Ind. Eng. Chem. Res. 2001, 40, 5923-5928
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Structure and Properties of Regenerated Cellulose Films Prepared from Cotton Linters in NaOH/Urea Aqueous Solution Lina Zhang,* Dong Ruan, and Jinping Zhou Department of Chemistry, Wuhan University, Wuhan 430072, China
Regenerated cellulose (RC) films having various viscosity-average molecular weights (Mη) ranging from 2.2 × 104 to 8.2 × 104 g/mol were prepared from cotton linters in 6 wt % NaOH/4 wt % urea aqueous solution by coagulation with 2 M acetic acid and 2% H2SO4 aqueous solution. The dissolution of cellulose and the structure, transparency, and mechanical properties of the RC films were investigated by 13C NMR, ultraviolet, and infrared spectroscopies; scanning electron microscopy; X-ray diffraction; and a strength test. The RC films exhibited the cellulose II crystalline form and a homogeneous structure with 85% light transmittance at 800 nm. 13C NMR spectroscopy indicated that the presence of urea in NaOH aqueous solution significantly enhanced the intermolecular hydrogen bonding between cellulose and the solvent, resulting in a higher solubility of cellulose and the complete transition of its crystalline form from I to II. The tensile strength (σb) of the RC films in the dry state increased with increasing Mη up to 6.0 × 104 g/mol and then hardly changed. The values of σb and the breaking elongation (b) of the RC film having Mη ) 6.0 × 104 g/mol by coagulation with 2% H2SO4 were found to be 106 MPa and 8.0%, respectively, in the dry state and 17.0 MPa and 10.7%, respectively, in the wet state, and the strength was much higher than that of commercially available cellophane. Therefore, a novel and nonpolluting process for the manufacture of cellulose film and fiber from cotton linters in 6 wt % NaOH/4 wt % urea aqueous solution is provided in this work. Introduction Cellulose and wood are most abundant in nature, and they are produced in a sustainable way and offer many possibilities for use, because they are renewable, biodegradable, biocompatible, and derivatizable.1 However, cellulose is difficult to process in solution or as a melt, because of the large amounts of intra- and intermolecular hydrogen bonds in cellulose, which interrupt the dissolution of cellulose solid into solution. In the regenerated cellulose fiber and cellophane industries, the viscose process has long occupied the leading position, although this process has problems with the discharge of toxic gases and substances.2 Therefore, many organic solvent systems such as N-methyl morpholine N-oxidewater,3 liquid ammonia-ammonium thiocyanate-water,4 LiCl-1,3-dimethyl-2-imidazolidinone (DMI),5 and LiCl-N,N-dimethyeacelamide (DMAc)6 have been investigated for regenerated cellulose fiber production. However, most of the systems still seem to be unsuccessful from an industrial viewpoint because of their toxicity and difficult solvent recovery.2 Recently, Kamide et al. reported7,8 that 8-10 wt % aqueous sodium hydroxide (NaOH) solution, one of the cheapest direct cellulose solvents, can dissolve an alkalisoluble cellulose, which was obtained by stream explosion treatment on soft and hard wood pulps. However, this approach is hardly effective on cotton linters, even under drastic stream explosion conditions.8 When cotton linters are placed in a solution of 8-10 wt % NaOH with intermittent mixing by a homegenizer stand at 4 °C for 8 h or longer,9 the mixture is only transformed into a highly swollen gel. Interestingly, the addition of urea * To whom correspondence should be addressed. Phone: +86-27-87219274. Fax: +86-27-87882661. E-mail: lnzhang@ public.wh.hb.cn.
or thiourea to the NaOH solution could have a substantial impact on the cellulose solubility.10-12 In our laboratory, 0.5 M NaOH/0.5 M urea aqueous solution has been used to dissolve a water-insoluble R-glucan, whose intermolecular hydrogen bonds were effectively broken by the solvent, resulting in the dissolution.13 Moreover, the solubility of cellulose in NaOH/urea aqueous solution has been extensively studied,14,15 revealing that 6 wt % NaOH/4 wt % urea aqueous solution is a stable solvent of cellulose and can completely dissolve cellulose I with a viscosity-average molecular weight (Mη) 6.7 × 104 g/mol and Bemliese (cellulose II) with Mη ) 11.2 × 104 g/mol. A new procedure for dissolving cotton linters in 6 wt % NaOH/4 wt % urea aqueous solution to prepare regenerate cellulose films was exploited in our laboratory.16 Compared with the viscose process,2 the advantage of this new solvent of cellulose is that the NaOH content is reduced by 3 times10 and it does not cause the same pollution as CS2. This could be a significant breakthrough in the development of routes for preparing regenerated cellulose that are environment friendly. The cellulose products obtained through the novel cellulose solvent system would have a great impact on the cellulose chemical industries. In this work, an attempt was made to prepare a new class of cellulose films from cotton linters in NaOH/urea solution and then to clarify their structure and properties. Celluloses with different molecular weight were obtained through treatments with sodium hypochlorite (NaClO) aqueous solution for different periods. The effects of the molecular weight of cellulose on the structure and properties of the cellulose films were investigated by 13C NMR, ultraviolet (UV), and infrared (IR) spectroscopies; X-ray diffraction (XAD); scanning electron microscopy (SEM); and a strength test.
10.1021/ie0010417 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001
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Experimental Section Materials. Cotton linters and cellophane were purchased from Hubei Chemical Fiber Group Ltd. (Hubei, China), and the viscosity-average molecular weight (Mη) of the linters was determined to be 10.1 × 104 g/mol. NaClO was purchased from Xilong Chemical Factory (Guangdong, China); it contained NaClO (calculated as effective chlorine) of not less than 13% and was of analytical grade. All chemical reagents were purchased from commercial resources in China and were of analytical grade. To obtain different molecular weights, the cotton linters were cut into small pieces, which were then soaked in NaClO aqueous solution at room temperature for the desired time. The resulting product was washed with distilled water and filtered under the reduced pressure to obtain degraded cellulose samples, which were coded as F1, F2, F3, F4, and F5 for degradation times from 1 to 25 h, respectively. Original cellulose from the cotton linters was coded as F0. Preparation of Cellulose Films. A 10-g sample of cellulose was dispersed into 190 g of 6 wt % NaOH/4 wt % urea aqueous solution at 4 °C, and the mixture was stirred for 5 min to obtain a slurry. The cellulose slurry was held at -5 °C for about 5 h and then thawed with intermittent mixing at 0-4 °C to obtain a transparent solution. A few insoluble parts of the cellulose and the gel in solution were removed by centrifuging at 8000 rpm for 30 min to give a clear cellulose solution with a polymer concentration (cp) of 4-5%. The resulting solution was cast onto a glass plate to give a thickness of 0.20-0.30 mm and then immediately coagulated in 2 M acetic acid (CH3COOH) or 2% H2SO4 aqueous solution for 5 min to obtain transparent films. The thickness of the cellulose solution was controlled to within 0.3 mm; otherwise, the cellulose film became a loose structure. The regenerated cellulose film was washed with running water and then dried in air. A series of films were prepared from cellulose samples F0, F1, F2, F3, F4, and F5 and were coded, resepectively, as RC-A0, RC-A1, RC-A2, RC-A3, RC-A4, and RC-A5 for coagulation with 2 M acetic acid and as RC-S0, RCS1, RC-S2, RC-S3, RC-S4, and RC-S5 for coagulation with 2% H2SO4. Characterizations. A 29 wt % aqueous solution of ethylenediamine was saturated with CdO at 0 °C under vigorous stirring and kept below 5 °C for 8 h. The solution was centrifuged at 8000 rpm for 20 min, and then the supernatant was filtered through a sand filter to obtain transparent cadoxen.17 The intrinsic viscosity [η] of the RC films in cadoxen solution at 25 °C was measured by using an Ubbelodhe viscometer, and the viscosity-average molecular weight (Mη) was calculated according to the equation18
[η] ) 3.85 × 10-2 Mw0.76 (mL g-1)
(1)
The Mη values were determined to be 2.2 × 104, 3.0 × 104, 4.5 × 104, 6.0 × 104, 7.1 × 104, and 8.2 × 104 g/mol for the films RC-A5, RC-A4, RC-A3, RC-A2, RC-A1, and RC-A0, respectively. The Mη value (8.2 × 104 g/mol) for RC-A0 was lower than that of the original linters because of degradation of the cellulose solution during storage.14 Solid-state 13C NMR spectra were recorded at ambient temperature on an Infinity Plus spectrometer (13C frequency ) 400.12 MHz) with a CP/MAS unit. The
spinning rate and the contact time were 5.0 kHz and 5.0 ms, respectively. The pulse width was 2.10 µs, the spectral width was 50.000 kHz, the acquisition time was 20.48 ms, and the spectrum was accumulated 2000 times. 13C NMR measurements of the cellulose solution were performed on a Varian Mercury 300 spectrometer (Palo Alto, CA) for cotton linters in a 6 wt % NaOH/4 wt % urea/D2O solution at ambient temperature. The polymer concentration was adjusted to 5 wt %. The spectrometer has a C/H dual-probe system operating at 300.07 MHz for 1H and 75.5 MHz for 13C. The relaxation delay was 2.00 s, the pulse was 32.7°, the spectral width was 13.98 kHz, and the acquisition time was 0.50 s. Scanning electron micrographs were taken with a Hitachi X-650 scanning electron microscope. RC films in the dry or wet state were frozen in liquid nitrogen, immediately snapped, and then vacuum dried. The free surface (side in direct contact with the coagulant) and the fracture surface of the films were sputtered with gold and then observed and photographed. The films were cut into powder and vacuum dried for 24 h, before the measurement of X-ray diffraction patterns and infrared (IR) spectra. The IR spectra of the samples were recorded with a Fourier transform IR (FT-IR) spectrometer (170SX, Nicolet, Madison, WI). The test specimens were prepared by the KBr-disk method. The X-ray diffraction patterns were measured with an X-ray diffractometer (D/MAX-1200, Rigaku Denki, Japan). The X-ray diffraction patterns with Cu KR radiation (λ ) 1.5406) at 40 kV and 30 mA were recorded in the range of 2θ ) 6-40°. The degree of crystallinity (χc) was calculated according to the usual method.19 The apparent crystal size (ACS) was estimated by Scherrer’s equation20
ACS ) 0.9λ/β(cos θ)
(2)
β ) (B2 - b2) 1/2
(3)
with
where λ is the wavelength of the incident X-rays (1.5406 Å); θ is the diffraction angle corresponding to the (11h 0), (110), and (200) planes; b is the instrumental constant (0.1°); and B is the half width in radians of the diffraction angle of the (11h 0), (110), and (200) planes. Measurements of Properties. The tensile strength (σb) and breaking elongation (b) of the dry and wet films were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co., Ltd., Shenzhen, China) according to ISO standard ISO6239-1986 (E) at a speed of 5 mm min-1. The size of the samples was 70 mm in length and 10 mm in width, and 50 mm was the distance between the two clamps. The wet films were measured immediately after being soaked in water for 10 min. Because the strength data are related to the environmental temperature and humidity, these data were obtained under the same conditions. The optical transmittance (Tr) of the films was measured with a UV-vis spectrophotometer (Shimadzu UV-160, Kyoto, Japan) in the wavelength range from 400 to 800 nm, and the thickness of the films used was about 20 µm. Results and Discussion Structure and Morphology of RC Films. SEM images of the RC films are shown in Figure 1. The dried
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Figure 1. SEM images of free surface of the RC-S0 film (A) in the dry state and (B) in the wet state and of the fracture surface of RC-S0 film (C) in the dry state and (D) in the wet state.
Figure 2. X-ray diffraction patterns of the RC-S films and cotton linters. Table 1. Crystalline Parameters of the Cellulose and RC Films ACS (Å) sample
crystallinity (%)
(11h 0)
(110)
(200)
cotton linters RC-S0 RC-S5
74 51 60
51.2 32.7 30.4
60.6 37.8 49.9
66.7 45.3 51.0
films displayed homogeneous structures from the interior to the surface, indicating a dense architecture. However, when a dried film was soaked in water for 6 h, its free surface exhibited a porous structure and an even distribution, in which no fibers from the cotton linters were detected. The X-ray diffraction patterns of the cellulose films are shown in Figure 2. The values of χc and ACS are listed in Table 1. The crystalline form of cellulose I has typical diffraction peaks at 2θ ) 14.8°, 16.3°, and 22.6°.21 The natural cotton linters exhibited a peak at 2θ ) 22.7° (200) and two broadened peaks within the range 2θ ) 16.4-14.7°, corresponding to the crystallographic form of cellulose I. The diffraction patterns of the RC films showed three peaks at 2θ ) 11.8°, 19.9°, and 21.6°, corresponding to the (11h 0), (110), and (200) planes, respectively, as observed for viscose rayon and cuprammonium rayon, which are attributed to the typical cellulose II crystalline form.22 Moreover, the crystallinity h 0), (110), and (200) planes of (χc) and ACS for the (11
Figure 3. FT-IR spectra of the RC-S films and cotton linters.
the RC films were obviously lower than those of the cotton linters. These results indicate that the change of cellulose I into cellulose II occurred in the regeneration process of the RC films. Figure 3 shows IR spectra of the original cotton linters and the cellulose films. The absorption band at 1422 cm-1 for the cellulose films, which is assigned to the CH2 scissoring motion, is weakened and shifted to low wavenumber compared to the 1431 cm-1 peak for the cotton linters, indicating the destruction of an intramolecular hydrogen bond involving O6.23 A new shoulder at 990 cm-1 belonging to the CO stretching vibration in the amorphous region emerges in the cellulose films but not in the cellulose I crystallites.24 The experimental results from SEM, X-ray diffraction, and IR spectroscopy indicate that the cotton linters were changed completely into cellulose II in the RC films by dissolving cellulose I in 6 wt % NaOH/4 wt % urea aqueous solution. To further understand the solubility of cellulose in the new solvent, 13C NMR spectra of cotton linters with Mη ) 5 × 104 g/mol in 6 wt % NaOH/4 wt % urea/D2O solution and of the original linters are shown in Figure 4; their chemical shifts together with those of other cellulose solutions are summarized in Table 2. The C4 peak for the cellulose solution was
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Table 2. Comparison of the
13C
Chemical Shifts of the Cellulose Solution, Cotton Linters, and Other Cellulose Solutions chemical shift (ppm)
sample
C1
C4
C3, C5, C2
C6
source
cotton linters wood pulp linters in 6 wt % NaOH/ 4 wt % urea cellulose in NaOH wood pulp in LiCl-DMI cellulose in LiCl-DMAc
105.0 105.4 103.9 104.7 103.2 103.0
89.3, 84.4 89.1, 84.5 79.2 80.0 79.3 79.6
75.2, 72.1 75.3, 72.7 75.7, 74.0 76.4, 75.0 76.3, 74.3 76.6, 74.4
65.6, 62.5 65.5, 63.0 60.7 61.9 60.5 60.7
this work ref 25 this work ref 25 ref 26 ref 27
located at 79.2 ppm, indicating that the intramolecular hydrogen bonds in cellulose were destroyed. 25 It is worth noting that the chemical shifts of C1 (103.9 ppm), C4 (79.2 ppm), C2 (74.0 ppm), and C6 (60.7 ppm) for the cellulose in 6 wt % NaOH/4 wt % urea were almost the same as those for cellulose in LiCl-DMI26 and LiCl-DMAc,27 which are true solutions of cellulose but differ slightly from the solution of cellulose in aqueous NaOH solution.25 This can be explained by the fact that urea, with its highly polar CO and NH2 groups, can be considered as a potential hydrogen-bonding donor and acceptor for cellulose. The presence of NaOH creates significant ion-pair interactions, which reduce the strong self-associated character of water, favoring the establishment of hydrogen bonds between urea molecules and cellulose chains. These new intermolecular interactions bring the cellulose into the aqueous solution and, at the same time, alter its crystalline conformation from cellulose I to II. Finally, upon coagulation with H2SO4 or acetic acid, the cellulose is regenerated into the RC film, where the intermolecular interactions in cellulose
Figure 4. (A) Solid-state CP/MAS 13C NMR spectrum for cotton linters and (B) proton-decoupled spectrum of 5% linters in 6 wt % NaOH/4 wt % urea/D2O.
form a dense homogeneous structure. Therefore, this work provides a novel process for the manufacture of cellulose film and fiber from cotton linters. The novel method for the RC films prepared from cotton linters in 6 wt % NaOH/4 wt % urea aqueous solution is superior to mercerization in 17.5-20% NaOH with carbon disulfide in xanthation, which consumes a great deal of NaOH and discharges toxic gases. Effect of Mη on Film Properties. The Mη dependences of the tensile strengths (σb) of the dry and wet RC films formed by coagulation with acetic acid and H2SO4 are shown in Figure 5. The results indicate that the σb values of the RC-A and RC-S films in the dry state increased with increasing Mη up to 6.0 × 104 g/mol and then hardly changed. However, the σb values of the films in the wet state slightly increased with increasing Mη. This indicates that the tensile strength of the films in the dry state is related to the molecular weight when Mη is lower than 3 × 104 g/mol. When Mη is larger than 6.0 × 104 g/mol, the effect of molecular weight was not obvious. It is noted that relatively high film strengths were reached, even for the molecular weight of 2.2 × 104 g/mol, suggesting strong intermolecular interactions in the film. As shown in Figure 6, the b values for the RC-A and RC-S films increased with increasing Mη, indicating that relatively high Mη values enhance the toughness of the films. Both σb and b of the films RCA2 and RC-S2 in the dry and wet states were obviously higher than those of other films with Mη below 4.5 × 104 g/mol, suggesting that cotton cellulose with Mη ) 6.0 × 104 g/mol is more suitable for preparing cellulose film in the NaOH/urea system. It is worth noting that the values of σb for film RCS2 in the dry and wet states were much higher than those of commercially available cellophane (unoriented
Figure 5. Mη dependence of the tensile strength (σb) of the RC-A films formed by coagulation with acetic acid in the dry state (O) and in the wet state (b) and of the RC-S films formed by coagulation with H2SO4 in the dry state (2) and in the wet state (4). ‚‚‚, data for commercial cellophane.
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slightly decreased with increasing Mη, but that of the RC-S films hardly changed. The Tr values of the RC-S films were higher than those of RC-A, corresponding with the results from mechanical properties. Relatively high light transmittance of the RC-S reflected relatively strong intermolecular interaction in the films. The light transmittance at 400-800 nm for cellophane having thickness of about 20 µm is 90% slightly higher than those of the RC-S and RC-A films. Conclusion
Figure 6. Mη dependence of the elongation (b) of the RC-A films formed by coagulation with acetic acid in the dry state (O) and in the wet state (b) and of the RC-S films formed by coagulation with H2SO4 in the dry state (2) and in the wet state (4). ‚‚‚, data for commercial cellophane.
Figure 7. Mη dependence of the light transmittance of the RC-A films formed by coagulation with acetic acid at the wavelengths of 400 (O) and 800 (b) nm and of the RC-S films formed by coagulation with H2SO4 at the wavelengths of 400 (2) and 800 (4) nm. ‚‚‚, data for commercial cellophane.
cross-stress), and the values of b were close to those of the cellophane, as shown in Figures 5 and 6. Therefore, the new cellulose films exhibit excellent mechanical properties , and a potential process for the preparation of cellulose films and fibers that can overcome the vital environmental problems of waste (toxic) gases in the current industrial processes for cellophane and viscose rayon is provided here. It is clear that the RC-S films formed by coagulation with 2% H2SO4 have higher tensile strengths and breaking elongations than the RC-A films formed by coagulation with acetic acid. Thus, 2% H2SO4 aqueous solution is a more suitable coagulant for the cellulose solution system. This suggests that the new molecular architecture of cellulose is formed more easily in H2SO428 than in acetic acid, resulting in an enhancement of the strength of the RC-S films. The RC films were transparent and colorless, rather than yellow like cellophane. Figure 7 shows the Mη dependence of the light transmittance at 400 and 800 nm for RC-A and RC-S films having thicknesses of about 20 µm. The light transmittance (Tr) of the RC-A films
Regenerated cellulose films were successfully prepared from cotton linters in 6 wt % NaOH/4 wt % urea aqueous solution. These cellulose films have a typical cellulose II crystalline form, and their degree of crystallinity and apparent crystal size were obviously lower than those of the original cellulose I. 13C NMR spectroscopy indicates that cellulose in 6 wt % NaOH/4 wt % urea aqueous solution forms a true solution. The urea plays an important role in increasing the intermolecular hydrogen bonding between cellulose and the solvent, thus bringing the cellulose into the aqueous solution and altering its crystalline conformation from cellulose I to II. σb values for the films having Mη values ranging from 2.2 × 104 to 8.2 × 104 g/mol were obtained as 77.0-106 MPa for RC-S films formed by coagulation with 2% H2SO4, thus indicating high strengths even for the films with Mη ) 2.2 × 104 g/mol. The film RC-S2 with Mη ) 6.0 × 104 g/mol has the best tensile strength, which is much higher than that of commercial cellophane. The Mη value of the cellulose hardly affected the tensile strength of the films in the dry and wet states when Mη was more than 6.0 × 104 g/mol. The RC-S films coagulated with 2 wt % H2SO4 displayed better mechanical properties and light transmittance than did those coagulated with acetic acid. The light transmittance of the films was close to that of commercially available cellophane. This novel process for cellulose provides a potential application in the field of cellulose fiber and films, as it involves a low cost and nontoxic system. Acknowledgment This work was supported by the National Natural Science Foundation of China (59773026 and 59933070) and Laboratory of Cellulose and Lignocellulosic Chemistry, Guangzhou Institute of Chemistry, Chinese Academy of Sciences. Literature Cited (1) Schurz, J. Trends in polymer science. A bright future for cellulose. Prog. Polym. Sci. 1999, 24, 481. (2) Yamashiki, T.; Matsui, T.; Kowsaka, K.; Saitoh, M.; Okajima, K.; Kamide, K. New class of cellulose fiber spun from the novel solution of cellulose by wet spinning method. J. Appl. Polym. Sci. 1992, 44, 691. (3) Graenacher, C.; Sallman. U.S. Patent 2179181, 1939. Johnson, D. British Patent 1144048, 1969. (4) Hudson, S.; Cuculo, J. A. The solubility of cellulose in liquid ammonia/salt solutions. J. Polym. Sci. A: Polym. Chem. 1980, 18, 3469. (5) Edgar, J.; Bogam, T. European Patent WO 9612096, 1996. (6) Turbak, A.; El-Kafrawy, A.; Snyder, F.; Auerbach, A. Solvent system for cellulose. U.S. Patent 4302252, 1981. (7) Kamide, K.; Okajima, K. U.S. Patent 4634470, 1987. (8) Yamashiki, T.; Matsui, T.; Saitoh, M.; Okajima, K.; Kamide, K.; Sawada, T. Characterization of cellulose treated by the steam explosion method. Part 1: Influence of cellulose resources on
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changes in morphology, degree of polymerization, solubility and solid structure Br. Polym. J. 1990, 22, 73. (9) Kamide, K.; Okajima, K.; Kowsaka, K. Dissolution of natural cellulose in aqueous alkali solution: Role of supermolecular structure of cellulose. Polym. J. 1992, 24, 71. (10) Laszkiewicz, B.; Wcislo, P. Sodium cellulose formation by activation process. J. Appl. Polym. Sci. 1990, 39, 415. (11) Laszkiewicz, B.; Cuculo, J. A. Solubility of cellulose III in sodium hydroxide solution. J. Appl. Polym. Sci. 1993, 50, 27. (12) Laszkiewicz, B. Solubility of bacterial cellulose and its structural properties. J. Appl. Polym. Sci. 1998, 67, 1871. (13) Zhang, P.; Zhang. L.; Cheng, S. Effect of urea and sodium hydroxide on the molecular weight and conformation of R-(1,3)D-glucan from lentinus adodes in aqueous solution. Carbonhydr. Res. 2000, 327, 431. (14) Zhou, J.; Zhang, L. Solubility of cellulose in NaOH/urea aqueous solution. Polym. J. 2000, 32, 866. (15) Zhang, L.; Zhou, J. Novel solvent system of cellulose. Chinese Patent Appl. CN 00114486.3, Apr 2000. (16) Zhang, L.; Zhou, J.; Ruan, D. Novel methods for manufacture of cellulose films. Chinese Patent Appl. CN 00114485.5, Apr 2000. (17) Zhang, L.; Ding, Q.; Zhang, P.; Zhu, R.; Zhou, Y. Molecular weight and aggregation behavior in solution of β-D-glucan from Poria cocos sclerotium. Carbohydr. Res. 1997, 303, 193. (18) Brown, W.; Wiskston, R. A. Viscosity-molecular weight relationship for cellulose in cadoxen and hydrodynamic interpretation. Eur. Polym. J. 1965, 1, 1. (19) Rabek, J. F. Experimental Methods in Polymer Chemistry: Applications of Wide-Angle X-ray Diffraction (WAXS) to the Study of the Structure of Polymers; Wiley-Interscience: Chichester, U.K., 1980; p 505.
(20) Cscherrer, P. Go¨ tinger Nachr. 1918, 2, 98. (21) Isogai, A.; Usuda, M.; Kato, T.; Uryu, T.; Atalla, R. Solidstate CP/MAS 13C NMR study of cellulose polymorphs. Macromolecules 1989, 22, 3168. (22) Zeronian, S. H.; Ryu, H. S. Properties of cotton fibers containing the cellulose IV crystal structure J. Appl. Polym. Sci. 1987, 33, 2587. (23) Nelson, M. L.; O’Connor, R. T. Relation of certain infrared bands to cellulose crystallinity crystal lattice type. Part 1. Spectra of lattice types I, II, III and of amorphous cellulose. J. Appl. Polym. Sci. 1964, 8, 1311. (24) Higgins, H. G.; Stewart, C. M.; Harrington, K. J. Infrared Spectra of cellulose and related polysaccharides. J. Polym. Sci. 1961, 51, 59 (25) Kamide, K.; Okajima, K.; Matsui, T.; Kowsaka, K. Study on the solubility of cellulose in aqueous alkali solution by deuteration IR and 13C NMR. Polym. J. 1984, 16, 857 (26) Takaragi, A.; Minoda, M.; Miyamoto, T.; Liu, H.; Zhang, L. Reaction characteristics of cellulose in the LiCl/DMI solvent system. Cellulose 1999, 6, 93 (27) McCormick, C. L.; Callais, P. A.; Hutchinson, B. H. Solution Studies of Cellulose in Lithium Chloride and N,NDimethylacetamide. Macromolecules 1985, 18, 2394 (28) Yang, G.; Zhang, L.; Feng, H. Role of poly(ethylene glycol) in formation and structure of regenerated cellulose microporous membrane. J. Membr. Sci. 1999, 161, 31.
Received for review December 1, 2000 Revised manuscript received April 27, 2001 Accepted August 15, 2001 IE0010417
