Biomacromolecules 2009, 10, 1315–1318
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Cellulose in Never-Dried Gel Oriented by an AC Electric Field Wolfgang Gindl,*,† Gerhard Emsenhuber,† Günther Maier,‡ and Jozef Keckes‡ Department of Material Sciences and Process Engineering, BOKU-University of Natural Resources and Applied Life Science, A-1190 Vienna, Austria, and Department of Materials Physics, University of Leoben and Erich Schmid Institute for Materials Science, Austrian Academy of Sciences, A-8700 Leoben, Leoben, Austria Received December 29, 2008 Revised Manuscript Received February 25, 2009
Never-dried cellulose gel obtained by slow coagulation from LiCl/N,N-dimethylacetamide (DMAc) solution was exposed to an alternating current electric field. Making use of the birefringence of oriented cellulose and by means of wide-angle X-ray scattering, it was demonstrated that preferred orientation of cellulose molecules parallel to the electric field lines is induced in the cellulose gel. The preferred orientation remained unchanged for several days after storage in water and persisted after drying of the cellulose gel.
Figure 1. Transparent film of cellulose gel under the action of an AC electric field between pin electrodes (electrode distance ) 10 mm). Preferred orientation in the cellulose film was visualized by birefringence between two linear polarization filters crossed at 90°. The bright area between the electrodes indicates preferred orientation of cellulose molecules parallel to electric field lines.
1. Introduction Cellulose is the most abundant biopolymer and is thus an important resource for present and future “green” products. With its many degrees of freedom, the technology of cellulose dissolution and regeneration allows the production of innovative products with specifically tailored structure, shape, and functionality. Cellulose fibers and films,1 aerogels,2 and recently proposed electroactive actuators3 may serve as examples. With respect to new products, techniques that allow influencing the structure of cellulose are of interest. One such innovative technique enabling controlled ordering of polymer molecules to a certain extent is the application of strong magnetic and electric fields.4,5 For cellulose, magnetic fields were successfully applied to induce preferred orientation in cellulose triacetate films cast from solution,6 and in suspensions of cellulose nanoand microfibrils.7-9 Significant orientation was also obtained by regenerating cellulose from LiCl/DMAc solution while exposed to a magnetic field,10 and by corona poling.11 Furthermore, native cellulose was successfully oriented in an alternating current (AC) electric field at both the macroscopic (Ramie fiber fragments) and colloidal level (tunicate cellulose nanocrystal suspensions).12,13 While cellulose shows orientation perpendicular to an applied magnetic field,6-10 orientation is parallel in an electric field.12,13 Also in electrorheological fluids, i.e., materials that switch from liquid-like to solid-like state upon application of an electric field,14 dispersed microcrystalline cellulose shows a strong response upon the application of an electric field.15 In the present study, we demonstrate for the first time that, by applying an AC electric field, the orientation of cellulose can not only be influenced in suspensions of highly mobile * To whom correspondence should be addressed. Tel.: ++43-1-476544255. Fax: + +43-1-47654-4295. E-mail:
[email protected]. † BOKU-University of Natural Resources and Applied Life Science. ‡ University of Leoben and Erich Schmid Institute for Materials Science.
macro- to nanosized fibrous objects, but also in never-dried regenerated cellulose gel.
2. Experimental Section Lyocell fibers (1.3 dtex, Lenzing R&D) were activated for 6 h in distilled H2O at room temperature. Subsequently, the cellulose was dehydrated in ethanol, acetone, and N,N-dimethylacetamide (DMAc) for 4 h each. In parallel, a solution of 8 g of LiCl in 100 mL of DMAc was prepared. After decanting DMAc from the dehydrated cellulose, 100 mL of LiCl/DMAc solution was poured onto 1 g of cellulose fibers. The solution was stirred until dissolution of the fibers (10 min) and then poured into Petri dishes (diameter ) 20 cm), and left at ambient atmosphere for 12 h. After this time, a 2 mm thick transparent gel had formed as a result of uptake of water from ambient air, which leads to the desolvation of cellulose. The gel was washed in distilled water, which was repeatedly renewed, for 24 h. The cellulose gel was positioned onto a linear polarization filter illuminated from below and contacted with two pin electrodes with a diameter of 1 mm at a distance of 10 mm, as shown in Figure 1. The space between the electrodes was covered with a second linear polarization filter rotated by 90° with respect to the first filter. The filters were positioned with respect to the cellulose gel in a way that the polarization plane of both filters was at an angle of +45° and -45°, respectively, with regard to the line connecting the tips of the pin electrodes. An AC electric voltage with a frequency of 800 Hz and an apparent intensity of 300 V cm-1 was applied to the electrodes. Because of the use of pin electrodes with 1 mm diameter, the effective voltage acting on the solution was presumably 2-5 times the apparent intensity. The frequency and intensity of the applied electric field were limited by the available equipment on the one hand, and by excessive electric current at field intensities >300 V cm-1 on the other hand. After the application of an electric field, some specimens were stored in distilled water, while others were gently dried between sheets of paper to a dry thickness of 0.05 mm. In order to analyze cellulose orientation in dry films, wide-angle X-ray scattering (WAXS) experiments were performed on a Bruker
10.1021/bm801508e CCC: $40.75 2009 American Chemical Society Published on Web 04/06/2009
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Notes
Figure 2. Sequence of images (from top left to bottom right) showing increasing degree of preferred orientation of cellulose molecules indicated by increasing brightness between crossed polarizers. The sequence starts with an image taken before switching the field on, and continues with images taken at intervals of 3 s within a total time of 30 s.
AXS Nanostar. An area of 12 mm × 13 mm was mapped in steps of 1 mm. By numerical integration of the intensity distribution of the most intense cellulose II (110/020) reflection, which is perpendicular to the crystallographic c-axis, on the obtained two-dimensional (2D) detector images, the degree of preferred orientation of cellulose II crystallites expressed by the orientation factor 〈cos2 θ〉 was calculated according to
〈cos θ〉 ) 2
∫
π/2
0
I(θ) cos2 θ sin θ dθ
∫
π/2
0
(1) I(θ) sin θ dθ
Herman’s orientation factor often used for cellulose was obtained from 〈cos2 θ〉 according to
fc )
3〈cos2 θ〉 - 1 2
(2)
Herman’s orientation factor fc is 0 at random orientation, and 1 at perfectly parallel orientation. The direction of preferred orientation was evaluated from the azimuthal position of peak scattering intensity on the 2D detector images.
3. Results and Discussion A significant change in the orientation of cellulose macromolecules in never-dried gel was observed immediately after applying an AC electric field. Figure 2 shows a sequence of images of cellulose gel placed between crossed polarizers taken in steps of 3 s within a total time of 30 s. The area between the electrodes was initially dark because of complete extinction of light by the crossed polarizers. After switching on the electric field, bright spots appeared first next to the electrodes. Very
soon a bright band, which finally spread to elliptic shape, connected both electrodes. The birefringence of cellulosic materials is related to the orientation of cellulose molecules located in both crystalline as well as in amorphous regions. The birefringence is zero at random orientation of molecules and can increase up to 0.062 at nearly parallel orientation of all cellulose macromolecules.16 It is therefore concluded that bright areas between the electrodes correspond to regions where cellulose orientation is significantly different from random. Increasing brightness in the present configuration of polarizers is related to an increasing degree of preferred orientation of cellulose macromolecules parallel to the connecting line between the pin electrodes. After switching off the electric field, the obtained birefringence pattern, i.e., orientation distribution, persisted. Even after storing treated specimens in distilled water for 6 days, no significant change in the birefringence pattern was observed. It should be mentioned that the structure of the cellulose gel used for the present experiments is different from the structure of never-dried cellulose produced by rapid coagulation in the spinning bath. It was shown that cellulose gel produced by slow coagulation over several hours is less than 20% crystalline.17 The gel can be deformed easily up to a draw ratio of 2.0, and the obtained plastic deformation is highly persistent along with concurrent macromolecular orientation.18 This agrees very well with the present observation of persisting orientation due to changes induced by an electric field. For further investigations, cellulose specimens were dried after exposure to an electric field. As shown in Figure 3, cellulose orientation was preserved in dry films. In the briefringence image, only cellulose oriented at an angle of +45° and -45°, respectively, with regard to the polarization planes of the crossed polarizers appears bright, which means that not all oriented cellulose is visible in Figure 3. Using a polarization microscope equipped with a Berek 5λ compensator, the birefringence ∆n next to the electrodes and at half-distance between the electrodes was quantified. Next to the electrodes, ∆n was 0.0032, whereas it was only 0.0012 at half-distance. Considering a maximum birefringence ∆nmax of 0.062 for perfectly parallel oriented cellulose, a modest total orientation factor ft of 0.052 and 0.019, respectively, was obtained by dividing ∆n by ∆nmax. So far, only specimen birefringence was considered for the imaging of preferred orientation in cellulose gels. Wide angle X-ray experiments were performed in order to validate birefringence results with an independent method. Detector images shown in Figure 4 obtained from untreated cellulose and cellulose exposed to an electric field, respectively, do not indicate a deviation from random orientation at first view. Only the difference image (Figure 4) obtained by subtraction of the two images confirms the presence of preferred orientation. The crystalline orientation factor fc calculated from the distribution of scattering intensity using eqs 1 and 2 varied between 0.013 next to the location of the electrodes and 0.005 at half-distance between the electrodes. Thus crystalline orientation amounted only to 25% of the total orientation calculated from birefringence. This observation agrees with the finding that, upon drawing, the change in preferred orientation of dry cellulose films produced according to the method used in the present study is significantly more pronounced in the noncrystalline part compared to the crystalline part.16 The values of fc and ft found in the present study are in the same magnitude as values found for dry cellulose films stretched to 3% for the area at halfdistance between the electrodes and 6% for the area next to the electrodes, where highest electric field intensity is presumed.
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Figure 3. Left: Image of a dry cellulose film between crossed polarizers. High brightness indicates a significant degree of preferred orientation, particularly in the vicinity of the electrodes (distance ) 10 mm). In the briefringence image, only cellulose oriented at an angle of +45° and -45°, respectively, with regard to the polarization planes of the crossed polarizers appears bright, which means that not all oriented cellulose is visible. Green lines added to the birefringence image indicate the direction of preferred orientation obtained by WAXS mapping. Centre: Plot of the intensity distribution along the cellulose II 110/020 reflection for all WAXS measurements with digitally brightened birefringence image as background. The 10 individual intensity distribution plots showing the highest degree of preferred orientation were highlighted in red color. Right: Finite element simulation of the distribution of electric field intensity and electric field lines (field lines are shown only for one-half of the image only for better viewing).
Figure 4. 2D detector images from WAXS experiments with untreated cellulose (left) and cellulose exposed to an electric field (center). The most intense reflection (cellulose II 110/020) was used for the evaluation of preferred orientation. The difference obtained by subtraction of the two images (right) reveals significant deviation from random orientation.
A plot of the direction of the preferred orientation of cellulose macromolecules evaluated from the WAXS detector images is shown in Figure 3. The distribution of orientation directions shown in a grid of 1 mm × 1 mm agrees well with the birefringence image, which serves as background for this plot. It is apparent that cellulose is oriented in a centripetal fashion around the two pin electrodes. Between the pin electrodes, the direction of preferred orientation is fairly parallel to the connecting line between the electrode tips. Figure 3 also shows a plot of the intensity distribution along the cellulose II 110/ 020 reflection for all WAXS measurements. The 10 individual intensity distribution plots showing the highest degree of preferred orientation (highlighted in red color) are found in the vicinity of the location of the pin electrodes, where the highest field intensity is presumed. In order to confirm assumptions on electric field intensity and electric filed lines, a finite element simulation of these parameters was performed using Maxwell SV software (Ansoft). The result of this simulation shown in Figure 3 indicates small variations of electric field intensity in the larger part of the specimen, but sharply increasing intensity in the vicinity of the pin electrodes. The electric field intensity is also slightly elevated in the area between the pin electrodes. As shown in Figure 3, there is a good agreement between the simulation result for the direction of electric field lines and the WAXS result for the direction of preferred orientation of cellulose macromolecules in regions of high electric field
intensity. Regions at a distance from the location of the pin electrodes and the area between the electrodes, respectively, do show a certain, mostly small degree of preferred orientation. However the direction of preferred orientation in these regions of the specimen does not coincide with the direction of electric field lines. This discrepancy is probably due to inhomogeneities in the cellulose gel (e.g., slight thickness variations) leading to localized deformations during drying. Such inhomogeneities were presumably present throughout the specimen, but only show up in the results from regions with low electric field intensity. Preferred orientation induced by an electric field was thus confirmed by two independent methods. It is important to note that, differently from earlier studies,12,13 where suspensions of fibrous cellulose were used, the object of the present study is regenerated cellulose gel, which opens up many new possibilities of new cellulosic products. For example, the fact that preferred orientation easily detected between crossed polarizers can be induced in initially random-oriented regenerated cellulose is of potential significance for the marking of cellulosic products. It is also conceivable that cellulosic templates, e.g. for the incorporation of minerals and subsequent ceramification, can be produced with this method.
4. Conclusion From the results obtained in the present study, it is concluded that preferred orientation of cellulose macromolecules can be obtained by applying an AC electric field to a never-dried gel produced by slow coagulation from LiCl/DMAc solution. The change in orientation persists for several days during storage of cellulose gel in distilled water and remains also after drying of the gel to a cellulose film.
References and Notes (1) Fink, H.-P.; Weigel, P.; Purz, H. H.; Ganster, J. Prog. Polym. Sci. 2001, 26, 1473–1524. (2) Jin, H.; Nishiyama, Y.; Wada, M.; Kuga, S. Colloid Surf., A: Physicochem. Eng. Aspects 2004, 240, 63–67. (3) Kim, J.; Yun, S.; Ounaies, Z. Macromolecules 2006, 39, 4202–4206.
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(4) Sardone, L.; Palermo, V.; Devaux, E.; Credgington, D.; de Loos, M.; Marletta, G.; Cacialli, F.; van Esch, J.; Samorı`, P. AdV. Mater. 2006, 18, 1276–1280. (5) Kakade, M. V.; Givens, S.; Gardner, K.; Lee, K. H.; Chase, D. B.; Rabolt, J. F. J. Am. Chem. Soc. 2007, 129, 2777–2782. (6) Kimura, T.; Yamato, M.; Endo, S.; Kimura, F.; Sata, H.; Kawasaki, H.; Shinagawa, Y. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1942–1947. (7) Sugiyama, J.; Chanzy, H.; Maret, G. Macromolecules 1992, 25, 4232– 4234. (8) Cranston, E. D.; Gray, D. G. Sci. Technol. AdV. Mater. 2006, 7, 319– 321. (9) Kimura, F.; Kimura, T.; Tamura, M.; Hirai, A.; Ikuno, M.; Horii, F. Langmuir 2005, 21, 2034–2037. (10) Kim, J.; Chen, Y.; Kang, K.-S.; Park, Y.-B.; Schwartz, M. J. Appl. Phys. 2008, 104, 096104.
Notes (11) Yun, S.; Kim, J. H.; Li, Y.; Kim, J. J. Appl. Phys. 2008, 103, 083301. (12) Bordel, D.; Putaux, J.-L.; Heux, L. Langmuir 2006, 22, 4899–4901. (13) Habibi, Y.; Heim, T.; Douillard, R. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 1430–1436. (14) Hao, T. AdV. Mater. 2001, 13, 1847–1857. (15) Zhang, S.; Winter, W. T.; Stipanovic, A. J. Cellulose 2005, 12, 135– 144. (16) Gindl, W.; Reifferscheid, M.; Martinschitz, K. J.; Boesecke, P.; Keckes, J. J. Polym. Sci. B: Polym. Phys. 2008, 46, 297–304. (17) Togawa, E.; Kondo, T. J. Polym. Sci. B: Polym. Phys. 1999, 37, 451– 459. (18) Togawa, E.; Kondo, T. J. Polym. Sci. B: Polym. Phys. 2007, 45, 2850–2859.
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