Langmuir 2001, 17, 21-27
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Steric Stabilization of a Cellulose Microcrystal Suspension by Poly(ethylene glycol) Grafting Jun Araki,* Masahisa Wada, and Shigenori Kuga Department of Biomaterials Science, Graduate School of Agriculture and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
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Received July 29, 2000. In Final Form: October 16, 2000 A sterically stabilized aqueous suspension of rodlike cellulose microcrystals was prepared by the combination of acid hydrolysis of native cellulose, oxidative carboxylation of microcrystals, and grafting of poly(ethylene glycol) having a terminal amino group on one end (PEG-NH2, MW ) 1000) using watersoluble carbodiimide. Chemical binding of PEG to the microcrystals was confirmed by weight increase, diminishment of carboxyl groups, thermogravimetry, and infrared spectroscopy, resulting in consumption of 20-30% of the initially introduced carboxyl groups. The amount of bound PEG was 0.2-0.3 g/g of cellulose. The PEG-grafted cellulose microcrystals showed drastically enhanced dispersion stability, that is, resistance to addition of 2 M sodium chloride, and ability to redisperse into either water or chloroform from the freeze-dried state. The concentrated aqueous suspension of PEG-grafted microcrystals formed a chiral nematic mesophase through a phase separation similar to that of the ungrafted sample, but with a reduced spacing of the fingerprint pattern.
Introduction Colloidal suspensions in general are stabilized by either electrostatic repulsion or steric hindrances by adsorbed or grafted polymers. In the latter case, direct contacts between particles are prevented by the polymer chains extending into the medium.1-3 If such polymers are covalently bound to the particle at one end and stretch into the surrounding medium, they are called “polymer brushes”.4,5 In practical applications of colloidal systems, the steric stabilization has several advantages over electrostatic stabilization: (a) they are stable at high ionic strengths, (b) the so-called electroviscous effect can be reduced by adding electrolyte, (c) higher solid contents can be achieved, and (d) hydrophilic particles can be dispersed in nonaqueous solvents.1-3 In addition, the steric stabilization provides useful models of the “hard repulsion” (particle interaction potential becomes infinite when two particles contact and is zero at greater distance) in contrast to the “soft repulsion” by the electrostatic stabilization described by the DLVO theory. This difference influences many aspects of colloidal behavior. For example, the electrostatically stabilized boehmite particles form various types of gel or a “birefringent glassy phase” according to electrolyte concentration,6 whereas the boehmite particles stabilized by poly(isobutene) grafts in cyclohexane form a nematic liquid crystal.7 The latter system is the first instance of isotropic-nematic transition by hard repulsion. Many instances of steric stabilization have been reported to date.3 Most of them, however, are those of spherical particle suspensions. To our knowledge, application of the steric stabilization to rodlike particles is limited to the * Corresponding author. E-mail:
[email protected]. (1) Napper, D. H. J. Colloid Interface Sci. 1977, 58, 390. (2) Vincent, B. Adv. Colloid Interface Sci. 1974, 4, 193. (3) Vincent, B. Chem. Eng. Sci. 1993, 48, 429. (4) Milner, S. T. Science 1991, 251, 305. (5) Milner, S. T. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 2743. (6) Philipse, A. P.; Nechifor, A.-M.; Pathmamanoharan, C. Langmuir 1994, 10, 2106. (7) Philipse, A. P.; Nechifor, A.-M.; Pathmamanoharan, C. J. Phys. Chem. B 1993, 97, 11510.
boehmite system mentioned above,6-8 the silica-coated boehmite grafted with poly(isobutene) or octadecyl alcohol,9 and the rodlike gold particles stabilized by polyvinylpyrolidone.10,11 Adding more varieties to this category of colloid will be useful in understanding its unique properties. In the present study we undertook the steric stabilization of cellulose microcrystals. The acid hydrolysis residue of native cellulose is known to form rodlike colloidal suspensions, and their interesting properties such as anomalous viscosity and chiral nematic phase separation have been investigated.12-18 While the cellulose microcrystal suspension obtained by sulfuric acid hydrolysis is stabilized by surface charge due to the introduction of sulfate esters,12-15 those obtained by hydrochloric acid have minimal surface charges and show some important differences from the former, such as anomalous viscosity.16-18 To prepare the sterically stabilized cellulose microcrystals, we chose the HCl-hydrolyzed microcrystal as the starting material and poly(ethylene glycol) (PEG) as the stabilizing polymer. An effective synthetic procedure for grafting PEG onto cellulose was developed. The properties of the obtained “polymer brush” type colloid were studied by infrared spectroscopy, thermogravimetry, (8) Buining, P. A.; Veldhuizen, Y. S. J.; Pathmamanoharan, C.; Lekkerkerker, H. N. W. Colloids Surf. 1992, 64, 47. (9) Philipse, A. P.; Nechifor, A.-M.; Pathmamanoharan, C. Langmuir 1994, 10, 4451. (10) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. J. Phys. Chem. B 1997, 101, 852. (11) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. Langmuir 2000, 16, 451. (12) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (13) Revol, J.-F.; Godbout, L.; Dong, X. M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (14) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir 1996, 12, 2076. (15) Orts, W. J.; Godbout, L.; Marchessault, R. H.; Revol, J.-F. Macromolecules 1998, 31, 5717. (16) (a) Hermans, J. J. Polym. Sci. C 1963, 2, 129. (b) Hermans, J. J. Polym. Sci. C 1963, 2, 145. (17) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Colloids Surf., A 1998, 142, 75. (18) Araki, J.; Wada, M.; Kuga, S.; Okano, T. J. Wood Sci. 1999, 45, 258.
10.1021/la001070m CCC: $20.00 © 2001 American Chemical Society Published on Web 12/07/2000
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Langmuir, Vol. 17, No. 1, 2001 Scheme 1. Procedures for Steric Stabilization of Cellulose Microcrystals
Araki et al. amidation reagent under mild conditions, such as pH ) 4.75 and at room temperature,23 and has been used, for example, for blocking carboxyl groups in pulp for studying the adsorption of papermaking chemicals.24 The conditions described by Danishefsky and Siskovic,23 however, were reported to fail to give the desired amide linkage because the reaction intermediate, O-acylisourea, irreversibly transforms into the stable N-acylurea.25,26 Bulpitt and Aeschlimann26 achieved the preparation of the desired amide by simultaneously adding N-hydroxysuccinimide (NHS) sulfonate, an esterification reagent, to the system and letting them react at pH ) 7.5-8.0. We followed their procedure to prepare the PEG-grafted microcrystals via amide linkage.
Experimental Section
electron microscopy, and X-ray analysis. The state of its colloidal dispersion was characterized by stability/dispersibility tests and mesophase formation. Experimental Scheme The procedure for the preparation of PEG-grafted cellulose microcrystals is shown in Scheme 1. First, the cellulose microcrystals prepared by HCl hydrolysis were carboxylated by NaClO oxidation catalyzed by 2,2,6,6,-tetramethyl-1-(pyperidinyloxy) radical (TEMPO). Due to the steric hindrance, TEMPO converts only primary hydroxyl groups of carbohydrates into carboxyl groups.19-22 This reaction has been applied to cellulose leading to partial21 or complete solubilization22 through the formation of “cellouronic acid”. The latter authors also noted that only the regenerated cellulose (cellulose II) was completely solubilized, while the native cellulose (cellulose I) was only partially converted, presumably because of poor accessibility of crystalline cellulose. This feature seems favorable for our present purpose, that is, to introduce carboxyl groups onto the surface of the microcrystals. The carboxylation of cellulose microcrystals was followed by amidation with a single teminally aminated PEG (PEG-NH2) using a water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC). EDC is an effective (19) de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Carbohydr. Res. 1995, 89. (20) de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H. Synthesis 1996, 1153. (21) Chang, P. S.; Robyt, J. F. J. Carbohydr. Chem. 1996, 15, 819. (22) Isogai, A.; Kato, Y. Cellulose 1998, 5, 153.
Materials. Whatman CF11 cellulose powder (Whatman, U.K.) was used as cellulose material. PEG-NH2 of MW ) 1000 (Sunbright MEPA-10H) was donated by NOF Corp., Tokyo. Sodium hypochlorite solution (min 5%) (Wako Pure Chemicals Ind., Tokyo) was used as received; its precise concentration was determined by iodometry27 as follows: 1 mL of NaClO solution was diluted to ∼50 mL by water and mixed with 2 g of potassium iodide and 15 mL of 4 N sulfuric acid, followed by titration of liberated iodine with 0.1 N sodium thiosulfate and starch indicator. All other chemicals were of reagent grade (Wako Pure Chemicals) and used without further purification. Deionized water was used for all experiments. Preparation of Cellulose Suspension. According to the method of Hermans,15a 10 g of CF11 was hydrolyzed with 100 mL of 2.5 N HCl at 100 °C for 15 min. After thorough washing with water by filtration, the hydrolyzate was homogenized with a Waring type blender for 30 min. The sample was repeatedly centrifuged at 1600g for 5 min, and turbid supernatant was collected. Carboxylation. Carboxylation of cellulose microcrystals was performed according to the method of Isogai and Kato19 based on de Nooy’s recipie16 as follows: 500 mL of the cellulose suspension of ∼0.5-1% solid content was mixed with 0.5 g of TEMPO and 5 g of NaBr. The oxidation was started by the addition of a desired amount of NaClO solution. The degree of carboxylation was controlled with the amount of added NaClO. The mixture was stirred for 4 h at room temperature with the pH adjusted to 10-11 by 3 N NaOH. The carboxylated microcrystals were collected by addition of ∼30 g of NaCl and centrifugation. The product was further washed with 0.5-1 M NaCl by redispersion and centrifugation. This washing procedure was repeated two more times for removing remaining NaClO. The carboxyls were converted to the free acid form by two more times of centrifuge washing with 0.1 N HCl. Finally, the product was dialyzed against deionized water. At this stage the microcrystals became a well-dispersed colloidal suspension. Similar to the results by Kitaoka et al.,28 the amount of introduced carboxyl groups could be controlled up to 1.2 mol/kg of cellulose by changing the amount of added NaClO as shown in Figure 1. The excess addition of NaClO, however, caused the decrease in mass yield, probably due to partial solubilization of cellulose. In the following experiments, except for thermogravimetry, the cellulose sample carboxylated by NaClO 50% weight to dry cellulose was used, giving a carboxyl content of 915 mmol/kg as determined by conductometric titration (see below). PEG Graft on Microcrystals. Grafting of PEG-NH2 onto microcrystals with EDC and NHS was performed according to Bulpitt and Aeschlimann26 with minor modifications. The typical procedure was as follows: To 400 mL of the carboxylated microcrystal suspension of ∼0.5-1% solid content, solid PEG(23) Danishefsky, I.; Siskovic, E. Carbohydr. Res. 1971, 16, 199. (24) Kitaoka, T.; Isogai, A.; Onabe, F. Nordic Pulp Pap. Res. J. 1995, 10, 253. (25) Kuo, J.-W.; Swann, D. A.; Prestwich, G. D. Bioconjugate Chem. 1991, 2, 232. (26) Bulpitt, P.; Aeschlimann, D. J. Biomed. Mater. Res. 1999, 47, 152. (27) Kolthoff, I. M.; Sandell, E. B. In Textbook of Quantitative Inorganic Analysis: 3rd ed.; the Macmillan Company: New York, 1952; p 597. (28) Kitaoka, T.; Isogai, A.; Onabe, F. Nordic Pulp Pap. Res. J. 1999, 14, 279.
Steric Stabilization of Cellulose Microcrystals
Langmuir, Vol. 17, No. 1, 2001 23 a concentrated suspension (>5% solid content) was prepared by concentrating a dilute suspension by osmotic compression using cellulose tubing and PEG 20 000. After sonication for 1 min (Nissei US-150 sonicator), the suspension was allowed to stand overnight to undergo the phase separation into upper isotropic and lower anisotropic phases. These phases were examined by a polarizing microscope.
Results and Discussion
Figure 1. Carboxyl contents and mass yields of the carboxylated microcrystals plotted versus the amount of added NaClO. NH2 of 2 times equivalent to the estimated carboxyl was added and stirred until dissolution. The pH of the mixture was adjusted to 7.5-8.0 by adding small amounts of 0.5-1 N NaOH or HCl. To this mixture 20 mL of water containing EDC and NHS was added, both 1.1-1.5 times equivalent to the carboxyl. The mixture was stirred overnight at room temperature while keeping the pH at 7.5-8.0. After the reaction, the sample was brought to pH ) 1 with the addition of HCl and dialyzed thoroughly against deionized water to remove excess reagents and free PEG-NH2. Characterizations of Microcrystal Suspension. The solid content of the samples was determined by weighing the sample after drying at 55 °C; drying of carboxylated sample at 105 °C should be avoided because of blackening due to thermal degradation. The size and shape of the microcrystals were examined by transmission electron microscopy. A drop of the diluted suspension was mounted on a bacitracin-pretreated surface of a carbon-coated grid and observed by JEOL JEM2000EX using the defocus contrast technique. FT-IR spectra of the dried microcrystal films formed on a Teflon sheet were recorded by Nicolet MAGNA-IR 860 with resolution of 2 cm-1 and 64 times accumulation. Only a spectrum for the pure PEGNH2 was obtained using the KBr pellet method due to the difficulty of preparing a solid film from gummy PEG. X-ray analysis of freeze-dried microcrystals was performed with a Rigaku RU-200BH rotating anode X-ray generator equipped with a flat-plate vacuum camera. Ni-filtered Cu KR radiation (λ ) 0.1542 nm) generated at 50 kV and 100 mA was collimated by a pinhole of 0.3 mm diameter. Diffraction images recorded on a Fuji imaging plate were converted into 2θ-intensity profiles using Rigaku R-AXIS software. Thermogravimetry of the freeze-dried sample was performed with Shinku-Riko TGD9600 at a heating rate of 2 °C/min under dry nitrogen. Only for this measurement, the sample prepared by addition of a large excess of NaClO (780% to cellulose) was used, since the 50% NaClO-treated sample did not give a clear two-step decrease, due to overlapping of the degradation of the carboxylated sample and that of PEG. The carboxyl content of microcrystals was determined by conductometric titration as follows: 50 mL of microcrystal suspension (containing ∼500 mg of microcrystals and 1 mM NaCl) was mixed with 2 mL of 0.1 N HCl, followed by titration with 0.1 N NaOH added at 0.1 mL/min. The titration curve showed the presence of strong and weak acid groups. The amount of strong acid corresponded with the added HCl, and that of weak acid gave the carboxyl content. The dispersion stability against electrolyte addition was examined for the carboxylated and the PEG-grafted suspensions. The suspension was mixed with NaCl solution to make a suspended content of 0.5% and a salt concentration of 0.02-2 M. After vigorous mixing the sample was allowed to stand for 1 week and the critical NaCl concentration to cause flocculation was determined. Liquid crystal formation was examined for the carboxylated and the PEG-grafted suspensions. For this purpose,
The PEG grafting by amidation was carried out successfully, with a mass yield of 124% based on the starting (carboxylated) cellulose. This corresponds to 26% of carboxyl consumption. Since the free PEG of molecular weight 1000 readily permeates the dialysis membrane (cellulose tubing), this increase must represent the bound PEG. Further evidence of effective PEG binding is the decrease in carboxyl content of the PEG-grafted sample determined by conductometric titration. As Figure 1 shows, the carboxyl contents of the starting cellulose and the carboxylated sample were 0 and 915 mmol/kg, respectively, while that of the PEG-grafted sample was 415 mmol/kg. After correcting the weight increase by PEG binding, the consumed carboxyl was 38.6% of the initial carboxyl content. Additional evidence of PEG binding was obtained from the decomposition behavior in thermogravimetry. Figure 2 shows the thermogravimetric curves of (a) carboxylated cellulose, (b) pure PEG-NH2, and (c) PEG-grafted cellulose. In general, thermal decomposition of unmodified cellulose proceeds at 320-360 °C. The shift of decomposition to lower temperature, to about 200 °C, is characteristic of the chemically modified cellulose.29,30 The present result for oxidized cellulose (curve a) is a typical example. Curve b shows the rapid decomposition of pure PEG at 340-380 °C, with a residue of 5% at 450 °C. The PEG-grafted cellulose (c) clearly shows a two-step decrease, namely a superposition of the features of modified cellulose and PEG. The amount of residue at 450 °C was about 18% of the starting material. The decrease in PEG weight in curve c was determined as ∼20%, by extrapolation of the PEG component (dotted line). By assuming that the grafted PEG decomposes in the same way as in curve b, the weight fraction of PEG in the PEG-grafted microcrystal was 21%. This value corresponds to the 30% consumption of the carboxyl groups on the carboxylated microcrystals. The amount of bound PEG evaluated from carboxyl consumption determined by conductometric titration was greater than that determined by thermogravimetry and the weight increase. This is possibly due to consumption of carboxyl groups by side reactions such as the formation of the N-acylurea derivative.25,26 Carboxyl consumption of 30% by PEG grafting corresponds to the PEG grafting density of one polymer/2 nm2, considering the cellulose particle dimensions of 7 × 7 × 120 nm. Figure 3 shows the X-ray diffraction profiles of PEGgrafted microcrystals, carboxylated microcrystals, and pure PEG-NH2. The profile for the PEG-grafted microcrystals was nearly identical to that of the carboxylated (ungrafted) sample. Two sharp diffraction peaks of the crystalline PEG at 2θ ) 19.1° and 23.3°31 were not observed in the profile of the PEG-grafted sample. This result shows that the grafted PEG molecules are amorphous, probably because the bound PEG molecules do not have enough mobility to crystallize. The lack of PEG crystallinity in the grafted sample also affected the FT-IR spectra. Figure (29) Varma, A. J.; Chavan, V. B. Cellulose 1995, 2, 41. (30) Lerdkanchanaporn, S.; Dollimore, D.; Alexander, K. S. Thermochim. Acta 1998, 324, 25. (31) Barnes, W. H.; Ross, S. J. Am. Chem. Soc. 1936, 58, 1129.
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Figure 3. X-ray profiles of (a) the PEG-grafted microcrystals, (b) the carboxylated microcrystals, and (c) PEG-NH2.
Figure 2. Thermogravimetric curves of (a) the carboxylated microcrystals, (b) PEG-NH2, and (c) the PEG-grafted sample.
4 shows the FT-IR spectra of the PEG-grafted microcrystals, the carboxylated microcrystals, and the pure PEGNH2. The spectrum of the carboxylated (ungrafted) microcrystal (Figure 4b) showed peaks at 1708 and 1640 cm-1 that are assigned to free and salt forms of carboxylic acid, respectively. The spectrum of PEG-grafted sample (Figure 4a) gave two new absorptions at 1657 and 1544 cm-1, corresponding to amide I and amide II absorptions. While the pure PEG has many characteristic peaks at 850-1500 cm-1 (Figure 4c), no corresponding peaks
appeared in the spectra of the PEG-grafted microcrystals. This is considered to be due to the amorphous nature of the bound PEG stated above, since the crystallinity is known to significantly affect the IR spectrum of PEG.32,33 Instead, the reported strong absorptions of amorphous PEG at 1107 and 2865 cm-1 (ref 33) should be observed for the PEG-grafted microcrystals. In fact, these peaks are seen in Figure 4a, though the change is obscured by the overlapping peaks of cellulose. The effective binding of PEG described above resulted in a drastic change in the stability of the microcrystal suspension, especially against addition of electrolyte. The ungrafted microcrystal suspension formed a loose precipitate when allowed to stand for 1 week under the presence of 0.04-0.1 M NaCl. This precipitate was readily redispersed by gentle shaking and showed flow birefringence. Under 0.5 M NaCl, the ungrafted microcrystal suspension became turbid immediately and lost flow birefringence. In contrast, the PEG-grafted microcrystal suspension was completely stable under 2 M NaCl, still showing flow birefringence (Figure 5a). Even after freezedrying, the PEG-grafted microcrystals could be readily redispersed in water and formed a stable suspension showing flow birefringence. The freeze-dried microcrystals could be dispersed also in chloroform by vigorous shaking or brief sonication. This suspension showed flow bire(32) Davison, W. H. T. J. Chem. Soc. 1955, 3270. (33) Matsuura, H.; Miyazawa, T. J. Polym. Sci. A-2 1969, 7, 1735.
Steric Stabilization of Cellulose Microcrystals
Figure 4. FT-IR spectra of (a) the PEG-grafted microcrystals, (b) the carboxylated microcrystals, and (c) PEG-NH2.
fringence (Figure 5b) and was stable for several days. To our knowledge this is the first instance of a stable cellulose suspension in a nonpolar solvent. Thus, the remarkable effect of steric stabilization by polymer grafting was shown here in terms of stability against electrolyte addition, as well as redispersibility similar to that for the case of boehmite by Buining et al.8 Figure 6 shows the electron micrographs of the original, carboxylated, and PEG-grafted microcrystals. The original microcrystals are seen to form bundlelike aggregates similar to those reported earlier.15 The carboxylated microcrystals show much better dispersion due to the electrostatic repulsion. The degree of dispersion is also high in the PEG-grafted sample. The carboxylated and the PEG-grafted microcrystals had the same size and shape; the isolated single microcrystal was 100-200 nm long and 5-10 nm wide. Above 5% solid content, both the carboxylated and the PEG-grafted suspensions separated into the upper isotropic and the lower anisotropic phases, similarly to previous observations with sulfuric acid-hydrolyzed cellulose12-14 and chitin34 microcrystals. The anisotropic phases of both carboxylated and PEG-grafted microcrystal (34) Revol, J.-F.; Marchessault, R. H. Int. J. Biol. Macromol. 1993, 15, 329.
Langmuir, Vol. 17, No. 1, 2001 25
Figure 5. (a) Carboxylated microcrystal suspension under a 0.5 M NaCl concentration (left) and the PEG-grafted microcrystal suspension under a 2 M NaCl concentration (right). The sample concentration was 0.5% (w/w) for both samples. (b) PEG-grafted microcrystals redispersed into chloroform after freeze-drying. The sample concentration is approximately 0.2% (w/v). Both photographs (a and b) were taken through crossed polarizers under gentle shaking of vials.
suspensions were a typical chiral nematic phase showing fingerprint textures (Figure 7). These patterns are similar to that of the microcrystal suspension prepared by direct sulfuric acid hydrolysis.12-14 Therefore, the PEG grafting and resulting decrease in carboxyl content does not seem to affect the formation of a chiral nematic phase. The fingerprint spacings of carboxylated and PEG-grafted microcrystals were 7.0 and 4.0 µm, respectively. This seems to contradict the intuitive expectation that steric shielding by PEG would lead to larger spacing. The molecular length of PEG 1000 used here, however, is estimated to be 8.5 nm. So, the actual thickness of the PEG layer, even in stretched form, is smaller than the interparticle distance in the chiral nematic phase (calculated as 20 nm, by assuming a solid content of 10% and a particle cross section of 5 × 5 nm2). Instead, the narrower spacing of the PEG-grafted suspension is probably due to the decrease in surface charge. Dong et al.14 also observed a decrease in the chiral nematic pitch by screening electrostatic repulsion with the addition of electrolyte to the liquid crystal phase of
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Figure 7. Fingerprint texture of (a) the carboxylated microcrystals and (b) the PEG-grafted microcrystals. The weight fraction of cellulose was 9.5% for both samples.
et al.12 argued, the three types of microcrystals stated above (directly H2SO4-hydrolyzed, carboxylated, and PEGgrafted) would be expected to give different types of a liquid crystalline phase. Therefore, the twisted particle geometry is the most likely mechanism of the formation of the chiral nematic phase. Figure 6. Electron micrographs of (a) the original (unoxidized) microcrystals, (b) the carboxylated microcrystals, and (c) the PEG-grafted microcrystals.
the H2SO4-hydrolyzed cellulose suspension. Although both their results and ours suggest that the reduced surface charge causes a decrease in the chiral nematic pitch, the appearance of the chiral nematic phase seems to be dependent on many other factors such as sonication or duration of liquid crystal growth. Two mechanisms have been suggested for the formation of the chiral nematic phase of the cellulose microcrystal suspension: the one is based on the spiral arrangement of charge groups on cellulose microcrystals,12 and the other is the mechanism with twisting of microcrystals together with charge envelopes.15,34 Our present results seem to reject the former mechanism for the following reasons. First, both the carboxylated microcrystals and the directly H2SO4-hydrolyzed microcrystals form almost the same chiral nematic phases, despite the difference in the type, amount, and manner of introduction of charge groups. Second, blocking of more than 20% of the carboxyl groups with PEG did not affect the formation of the chiral nematic phase. If the arrangement of charge groups significantly affects the formation of chiral nematic phases, as Revol
Conclusion In this study we successfully prepared a sterically stabilized cellulose microcrystal suspension by grafting of PEG using a carboxylation-amidation procedure. Grafting PEG to cellulose by an amide linkage has the advantage of greater stability than that of an ester linkage or adsorptive binding against severe chemical conditions, such as acid, base, or heating. The freeze-dried PEGgrafted microcrystal could be redispersed in water or nonaqueous solvents. These features seem favorable for industrial applications of microcrystalline cellulose. The PEG-grafted microcrystals, however, did not show a significant difference from the ungrafted sample in liquid crystal formation, probably because the PEG we used here was too short to form a steric barrier shielding the effect of surface charge. In addition, the present PEG-grafted microcrystals contained a certain amount of unreacted carboxyl groups. For elucidating the effect of PEG grafting on the liquid crystalline behavior of the cellulose suspension, it is necessary to use longer PEG molecules and reduce the amount of remaining carboxyl groups by improved reaction conditions. Acknowledgment. We greatly thank NOF Corp., Tokyo, for donation of the aminated poly(ethylene glycol)
Steric Stabilization of Cellulose Microcrystals
sample. We also thank Dr. A. Isogai of the University of Tokyo for his critical and helpful comments, especially about TEMPO-mediated oxidation and amidation procedures, and Dr. K. Iiyama, Dr. A. Takemura, and Mr. A. Ikeda, of the University of Tokyo, for their helpful advice for experiments. Mr. U.-J. Kim provided assistance in thermogravimetry. One of the authors (J.A.) is a research fellow of the Japan Society of the Promotion of Science. This work was partly supported by a Grant-in-Aid for Scientific Research from Ministry of Education, Science, Sports and Culture, Japan (No. 11460077).
Langmuir, Vol. 17, No. 1, 2001 27
Note Added in Proof. After submission of this paper, Heux et al.35 reported a steric stabilization technique of cellulose microcrystals in toluene and cyclohexane. Their method is based on coating microcrystals with surfactant by freeze-drying of aqueous suspension. In this case the stabilizing molecules are physically adsorbed, instead of covalently bound, onto cellulose particles. LA001070M (35) Heux, L.; Chauve, G.; Bonini, C. Langmuir 2000, 16, 8210.
