Parabolic Focal Conics in Self-Assembled Solid Films of Cellulose

Langmuir 2005, 21, 5555-5561

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Parabolic Focal Conics in Self-Assembled Solid Films of Cellulose Nanocrystals Maren Roman† and Derek G. Gray* Department of Chemistry, Pulp and Paper Research Centre, McGill University, 3420 University Street, Montreal, Quebec, Canada H3A 2A7 Received December 23, 2004. In Final Form: March 9, 2005 Suspensions of cellulose nanocrystals form colloidal chiral nematic phases. The liquid crystalline order in these suspensions can be captured in solid films by slow evaporation of the liquid. Studies of the microstructure of such chiral nematic solid films revealed parabolic focal conic (PFC) defects, a symmetric form of focal conic defects in which the line defects form a pair of perpendicular, antiparallel, and confocal parabolas. The cellulose films with PFC defects were characterized by polarized-light and atomic force microscopy. The film surface showed a regular array of large and small elevations resulting from the displacement of the structural layers. Film fracture lines showed a series of layered half-cones. The microstructure of the films was modeled by computer. The model revealed that many structural layers terminate at the film surface.

1. Introduction Cellulose nanocrystals (CNCs), like many other rodlike microscopic particles, form colloidal liquid crystalline phases.1-7 The phase that is commonly observed in CNC suspensions is the chiral nematic phase.2 The parallel alignment of the crystals above a critical concentration is a result of favorable excluded volume interactions leading to a higher packing entropy than in the disordered phase.8 Explanations for the surprising observation that these aqueous suspensions form a chiral nematic phase remain speculative.2,9,10 The liquid crystalline order in CNC suspensions can be preserved in solid films by slow evaporation of the liquid.2,11,12 The microstructure of such films depends delicately on the drying conditions. Films that are prepared at ambient conditions generally show a polydomain structure with the helical axes of different chiral nematic domains pointing in different directions. Applying a magnetic field during the drying can increase the size of the chiral nematic domains and affect the orientation of the helical axis with respect to the plane of the film.12 In an effort to elucidate the microstructure of chiral nematic CNC films, we studied films of different thickness * Author to whom correspondence should be addressed: e-mail [email protected]; phone 514-398-6182; fax 514-398-8254. † Present address: Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. (1) Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Nature 1959, 184, 632. (2) Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170. (3) Revol, J.-F.; Godbout, L.; Dong, X.-M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst. 1994, 16, 127. (4) Dong, X. M.; Revol, J.-F.; Gray, D. G. Cellulose 1998, 5, 19. (5) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Colloids Surf. 1998, 142, 75. (6) Araki, J.; Wada, M.; Kuga, S.; Okano, T. Langmuir 2000, 16, 2413. (7) Heux, L.; Chauve, G.; Bonini, C. Langmuir 2000, 16, 8210. (8) Onsager, L. Ann. N.Y. Acad. Sci. 1949, 51, 627. (9) Revol, J.-F.; Marchessault, R. H. Int. J. Biol. Macromol. 1993, 15, 329. (10) Orts, W. J.; Godbout, L.; Marchessault, R. H.; Revol, J.-F. Macromolecules 1991, 24, 5747. (11) Revol, J.-F., Godbout, L.; Gray, D. G. J. Pulp Paper Sci. 1998, 24, 146. (12) Edgar, C. D.; Gray, D. G. Cellulose 2001, 8, 5.

by polarized-light microscopy. Besides planar and polydomain areas, we were surprised to note that several films contained areas with parabolic focal conic (PFC) defect structure. PFC defect structures have been observed in thermotropic smectic phases of small molecules13-15 and polymers16 and in lyotropic lamellar phases of lowmolecular-weight17-20 and polymeric21 surfactants and lipids.22,23 To our knowledge this is the first indication that a simple rodlike species can self-assemble into such a complex yet symmetrical structure. The fact that the PFC structure is captured in a solid film presents a possibility to study this highly symmetric defect structure by means other than optical microscopy. In addition, a chiral nematic system with a helical pitch large enough to be resolved by an optical microscope allows visualization of the individual structural layers. This paper reports our results of the characterization of CNC films with PFC defect structure by polarized-light and atomic force microscopy (AFM). 2. Experimental Section 2.1. Nanocrystal Suspension. Cellulose nanocrystals were prepared from dissolving-grade softwood sulfite pulp lapsheets (Temalfa 93, Tembec Inc, Temiscamingue, QC). The sheets were cut into pieces of 1 × 1 cm and milled in a Wiley mill to pass a 20-mesh screen. The milled fibers were hydrolyzed for 45 min at 45 °C with 8.75 mL of 64 wt % sulfuric acid (Fisher Scientific, 95.0-98.0%, reagent grade)/g of cellulose. The hydrolysis was quenched by diluting 10-fold with cold water. The crystals were (13) Rosenblatt, C. S.; Pindak, R.; Clark, N. A.; Meyer, R. B. J. Phys. 1977, 38, 1105. (14) Nawa, N. Jpn. J. Appl. Phys. 1995, 34, 2423. (15) Rout, D. K.; Choudhary, R. N. P. Mol. Cryst. Liq. Cryst. 1989, 166, 75. (16) Hudson, S. D.; Lovinger, A. J.; Larson, R. G.; Davis, D. D.; Garay, R. O., Fujishiro, K. Macromolecules 1993, 26, 5643. (17) Benton, W. J.; Toor, E. W.; Miller, C. A.; Fort, T., Jr. J. Phys. 1979, 40, 107. (18) Benton, W. J.; Miller, C. A. Prog. Colloid Polym. Sci. 1983, 68, 71. (19) Hirsch, E.; Wittmann, J. C.; Candau, F. J. Dispersion Sci. Technol. 1982, 3, 351. (20) Platz, G.; Thunig, C. Langmuir 1996, 12, 1906. (21) Candau, F.; Ballet, F.; Debeauvais, F.; Wittmann, J.-C. J. Colloid Interface Sci. 1982, 87, 356. (22) Asher, S. A.; Pershan, R. S. J. Phys. 1979, 40, 161. (23) Asher, S. A.; Pershan, P. B. Biophys. J. 1979, 27, 393.

10.1021/la046797f CCC: $30.25 © 2005 American Chemical Society Published on Web 04/29/2005

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Figure 1. AFM deflection image of cellulose nanocrystals. Scan size 5 µm. The apparent width of the cellulose nanocrystals is exaggerated due to tip-sample convolution. collected and washed once by centrifugation for 10 min at 5000 rpm and then dialyzed in SpectraPor 4 membranes against ultrapure water (Millipore Milli-Q UF Plus) until the pH was neutral. Crystal aggregates were disrupted by sonicating the suspension five times for 7 min under ice-bath cooling with a Vibracell ultrasonic processor (VC-1500, output control setting 60). The suspension was kept over ion-exchange resin for 4 days and then filtered through Whatman 541 filter paper. The final concentration of the suspension was 0.7 wt %. Figure 1 shows an AFM deflection image of the nanocrystals in the suspension. The lengths of the particles ranged from several tens to a few hundred nanometers. 2.2. Solid CNC Films. Solid films of the nanocrystals were prepared by drying down 10 mL of the suspension under ambient conditions in small polystyrene Petri dishes (50 mm diameter). The resulting films had a thickness of approximately 22 µm, as measured by optical microscopy on film cross-sections. 2.3. Atomic Force Microscopy. For images of the nanocrystals, round glass coverslips (Ted Pella, 12 mm diameter) were immersed into hot, concentrated sulfuric acid and rinsed three times with ultrapure water. Four drops of a 0.001 wt % suspension of the nanocrystals were deposited onto a coverslip and left to dry for a period of 3 days inside a polystyrene Petri dish. The coverslips were mounted onto AFM specimen disks (Ted Pella, 15 mm diameter) with standard adhesive tabs (Ted Pella). To investigate films of the nanocrystals by both AFM and light microscopy, pieces of the films were mounted with doublesided sticky tape onto cleaned glass coverslips (15 mm diameter) so that the area in the center of the coverslip is free of tape. For AFM studies, the cover slips were affixed lightly at the rim onto AFM specimen disks. AFM images were recorded with a Digital Instruments Multimode scanning probe microscope with a Nanoscope IIIa controller. 2.4. Polarized-Light Microscopy. The CNC films or the AFM specimen, described above, were placed onto standard microscopy slides for observation. Pictures were taken with a Nikon Coolpix 990 digital camera on a Nikon Microphot-FXA polarized-light microscope with a 530-nm retardation plate.

3. Results 3.1. Structure Identification. When viewed between crossed polarizers, significant areas of the CNC films (∼520%, depending on the perfection of the pattern) showed square lattices with a period of 50-60 µm (Figure 2). Similar lattices have been observed in aqueous solutions

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Figure 2. Square lattice in a solid film of cellulose nanocrystals between crossed polarizers. Scale bar 40 µm.

of hydroxypropyl cellulose24-27 and have controversially been attributed to either a parabolic focal conic structure,26 as first described by Rosenblatt et al.,13 or a polygonal structure,27 first described by Bouligand.28,29 Due to their similar appearance, the parabolic focal conic and polygonal structures are not easily distinguished. One method that yields different results for the two structures is rotation of the crossed polarizers. For the parabolic focal conic structure, rotation of the crossed polarizers leads to a rotation of the dark crosses in the square lattice.22 For the polygonal structure, rotation of the crossed polarizers results in a change in contrast between neighboring squares.25-27 Figure 3 shows the effect of a rotation of the crossed polarizers on our films. We observed a counterclockwise rotation of the crosses corresponding to the intersection of the parabolas with the surfaces of the film. An example of a cross generated by the intersection of four upwardopening parabolas with the top surface of the film (see discussion below) is circled in Figure 3. The counterclockwise rotation is evident from a careful comparison of the position of the arms in the white and black circles. We did not see any change in contrast between neighboring squares. Thus, the structure in our films is the parabolic focal conic structure. 3.2. Local Orientation of Nematic Planes. To identify the local orientation of the quasi-nematic planes in the chiral nematic film, we inserted a full-wave retardation plate into the optical path of the microscope. The result was a checkered pattern as shown in Figure 4. With a full-wave retardation plate, a blue color appears where the slow axis of the birefringent sample is aligned with the slow axis of the wave plate and a yellow color appears where the fast axis of the sample is aligned with (24) Werbowyj, R. S.; Gray, D. G. Mol. Cryst. Liq. Cryst. Lett. 1976, 34, 97. (25) Shimamura, K. Macromol. Chem., Rapid Commun. 1983, 4, 107. (26) Donald, A. M.; Viney, C.; Ritter, A. P. Liq. Cryst. 1986, 1, 287. (27) Meeten, G. H.; Navard, P. J. Polym. Sci., Part B: Polym. Phys. 1988, 26, 413. (28) Bouligand, Y. J. Phys. 1972, 33, 715. (29) Bouligand, Y. In Physical Properties of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: New York, 1999; p 304.

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Figure 3. Effect of rotation of crossed polarizers: (left) crossed polarizers in standard position; (right) crossed polarizers rotated counterclockwise by 25°. Scale bar 20 µm. Circles indicate a typical dark cross, generated by the intersection of parabolas with the surface of the film (see text).

Figure 4. Square lattice with crossed polars and full-wave retardation plate inserted into the microscope. Scale bar 20 µm.

the slow axis of the wave plate. By convention, the slow axis of the wave plate is oriented in the northeastsouthwest direction. Crystalline cellulose is optically biaxial. The slow axis, in the direction of largest refractive index, is oriented along the cellulose backbone and thus along the long axis of the nanocrystals. The two refractive indexes perpendicular to the long axis differ only slightly, so that we can treat cellulose nanocrystals as optically uniaxial with the fast axis perpendicular to the long axis. Within the quasinematic planes of the chiral nematic superstructure, the nanocrystals are aligned parallel to each other. As a result, each nematic plane has a slow axis in the direction of the nematic director and a fast axis perpendicular to it. In the chiral nematic superstructure, the quasi-nematic planes are stacked so that the orientation of the nematic director changes progressively from plane to plane. As one moves

through the stack of planes, the nematic director, and therefore the slow axis, rotates about an axis normal to the planes, the helical axis. The direction of the fast axis, being perpendicular to the slow axis, remains the same for all planes. Consequently, the fast axis, in the direction of the helical axis, becomes the optical axis of the chiral nematic superstructure. Thus, a blue color in Figure 4 indicates a local helical axis with a northwest or southeast projection onto the plane normal to the viewing direction, and a yellow color indicates a local helical axis with a northeast or southwest projection. The rotation of the slow axis about the helical axis is evidenced in Figure 4 by periodic dark lines resembling fingerprints, which appear where the local slow axis, that is, the local nematic director, has an upward or downward projection onto the plane parallel to the viewing direction. Thus, in areas that show fingerprint lines, the nematic planes of the chiral nematic superstructure are parallel or oblique, but not normal, to the viewing direction. In these areas, the local helical axis, which is normal to the nematic planes, is oriented perpendicular or oblique to the viewing direction. The junctures of four squares fall into one of two categories: those that are centers of concentric fingerprint lines, and those at which the fingerprint lines meet. The distance along the helical axis between two nematic planes with the same director is called the chiral nematic pitch. By measuring the distance between one fingerprint line and the next but one in Figure 4, we can determine the pitch in our films. We counted between 13 and 18 lines over a length of 20 µm. Thus, the chiral nematic pitch in these solid films of cellulose nanocrystals is between 2.2 and 3.1 µm. When the local optical axis in the sample is oriented parallel to the viewing direction, the linearly polarized light passing through the sample will be completely extinguished by the analyzer. As a result, regions with an optical axis along the optical path of the microscope will appear black. With a full-retardation plate inserted between polarizer and analyzer, instead of black those regions will appear magenta because now the analyzer acts as a filter for light of wavelength 530 nm (green); that is, light that has practically not been affected by the

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Figure 5. Film topography as observed by AFM: (left) height image (z-range 800 nm); (right) deflection image (z-range 5 nm). Scan size 100 µm; high areas are light-colored.

Figure 6. Same area as in Figure 5 by polarized-light microscopy (crossed polars, full-wave retardation plate): (left) focused near the top surface of the film; (right) focused near the bottom surface of the film. Scale bar 20 µm.

retardation plate. Because the optical axis is parallel to the helical axis and the helical axis is normal to the nematic planes, a magenta color indicates regions in which the nematic planes are normal to the viewing direction. More importantly, the parabolic line defects (see discussion below) are disordered and so the black crosses observed with crossed polars also appear magenta when a retardation plate is inserted (Figure 4). 3.3. Film Topography. Both Rosenblatt et al.13 and Asher and Pershan22 considered the interactions of fluid PFC arrays with bounding planes. However, the situation for our nanocrystal films is somewhat different from classical smectic or chiral nematic liquid crystals. As the water evaporates from the anisotropic suspension, the upper surface is not constrained to a plane, and the underlying structural anisotropy may be reflected in the surface topography of the final dry film.

Studies of the film topography by AFM revealed a regular array of larger elevations, approximately 800 nm high, with smaller elevations of ca. 300 nm height between (Figure 5). Figure 6 shows the same area as Figure 5 by polarized-light microscopy. In the left panel, the microscope is focused near the top surface of the film, and in the right panel, near the bottom surface of the film. With focus near the top surface of the film (Figure 6, left panel), we see fingerprint lines in regions that correspond to low surface areas (compare with Figure 5). Some of the lines can also be seen in the AFM deflection image (Figure 5, right panel). As explained above, fingerprint lines appear where the nematic planes of the chiral nematic superstructure are parallel or oblique to the viewing direction. Thus, in low topographic regions, the nematic planes are tilted at or near the film surface.

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The fact that the layer lines are visible in the fractured edge indicates that the chiral nematic orientation of the nanocrystals in the film results in anisotropic mechanical properties at the scale of the pitch. Intuitively, it would be expected that the material would fracture more readily when the constituent nanocrystals are oriented along the fracture line than across it. On a larger scale, the conical structures suggest that the parabolic-shaped line defects, where the orientation of the layer structure is discontinuous, introduce mechanical weakness in the film, resulting in the half-cones observed in Figure 7. 4. Discussion

Figure 7. Layered conical structures (white arrows) observed at a film fracture line. Crossed polars, full-wave retardation plate. Scale bar 30 µm.

The high topographic regions (white in Figure 5) are featureless in the optical microscope with focus near the top surface of the film, even though these regions are within the focal range. For features to be visible by optical microscopy, they need to have edges that diffract, refract, or reflect light. The fact that the high topographic regions do not exhibit any optical edge effects implies that in these regions the nematic planes of the chiral nematic superstructure are normal to the viewing direction; that is, we are viewing those regions along their local helical axes. Areas that are viewed along their optical axis should be magenta in color. The high topographic regions, however, still show blue or yellow color because the orientation of the nematic planes is different at different depths in the film. At the top surface the nematic layers are normal to the viewing direction but at some depth they are oblique or parallel to the viewing direction. The conclusion that the orientation of nematic layers is different at different depths of the film is supported by the right panel in Figure 6. When the optical microscope is focused near the bottom surface of the film, we see fingerprint lines in regions that are featureless when the focus is near the top surface of the film. Thus, in high topographic regions, the nematic planes are tilted at or near the bottom surface of the film. 3.4. Fracture Lines. The cellulose films are quite brittle and fracture easily on bending. Figure 7 shows a picture of a film fracture line from above. Along the edge of the fracture, a series of adjacent half-cones with layerlike structure are visible.

Parabolic focal conics are a special, highly symmetric form of focal conics in which the line defects form a pair of perpendicular, antiparallel, and confocal parabolas.13,22 Depending on the substance and sample preparation, the parabolas lie either horizontally or vertically in the sample plane.13 If they are vertical, as in our case, the parabolas intersect the top and bottom sample surfaces with their ends. The focal regions are located in midplane. The square lattice, shown by vertical PFCs in the optical microscope, is a result of a space-filling square array of PFCs so that parabolas of adjacent PFCs meet at the sample surfaces (see Figure 3). From the thickness of the film, t, and the width of the parabolas at the film surface, 2R, we can calculate the focal length, f, of the confocal parabolas by using the definition of a parabola:

f R2 t + ) 2 2 4f Solving for f gives

f)

(x2R2 + t2 t 2 2

The parabola width at the surface is directly related to the period of the optical texture. Taking the parabola width at the surface as 54 µm, the film thickness as 22 µm, and ignoring the negative root, we get a focal length of 11 µm. A focal length of half the sample thickness signifies very wide parabolas. With a helical pitch of around 2.75 µm, a film of thickness 22 µm accommodates 8 pitches or 16 half-pitches. Using the equations given by Rosenblatt et al.,13 we created a computer model of the PFC defects as they exist in our films. Figure 8 shows the xz and yz planes of a PFC with a focal length of 11 µm containing 17 layers with a layer spacing of 1.375 µm. From Figure 8 it is evident that a sample of thickness 22 µm accommodates little more than the focal region of the PFC. The focal region lies between z ) 5.5 and z )

Figure 8. Cross-sections of layers in a parabolic focal conic (PFC) structure showing the parabolic line defects in the xz and yz planes. The PFC shown has a focal length of 11 µm, a layer spacing of 1.375 µm, and a height (sample thickness) of 22 µm.

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Figure 9. Illustration of a quarter of a PFC structure with a focal length of 11 µm in a sample of 22 µm thickness, layer spacing 1.375 µm. The red and green lines indicate where two typical curved surfaces intersect the surfaces of the cube.

-5.5. As a consequence, 16 out of the 17 structural layers are incomplete and terminate at the sample surface. The necessity for some layers to terminate at the sample surface in order to accommodate the layer tilt and increased number of layers due to the multiple connectivity of the layers has already been recognized by Rosenblatt et al.13 Originally, the parabolic focal conic structure was proposed as a nearly planar alternative to Bouligand’s polygonal structure, in which the layers lie vertically at

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the upper and lower faces of the mesophase. However, in our sample the planar component of the PFC structure has almost disappeared since nearly all of the layers are tilted and terminate at the sample surface. The tilt of individual layers in the PFCs is apparent in Figure 9. Figure 9 and the following figures show the structural layers as they would be shaped in an isolated PFC. However, PFCs usually exist in square arrays in which each parabola meets three other parabolas, two of which are perpendicular and one is parallel, at the sample surface. The structural layers of a PFC that is part of an array differ from those of an isolated PFC. The differences are most pronounced at the edges of the layers, where the layers of a PFC in an array form the adjacent PFCs while those in an isolated PFC flatten out. Figure 10 shows a slice of the center of an isolated PFC. The tilt of the layers, reason for the blue and yellow color and the fingerprint lines in Figure 4, is clearly visible. Figure 10 shows that the centers of the PFCs are located at those points between four squares in Figure 4 at which the fingerprint lines meet, as opposed to those points that are centers of concentric fingerprint lines. From Figures 5 and 6 we know that the junctures of four squares that are centers of concentric fingerprint lines correspond to the larger surface elevations and low surface areas. Consequently, the junctures of four squares at which the fingerprint lines meet, which we identified as the centers of the PFCs, must correspond to the smaller surface elevations. These small surface elevations might be a result of an increased sample thickness due to the multiple connectivity of the layers resulting in an increase in the number of layers. Figure 11 shows a complete isolated PFC with focal length of 11 µm. The upward-opening parabola, along the y-axis, intersects the top surface at x ) 0, y ) -27 and x ) 0, y ) 27. The downward-opening parabola, along the

Figure 10. Layer tilting in the center of a PFC structure with a focal length of 11 µm, layer spacing 1.375 µm. For clarity, only every fourth layer is shown. The corresponding area of a PFC is shown in the photomicrograph on the right.

Figure 11. Surface features of a PFC structure with a focal length of 11 µm in a sample of 22 µm thickness, layer spacing 1.375 µm. On the right is the AFM false color scan (red high, blue low) of the top surface of a corresponding area of a PFC, taken from the height image in Figure 5.

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x-axis, intersects the bottom surface at x ) -27, y ) 0 and x ) 27, y ) 0. The PFC structure in Figure 11 may be compared with the AFM measurement of surface topography, shown in Figure 5, and polarizing microscope images of the same area, shown in Figure 6. Comparison of Figures 11 and 6 reveals that concentric fingerprint lines observed at the top surface correspond to upwardopening nests of chiral nematic layers, and concentric fingerprint lines observed at the bottom surface correspond to downward-opening nests of chiral nematic layers. To aid comparison of structure and surface topography, a false-color height image in the appropriate orientation is shown in Figure 11. Presumably, orientationally dependent shrinkage of the PFC structures leads to the surface topography observed here for the first time. High spots correspond to areas where the chiral nematic axis is close to perpendicular to the surface. Conclusions Films formed by evaporation of water from colloidal suspensions of cellulose nanocrystals maintain the chiral

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nematic orientational order of the liquid crystal. In particular, we found clear evidence that a parabolic focal conic (PFC) structure is trapped in the film. Although this texture with its symmetrical array of line defects is well-known for thin layers of smectic and chiral nematic molecular and polymeric liquid crystalline phases between flat surfaces, we think that this is the first time that it has been observed in a self-assembled solid film. The square array of parabolic line defects is in accord with polarized-light microcopy of the films, with the focal length of the parabolas being around half the thickness of the film. The surface topography of the film shows a regular pattern reflecting the underlying bulk structure. It seems remarkable that simple rodlike species can self-assemble into such a regular complex extended structure. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial support and the Center for Self-Assembled Chemical Structures, FQRNT, for infrastructure support. LA046797F