Langmuir 2006, 22, 4899-4901
4899
Orientation of Native Cellulose in an Electric Field Damien Bordel, Jean-Luc Putaux, and Laurent Heux* Centre de Recherches sur les Macromole´ cules Ve´ ge´ tales, CERMAV-CNRS, ICMG, Affiliated with UniVersite´ Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France ReceiVed January 5, 2006. In Final Form: March 15, 2006 Native cellulose has been oriented in an ac electric field at both the macroscopic and colloidal level. Ramie fiber fragments suspended in chloroform have been shown to point along the field. Cellulose microcrystal suspensions in cyclohexane have also been allowed to evaporate in an electric field and have exhibited a high degree of orientation when further examined by TEM and electron diffraction. Similarly, cellulose whisker suspensions showed increasing birefringence with increasing field strength and displayed interference Newton colors that saturated at around 2000 V cm-1. A high degree of order of this suspension was also obtained by evaluating the induced birefringence with color charts.
Introduction Cellulose is biosynthesized in the form of slender crystalline microfibrils that, upon acid treatment, yield short microcrystals that are commonly called cellulose “whiskers”. Depending on their origin, these whiskers adopt a variety of dimensions ranging from 3 to 20 nm in width and from more than 1 µm to 1 m in length. These monocrystalline elements, which are relatively easy to prepare, display interesting properties leading to a number of applications.1 When hydrolysis is achieved with sulfuric acid, aqueous suspensions of cellulose whiskers do not flocculate. Above a critical concentration, these stable colloidal suspensions spontaneously self-organize into chiral nematic structures.2,3 More recently, it was shown that stable chiral nematic suspensions of cellulose whiskers, mixed with specific surfactants, could also be prepared in apolar solvents.4 Nonflocculated cellulose whiskers in an aqueous supension under an external field are oriented when they are subjected to a shear force5 or to a magnetic field.6 In the latter case, each whisker orients with its long axis perpendicular to the field, owing to the negative diamagnetic anisotropy of cellulose. Thus, when a magnetic field is applied to chiral nematic suspensions, an overall orientation is achieved where the cholesteric axis becomes parallel to the magnetic field.7 The resulting helical structure becomes unwound if the magnetic field is rotated.8 Under shear flow, the whiskers orient vertically in a plane perpendicular to the shear direction for low shear rates and along the shear direction for higher rates.5 Among the different techniques used to induce an alignment of anisotropic particles in a suspension, the use of an electric field is a versatile and easily implemented tool. In fact, a number of anisometric particles have been aligned in an electric field, namely, TMV viruses,9 PTFE whiskers,10 gold rods,11 FeOOH,12,13 * Corresponding author. E-mail:
[email protected]. Tel: 33-476 03 76 08. Fax: 33-476 54 72 03. (1) De Souza Lima, M. M.; Borsali, R. Macromol. Rapid Commun. 2004, 25, 771-787. (2) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170-172. (3) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir 1996, 12, 2076-2082. (4) Heux, L.; Chauve, G.; Bonini, C. Langmuir 2000, 16, 8210-8212. (5) Ebeling, T.; Paillet, M.; Borsali, R.; Diat, O.; Dufresne, A.; Cavaille, J-Y.; Chanzy, H. Langmuir 1999, 15, 6123-6126. (6) Sugiyama, J.; Chanzy, H.; Maret, G. Macromolecules 1992, 25, 42324234. (7) Revol, J.-F.; Godbout, L.; Dong, X. M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127-134. (8) Kimura, F.; Kimura, T.; Tamura, M.; Hirai, A.; Ikuno, M.; Horii, F. Langmuir 2005, 21, 2034-2037.
and attapulgite particles.14 Surprisingly, as far as we know, the alignment of cellulose in an electric field has not been reported in the literature. Most studies on the orientation of suspended particles in an electric field have dealt with aqueous suspensions, and there are several limitations arising mainly from the high conductivity of water. Here, we took advantage of the dispersion of cellulose whiskers in organic solvents to provide evidence of the possibility of cellulose orientation at the colloidal level. In this letter, we will show first that it is possible to orient macroscopic ramie cellulose fibers along the field. The orientation of colloidal cellulose whiskers in a Kerr cell will then be demonstrated and visualized by optical and transmission electron microscopy (TEM). Materials and Methods Aqueous cellulose whisker suspensions were prepared by sulfuric acid hydrolysis following the method of Revol et al. for cotton3 (Tubize Plastics) and Favier et al. for tunicin.15 The dispersions were transferred to organic solvents following a method described elsewhere.4 Ramie fibers were cut with a razor blade to an approximately 2 mm length and were suspended in chloroform. The suspensions were introduced into a Kerr cell consisting of a Teflon container and two parallel 7 cm × 4 cm stainless steel plates. A distance of 1 cm between the plates was kept for all of the experiments. Two glass windows were located at the top and the bottom of the cell for optical observation of the suspension between crossed polars. The electric field was produced by a 0-4 V ac generator (Centrad GF265) delivering a sinusoidal signal with a frequency ranging from 10 to 10 kHz. This signal was sent to a high-voltage amplifier (Trek 10/10B). A 0.5% w/v suspension of tunicin whiskers in cyclohexane was poured into the Kerr cell, operating under a 2000 V cm-1 electric field at 1 kHz. The suspension was then allowed to evaporate onto thin Formvar supporting films. The specimens were observed under low-dose illumination using a Philips CM200 Cryo TEM operating at 80 kV for imaging and 200 kV for diffraction. Images were recorded on Kodak SO163 films, and diffraction patterns were captured using a KeenView CCD camera (Soft Imaging Systems, Germany). (9) Asai H.; Watanabe N. Biopolymers 1976, 15, 383-392. (10) Foster, K. R.; Osborn, A. J.; Wolfe, M. S. J. Phys. Chem. 1992, 96, 5483-5487. (11) Van der Zande, B. M. I.; Koper, G. J. M.; Lekkerkerker, H. N. W. J. Phys. Chem. B 1999, 103, 5754-5760. (12) Baloch, M. K.; Van de Ven, T. G. M. J. Colloid Interface Sci. 1990, 135, 594-597. (13) Radeva, T. J. Colloid Interface Sci. 1997, 187, 57-61. (14) Fairey, R. C.; Jennings, B. R. J. Colloid Interface Sci. 1982, 85, 205-215. (15) Favier, V.; Chanzy, H.; Cavaille´, J. Y. Macromolecules 1995, 28, 63656367.
10.1021/la0600402 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/19/2006
4900 Langmuir, Vol. 22, No. 11, 2006
Letters
Figure 2. Low-dose electron micrograph of a film of 0.5% (w/v) tunicin whisker suspension in cyclohexane oriented with a 1 kHz electric field of 2000 V cm-1 and allowed to dry on a Formvar supporting film (scale bar 2 µm). (Insert) electron diffraction pattern correctly oriented with respect to the specimen, recorded on 1 µm2 of the specimen. The arrow indicates the direction of the electric field.
Figure 1. Optical micrograph in polarized light with cross nicols of ramie fiber fragments (a) in the absence of field and (b) in the presence of a 1 kHz electric field of 1200 V cm-1.
Results and Discussion Orientation of Ramie Fibers. Ramie fibers consist of an array of cellulose microfibrils that are highly aligned along the axis of the fiber.16 Considering that a ramie fiber has a diameter of around 50 µm and that the fiber fragments used here were about 2 mm long, these samples can be considered to be made of several million tightly packed cellulose whiskers with their long direction pointing along the axis of the fiber. In chloroform, the ramie fiber fragments gently float on the time scale of the experiment because of the almost matching density of cellulose and chloroform. When introduced in the Kerr cell, the fragments were randomly distributed (Figure 1a) and did not show any sign of motion until a certain voltage was attained. At 1200 V cm-1, the fragments suddenly became aligned perpendicular to the electrodes (Figure 1b), thus pointing in the direction of the electric field. The ramie fibers most probably orient along the field when the electrical torque exceeds the frictional forces exerted by the fluid. Because of the absence of Brownian motion on this macroscopic scale, these fragments remained aligned along the field after we switched off the generator current. From this simple experiment, it can be deduced that cellulose crystals have a natural tendency to align with their long axis along an electric field. This dielectric anisotropy is consistent with the positive birefringence of cellulose, with the parallel refractive index being higher than the perpendicular one.16 Orientation of the Cellulose Whiskers. Figure 2 shows a TEM image of the specimen prepared by allowing tunicin whiskers to dry onto a Formvar film in the Kerr cell under a 2000 V cm-1 electric field at 1 kHz. The first inspection of this image indicates that most of the cellulose whiskers are aligned along the direction of the field. Despite this overall alignment, some of the rods appear to be disoriented with respect to the field (16) Herman, P. H. In Physics and Chemistry of Cellulose Fibres; Elsevier: New York, 1949.
Figure 3. Sequence of Newton colors obtained between crossed polars on a suspension of cotton whiskers in toluene (1% w/v). Each band corresponds to an increase of ca. 60 V cm-1. The numbers indicate the applied field in V cm-1.
direction. Upon close inspection of Figure 2, one notices that a percentage of the whiskers present some kinks along their longitudinal axes due to prior sonication during the whisker extraction process (data not shown). This perturbed geometry or the possible entanglement of very long rods most probably results in a lower orientation ability along the field. However, the electron diffraction shown as the inset in Figure 2 indicates that this relative disorganization of a few elements does not significantly change the overall orientation of the whiskers. Indeed, the reflections appear as arcs with a relatively small angular spread (less than 15°). The presence of a weak 11h0 reflection as opposed to a strong 110 reflection (according to the index of the monoclinic unit cell of Iβ cellulose17) also indicates a marked uniplanar orientation, which could be due to either the effect of the field or the natural orientation of cellulose crystals on flat surfaces.18 (17) Sugiyama, J.; Vuong, R.; Chanzy, H. Macromolecules 1991, 24, 41684175. (18) Mukherjee, S. M.; Woods, H. J. Biochim. Biophys. Acta 1953, 10, 499511.
Letters
Langmuir, Vol. 22, No. 11, 2006 4901
Orientation of the Cellulose Whisker Suspensions. When positioned inside the Kerr cell without applying any field, a cellulose whisker suspension appears dark between crossed polars. When turning on the electric field, a homogeneous birefringence appears throughout the cell for values as low as 100 V cm-1. Interestingly, for cellulose suspensions at cellulose concentrations higher than 0.5% (w/v), increasing values of the field lead to a succession of sharp Newton colors (Figure 3) similar to those obtained for thin films of thermotropic liquid crystals. However, one has to notice that at these concentrations the cellulose whiskers suspensions are far from their isotropic/nematic transition. In that regime, the coupling between the electrical field and a possible anisotropic phase is not expected, excluding cooperative effects. The sequence of colors with increasing field up to three orders indicates an increase in the overall birefringence of the suspension with increasing field strength. One also notices that when the field is switched off the birefringence of the suspension quickly vanishes. At around 2200 V cm-1, third-order orange is obtained, which does not evolve with further increases in the field up to 3000 V cm-1, indicating a saturation of the orientation. Knowing the thickness of the liquid in the cell, e, we can estimate the absolute birefringence with the relation
δ ) ∆ne where δ is the retardation and ∆n is the absolute birefringence. For the third-order orange, the retardation is approximately 900 nm,19 giving for a 1.5 cm height of liquid suspension an absolute birefringence of ∆n ) 6 × 10-4. When scaled by the cellulose concentration in the suspension, which is 10-2 g cm-3, this gives a birefringence saturation of ∆n/c ) 0.06 g-1 cm3. This estimated value is not far from the commonly accepted value of 0.074 expected for a perfectly aligned crystal of native
cellulose.16 This indicates a high degree of order in the aligned suspension.
Conclusions In this letter, we have shown that cellulose in its native form can be successfully oriented by relatively modest electric fields, both at macroscopic and colloidal levels, as evidenced by optical and TEM observations together with a birefringence appraisal. Interestingly, the dispersion of cellulose in apolar solvents allows the application of the electric field for times compatible with the visual observation of the orientation with optical techniques and the preparation of aligned dried samples. Both ramie fiber fragments and cellulose whiskers orient parallel to the electric field, in contrast to their perpendicular orientation in a magnetic field. This axial orientation may originate from either a positive dielectric anisotropy and/or from the form of birefringence arising from the elongated shape of the particles. However, from the high anisotropy of the cellulose I crystals, one may suspect that it is the dielectric anisotropy that plays a dominant role. Finally, because of their strong intrinsic birefringence, cellulose whiskers are good candidates for electric birefringence measurements. Work is in progress to address quantitatively the orientational behavior of these rods in an electric field. Acknowledgment. We thank Henri Chanzy for the gift of a sample of ramie cellulose and for valuable discussions and suggestions. We also thank Patrick Perez for the fabrication of the Kerr cell and Patrice Ballet for the high-voltage setup. We acknowledge Eloı¨se SARL for the use of the KeenView CCD camera. LA0600402 (19) Bloss, F. D. In An Introduction to the Methods of Optical Crystallography; Holt, Rinehart and Winston: New York, 1961.
