis. The aim was to optimize the hydrolysis conditions for the application of the crystals as reinforcements in polymer nanocomposites. Bacterial cellulose was hydrolyzed under different conditions and then characterized by potentiometric titration and thermogravimetric analysis. Even small amounts of sulfate groups caused a significant decrease in degradation temperatures. Larger amounts resulted in a stepwise degradation behavior. Experimental Section Materials. Bacterial cellulose (PrimaCel) was provided by the Nutrasweet Kelco Company, Chicago, IL (Now CPKelco, Wilmington, DE). Sodium hydroxide for the cellulose purification was purchased from MCB, acetic acid from EM Science, and sulfuric acid from Fisher Scientific.
10.1021/bm034519+ CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
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Table 1. Amounts of Introduced Sulfate Groups, nSO3H, and Surface Charge Densities, σ, at Different Reaction Conditions for the Sulfuric Acid Hydrolysis of Bacterial Cellulose hydrolysis conditionsa sample
(%(w/v))
(mmol‚g-1)
A B C D E F G H
unhydrolyzed 12 65 65 65 65 61 65
188 66 199 663 66 618 66
(°C)
(h)
nSO3H ( 0.8 (mmol‚kg-1)
σ ( 0.003 (e‚nm-2)b
104 40 40 40 40 40 60
2 1 1 1 3 3 2
0 2.1 6.5 8.5 9.8 10.3 50.8 73.0
0.000 0.007 0.021 0.027 0.031 0.033 0.162 0.233
a Sulfuric acid concentration, acid-to-cellulose ratio, hydrolysis temperature, hydrolysis time. b We calculated a specific surface area of 189 m2 g-1 using an assumed average rectangular-solid particle with dimensions 8 × 40 × 1000 nm.
Sodium hydroxide for the titration was purchased from Mallinckrodt and the hydrochloric acid standard solution (0.2015 N) from J. T. Baker. All chemicals were reagent grade or higher in purity and used as received. The water for the titrations was of ASTM type I. Sulfuric Acid Hydrolysis. Bacterial cellulose was purified by dispersing 10 g (dry weight basis) for 6-7 days in 1 L 0.5% aqueous NaOH solution and repeatedly washed with water and filtered until the filtrate pH reached neutrality. Then the cellulose was redispersed for 1-4 days in 1 L 0.5% acetic acid, and the washing/filtration process was repeated.17 The cellulose was freeze-dried from aqueous suspension prior to hydrolysis. The purified cellulose was placed in sulfuric acid of known concentration and acid-to-cellulose ratio at a constant temperature for a period of time. The applied hydrolysis conditions are listed in Table 1. After completion of the hydrolysis, the reaction was quenched by addition of ice-cold water except for the 12% (w/v) sulfuric acid for which the reaction mixture was cooled externally with an ice bath. The crystals were collected in a filter and washed with water by the washing/filtration process described above. Sample F was washed by successive centrifugation until peptization occurred prior to filtration. Centrifugation was not used for the other samples because it was more tedious than filtration. Finally, the crystals were freeze-dried from aqueous suspension. Potentiometric Titration. A fine aqueous suspension (0.1%) of the sample was prepared by ultrasound treatment in a plastic jar, to avoid release of ions from glass-jar walls, and under ice-bath cooling, to avoid desulfation upon heating. The suspension (50 g) was weighed into a 100-mL threeneck flask and 1 mL of 0.05 M aqueous NaCl solution was added. An ORION combination electrode (ROSS) and a thermometer were inserted through the side-necks. Before each measurement, the electrode was calibrated with both pH 4 and 7 buffers. The suspension was held at 25 °C and stirred under argon gas flow, to minimize carbon dioxide interference, until the reading of the pH meter (Corning, model 440) was stable. Then aqueous NaOH solution (0.0011 N, determined by titration with a hydrochloric acid standard solution) was added dropwise from a microburet and the pH recorded after each drop. An example of the resulting titration curves is shown in Figure 1a. The equivalence points in the titrations were determined using Gran’s Plot procedure.18 Figure 1b shows the Gran plot for the titration curve in Figure 1a.
Figure 1. Determination of the amount of sulfate groups in sulfuric acid hydrolyzed bacterial cellulose by potentiometric titration (sample H): (a) titration curve, (b) Gran plot; V0: initial volume of suspension; V: volume of NaOH added; k and k′: arbitrary constants.
The equivalence-point volume is given by the abscissa intercepts of the linear regression lines. Ideally, the lines for the two branches of the titration curve intercept the volume axis at the same value so that either branch can be used for the determination of the equivalence-point volume. In our titrations, the abscissa intercepts, and thus the equivalencepoint volumes for the left and right branch differed by up to 0.82 mL. We attributed this difference to carboxylic acid groups in the samples, which start to dissociate once the stronger sulfate groups have been neutralized. The actual equivalence-point volume was taken as the average of the two volumes determined from both branches of the titration curve. Titration of the unhydrolyzed bacterial cellulose gave an equivalence-point volume of 0.27 mL translating into a carboxyl group content of 6.0 mmol kg-1. The sulfate content in the hydrolyzed samples was calculated from the additional base required for neutralization. Thermogravimetric Analysis. Thermogravimetric (TG) curves were recorded with a TA Instruments model 2950 Hi-Res TGA. The sample (20 mg) was heated to 500 °C in
Hydrolysis of Bacterial Cellulose
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FTIR Spectroscopy. FTIR spectra (500 scans, data spacing 1.928 cm-1) were recorded with a Nicolet Impact 400 FTIR spectrometer. KBr pellets contained 2% of sample and were dried under vacuum in a desiccator for 2 days. Results and Discussion
Figure 2. Deconvolution of the DTG curve between 150 and 350 °C of sulfuric acid hydrolyzed bacterial cellulose (sample H) using split Pearson VII line shapes.
an air current of 100 cm3 min-1. The heating rate was 10 °C min-1 up to 110 °C and 2 °C min-1 from 110 to 500 °C. Derivative TG curves (DTG) express the weight-loss rate as a function of temperature. Profile analysis of DTG curves was done with a peak fitting program using asymmetric split Pearson VII line shapes.19 A deconvolution example is shown in Figure 2. The activation energies, Ea, of the degradation processes were determined with the Broido method using the equation20 Ea
( 1y) ) - R T1 + C
ln ln
where R was the gas constant, C was a temperature independent term, and y was the fraction of undecomposed material remaining at time t, (W - Wf)/(Wi - Wf). Wi and Wf are the initial and final sample weight, and W is the sample weight at time t. The activation energy is obtained from the slope of a plot of ln(ln 1/y) versus 1/T. Transmission Electron Microscopy (TEM). One drop (8 µL) of a 0.001% aqueous suspension of crystals was allowed to dry on a Formvar and carbon coated grid (200 mesh, treated with poly-L-lysine solution). The crystals were stained with uranyl acetate. Micrographs were taken at 80 kV with a JEOL JEM-2000 EX electron microscope. X-ray Diffraction. X-ray diffraction was carried out with a Rigaku DMAX-1000 diffractometer and fiber attachment using Ni-filtered Cu KR radiation, generated with a rotating anode generator operating at 50 kV and 120 mA. The diffraction angle was calibrated with sodium fluoride. The crystals were loosely inserted into the sample holder. Before each measurement, the random orientation of the crystals in the sample holder was verified by rotation of the sample about the transmitted beam direction and also from rotation in θ while recording intensities at fixed 2θ values corresponding to the principal reflections in a native cellulose pattern (2θ ) 14.4, 16.6, 22.5°). To determine the sample crystallinity, profile analysis was carried out with a peak fitting program using Gaussian line shapes.19 The deconvolution included the interference maxima at 2θ ) 14.4, 16.6, 20.3, and 22.5° and a broad peak at the interference minimum at 2θ ) 18.5° accounting for the amorphous scattering. The crystallinity was taken as the ratio of the sum of areas under the crystalline diffraction peaks to the total area under the curve between 2θ ) 10 and 27°.
Sulfate Content and Crystal Aggregation. The sulfate contents of the samples, after hydrolysis, were determined by potentiometric titration, and the values are listed in Table 1. Sample B, hydrolyzed under the conditions used in ref 1, contained a very small number of sulfate groups. The amount was much smaller than, for example, in sample D, which was hydrolyzed at roughly the same acid-to-cellulose ratio but with a higher acid concentration. At low acid concentrations, water is present in large amounts. Because water is a reaction product in esterifications, an excess of it pushes the equilibrium of the reversible reaction to favor the reactants. Thus, higher acid concentrations lead to larger amounts of sulfate groups in the crystals. The amounts of sulfate groups were also larger for larger acid-to-cellulose ratios (samples C-E and samples F and G) and longer hydrolysis times (samples C and F and samples E and G). The effect of the hydrolysis time was more pronounced at a higher acid-tocellulose ratio. Hydrolysis at 60 °C (sample H) gave higher sulfation than hydrolysis at 40 °C (sample F). The sulfate content of sample H was 73 mmol kg-1. Araki and Kuga hydrolyzed bacterial cellulose with 65% sulfuric acid (by weight) at 70 °C for 30 min and measured a sulfate content of 5 mmol kg-1.21 The acid-to-cellulose ratio was not specified but was 103 mmol g-1 in the author’s reference to previously published experimental details. Considering that the acid concentration and probably the acid-to-cellulose ratio was higher by about one-third and the temperature by 10 °C, the value of Araki and Kuga seemed to be low even after allowing for the 75% decrease in the hydrolysis time. TEM pictures of samples B and G are shown in Figure 3. In sample B, the crystalline fragments were aggregated to a great extent. Sample G still showed some aggregation, but a larger number of isolated fragments could be found. Our results agree with the observation of Mukherjee and Woods that cellulose hydrolyzed under more drastic conditions aggregates less.22 Compared to cellulose crystals from ramie,22 cotton,22 filter paper,6,8 or bleached kraft wood pulp,4,7 the fragments from bacterial cellulose appeared to be less uniform, which was also apparent in pictures of ref 2, 21, and 23. The length of the fragments ranged from about 200 nm to several micrometers. The reason for the nonuniform fragmentation could be related to the predominance of the cellulose IR allomorph in bacterial cellulose or to morphological differences in the microfibrils.24 Thermal Degradation Behavior and Morphology. The TG and DTG curves of unhydrolyzed bacterial cellulose are shown in Figure 4. Three maxima in the weight-loss rate were apparent. The broad DTG peak centered at 50 °C, corresponding to a weight loss of 3.8%, was due to the evaporation of adsorbed water.
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Figure 5. (a) TG and (b) DTG curves of unhydrolyzed (s) and sulfuric acid hydrolyzed bacterial celluloses, samples: B (- - -), F (- -), G (s -), H (- - -).
Figure 3. TEM pictures of sulfuric acid hydrolyzed bacterial celluloses: sample B (top), sample G (bottom). Arrows indicate isolated fragments, bar: 200 nm.
Figure 4. TG (s) and DTG (- -) curves of unhydrolyzed bacterial cellulose.
The DTG peak between 250 and 325 °C was caused by concurrent cellulose degradation processes such as depolymerization, dehydration, and decomposition of glycosyl units followed by the formation of a charred residue. The DTG peak above 425 °C was attributed to the oxidation and
breakdown of the charred residue to lower molecular weight gaseous products. The TG and DTG curves between 150 and 400 °C of various hydrolyzed samples are shown in Figure 5. The degradation behavior of the hydrolyzed samples showed significant differences from that of unhydrolyzed bacterial cellulose. With increasing sulfate content degradation started at lower temperature and occurred over a broader temperature range. Similar changes in the degradation behavior of cellulose were observed with an increase in amorphous content.25 Such an increase could occur during the hydrolysis because sulfuric acid, above a certain concentration (g 65 wt %),26 is a cellulose solvent and at lower concentration still causes swelling. To rule out that an increase in amorphous content was the reason for the changes in degradation behavior, the morphology of the crystals was investigated by X-ray diffraction. The diffraction patterns of selected samples are shown in Figure 6. The diffraction patterns of the hydrolyzed samples still showed the typical reflections of cellulose I indicating that the crystal integrity had been maintained. The crystallinity in the unhydrolyzed sample was 85%. In most hydrolyzed samples, the same amount of or a slight increase in crystallinity was observed. Sample F had a lower crystallinity (72%) than the unhydrolyzed sample. A possible explanation is that the sample may have started to dissolve during hydrolysis. However, neither sample G or H showed a similar decrease in crystallinity and they were hydrolyzed under more drastic conditions. Hence, we attributed the increase in amorphous content in sample F to swelling during the time-consuming washing process by centrifugation, which was only applied to sample F. The fact that the amorphous content decreased in most samples during the hydrolysis
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Hydrolysis of Bacterial Cellulose
Figure 6. X-ray diffraction patterns of unhydrolyzed (sample A) and sulfuric acid hydrolyzed bacterial celluloses. Table 2. Water Content and Char at 350 °C of Sulfuric Acid Hydrolyzed Bacterial Celluloses, from TG Curves sample
water contenta (%)
char at 350 °C (%)
A B C D E F G H
3.8 3.1 2.8 2.7 2.8 2.8 3.1 4.0
11.6 16.5 15.3 15.6 14.1 17.5 21.1 21.9
a
Weight loss at 110 °C.
Table 3. Degradation Temperatures, Tmax, and Maximum Weight-Loss Rates, WLRmax, of the Thermal Degradation Processes of Sulfuric Acid Hydrolyzed Bacterial Celluloses, from Profile Analysis of DTG Curves process 1 sample A B C D E F G H
Tmax (°C)
255 262 260 249 239 226
WLRmax (% min-1)
1.29 1.43 1.27 1.03 1.07 0.79
process 2
Tmax (°C)
WLRmax (% min-1)
297 288 280 284 283 280 269 256 283
7.45 5.10 3.13 3.23 3.33 2.78 1.66 1.01 0.52
suggested that the changes in thermal degradation behavior were due to the sulfate groups. The water content and char at 350 °C of the different samples, obtained from the TG curves, are listed in Table 2. The water content in all samples was around 3%. In sample H, degradation started below 150 °C. Therefore, the weight loss at 110 °C might be due, in part, to degradation processes, and the actual weight loss from water evaporation might be less than the value shown in Table 2. The amount of charred residue at 350 °C was larger in samples with a larger number of sulfate groups, confirming that the sulfate groups act as flame retardants.
Between 150 and 350 °C, the DTG curves were investigated by profile analysis. The degradation temperatures (temperatures at maximum weight-loss rate), Tmax, and maximum weight-loss rates, WLRmax, of the degradation processes, obtained from profile analysis, are listed in Table 3. With progressively higher sulfation, a more complex degradation pattern was observed. Starting with sample C, the degradation between 150 and 350 °C is best described in terms of two processes. We propose that the lower temperature process (termed “process 1” in Table 3) corresponds to degradation of the more accessible, and therefore more highly sulfated, amorphous regions, whereas the higher temperature process (“process 2” in Table 3) relates to the breakdown of the unsulfated crystal interior. At the highest sulfation levels (samples G and H), a two-process degradation fails to totally explain the experimental data. In the case of sample H, we tested this possibility by adding an intermediate temperature process and obtained an excellent fit to the observed DTG data. For convenience, the third process in sample H is listed in Table 3 as a second entry under process 2. Under the more drastic hydrolysis conditions, applied in the cases of samples G and H, sulfation might not only occur in the amorphous regions but might also affect cellulose chains at the splayed ends of the crystals. Accordingly, the additional degradation process could be related to enhanced access to the crystal interior due to degradation of sulfated regions at the ends of crystals. The degradation temperatures and maximum weight-loss rates of the different degradation processes decreased with increasing sulfate content. The differences in sulfate content in the samples C, D, and E were too small to define a noticeable trend. A stepwise degradation behavior was also observed by Kim et al. in TGA curves of sulfuric acid impregnated cellulose.15 They concluded that cellulose pyrolysis in the presence of sulfate is divided into a low-temperature process between 110 and 200 °C and a high-temperature process between 300 and 600 °C. Sample impregnation was done by immersing the cellulose for a few minutes in sulfuric acid with concentrations ranging from one to 20%. Under these conditions, breakdown of the cellulose fibers is not expected and the sulfate will be located primarily in the amorphous regions on the fiber surface. Thus, the interpretation of our three-step degradation behavior is still consistent with the two-step behavior observed by Kim et al. To elucidate the chemical events associated with process 1, sample H was heated to 228 °C and held isothermally for 10 min. The residue was then investigated by FTIR spectroscopy. The FTIR spectra of the sample before and after heating are shown in Figure 7. The final weight loss after 10 min at 228 °C was 37%. The spectrum, after heating, showed bands at 1732 and 1631 cm-1 indicating the presence of carbonyl groups and unsaturated carbon-carbon bonds in the sample. During thermooxidative degradation of cellulose, there are a number of reactions that lead to CdO and CdC bond formation. At low temperatures, CdC bonds arise from elimination of water involving the ring hydroxyl groups. Carbonyl groups are formed by rearrangement of intermediary enols.27 A
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Roman and Winter Table 4. Onset Temperatures and Activation Energies, Determined with the Broido Method, of the Thermal Degradation Processes in Sulfuric Acid Hydrolyzed Bacterial Celluloses process no. 1
Figure 7. FTIR spectra of sulfuric acid hydrolyzed bacterial cellulose (sample H): (a) before and (b) after heating in air to 228 °C for 10 min, normalized to the same intensity of the C(3)-O(3)H stretching vibration at 1059 cm-1.
decrease in the size of the OH band between 3000 and 3800 cm-1 upon heating indicated that partial dehydration had taken place. Since the elimination of sulfuric acid in sulfated anhydroglucose units requires less energy than elimination of water,14 desulfation probably also occurred. The released sulfuric acid molecules stay in the heated zone and do not decompose or volatize until higher temperatures (g380 °C).14 If behaving like phosphoric acid, sulfuric acid facilitates removal of the ring hydroxyl groups by either direct catalysis of the elimination of water or esterification of the hydroxyl groups and subsequent elimination of sulfuric acid.13 Hence, process 1 probably includes both desulfation and acidcatalyzed dehydration. The large weight loss of 37% after 10 min at 228 °C suggests that partial decomposition or depolymerization and volatilization of the products had also taken place. Removal of the ring hydroxyl groups reduces depolymerization through intramolecular transglycosylation27 and, thus, the amount of volatile anhydro sugars formed, constituting the tar fraction. A suppression of intramolecular transglycosylation could be the reason for the larger char fraction observed in samples with larger amounts of sulfate groups. However, the char fraction also increases through decomposition of anhydro sugars after their formation before volatilization and through thermal auto-cross-linking of cellulose chains. Both processes are also acid catalyzed.28,29 In an oxidative atmosphere, degradation processes involving radicals, such as the thermal auto-oxidation of cellulose via hydroperoxide groups, also have to be considered. The effect of sulfuric acid on radical reactions is not obvious. The activation energies of the degradation processes between 150 and 350 °C were determined with the Broido method. The calculated values are listed in Table 4 together with the temperature ranges and R2 values for the linear regression. The onset temperatures of the degradation peaks, obtained from profile analysis of the DTG curves, are also included. The activation energies for both processes were significantly lower for larger amounts of sulfate groups, suggesting that degradation reactions had been catalyzed by sulfuric acid. Catalysis could either be direct through the acid molecules or indirect by promoting dehydration reactions and increasing the amount of water released. Water is known to catalyze
2
sample C D E F G H A B C D E F G H
onset tempa (°C)
temp range (°C)
activation energy (kJ mol-1)
R2 valueb
212 220 215 203 183 180 272 258 254 260 257 252 243 212 258
205-264 212-269 206-268 200-247 191-241 198-235 284-300 262-292 264-283 269-288 268-287 265-284 254-277 240-260 270-287
123.0 130.3 122.1 111.0 74.5 71.2 283.4 297.5 138.9 148.2 146.6 127.1 74.1 60.8 34.5
1.0000 0.9999 1.0000 0.9997 0.9998 0.9998 0.9998 0.9999 0.9998 0.9998 0.9998 0.9998 0.9999 0.9995 0.9995
a From profile analysis of DTG curves. b Regression coefficient from linear regression analysis of a plot of ln(ln 1/y) vs 1/T, see Experimental Section.
cellulose degradation reactions.30 In addition, removal of the ring hydroxyl groups leads to a loss of the stabilizing intraand intermolecular hydrogen bonds, which could also lower the activation energy for degradation. Conclusions The objective of this study was to optimize the hydrolysis conditions for the application of the crystals as reinforcements in polymer composites. In view of the considerable decrease in degradation temperatures caused even by small amounts of sulfate groups, esterification during hydrolysis should be kept at a minimum. Thus low acid concentrations, low acid-to-cellulose ratios, and short reaction times are preferred. An alternative is to use hydrochloric acid, which does not introduce any acidic groups. The aggregation of cellulose crystals from hydrochloric acid hydrolysis,7 due to the lack of surface charges, was a disadvantage of hydrochloric acid with respect to sulfuric acid. However, because the suppression of aggregation by introduction of charged sulfate groups compromises the thermostability of the crystals, aggregation of the crystals cannot be avoided even with sulfuric acid. Acknowledgment. The authors gratefully acknowledge generous support by USDA-CSREES awards (W.T.W.) 9635501-3454 and 98-35504-6358 and fellowship support for M.G. by the German Academic Exchange Service (DAAD). Assistance by Dr. R. B. Hanna and Dr. S. E. Anagnost with the TEM is also acknowledged. References and Notes (1) Rånby, B. G. Acta Chem. Scand. 1949, 3, 649. (2) Marchessault, R. H.; Morehead, F. F.; Joan Koch, M. J. Colloid Sci. 1961, 16, 327. (3) Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Nature 1959, 184, 632. (4) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170.
Hydrolysis of Bacterial Cellulose (5) Revol, J.-F.; Godbout, L.; Dong, X.-M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (6) Dong, X. M.; Revol, J.-F.; Gray, D. G. Cellulose 1998, 5, 19. (7) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Colloids Surf. 1998, 142, 75. (8) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Langmuir 2000, 16, 2413. (9) Favier, V.; Chanzy, H.; Cavaille´, J. Y. Macromolecules 1995, 28, 6365. (10) Helbert, W.; Cavaille´, J. Y.; Dufresne, A. Polym. Compos. 1996, 17, 604. (11) Chazeau, L.; Cavaille´, J. Y.; Canova, G.; Dendievel, R.; Boutherin, B. J. Appl. Polym. Sci. 1999, 71, 1797. (12) Tang, W. K.; Neill, W. K. J. Polym. Sci., Part C 1964, 6, 65. (13) Barker, R. H.; Drews, M. J. In Cellulose chemistry and its applications; Nevell, T. P., Zeronian, S. H., Eds.; Ellis Harwood: Chichester, U.K., 1985; p 423. (14) Julien, S.; Chornet, E.; Overend, R. P. J. Anal. Appl. Pyrolysis 1993, 27, 25. (15) Kim, D.-Y.; Nishiyama, Y.; Wada, M.; Kuga, S. Cellulose 2001, 8, 29. (16) Parks, E. J. Tappi J. 1971, 54, 537. (17) Verlhac, C.; Dedier, J.; Chanzy, H. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 1171.
Biomacromolecules, Vol. 5, No. 5, 2004 1677 (18) Gran, G. Analyst 1952, 77, 661. (19) Krumm, S. WinFit, Beta Release 1.2.1; Geology Department, University of Erlangen: Erlangen, Germany, 1997. (20) Broido, A. J. Polym. Sci., Part A-2 1969, 7, 1761. (21) Araki, J.; Kuga, S. Langmuir 2001, 17, 4493. (22) Mukherjee, S. M.; Woods, H. J. Biochim. Biophys. Acta 1953, 10, 499. (23) Rånby, B. G. Ark. Kemi 1952, 4, 249. (24) Yamamoto, H.; Horii, F. Cellulose 1994, 1, 57. (25) Cabradilla, K. E.; Zeronian, S. H. In Thermal Uses and Properties of Carbohydrates and Lignins; Shafizadeh, F., Sarkanen, K. V., Tillman, D. A., Eds.; Academic Press: New York, 1976; pp 73-96. (26) Jayme, G.; Lang, F. In Methods in Carbohydrate Chemistry; Whistler, R. L., Ed.; Academic Press: New York, 1963; Vol. 3, pp 75-83. (27) Shafizadeh, F. In Cellulose chemistry and its applications; Nevell, T. P., Zeronian, S. H., Eds.; Ellis Harwood: Chichester, U.K., 1985; pp 266-289. (28) Fung, D. P. C.; Tsuchiya, Y.; Sumi, K. Wood Sci. 1972, 5, 38. (29) Black, E. L. Pulp Paper Magn. Can. 1967, 68, T-165. (30) Scheirs, J.; Camino, G.; Tumiatti, W. Eur. Polym. J. 2001, 37, 933.
BM034519+
