1702
Ind. Eng. Chem. Res. 2008, 47, 1702-1706
Removal of Impurities from Cellulose Films after Their Regeneration from Cellulose Dissolved in DMAc/LiCl Solvent System Jyoti N Nayak, Yi Chen, and Jaehwan Kim* Centre for EAPap Actuator, Department of Mechanical Engineering, Inha UniVersity, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, South Korea
Solvents and ions entrapped in a cellulose matrix during regeneration process have a significant effect on the performance and durability of electroactive paper actuators made using regenerated cellulose. This article describes a simple approach to remove solvents and Li+ ions from the regenerated cellulose films by combining distillation process, washing with isopropyl alcohol (IPA)-deionized (DI) water mixture and running-water technique. It is possible to obtain a good surface morphology and the highest transmittance (90%) as well as a negligible amount of DMAc when films are washed with a DI/IPA mixture ratio equal to 60:40. Li+ ion concentration drastically reduced from 15 380 to 10.61 ppm upon exposure to running water continuously for 24 h. This process enables us to develop e-paper, flexible thin film transistors, IDT, and SAW sensors based on cellulose paper. 1. Introduction Cellulose is the most abundant natural polymer on earth, consisting of glucose-glucose linkages arranged in linear chains.1 Cellulose is found in plants as micro fibrils (2-20 nm in diameter and 100-40 000 nm long). These form the structurally strong framework in the cell walls, making it an inexhaustible source of raw material for environmental friendly and biocompatible products.2 Cellulose derivatives are used for coatings, laminates, optical films, pharmaceuticals, food, and textile industries.3-6 Despite many studies and research on cellulose, the potential of cellulose has not been fully explored. Cellulose is a versatile framework for supermolecular chemistry, and there is plenty still to discover from cellulose. An interesting phenomenon has been discovered with cellulose paper.7 When an electric field was applied across the thickness of the paper, it showed a bending deformation. This paper was termed cellulose electroactive paper (cellulose EAPap).8 The actuation principle was found to be a combination of piezoelectric and ion migration effects. Cellulose EAPap has merits in terms of its light weight, biodegradability, low cost, large displacement output, and low actuation voltage. This cellulose EAPap will enable us to develop lightweight biomimetic actuators, sensors, MEMS devices, electronic displays, and so forth. Cellulose EAPap is made by dissolving cellulose fibers into a solution and regenerating a cellulose film, followed by gold electrode coating on both sides of it. However, perfect regeneration of cellulose is necessary for cellulose EAPap. Various solvent systems like sodium hydroxide/urea (NaOH/ urea), NNMO, DMAc/LiCl, and so on have been used for regenerating cellulose. Among these solvent systems, dissolution of cellulose in a DMAc/LiCl system (principle of cellulose dissolution in DMAc/LiCl solvent system is shown in Figure 1) offers the following advantages: (i) A great level of polymer-solvent interactions and a complete dissolution. (ii) Negligible reduction in intrinsic viscosity of the polymer in a solution after a long period of storage.9 (iii) In addition, it has better performance at room humidity, that is the actuator made by dissolving cellulose in a DMAc/LiCl system10 has higher displacement output compared to that of an actuator based on * To whom correspondence should be addressed. Tel.: +82-32-8607326. Fax: +82-32-868-1716. E-mail:
[email protected].
Figure 1. Dissolution mechanism of cellulose in the LiCl/DMAc solvent system.
cellulose/NaOH/urea.11 Although an actuator made with cellulose in a DMAc/LiCl system exhibits a high displacement output, there are some issues to be resolved: (i) Damage of the electrode of the actuators during testing and (ii) low durability due to presence of DMAc solvent and ions (Li+) in the cellulose matrix. Further improvement in the performance of the actuator is possible by increasing its piezoelectric constants that can be improved by electric poling of wet cellulose films. However, during the poling process the entrapped solvents and ions in the matrix of films may affect its morphology and also in applying this EAPap material to various devices like e-paper, flexible thin film transistors, humidity sensors, and SAW sensors. Thus, it is necessary to eliminate solvents and ions from the matrix of regenerated cellulose films without affecting its morphologies. In the present investigation, we made an attempt to eliminate solvent and Li+ ions from regenerated cellulose films by employing a simple process: by combining distillation, washing with isopropyl alcohol (IPA), and a deionized (DI) water mixture and running-water technique. The effect of running-water exposure time and different ratios of IPA and DI water on morphology, and the amount of solvent and ions in the regenerated cellulose films were discussed by means of scanning electron microscopy (SEM), infrared spectroscopy (FTIR), and inductively coupled plasma-mass spectrometry (ICP-MS). 2. Experimental Details 2.1. Materials. Cotton cellulose (MVE, DPw 4580) was purchased from Buckeye Technologies Co. and N,N-dimethy-
10.1021/ie0710925 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/07/2008
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1703
Figure 2. Running-water system setup to remove Li+ ions. Table 1. Details of Distillation by Rotary Evaporation Technique Weight of Solution (g)
trial
pressure (Pa)
temp (°C)
before
after
time
removal rate (g/min)
1 2 3
0.13 0.13 0.13
25 45 65
100 80 100
85 68 65
1 h 15 min 5 min 56 s 3 min 17 s
0.2 2.16 11.04
lacetamide-DMAc (anhydrous, 99.8%) was from Sigma Aldrich. The anhydrous DMAc was carefully dried with molecular sieves for 1 week before use. Extra-pure lithium chloride was purchased from Junsei Chemicals Co., Japan. Preparation of Cellulose Solution and Distillation. Cotton cellulose (16 g) and LiCl (80 g) were heated at 110 °C for an hour, and the LiCl was dissolved in DMAc (720 mL). Then, cellulose was mixed with LiCl-DMAc solution and heated at 150 °C for 30 min followed by cooling to 40 °C for 2 h. The detailed procedure of preparing a cellulose solution may found elsewhere.10 Prepared cellulose solution was subjected to distillation at three different temperatures (25, 45, and 65 °C) by maintaining the pressure at 0.13 Pa using Eyela rotary evaporator N-1000. The objective of the distillation is to remove an additional amount of DMAc from the cellulose solution without disturbing its structure. 2.2. Sample Preparation and Running-Water Technique. (i) The cellulose solution (50 mL) remaining from the distillation was spin coated on an 8 in. silicon wafer using spin coater EDCZ-100 at 300 rpm for 10 s in the first step and 500 rpm for 10 s in the next step. Spin-coated films were allowed to cure/ gel for 3 h. (ii) To remove the DMAc solvent from the films, they were further washed in a solvent composed of deionized water (DI) and isopropyl alcohol (IPA) under a pressure load to impove the flatness of film. The IPA:DI water ratios used in the present investigation were 0:100, 20:80, 80:20, 70:30, and 60:40. (iii) After washing with a DI/IPA mixture, films were further soaked in DI water to remove residual IPA on the films. (iv) Further, to remove the Li+ ions from the regenerated cellulose films, the washed films were exposed to running water (for 6, 12, and 24 h) under a pressure load to maintain flatness. Figure 2 shows the running-water washing system. Wet cellulose films were put into a reservoir, and DI water flew into the reservoir by adjusting approximately equal to the outflow to remove the Li+ ions from the regenerated cellulose films. (v) Upon exposure to running water, the films were completely dried in room conditions and are used for analysis. 2.3. Characterization. The surfaces of the films were examined with a scanning electron microscope (Hitachi S4300)
Figure 3. Microphotograph of cellulose after distillation at 0.13 Pa and 65 °C.
Figure 4. FTIR spectra of samples after distillation at 25 and 45 °C.
to evaluate the effect of running-water exposure time and different ratios of the IPA:DI water mixture on the morphology of films. The films were coated with platinum under vacuum conditions before the SEM experiments. The IR spectra were obtained using a Nicolet 6700 FTIR spectrometer with a KBr beam splitter detector at 4 cm-1 resolution with 32 scans per sample. The films were cut into very small particles and characterized by FTIR for evaluation of the chemical structure using the KBr pellet. Li+ ion concentration in cellulose films was analyzed using an inductively coupled plasma-mass spectrometer (ICP-MS) (PerkinElmer, ELAN6100). Cellulose films (100 mg) were dissolved in strong acid (HCl/HNO3 ) 1:3) by heating at 200 °C, and the solutions were analyzed after dilution with DI water. 3. Results and Discussions 3.1. Removal of DMAc. After distillation of the cellulose solution by rotary evaporation, gas chromatography-mass spectrometry (GC-MS) analysis was conducted on the distillated liquid. It indicates that the liquid obtained after distillation was DMAc. Because the distilled DMAc contains no other impurities it can be reused. Table 1 shows the distillation details in terms of the DMAc removal rate with different temperature. As the temperature increased, the DMAc removal rate increased. It is observed that the cellulose solution remains unchanged when distillation is carried out at 25 and 45 °C, which indicates
1704
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008
Figure 5. FTIR spectra of cellulose samples washed with different DI/IPA ratios, (a) 0:100; (b) 20:80; (c) 80:20; (d) 70:30; and (e) 60:40.
Figure 6. UV transmittance of cellulose samples washed with different DI/IPA ratios, (a) 0:100; (b) 20:80; (c) 80:20; (d) 70:30; and (e) 60:40.
the presence of sufficient DMAc. However, it changed to a white thick solution (as shown in Figure 3) when distilled at 65 °C. It may be due to the fact that the pressure and temperature conditions were near the boiling point of DMAc (165 °C), and the molecular weight degradation became severe and showed pronounced pulp degradation in the presence of LiCl and also the separation of the Li+-DMAc macrocation complex.12,13 Because the solution was unchanged (remains as a clear solution) and the DMAc removal rate was also reasonably high, 45 °C and 0.13 Pa were considered as the optimum temperature and pressure for removing DMAc. FTIR spectra of samples distilled at 25 and 45 °C are shown in Figure 4. A peak at 1647 cm-1 represents the DMAc.14 It is observed that spectra in the finger print region of 900-1250 cm-1 overlapped each other. However, the spectrum beyond 1250 cm-1 shows a difference in intensities; it may be due to the removal of DMAc from the cellulose/DMAc/ LiCl system. Also, the absorbance peak at 1647 cm-1 appreciably reduced
after distilling the solution at 45 °C, and the sharpness of the peak remained. This indicates the presence of DMAc residue in the film. 3.2. Removal of Residual DMAc. Because DMAc is soluble in water as well as in alcohols, we tried several combinations of DI water and IPA ratios to remove the residual DMAc in the cellulose films. Figure 5 depicts the FTIR spectra for the samples washed with different ratios of DI water and IPA. FTIR has often been used as a useful tool in determining specific functional groups or chemical bonds that exists in a material. The presence of a peak at a specific wave number would indicate the presence of a specific chemical bond.10,14,15 The bands at 1431 and 1319 cm-1 were assigned as symmetric CH2 bending16-20 and CH2 wagging,20,21 respectively, and the band at 1076 cm-1 may be due to a secondary alcohol. The transmittance peak at 1076 cm-1 was reduced for the samples (c), (d), and (e), and no change in the transmittance of the films (a) and (b) was observed. In addition to the transmittance change, wave number shifts were also shown for (a) 1647 f 1657 cm-1, (b) 1647 f 1650 cm-1, (c) 1647 f 1639 cm-1, and (d) 1647 f 1627 cm-1. However, for the film (e) washed with 60:40 (DI/IPA) mixture, no change in the wave number (1647 cm-1) was observed, but the transmittance peak reduced greatly and became broader. For a rough estimation of the amount of residual DMAc present in the film after washing with the DI/IPA mixture, the intensity of the band at 1647 cm-1 obtained from a local baseline between adjacent valleys was automatically calculated at the maximum absorbance found by a sensitivity of 100 using OMNIC 4.0 software.22 Intensity of the peak at 1647 cm-1 for films washed at 0:100, 20:80, 80:20, 70:30, and 60:40 ratios of DI/IPA mixtures were 0.184, 0.175, 0.154, 0.126, and 0.053, respectively. It is found that the intensity of the DMAc peak reduced with varying the ratio of DI/IPA mixture, that is, when intensity reduced from 0.184 to 0.053 upon changing the DI/IPA ratio from 0:100 to 60:40. This result indicates the ability of the DI/IPA mixture (60:40) to remove most of the DMAc solvent from regenerated films. To evaluate the effect of the DI/IPA mixture on the optical properties of the cellulose films, they were subjected to a UVtransmittance test. Figure 6 shows the UV transmittance of cellulose samples washed with different DI/IPA ratios. For the
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1705
Figure 7. SEM image of cellulose samples washed with different DI/IPA ratios, (a) 0:100; (b) 20:80; (c) 80:20; (d) 70:30; and (e) 60:40.
Figure 8. SEM image of the films after exposed to running water for (a) 6 h and (b) 24 h.
sample (a), the transmittance increased linearly and reached the maximum of 49% at 350 nm, whereas the transmittance of the film (b) remained constant at 82% in the range of 200-350 nm. However, the transmittance of the samples (c), (d), and (e) increased linearly and reached a maxima of 87% for samples (c) and (d), 84% for sample (d) at 270 nm, and remained almost same thereafter. Among all, sample (e) showed the highest transmittance of 90% at 350 nm, which is a promising property for optical application. Further, to verify why UV-transmittance patterns were different for films washed with different ratios of DI/IPA mixtures, the surfaces of the films were observed under SEM. Figure 7 shows the SEM images of the cellulose samples washed with different ratios of the DI/IPA mixture after distilling the cellulose solution at 45 °C. The surface of sample (a) washed with 100% IPA shows a microcrack along with shrinkages; however, no cracks but shrinkages were observed when washed with an 80:20 (DI/IPA) mixture. However, samples (b) and (d) exhibited small shrinkages compared to sample (e), which shows a very good surface morphology. This result indicates that DI/ IPA ratios have a significant effect on the surface morphology and transmittance of the cellulose films. From the results of FTIR, UV transmittance, and SEM images, we arrived at the following conclusion. Because it is
possible to obtain a good surface morphology and the highest transmittance (90%) as well as negligible DMAc content in the cellulose film, 60:40 ratios was considered as an optimum DI/ IPA ratio to remove residual DMAc from the films after distillation. 3.3 Removal of Li+ Ions. To remove Li+ ions from the regenerated cellulose films, a simple running-water setup as shown in Figure 2 was employed in the present investigation. Three films were exposed to running water for different washing times. ICP-MS results showed that the Li+ ions’ contents for cellulose film not exposed to running water and exposed running water for 6, 12, and 24 h were 15 380, 26.25, 14.07, and 10.61 ppm, respectively. According to the mechanism for dissolving cellulose in the LiCl/DMAc solvent system, hydroxyl protons of the anhydroglucose units are associated with the chloride anion by hydrogen bonding. The chloride ion is associated with a Li+(DMAc)x macrocation as shown in Figure 1.12 Because DMAc is completely removed by distillation and washing with an IPA:DI water mixture, Li+ cations may reunite with chloride anion, and thus forms a cellulose-LiCl complex. When the films are subjected to running water, running water may pull out the Li+ ions from the cellulose-LiCl complex, as the solubility of LiCl in DI water is very high (63.7 g) compared to ethanol (42.4 g) and acetone (4.11 g) per 100 mL of water.
1706
Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008
Also, Li+ ions are highly polar. Furthermore, LiCl behaves as a fairly typical ionic compound, although Li+ ion is very small.23 Figure 8 shows SEM images of the films exposed at 6 and 24 h in the running water. 4. Conclusion This study investigated the possibility of regenerating cellulose that is solvent- and ion-free by combining distillation, washing with DI water and IPA mixture, and running-water technique. Because the cellulose solution was unchanged (remained as a clear solution) and the DMAc removal rate was also reasonably high, 45 °C and 0.13 Pa are considered as the optimum temperature and pressure for removing DMAc by distillation. Because it was possible to obtain a good surface morphology and highest UV transmittance as well as very low DMAc, 60:40 is considered as an optimum DI/IPA ratio to remove residual DMAc from the films after distillation. ICPMS results of cellulose films indicated that running-water washing of cellulose films could remove Li+ ions, although a negligible amount of DMAc and Li+ (10.61 ppm) ions were present in the films even after washing with a DI/IPA mixture and subjecting it to running water. This process proved to be reliable for regenerating solvent- and ion-free cellulose films. Acknowledgment This work was supported by the Creative Research Initiatives (EAPap Actuator) of KOSEF/MOST. Literature Cited (1) Kadla, J.; Gilbert, R. Cellulose Structure: A Review. Cellul. Chem. Technol. 2000, 34, 197. (2) Yen-Ning, K.; Juan, H. A New Method for Cellulose Membrane Fabrication and Determination of Its Characteristics. J. Colloid Interface Sci. 2005, 285, 232. (3) Vaquez, M. I.; Galan, P.; Casado, J.; Ariza, M. J. Effect of Radiation and Thermal Treatment on Structural and Transport Parameters for Cellulose Regenerated Membreanes. Appl. Surf. Sci. 2004, 238, 415. (4) Kovalev, G. V.; Bugaenko, L. T. On Cross Linking of Cellulose under Exposure to Radiation. High Energy Chem. 2003, 37 (4), 209. (5) Potthast, A. The Cellulose Solvent System N,N-Dimethylacetamide/ LiCl Revisited: The Effect of Water on Physiochemical Properties and Chemical Stability. Cellulose 2002, 9, 41. (6) DuPont, A. L. Cellulose in Lithium Chloride/N,N-Dimethylacetamide, Optimization of a Dissolution Method Using Paper Substrates and Stability of the Solution. Polymer 2003, 44, 4117.
(7) Kim. J.; Seo, Y. B. Electro-Active Paper Actuators. Smart Mater. Struct. 2002, 11, 355. (8) Kim, J.; Yun, S.; Ounaies, Z. Discovery of Cellulose as Smart Material. Macromolecules 2006, 39, 4202. (9) Kim, J.; Wang, N.; Chen, Yi.; Lee, S. K.; Yun, G. Y. ElectroactiveActive Actuators Made with Cellulose/NaOH/Urea and Sodium Alginate. Cellulose 2007, 14, 217. (10) Wang, N.; Kim, J.; Chen, Yi.; Yun, S. R.; Lee, S. K. ElectroactivePaper Actuator Made with LiCl/Cellulose Films: Effect of LiCl Content. Macromol. Res. 2006, 14, 624. (11) Kasaai, M. R. Comparison of Various Solvents for Determination of Intrinsic Viscosity and Viscometric Constants for Cellulose. J. Appl. Polym. Sci. 2002, 86, 2189. (12) McCormick, C. L.; Callais, P. A.; Hutchnson, B. H., Jr. Solution Studies of Cellulose in Lithium Chloride and N,N-Dimethylacetamide. Macromolecules 1985, 18, 2394. (13) Potthast, A.; Rosenau, T.; Sartori, J.; Sixta, H.; Kosma, P. Hydrolytic Processes and Condensation Reactions in the Cellulose Solvent System N,NDimethylacetamide/Lithium Chloride. Part 2: Degradation of Cellulose. Polymer 2003, 44 (1), 7. (14) Silverstein, R. M.; Webster, F. X. Spectroscopic Identification of Organic Compounds, 6th ed.; John Wiley & Sons: New York, 1997. (15) Liu, C.; Bai, R. Preparation of Chitosan-Cellulose Acetate Blend Hollow Fibres for Adsoprtive Performances. J. Membr. Sci. 2005, 267, 68. (16) Cael, J.; Gardner, K. H.; Koemig, J. L.; Blackwell, J. Infrared and Raman Spectroscopy of Carbohydrates. J. Chem. Phys. 1975, 62, 1145. (17) Kondo, T.; Sawatari, C. A Fourier Transforms Infrared Spectroscopic Analysis of the Character of Hydrogen Bonds in Amorphous Cellulose. Polymer 1996, 37, 393. (18) Nelson, M. L.; O’Connor, T. Relation of Certain Infrared Bonds to Cellulose Crystallinity and Crystal Lattice Type. Part 1 Spectra of Lattice Types I, II, III and of Amorphous Cellulose. J. Appl. Polym. Sci. 1964, 8, 1311. (19) Ruan, D.; Zhang, L.; Mao, Y.; Zeng, M.; Li, X. Microporus Membranes Prepared from Cellulose in NaOH/Thiourea Aqueous Solution. J. Membr. Sci. 2004, 241, 265. (20) Colom, X.; Carrillo, F. Crystallinity Changes in Lyocell and Viscose Type Fibres by Caustic Treatment. Eur. Polym. J. 2002, 38, 2225. (21) Cao, Y.; Tan, H. Structural Characterization of Cellulose with Enzymatic Treatment. Mol. Struct. 2004, 705, 189. (22) Oh, S. Y.; Yoo, D. I. Seo, G. FTIR Analysis of Cellulose Treated with Sodium Hydroxide and Carbon Dioxide. Carbohydr. Res. 2005, 340, 417. (23) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997.
ReceiVed for reView August 10, 2007 ReVised manuscript receiVed November 30, 2007 Accepted December 30, 2007 IE0710925
