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MATERIALS AND INTERFACES Effects of Coagulation Conditions on the Properties of Regenerated Cellulose Films Prepared in NaOH/Urea Aqueous Solution Lina Zhang,* Yuan Mao, Jinping Zhou, and Jie Cai Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
Effects of coagulant components and coagulation conditions on the structure and properties of regenerated cellulose (RC) films prepared from cellulose in 7.5 wt % NaOH/11 wt % urea aqueous solution were investigated by 13C NMR, X-ray diffraction, scanning electron micrograph, tensile testing, etc. The uniform design method based on theoretical accomplishments in the numbertheoretic method was used to optimize the coagulation conditions of H2SO4 aqueous solution with various concentrations (from 1 to 15 wt %), time (from 1 to 15 min), and temperature (from 25 to 55 °C). Moreover, a series of RC films coagulated, with 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4, respectively; different concentrations (from 1 to 20 wt %) and times (from 1 to 20 min) at 25 °C were also investigated. The results indicated that the optimal coagulant concentrations and coagulation times of the RC films are 5 wt % H2SO4 for 5 min, 5 wt % H2SO4/5 wt % Na2SO4 for 5 min, 5 wt % Na2SO4 for 15 min, 3 wt % HOAc for 5 min, and 5 wt % (NH4)2SO4 for 3 min, respectively, at 25 °C. The RC film that coagulated with 5 wt % H2SO4/5 wt % Na2SO4 aqueous solution exhibited a higher optical transmittance, more homogeneous structure, and better mechanical properties than that coagulated with others on the whole. Moreover, the tensile strength of the RC films in the wet and dry states increased simultaneously with a drop in coagulation temperature. The coagulation mechanism can be described as a two-phase separation, namely a cellulose-rich phase in the gel and a cellulose-poor phase in solution. Introduction Cellulose as an environmentally friendly material can be regenerated or derivatized to yield various useful products as a result of its renewability, biodegradability, and derivatizability.1 Recently, the research and development of regenerated cellulose (RC) films have attracted much scientific and practical interest because of their excellent mechanical performance, chemical stability, permeation characteristics, and biological compatibility, which are important requirements in dialysis technology, food processing, medical applications, etc.2-4 The properties of RC films are intimately related to their structure and morphology, which are mainly controlled by coagulation conditions, coagulant nature, and coagulation mechanism. Moreover, the information about the coagulation conditions is very important in the cellulose industry. Therefore, many attempts have been carried out by investigating the coagulation/regeneration process to design the desired morphology and properties of the RC films.5-7 Extensive studies have been done on the basis of the viscose and cuprammonium process, which still occupies a sole and exclusive position in the preparation of commercial products of cotton or wood celluloses in the form of films and fibers.8 Usually, there are different coagulation conditions in the viscose process, such as 130 g/L H2SO4, * To whom correspondence should be addressed. Phone: +86-27-87219274. Fax: +86-27-68756661. E-mail: lnzhang@ public.wh.hb.cn.
265 g/L Na2SO4, and 11.5 g/L ZnSO4 at 50 °C for a multifilament. In this case, the coagulation process involves a complicated chemical reaction between cellulose xanthate and various mobile ions such as H+, OH-, and Zn2+ in the H2SO4/Na2SO4/ZnSO4 coagulation bath. Hermans et al. have extensively analyzed the coupled diffusion and chemical reaction in a coagulation/ regeneration process for cellulose xanthate solution.9 In the cuprammonium method, very complicated coagulation/regeneration processes such as water coagulation/ acid regeneration, aqueous alkaline coagulation/acid regeneration, and organic-water mixture coagulation/ acid regeneration systems have been adopted for the fiber and film formation. A film with a relatively densely coagulated morphology in the cross-sectional direction and relatively large pore size with some gradient structure in pore size distribution has been obtained, respectively.5,6 The coagulation/regeneration action of the cellulose solution is attributed to three types, namely copper removal, the Norman reaction, and ammonia removal, which are determined by interfacial potential.10 In recent years, the new and powerful organic solvent N-methylmorpholine N-oxide (NMMO), which dissolves cellulose directly without formation of the complex with cellulose, has been developed.11 The RC fibers and films prepared from cellulose/NMMO/H2O solution exhibited excellent mechanical properties and permeation characteristics when the 8-12 wt % cellulose dope was coagulated in water at lower temperature by using the phase-inversion method.11-13 This coagulation process is merely a physical desolvation
10.1021/ie0491802 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/06/2005
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process controlled by diffusion action between the cellulose solution and water as nonsolvent.11 The method simplifies the coagulation/regeneration process and overcomes the vital environmental problems compared with the traditional regenerated-cellulose industry.11,14 In our laboratory, a cheap and nonpolluting direct solvent of cellulose, NaOH/urea aqueous solution, has been developed and studied.15-19 Cellulose could be completely dissolved in 7-8 wt % NaOH/11-12 wt % urea aqueous solution precooled to -10 to -13 °C within 5 min at the ambient temperature.18 Moreover, RC fibers with excellent mechanical properties have been prepared by a wet spinning method.19 This finding has provided considerable potential for RC film and fiber production with respect to both environmental and economic issues. In this work, we attempted to prepare a series of RC films from cellulose in 7.5 wt % NaOH/ 11 wt % urea aqueous solution by coagulating with various coagulants such as H2SO4, 5 wt % H2SO4/ Na2SO4, Na2SO4, HOAc, and (NH4)2SO4. The effects of coagulation conditions and coagulant nature on the mechanical properties, structure, and morphology of the RC films were investigated by using the uniform design method, X-ray diffraction, scanning electron micrography, and tensile testing. Experimental Section Materials. The cellulose (cotton linter pulp) and commercially available cellophane were supplied by Hubei Chemical Fiber Group Ltd. (Hubei, China), and the viscosity-average molecular weight (Mη) of the cellulose was determined by viscometry in cadoxen to be 10.0 × 104. All chemicals employed were of analytical grade and were purchased from commercial sources in China. Film Preparation. Cellulose solution was prepared according to previously reported methods.18,19 The sheets of the cotton linter pulp were shredded and vacuumdried at 80 °C for 24 h before being used. Into a 250 mL beaker, an adequate amount of NaOH, urea, and distilled water (7.5:11:81.5 by weight) were added, and the resulting aqueous solution mixture was stored in a refrigerator. After the solution was precooled to -12 °C, cellulose was added immediately to it with vigorous stirring for 5 min at ambient temperature to obtain a transparent cellulose dope containing 4 wt % cellulose. The cellulose dope was subjected to centrifugation at 10 000 rpm for 20 min at 5-10 °C in order to exclude the slightly remaining undissolved part and to carry out the degasification. The resulting transparent solution was immediately cast on a glass plate at a thickness of 250 µm and then immersed into coagulants (2000 mL) for the desired time at the desired temperature. The resulting wet films were washed with running water and finally air-dried at ambient temperature. A series of aqueous solutions of H2SO4, HOAc, 5 wt % H2SO4/ Na2SO4, Na2SO4, and (NH4)2SO4 as coagulants were examined. The coagulant concentration, coagulation time, and temperature of the H2SO4 solution were varied from 1 to 15 wt %, 1 to 15 min, and 25 to 55 °C, respectively, and the RC films that coagulated at 25 °C were coded as I-ck-tn, where k is the coagulant concentration and n is the coagulation time. The RC films that coagulated with 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4 with various concentrations (c, from 1 to 20 wt %) for desired times (t, from 1 to 20 min) at 25
°C were coded as II-ck-tn, III-ck-tn, IV-ck-tn, and V-cktn, respectively. Characterization. The solution 13C NMR spectrum of the cellulose in 7.5 wt % NaOH/11 wt % urea/D2O solution was recorded on a Mercury-VX 600 spectrometer (Varian, Inc., USA) at ambient temperature. The cellulose concentration was 4 wt %. Deuterated dimethyl sulfoxide (DMSO-d6) was used as the internal standard. The films were cut into powder and vacuum-dried overnight before the measurement of X-ray diffraction. The wide-angle X-ray diffraction was carried out with an X-ray diffractometer (D/MAX-1200, Rigaku Denki, Japan). X-ray diffraction patterns with Cu KR at 40 kV and 50 mA were recorded in a range of 2θ from 4 to 40°. The degree of crystallinity (χc) was calculated according to the peak separation method.20 Scanning electron micrographs (SEM) were performed on a Hitachi S-570 microscope (Japan). The wet films were frozen in liquid nitrogen, snapped immediately, and vacuum-dried. The free surface (side contacting the coagulant) and cross-section of the films were sputtered with gold for SEM measurements. The optical transmittance (Tr) of the films was measured by a UV-vis spectroscope (Shimadzu UV-160A, Japan) at wavelengths ranging from 400 to 800 nm. The thickness of the films was approximately 25 µm. The tensile strength (σb) and elongation at break (b) of the films in both dry and wet states were measured on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co., Ltd., Shenzhen, China) according to ISO 6239, 1986 (E) at speeds of 5 and 10 mm min-1, respectively. The wet films were measured immediately after soaking in water for 30 min. The σb and b values are the averages of five measurements. Uniform Design Method. On the basis of theoretical accomplishments in the number-theoretic method (NTM), the uniform design method (UDM) proposed by Wang and Fang21,22 was used to optimize the coagulation conditions for H2SO4 as coagulant. Three factors influencing the tensile strength of the RC films were considered: (1) coagulant concentration, denoted by X1 (wt %); (2) coagulation time, denoted by X2 (min); (3) coagulation temperature, denoted by X3 (°C). Two factors (X1 and X2) taking eight levels with the domain [1, 15] and one factor (X3) taking four levels with the domain [25, 55] were chosen for the experiment to get more information. It is known that there are some interactions among the factors, and we can ignore highorder interactions. Therefore, U8(82 × 41) was adopted for experimental design.22 The design of the experiments and corresponding results obtained are given in Table 1. All treatments were performed and data were analyzed using the Data Processing Software of Uniform Design 1.00, which was developed by the Uniform Design Association of the Chinese Mathematical Society, and the generalized regression model of the wet and dry films coagulated from H2SO4 is given as
Y ) b0 + b1X1 + b2X2 + b3X3 + b4X12 + b5X22 (1) where Y is the tensile strength, X1 is the coagulant concentration, X2 is the coagulation time, X3 is the coagulation temperature, b0 is the intercept, and bn is the regression coefficient with n from 1 to 5. Thus, threedimensional (3D) plots were produced from regression equations by holding one variable fixed. MATLAB
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Table 1. Uniform Design Data of U8(82 × 41)a for the RC Films from an H2SO4 Coagulation Bath
coagulant no. of concn coagulation coagulation expt (wt %) time (min) temp (°C) 1 2 3 4 5 6 7 8
1 3 5 7 9 11 13 15
7 15 5 13 3 11 1 9
55 45 35 25 55 45 35 25
tensile strength (σb) of the RC films (MPa) wet
dry
11.0 12.5 14.1 14.9 12.0 11.8 10.4 9.7
69.5 70.5 88.7 82.1 82.3 75.4 72.3 65.1
a U (pt × qs), where U denotes the UD, n the number of n experiments, p and q the number of levels of each factor, and t and s the number of factors of experiments.
Figure 1. Solid-state CP/MAS 13C NMR spectrum for cellulose (a) and 13C NMR spectrum of cellulose in 7.5 wt % NaOH/11 wt % urea aqueous solution (b).
Version 6.5 (MathWorks, Inc., USA) was used to conduct the statistical analyses and surface plotting. Results and Discussion Dissolution of Cellulose. Figure 1 shows the solidstate CP/MAS 13C NMR spectrum for original cellulose (a) and the 13C NMR spectrum of cellulose in 7.5 wt % NaOH/11 wt % urea/D2O solution (b). All the signals of cellulose solution in Figure 1b exhibit a sharp peak. This phenomenon suggests that the distance between the molecular chains increases as a result of the complete dissolution of cellulose in NaOH/urea aqueous solution, leading to a single and sharp time-averaged NMR peak due to each carbon’s relaxation time.23 Moreover, the chemical shifts of cellulose in this solvent almost shift
Table 2. Regression Equation Coefficients and Correlation Coefficients of Second-Order Polynomials for the Wet and Dry Films Coagulated with H2SO4 sample
b0
b1
b2
b3
b4
b5
R
wet film 13.6 0.838 0.190 -0.076 -0.069 -0.011 0.9998 dry film 76.5 5.12 1.03 -0.228 -0.372 -0.108 0.9911
upfield, except for C3, C2, and C5, and, in particular, the variation in the C4 carbon peak is remarkable. The C4 peak corresponds to the region in which the intramolecular hydrogen bonds (O3-H‚‚‚O6′) are strongly formed.24 The C4 carbon peak of cellulose/NaOH/urea solution at 79.7 ppm shows that the intramolecular hydrogen bonds have been destroyed in this solution.23 The results from NMR indicate that the presence of NaOH creates a significant ion-pair interaction, which reduces the strong self-associated character of water or cellulose to favor the establishment of new intermolecular interactions between urea and cellulose, bringing the cellulose into the aqueous solution.17 The chemical shifts of cellulose in this solvent are almost the same as those of cellulose in 6 wt % NaOH/4 wt % urea,17 LiCl-DMI,25 and LiCl-DMAC,26 which are true solvents. Moreover, no new peaks appear in the solution 13C NMR spectrum, suggesting the absence of derivation of cellulose: i.e., 7.5 wt % NaOH/11 wt % urea is a direct solvent of cellulose rather than a derivative aqueous solution system. Effect of Coagulation Conditions for the Aqueous H2SO4 System. Tensile strength and elongation at break are important mechanical properties of the RC films, which are related to coagulation conditions such as coagulant concentration (c), coagulation time (t), and temperature (T). Here, the uniform design method (UDM) was used to conduct the statistical analyses for coagulants of the H2SO4 system. Table 2 shows the regression equation coefficients and correlation coefficients of second-order polynomials for the wet and dry films coagulated with H2SO4. The three-dimensional plots of σb-c-t, σb-c-T, and σb-t-T for the RC films in the wet and dry states are presented in Figures 2 and 3, respectively. The results indicate that the σb values of the RC films in wet and dry states show an almost similar dependence on the coagulant concentration, coagulation time, and temperature. As the H2SO4 concentration increases, the σb values of the films increase slightly at first, but a continuing increase of the H2SO4 concentration causes a decrease in σb. Similarly, with an increase of coagulation time, the σb values of the films also undergo the same tendency from a slight increase at first to a decrease. However, with an increase of the coagulation temperature, the σb values of films decrease monotonically. As shown in Figures 2 and 3, the optimal coagulation conditions for the films are 5 wt % H2SO4 and 5 min at 25 °C, in which the σb values of the film I-c5-t5 in wet and dry states are 15.1 and 90.8 MPa, respectively. Figure 4 shows the effect of the coagulation temperature from 5 to 55 °C on the σb values of the RC films in the wet and dry states coagulated with 5 wt % H2SO4 for 5 min. The theoretical values of tensile strength calculated from eq 1 are consistent with the experimental data, which suggests that the uniform design method could be able to adequately model the coagulation parameters of RC films. The results indicate that the films coagulated at relatively low temperature possess better mechanical properties as compared to those at relatively high temperature, which can be explained by the notion that
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Figure 2. Effects of coagulant concentration (c), coagulation time (t), and temperature (T) on the tensile strength (σb) of wet films coagulated with an H2SO4 bath: (a) σb-c-t; (b) σb-c-T; (c) σb-t-T.
Figure 3. Effects of coagulant concentration (c), coagulation time (t), and temperature (T) on the tensile strength (σb) of dry films coagulated with an H2SO4 bath: (a) σb-c-t; (b) σb-c-T; (c) σb-t-T.
Figure 4. Effect of coagulation temperature (T) on the tensile strength (σb) of RC films in the dry (b) and wet (9) states coagulated with 5 wt % H2SO4 for 5 min. Solid lines represent experimental data, and dotted lines denote theoretical values calculated from eq 1.
the higher temperature could accelerate the mobility of all components in the coagulation system and increase the coagulation rate. This rapid precipitation gives the cellulose chains less time to pack themselves into an ordered structure, resulting in a drop of the mechanical properties.27,28-30 In addition, the low concentration and short time of coagulation could restrain the rate of the counter-diffusion between coagulant and NaOH/urea solvent, leading to difficulties in the cellulose regeneration process from the coagulation.27 The high concentration and long time of coagulation could be not only capable of facilitating decomposition of cellulose but also mainly responsible for an increase of the pore size of the films.27,31 As shown in Figures 2 and 3, the change of the σb values caused by coagulant concentration is more remarkable than that by coagulation time and temperature, suggesting that the coagulant concentra-
tion is the main factor determining the mechanical properties of the films. Effect of Coagulant Components. Figures 5-8 show the dependences of the σb and b values of the RC films in the wet and dry states on the coagulant concentration and coagulation time of various coagulants (5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4) at 25 °C, respectively. The results indicate that the optimal coagulant concentration and coagulation time of the coagulants at 25 °C are respectively 5 wt % H2SO4/5 wt % Na2SO4 for 5 min, 5 wt % Na2SO4 for 15 min, 3 wt % HOAc for 5 min, and 5 wt % (NH4)2SO4 for 3 min. Table 3 gives the σb and b values of all RC films in the wet and dry states. Both σb and b of the wet and dry films were obviously higher than those of commercially available cellophane. Therefore, a wide variety of coagulants can be employed for this cellulose solution to prepare RC films with excellent mechanical properties. The addition of the strong electrolyte Na2SO4 to H2SO4 could reduce the H+ concentration in the coagulant, leading to a counter-diffusion rate and acid-alkali neutralization process slower than that of H2SO4. Thereby, the RC films coagulated with 5 wt % H2SO4/Na2SO4 exhibit a tensile strength considerably better than that of the films coagulating from H2SO4. For HOAc aqueous solution as coagulant, because of its weak dissociation action, the film also exhibits good mechanical properties. For a pure salt system such as Na2SO4 and (NH4)2SO4 aqueous solutions as coagulants, the former may involve a salt coagulation/water regeneration process because of the slower coagulation rate. Therefore, the longer coagulation time of Na2SO4 of about 15 min is required to prepare a film with excellent mechanical properties, but (NH4)2SO4 only requires 3 min. It is worth noting that the RC films from these
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Figure 5. Dependence of tensile strength (σb) and breaking elongation (b) of the RC films on the concentration (c) of various coagulants for 5 min at 25 °C in the wet state.
Figure 6. Dependence of tensile strength (σb) and breaking elongation (b) of the RC films on the coagulation time (t) of various coagulants at 25 °C in the wet state.
Figure 7. Dependence of tensile strength (σb) and breaking elongation (b) of the RC films on the concentration (c) of various coagulants for 5 min at 25 °C in the dry state.
coagulants also have a dependence of tensile strength on the coagulant concentration and coagulation time similar to that of H2SO4 solution, suggesting that the formation of the films from different kinds of coagulants could have the same coagulation process and mechanism. Coagulation Process and Mechanism. In view of the results from Table 3 and Figures 1-8, the coagulation rate, namely, the counter-diffusion rate between casting solution and coagulant, mainly determines the mechanical properties of the films in nature, suggesting that the coagulation process in this solvent is a twophase separation, similar to that of the NH3/NH4SCN and NMMO/H2O systems, which is just a diffusioncontrolled process.11,29-30,32 In the case of the NMMO/ H2O system, the coagulation of cellulose involves the
diffusion-driven precipitation of the cellulose in the coagulation bath. The exchange of solvent with nonsolvent, namely a coagulant such as water, leads to a desolvation of the cellulose molecules and to a reformation of the intra- and intermolecular hydrogen bonds. During the coagulation of the cellulose solution, the NMMO molecules attract the precipitant molecules, resulting in the formation of a swollen cellulose gel, and finally regenerate from the solution.11 Figure 9 shows the X-ray diffraction patterns of cellulose and the RC films coagulated under the optimal coagulation conditions, and the values of χc are listed in Table 3. The results indicate that the transition from cellulose I to cellulose II has taken place in the RC film formation process.33,34 The values of the crystallinity index of the RC films range from 0.48 to 0.56, which are lower than
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Figure 8. Dependence of tensile strength (σb) and breaking elongation (b) of the RC films on the coagulation time (t) of various coagulants at 25 °C in the dry state. Table 3. Physical Properties of Cellulose, Cellophane, and the RC Films from Various Coagulants with the Optimal Coagulation Conditions σb (MPa) sample native cellulose I-c5-t5 II-c5-t5 III-c5-t15 IV-c3-t5 V-c5-t3 cellophane (unoriented cross-stress)
b (%)
Tr (%)
coagulant
χc (%)
dry
wet
dry
wet
800 nm
400 nm
H2SO4 5 wt % H2SO4/Na2SO4 Na2SO4 HOAc (NH4)2SO4
73 48 50 53 55 56 35
90.8 94.9 98.1 94.0 93.0 80.5
15.1 20.0 24.9 24.7 19.3 10.0
17.1 14.0 10.8 10.5 10.1 8.0
30.6 22.6 29.6 32.8 27.9 12.0
87.4 88.2 87.2 84.2 81.4 89.5
80.0 82.0 80.7 75.6 70.3 85.9
that of original cellulose I (0.73) but higher than that of commercially available cellophane (0.35). It is worth noting that the χc values are dependent on the coagulant
Figure 9. X-ray diffraction patterns of native cellulose and RC films from various coagulants.
nature and a stronger coagulant corresponds to a lower value of χc. For a stronger coagulant, on the whole, a faster coagulation rate leads to the cellulose chains not being able to form a fully highly ordered state, resulting in lower crystallinity.13,35 Figure 10 shows the SEM images of the free surface and cross-section of the wet films. All of the RC films display a homogeneous porous structure with a mesh network in the free surface and a microporous structure in the cross-section. It is noted that the II-c5-t5 film coagulated with 5 wt % H2SO4/ Na2SO4 exhibits a smaller pore size in the free surface and better mechanical properties than others, proving further that 5 wt % H2SO4/Na2SO4 aqueous solution is the optimal coagulant for the preparation of the films in this solvent system. Usually, optical transmittance (Tr) reflects the homogeneity of the material. The Tr values at 800 and 400 nm of the RC films are summarized in Table 3. The Tr results indicate that almost all of the RC films exhibit the good optical transmittance at 400-800 nm, and the film II-c5-t5 from 5 wt % H2SO4/Na2SO4 aqueous solution gives the best optical transmittance at 400 nm (82.0%) and 800 nm (88.2%), as a result of a relatively dense and homogeneous structure, which supports the conclusions from the SEM study. In view of these results, the whole coagulation process can be described as follows. When the cellulose solution contacts the coagulant, a counter-diffusion and chemical neutralization process between solvent in the cellulose solution and nonsolvent in the coagulant occur. The removal of solvent from cellulose solution and penetration of nonsolvent into the cellulose solution result in the cellulose losing its solubility, and the freshly formed fluid gel containing cellulose, water, etc. is gradually converted into a semisolid state to precipitate and regenerate. Therefore, the coagulation mechanism can
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Figure 10. SEM images of the free surface of the (a) I-c5-t5, (b) II-c5-t5, (c) III-c5-t15, (d) IV-c3-t5, and (e) V-c5-t3 films and crosssections of the (f) I-c5-t5, (g) II-c5-t5, (h) III-c5-t15, (i) IV-c3-t5, and (j) V-c5-t3 films coagulated with H2SO4, 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4, respectively.
contribute to a two-phase separation of the celluloserich phase in gel and cellulose-poor phase in solution during the coagulation. The coagulant penetrates into the cellulose solution, leading to physical cross-linking networks caused by the self-association force of the cellulose macromolecule, in which its hydroxyl serves as a junction, thus resulting in a cellulose-rich phase.36 In the cellulose-rich phase, as a result of the weave by nonsolvent penetrating in and solvent coming out from the cellulose solution, the mesh structure of the surface for the RC film has formed, as shown in Figure 10. A stronger coagulant causes an increase in the counterdiffusion rate, namely the phase separation rate, resulting in a film with a less dense structure. As shown in Figure 10, the films coagulated with H2SO4 aqueous solution exhibit a loose structure and larger pore size, leading to the relatively poor mechanical properties. However, the films coagulated with a milder coagulant display a relatively dense structure and better mechanical properties, which could be seen from the SEM photos of RC films coagulated with other coagulants such as 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4. Conclusions Cellulose was dissolved directly in 7.5 wt % NaOH/ 11 wt % urea aqueous solution within 5 min to obtain a transparent solution. A series of RC films were prepared from the cellulose solution by coagulating with H2SO4, 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4, respectively. The uniform design method was used successfully to optimize and evaluate the coagulation conditions. The best coagulant concentrations and coagulation times of the RC films are 5 wt % H2SO4 for 5 min, 5 wt % H2SO4/5 wt % Na2SO4 for 5 min, 5 wt % Na2SO4 for 15 min, 3 wt % HOAc for 5 min, and 5 wt % (NH4)2SO4 for 3 min at 25 °C. Moreover, the RC films coagulated at relatively low temperature possessed better mechanical properties as compared to those at relatively high temperature. The values of σb and b of the RC films are better than those of commercially available cellophane under a wide range of coagulant and coagulation conditions. The optimal coagulant is 5 wt % H2SO4/Na2SO4 aqueous solution, resulting in the
films possessing a more homogeneous structure and better mechanical properties on the whole. All of the RC films prepared from various coagulants exhibited good light transmittance. The results from XRD, SEM, optical transmittance, and tensile testing indicated that the coagulation was related to the counter-diffusion between solvent in the cellulose solution and coagulant. The coagulation mechanism can be described as a twophase separation, namely a cellulose-rich phase in the gel and a cellulose-poor phase in solution, which shows that cellulose in the gel was precipitated and regenerated with the coagulation process to form the RC films. Acknowledgment This work was supported by the National High Technology Research and Development Program of China (863 Program; 2003AA333040 and 2004AA649250) and the National Natural Science Foundation of China (59933070 and 20204011). Literature Cited (1) Schurz, J. Trends in polymer science a bright future for cellulose. Prog. Polym. Sci. 1999, 24, 481. (2) Zhang, L.; Yang, G.; Xiao, L. Blend membranes of cellulose cuoxam/casein. J. Membr. Sci. 1995, 103, 65. (3) Kamide, K.; Iijima, H. Recent Advances in Cellulosic Membranes, Cellulosic Polymer, Blends and Composites; Hanser: Munich, 1994; Chapter 10, p 189. (4) Nues, S. P.; Peinemann, K. V. Membranes Technology in the Chemical Industry; Wiley-VCH: Weinheim, Germany, 2001. (5) Hongo, T.; Inamoto, M.; Iweata, M.; Matsui, T.; Okajima, K. Morphological and structural formation of the regenerated cellulose membranes recovered from its cuprammonium solution using aqueous sulfuric acid. J. Appl. Polym. Sci. 1999, 72, 1669. (6) Inamoto, M.; Miyamoto, I.; Hongo, T.; Iwata, M.; Okajima, K. Morphological formation of the regenerated cellulose membranes recovered from its cuprammonium solution using various coagulants. Polym. J. 1996, 28, 507. (7) Manabe, S. I.; Kamata, Y.; Iijima, H.; Kamide, K. Some morphological characteristics of porous polymeric membranes prepared by “micro-phase separation method”. Polym. J. 1987, 19, 391. (8) 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.
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Received for review September 1, 2004 Revised manuscript received October 28, 2004 Accepted November 15, 2004 IE0491802
