Self-Assembly of Cellulose Nanocrystals: Parabolic Focal Conic Films

Chapter 3

Self-Assembly of Cellulose Nanocrystals: Parabolic Focal Conic Films 1

Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch003

Derek G. Gray and Maren Roman

2

1

Department of Chemistry, Pulp and Paper Building, McGill University, Montréal, Québec H3A 2A7, Canada Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

2

Evaporation of aqueous suspensions of cellulose nanocrystals gives solid films which retain the orientational order of the liquid crystalline phase observed in the liquid state. At a given film thickness, a parabolic focal conic texture is fixed in the solid film. A brief explanation of this structure, and a tentative rationale for its formation are presented.

Introduction In nature, cellulose is the main structural polymer in the plant cell wall, where it exists as partially crystalline fibrous microfibrillar aggregates. Under suitable conditions, acid hydrolysis breaks down the aggregates into individual needle-shaped crystalline rods of colloidal dimensions (7) whose length and breadth depend on the cellulose source and the hydrolysis conditions. The width of the rods is of the order of a few nanometers, and so the resultant highly crystalline cellulose I particles are referred to as nanocrystals. It is convenient to 26

© 2006 American Chemical Society

In Cellulose Nanocomposites; Oksman, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.


27 distinguish between these nanocrystals, whose length is of the order of tens or hundreds of nanometers, and the much longer elementary fibrils or microfibrils of cellulose that are the primary biosynthesized species in plant cell walls. When suitably stabilized, aqueous suspensions of cellulose nanocrystals may form chiral nematic liquid crystalline phases (2). When such a suspension is allowed to dry down slowly on a flat surface, the chiral nematic order of the crystals is preserved in the resulting solid film. The iridescent properties of these films suggest decorative and security applications (5). The basic structure of these films depends on the self-orientation of the nanocrystals into the wellknown twisted or hélicoïdal arrangement. However, this idealized arrangement is seldom observed for thick samples. To elucidate the arrangement of nanocrystals in these self-assembled films, we investigated films of different thickness by polarized-light and atomic force microscopy.

Experimental Cellulose nanocrystals were prepared from dissolving-grade softwood sulfite pulp lapsheets (Temalfa 93, Tembec Inc., Temiscaming, QC, milled in a Wiley mill to pass a 20-mesh screen. The milled fibers were hydrolyzed for 45 mm at 45 °C with 8.75 mL of 64 wt% sulfuric acid per gram of cellulose. The hydrolysis was quenched by diluting 10-fold with cold water. The crystals were collected and washed once by centrifugation for 10 min at 5000 rpm and then dialyzed against ultrapure water (Millipore Milli-Q UF Plus) until the pH was neutral. Crystal aggregates were disrupted by sonication. 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.%. 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 dia.). The thickness of the films was measured by optical microscopy on film crosssections.

Results and Discussion Figure 1 shows images of films of different thickness cast from aqueous cellulose nanocrystal suspensions. On the left, the thinnest film is relatively featureless, showing only a few isolated disclination lines (4). This optical texture is characteristic of a planar texture, where the hélicoïdal axis is normal to the plane of the film. The thickest sample, on the right in Figure 1, shows a more complex texture resembling the polygonal focal texture often observed for smectic liquid crystals (4).



Figure 1. Polarizing microscope images (area 300 μπι χ 300 μm) of cellulose nanocrystal films, thickness (left to right), 12 μm, 24 μνη, 36 μm. The parabolic focal conic texture is shown in the centre image.


29


In films of intermediate thickness (Figure 1, centre image), we unexpectedly observed regular arrays of parabolic focal conies (PFCs), a special case of focal conic defect structures (5). PFCs are well known in thermotropic smectic phases of small molecules and polymers, and in lyotropic lamellar phases of low-molecular-weight and polymeric surfactants and lipids. To our knowledge, this is the first occurrence of the PFC structure in a colloidal liquid crystal. In addition, our solid films offer a unique opportunity for the understanding of these defects. Here, we indicate qualitatively the nature of the film structure, speculate on why it forms, and suggest its significance.

Why parabolic? Imagine, for the moment, that the chiral nematic structure can be viewed simply as a layered fluid structure, where the layers correspond to a parallel orientation of the nanocrystal orientation in the layer. The layers will tend to arrange themselves parallel to a neighbouring flat surface, such as a microscope slide and a cover slip. In Figure 2, the relationship between flat and curved planes in a parabolic focal conic defect is sketched in cross-section. A parabola, defined as the set of points in a plane that are equidistant from a fixed point (the focus) and a fixed line (the directrix), is found at the intersection between a set of flat parallel and nested spherical planes. The orientation is indeterminate at the line of defects, which thus appears dark under the polarizing microscope. The parabolic focal conic arrangement consists of confocal pairs of upward and downward facing parabolas at right angles to each other. Viewed from above along the z-direction, these appear as dark crosses, as shown in the center image of Figure 1. A full description of defects and focal conic textures in liquid crystals is given in many texts (6, 7).

Why does the parabolic focal conic texture form? The formation of a regular parabolic focal conic liquid crystalline texture during the evaporation of water from the chiral nematic suspension of cellulose nanocrystals is surprising. We speculate that the change in volume on evaporation from the liquid surface in the Petri dish causes compressive strain within the liquid that results in layer buckling. The situation is sketched in Figure 3.



30

Figure 2. Sketch of the parabola-shaped intersection between a set offlat parallel and nested spherical planes.

Figure 3. Sketch ofpossible sequence leading to formation of a parabolic focal conic texture from a planar texture(5). The numbers on the axes correspond to dimensions in μm for a typical chiral nematic cellulose film.


31


Most focal conic structures have been observed for smectic liquid crystals, where the fluid layered structure is evidently conducive to this type of defect formation. The "layering" of chiral nematics is less well-defined; the fingerprint texture observed for chiral nematics in general (and also in the cross-section of these cellulose films) is due to the alternating refractive index orthogonal to the hélicoïdal axis as the nanocrystals are oriented along and across the viewing direction. But the layers in chiral nematics are not real; there is a smooth gradation of properties normal to the hélicoïdal axis. So other factors may be involved in the formation of this texture from aqueous colloidal suspensions of cellulose nanocrystals.

Concluding Comments Our PFC films demonstrate that, starting from a simple rod-like colloid, it is possible to spontaneously form regular and complex structures that can be trapped in a solid film. The dimensions of the observed defect structure in these cellulose nanocrystal films have been fitted to a calculated model of parabolic focal conic structure (#). The film structure is not purely of academic interest; the iridescence observed from these films depends both on the pitch length of the chiral nematic structure (which governs the basic reflection colour) and also on the texture. An ideal planar texture reflects a single sharp wavelength of light at a given angle. The parabolic focal conic texture reflects over a range of angles and wavelengths, giving a more sparkling reflection. The solid film is essentially homogeneous, being pure Cellulose I. The structure is due to the variation in nanocrystal orientation in different regions of the film. However, the film material may be combined or embedded in a matrix such as an epoxy resin (J) to enhance optical and material properties.

Acknowledgements We thank the Natural Sciences and Engineering Research Council of Canada for financial support and the Center for Self-Assembled Chemical Structures. FQRNT (Québec), for infrastructure support.

References 1. Mukherjee, S. M.; Woods, H. J. Biochim. Biophys. Acta 1953,10,499-511. 2. Revol, J. F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol. 1992, 14, 170-172.


32 3. 4.

5. 6.


7.

8.

Revol, J.-F.; Godbout, J. D. L.; Gray, D. G. U.S. Patent 5,629,055, 1997. Collings, P.; Hird, M . An Introduction to Liquid Crystals: Chemistry and Physics; The Liquid Crystals Book Series; Taylor & Francis: London, UK, 1997. Rosenblatt, C. S.; Pindak, R.; Clark, Ν. Α.; Meyer, R. B. J. Phys. (Paris) 1977, 38, 1105-1115. Chandrasekhar, S. Liquid Crystals; 2nd ed.; Cambridge University Press: Cambridge, UK, 1992. Bouligand, Y. In Physical Properties of Liquid Crystals; Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H. W.; Vili, V., Eds.; Wiley-VCH: New York, NY, 1999; pp 304-351. Roman, M.; Gray, D. G. Langmuir 2005, 21, 5555-5561.


Self-Assembly of Cellulose Nanocrystals: Parabolic Focal Conic Films

Recommend Documents