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Controlling the Reflection Wavelength of Iridescent Solid Films of Nanocrystalline Cellulose Stephanie Beck,* Jean Bouchard, and Richard Berry FPInnovations, 570 St. Jean Boulevard, Pointe-Claire, QC H9R 3J9 Canada Received September 13, 2010; Revised Manuscript Received November 3, 2010

Nanocrystalline cellulose (NCC) self-assembles in suspension to form iridescent chiral nematic films upon drying that can reflect circularly polarized light at specific wavelengths. Ultrasound treatment has now been found to increase the chiral nematic pitch in suspension and red-shift the reflection wavelength of NCC films as the applied energy increases. Sonication and electrolyte addition combined allow the reflective properties of the film to be predictably tuned. The effects of sonicating an NCC suspension are cumulative and permanent. Suspensions sonicated with different energy inputs may be mixed to give an NCC film having a reflection band intermediate between those obtained from the individual suspensions. The data suggest that the ultrasound-induced red-shift is electrostatic in nature.

Introduction Cellulose is the main structural constituent of wood and higher plants and is therefore the most abundant renewable organic material on earth.1 Many products are derived from cellulose, including cellulose nanocrystals or nanocrystalline cellulose (NCC), also called whiskers. This nanomaterial has received a great deal of attention in the last two decades, particularly for its potential applications as reinforcement or nanofillers in polymer composites and its unique self-assembly and optical properties.2,3 Nanocrystalline cellulose can be extracted from native cellulose sources such as bleached wood pulp by controlled acid hydrolysis.4,5 Wood cellulose nanocrystals average 100-200 nm in length with a cross-section of 3-5 nm (an aspect ratio of 30-67).6,7 The use of sulfuric acid to produce NCC imparts sulfate ester groups to the cellulose nanocrystal surfaces, resulting in electrostatically stabilized aqueous NCC suspensions.8-12 The anisometric rodlike shape and negative surface charge of NCC particles result in suspensions which phase separate into an upper isotropic phase and a lower ordered phase, above a critical NCC concentration.10 Revol and co-workers proposed that the ordered phase is a cholesteric or chiral nematic liquid crystal.10 Chiral nematic liquid crystals contain rodlike particles arranged in pseudolayers with their long axes parallel to the plane of the layers.13 The average particle axis direction in each layer (the director) is rotated at a small angle to the layers above and below it, producing a helical distribution of the pseudolayers. The chiral nematic pitch P is defined as the distance required for the director to make one full rotation about the cholesteric axis. Aqueous NCC suspensions can be evaporated to produce solid semitranslucent NCC films that retain the self-assembled chiral nematic liquid crystalline order formed in the suspension. These films are iridescent and reflect left-handed circularly polarized light in a narrow wavelength band determined by the chiral nematic pitch of the liquid crystal structure. The reflected wavelength gives rise to visible iridescence colors when the * To whom correspondence should be addressed. Tel.: 1 514 630-4100. Fax: 1 514 630-4134. E-mail: [email protected].

pitch of the helix is on the order of the wavelengths of visible light (around 400-700 nm). Previous studies have found that increasing the electrolyte concentration in the NCC suspension prior to film casting shortens the peak wavelength of reflection of the solid films.14 The electrolyte partially screens the negative charges of the sulfate ester groups on the NCC surfaces, reducing interparticle electrostatic repulsion. The rodlike particles therefore approach each other more closely, which shortens the chiral nematic pitch of the liquid crystal phase and shifts the reflection band to shorter wavelengths. This method of controlling NCC film iridescence color is limited by the amount of salt which can be added before the colloidal suspension is destabilized and gelation occurs.11,14 A second limitation lies in the unidirectional shift of the reflection toward the UV region of the spectrum. NCC film iridescence colors also depend on the cellulose source and the acid hydrolysis conditions used to extract the NCC;14 shorter and thinner NCC particles reduce the chiral nematic pitch and give films with shorter reflection wavelengths. The sulfate ester content of NCC was also found to affect the pitch of the final films; desulfation by heating the aqueous NCC suspension results in a blue-shift to shorter pitches.14 In the laboratory, ultrasound treatment (sonication) is used as a final step to obtain a well-dispersed colloidal NCC suspension.11 Sonication is thought to break up side-by-side NCC aggregates in suspension.12 The effects of sonication on NCC suspension properties have been studied by Dong and coworkers,12 who found that brief sonication was sufficient to disperse the cellulose nanocrystals and that further sonication was counterproductive, in that it increased the critical concentration required for liquid crystalline phase formation. A more recent study corroborates this observation.15 To our knowledge, there is no method reported in the literature which can control the peak reflection wavelength of solid chiral nematic NCC films without the use of additives, or which shifts the reflection band toward longer wavelengths. A method to fine-tune the reflection wavelength of NCC films with minimal time and energy input, and without altering the desirable physicochemical properties of the NCC, would allow the fabrication of a range of products incorporating the unique optical properties of self-assembled chiral nematic NCC. We

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have evaluated the effects of ultrasonic energy treatment of aqueous NCC suspensions prior to film casting in order to develop practical methods of changing and controlling NCC film iridescence color.

Experimental Section NCC Suspension Preparation and Characterization. Aqueous NCC suspensions were prepared at FPInnovations by controlled sulfuric acid hydrolysis of a commercial bleached softwood kraft pulp according to a patent-pending procedure scaled up from the literature.12 The final pH of the NCC suspensions was around 2.5-2.9, and the concentration was around 2.8 wt %, measured by gravimetry. The sulfur content (0.80 ( 0.05 wt % S on NCC) was measured by elemental analysis (inductively coupled plasma spectroscopy-atomic emission spectroscopy, ICP-AES) or by conductometric titration. Particle size was determined by photon correlation spectroscopy (Zetasizer 3000, Malvern Instruments, Worcestershire, U.K.). Photon correlation spectroscopy (PCS) is a light scattering method; the measured NCC particle size values are the hydrodynamic radii of equivalent spheres and do not represent actual physical dimensions of the rodlike NCC particles. However, they are valid for comparison purposes. Measurements were performed on samples containing 10 mM NaCl and a final NCC concentration of 1.0-1.5 wt %. Samples were filtered with 0.45 µm nylon Whatman syringe filters to remove dust or other large particles. Ultrasound Treatment. Samples of NCC suspension were sonicated using a Sonics vibra-cell 130 W 20 kHz ultrasonic processor with a 6 mm diameter probe: Typically, 15 mL of a 2-3 wt % NCC suspension was placed in a 50-mL plastic tube and sonicated at 60% of the maximum power. Prolonged sonication (to an energy input of over 3600 J/g NCC) was performed in an ice bath to prevent desulfation caused by heating of the suspension. Film Casting. The treated suspension was poured into a polystyrene Petri dish (90 mm diameter) and allowed to evaporate undisturbed at ambient conditions (20-25 °C, 20-60% relative humidity) or in an oven at 30-70 °C. Solid NCC films with an average thickness of 70-80 µm and basis weight around 65 g/m2 were obtained. All photographs of NCC films are taken normal to the films, with the films placed against a matte black background in order to view the reflected iridescence color in diffuse incident light. Spectrophotometric Analysis of Iridescent Films. A goniospectrophotometric color measurement system (GCMS-3B, Murakami Color Research Laboratory, Tokyo, Japan) was used to quantify the differences in NCC film reflection colors. Films were conditioned at 50% relative humidity and 23 °C and then placed against a matte black background with their edges taped down. A 12 V, 100 W unpolarized halogen light source was used. The instrument was calibrated against a barium sulfate tile. Reflection spectra at wavelengths 390 to 730 nm were measured under 45° incident illumination and angular detection range from 0 to 75°. Viewed area on the sample ranged from ∼8 × 16 mm at 0° receiving angle to ∼8 × 94 mm at 80°. Although several reflection angles were measured, comparisons of the different films were generally made at the specular reflection angle (45°), owing to the high intensity of reflection at that angle and the margin it afforded in monitoring the red-shift of samples under different conditions. Unless otherwise noted, the dominant wavelengths of reflection were compared at 45° reflection angles. Aqueous NCC Suspension Properties after Sonication. Aliquots (15 mL) of 5 wt % NCC suspension were sonicated with increasing energy inputs and allowed to phase separate in sealed glass vials at ambient conditions, over a period of 48 to 72 h to ensure that equilibrium between the phases was established. Upon phase separation, the volume fraction of each phase was calculated from their heights and the concentration of each phase determined by gravimetry. Samples taken from each suspension immediately after sonication were placed in flat glass tubes of rectangular cross-section (0.4 mm optical path length) and allowed to phase separate as well. The chiral nematic (CN)

Figure 1. NCC films produced from suspensions treated with increasing applied ultrasonic energy (0, 250, 700, 1800, and 7200 J/g NCC) from left to right. Viewing is normal to the film surface under diffuse lighting. Scale marker ) 1 cm.

Figure 2. Specular reflection spectra (45° incidence and detection angles) of NCC films produced with increasing applied sonication energy: 507 (0), 1267 (]), 1521 (4), 1901 (×), and 2535 J/g NCC (O).

Figure 3. Peak reflection wavelength for solid NCC films prepared from a 2.7 wt % NCC suspension treated with applied ultrasonic energy up to 5000 J/g NCC.

pitch P of the liquid crystalline phase was determined by polarized light optical microscopy, following Dong et al.’s procedure.11

Results and Discussion Increasing Applied Ultrasonic Energy. Aliquots of 2.8 wt % NCC suspension were sonicated with increasing energy inputs (measured in J/g NCC) and cast into solid films. The films exhibit reflected iridescence in normal viewing under diffuse incident light with colors ranging from blue-violet to red (Figure 1). As measured by goniospectrophotometry, the dominant wavelength of reflection of the films at 45° incident illumination and 45° reflection increased from 0.9 0.70 0.65 0.57

4.8 5.2 5.3

4.3 4.7 4.7

3.4 6.6 7.2 15.7

a

Figure 6. (Top) From left to right, NCC films prepared from suspensions E (750 J/g NCC), F (1:1 ratio of 750 J/g NCC to 2225 J/g NCC), and G (2225 J/g NCC). Reflection is normal to the film surfaces. Scale marker ) 1 cm. (Bottom) Specular reflection spectra (45° incidence and detection angles) for NCC films E (0), F (4), G (]), and H (1490 J/g NCC; ×).

with an intermediate amount of sonication energy. NCC films have a polydomain structure in which the helical axes of different chiral nematic domains point in different directions,16 with the pitch of each domain giving an average pitch. Because pitch controls the reflection wavelength, the chiral nematic domains in sample F must have a pitch intermediate between those of samples E and G. Effects of Sonication on Intrinsic Properties of Cellulose Nanocrystals. In aqueous suspension, NCC has three essential properties that govern its liquid crystal behavior: the geometry (dimensions and aspect ratio) of the rodlike cellulose nanocrystals, the surface charge density, and the electrical double layer of the nanocrystals along with any other ions in the surrounding aqueous media. Atomic force microscopy measurements show that the NCC produced at the pilot plant scale has an average length of 140 ( 53 nm, in agreement with literature values of 180 ( 75 nm.6 The pitch of chiral nematic phases formed in suspensions of rodlike polyelectrolytes other than NCC has not been found to vary systematically with particle length L.17-19 However, films cast from NCC suspensions containing particles of longer average length (obtained by fractionation) have reflection bands shifted toward the red end of the visible spectrum, while shorter NCC particles produce films with reflection wavelengths shifted toward the blue end.14 Because sonication can only shorten and not lengthen the NCC particles, it is clear that it cannot cause the observed pitch increase in this manner. It has been proposed that prolonged sonication breaks the NCC particles, lowering their mean axial ratio, thereby explaining the observed increase in the critical concentration c* of NCC suspensions with sonication (see below).12 However, sonication above energy inputs of ∼2000 J/g NCC does not significantly change the particle size as measured by photon correlation spectroscopy (PCS), while still increasing the pitch and causing significant changes in the optical properties of the resulting films, leaving it unclear what changes occur at large sonication energy

CN ) chiral nematic.

b

I ) isotropic.

inputs. This is in agreement with the results of Dong et al., who found that sonication treatments longer than 5 min did not further decrease particle length as measured by TEM and PCS, but did affect the liquid crystal behavior of NCC suspensions.12 Sonication, therefore, likely does not change the reflection wavelength by altering the NCC particle dimensions. The degree of NCC sulfation determines the surface charge density σ of the cellulose nanocrystals and affects the reflection wavelength of dried NCC films.14 However, heat-induced desulfation of NCC suspensions occurs to a significant extent only at temperatures above 40-50 °C, over a period of at least several hours.20 Sonication times were less than 7 min, with the suspension temperature reaching a maximum of 40 °C. Sonication energy inputs up to 3700 J/g NCC did not affect the sulfur content (measured by ICP-AES) or the surface charge density (measured by conductometric titration) of the NCC used in these experiments. The energy supplied by typical ultrasound treatment is therefore insufficient to break the covalent sulfate ester-cellulose bonds at room temperature. Furthermore, we have determined by size exclusion chromatography coupled with multiangle laser light scattering (SEC-MALLS) that the weightaverage DP (140-150) of the cellulose chains within our NCC is unaffected by the sonication energy applied in these experiments. Effects of Sonication on NCC Suspensions. The effect of sonication on NCC film optical properties must first arise in the bulk aqueous suspension to which the sonication is applied. A change in chiral nematic pitch in the ordered phase of a suspension translates into a corresponding change in pitch and, hence, in the color of the final solid NCC film. This is exemplified by the decreasing pitch of films cast from NCC suspensions containing increasing concentrations of salt.14 Accordingly, we examined the effect of sonication on the behavior and properties of NCC suspensions of equal concentration, to better understand its effects on NCC film iridescence. The results are summarized in Table 3. The data show that sonication hinders anisotropic phase formation in the NCC suspensions. Increasing the energy input raises the critical concentration for phase separation, decreasing the volume fraction of the chiral nematic (CN) phase, in agreement with previous findings.12 Sonication also causes a slight increase in NCC concentration in both the CN and isotropic (I) phases while simultaneously increasing P. Although increasing the suspension ionic strength by adding salt also hinders anisotropic phase formation and increases the NCC concentration in both phases,11 it acts in the opposite direction to sonication by decreasing P, as discussed above. Therefore, sonication does not induce a large change in the bulk ionic strength of the suspension. However, by measuring small changes in suspension conductivity as a function of sonication, it was found that the NCC suspension conductivity increases with sonication. The changes in conductivity appear to be correlated with the peak reflection wavelength red-shift (Figure

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Figure 7. Evolution of the conductivity (0) and peak reflection wavelength of the resulting film (2) for a 2.8 wt % NCC suspension as a function of the applied ultrasonic energy.

7); it is evident that the conductivity increase and red-shift both occur immediately upon application of ultrasonic energy. In the following section, we will propose a mechanism to explain the sonication-induced increase of the chiral nematic pitch of NCC suspensions and the associated red-shift of the NCC film reflection wavelength by combining the observations described above. Mechanism of Sonication-Induced Pitch Increase. Although sonication does not significantly change many important properties of cellulose nanocrystals, it increases the critical concentration c* required for phase separation of an NCC suspension, the chiral nematic pitch P of the liquid crystal phase and the reflection wavelength of the corresponding dry NCC film. Any proposed mechanism for sonication-induced pitch increase must also account for the other effects of sonication that have been observed in NCC suspensions and films. The results given above suggest an electrostatic contribution to the sonication-induced pitch increase in NCC suspensions. The electrostatic nature of sonication-induced pitch increase in NCC suspensions is further supported by the increase in suspension conductivity seen in sonicated suspensions (Figure 7), which appears to be correlated with iridescence wavelength red-shift with increasing sonication. Mechanisms by which sonication may increase the chiral nematic pitch of NCC suspensions are proposed below (Figure 8). Ions Trapped in the Bound-Water Layer. The hydrolysis used to extract the NCC from the wood pulp is performed at very high acid concentration; protons and sulfate ions may remain trapped in the bound-water layer (BWL). These ions are in addition to the proton counterions associated with the sulfate ester groups imparted to the cellulose chains at the surface of the NCC particles during hydrolysis. When an NCC suspension is sonicated, the energy may be sufficient to eject some of the ions trapped in the BWL, where they are free to diffuse in the bulk suspension, thereby preventing or hindering their return to the BWL. The electrical double layer (EDL) is thought to screen the “chiral interaction” between NCC rods;12 if it were no longer compressed by the surrounding ions in the BWL, the NCC particles would experience weaker chiral interactions, leading to a larger pitch and a red-shift of the iridescence wavelength of the resulting solid film. Oligosaccharide Gel Layer. SEC-MALLS carbanilation kinetics studies (to be published) suggest that the cellulose chain DP increases toward the interior of the NCC particles, a higher proportion of oligosaccharides being located at the particle

Figure 8. A bound-water layer or a gel layer of sulfated oligosaccharides surrounds the cellulose nanocrystal, which contain ions originating from the hydrolysis process that have not been removed during the purification steps. These ions compress the electrical double layer and screen the electrostatic repulsions between the oligosaccharide molecules. Sonication ejects some of the ions into the bulk suspension, resulting in a larger electrical double layer and weaker chiral interactions between particles or in stronger repulsions between the oligosaccharides and swelling of the oligosaccharide gel layer. Covalently bound sulfate ester groups on the NCC surface and their proton counterions are omitted for clarity, as are the ion valencies.

surface. This suggests that there could be a gel-like layer of anionic sulfated oligosaccharides surrounding the NCC particles in which the protons and sulfate ions are trapped. A mechanism similar to the BWL mechanism may occur during sonication of NCC suspensions; the trapped ions may be partially released into the bulk suspension, and the oligosaccharide gel layer may swell and increase the excluded volume of the NCC particles, leading to an increase in P. In addition, extensively purifying an NCC suspension by dialysis red-shifts the resulting film iridescence compared to that of the original suspension. This observation is in agreement with the proposed mechanisms. Applications. The method for developing NCC film iridescence color discussed in this article can precisely tailor the desired properties without the need for additives. Ultrasonic energy treatments provide a new method to increase the peak reflection wavelength of iridescent NCC films. This feature has been promoted as a means to provide color benignly in consumer products, to develop authenticating devices in security papers and to make UV and IR reflective barriers or optical filters.

Conclusions Providing ultrasonic energy input to an NCC suspension prior to film casting increases the pitch of the chiral nematic phase in suspension and moves the reflection band of the final iridescent film to longer wavelengths. The energy input per gram of NCC in the sample controls the magnitude of the reflection band red-shift. The effects of sonication on treated NCC suspensions are permanent and cumulative. NCC suspensions treated with different levels of sonication can be mixed together

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to give solid iridescent NCC films with reflection wavelengths corresponding to an intermediate level of sonication. The effects of sonication are reversed by the addition of an electrolyte to the suspension, which induces a blue-shift in reflection wavelength. Cumulative evidence suggests that the sonication-induced redshift of NCC film iridescence is electrostatic in nature and may involve expulsion of ions from the bound-water layer or a gel layer of sulfated oligosaccharides at the NCC particle surface. Other NCC properties such as particle length, cellulose chain DP, and surface charge density do not appear to contribute to this effect. Acknowledgment. The authors thank George Sacciadis and Guy Njamen for production of the NCC suspensions. The technical expertise and valuable contributions of Dr. Lyne Cormier, Maureen O’Neill, Craig Muirhead and Dr. Greg Chauve are gratefully acknowledged. One of the authors (SB) was supported by an Industrial Research and Development Fellowship from NSERC.

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(4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

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