Tactoid Annealing Improves Order in Self-Assembled Cellulose

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Tactoid Annealing Improves Order in Self-Assembled Cellulose Nanocrystal Films with Chiral Nematic Structures Andy Tran,† Wadood Y. Hamad,‡ and Mark J. MacLachlan*,† †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada FPInnovations, 2665 East Mall, Vancouver, British Columbia V6T 1Z4, Canada

‡

ABSTRACT: The self-assembly process in cellulose nanocrystal (CNC) film formation was studied as a function of evaporation time. It is known that the total evaporation time of CNC dispersions affects the structure of the film obtained, but the extension of different phases of the evaporation has not been explored. By extending the evaporation time of CNC suspensions after the onset of liquid crystallinity, the homogeneity of the resulting films could be improved as observed by polarized optical microscopy and scanning electron microscopy. Here, we show that an intermediate stage of self-assembly, between phase separation and gel vitrification, called tactoid annealing, helps explain the discrepancies in order for chiral nematic CNC films dried at varying evaporation times. This intermediate stage of self-assembly may be useful for designing highly ordered and homogenous CNC-based materials.

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INTRODUCTION Cellulose nanocrystals (CNCs) have been extensively studied owing to the natural abundance of cellulose as well as their brilliant optical and mechanical properties.1 The use of cellulose from plants, bacteria, or tunicates represents an avenue toward sustainable and inexpensive functional materials. CNCs can be extracted from cellulose fibers through sulfuric acid-mediated hydrolysis,2,3 leaving negatively charged sulfate groups on the CNC surface that enable them to form a stable colloidal dispersion in water. The sizes of the spindle-shaped nanocrystals vary depending on the cellulosic source, but typical dimensions are 5−30 nm by 100−500 nm. When diluted in water, a dispersion of CNCs forms an isotropic phase with CNCs well-separated.4 Above a critical concentration, however, CNCs aggregate and form short-range ordered structures called tactoids, which subsequently fuse and precipitate to form an anisotropic phase.5 Within tactoids, the CNCs have a chiral nematic order where CNCs organize into pseudoplanes oriented along a common axis, known as the director. The director rotates through the liquid crystal along an axis perpendicular to the director, forming a helix.4 The pitch of the helix is defined as the distance for the director to complete a full revolution about the elongation axis. In the case of CNCs, the helical twist of the chiral nematic structure is always left-handed. Interestingly, the hierarchical structure found in the anisotropic phase can be preserved in solid films upon complete evaporation of water (Figure 1).4,6 These pure films of CNCs appear iridescent when the pitch is similar to the wavelengths of visible light, and the reflected light is lefthanded circularly polarized, corresponding to the helicity of the structure. CNC films have iridescent colors whose wavelengths depend on the average refractive index of the film n, the half-helical © XXXX American Chemical Society

pitch P/2, and the reflection angle with respect to the surface of the film θ (eq 1).7 Thus, CNC films demonstrate structural color, similar to the many examples found in nature, including plants, insects, and marine animals.8−10 λ=n

P sin θ 2

(1)

There has been considerable recent activity on iridescent films of CNCs, and the control of the chiral nematic structures found in CNCs has garnered much interest over the last decade.1,11−15 We and others have been exploring the transfer of this chiral nematic organization to other materials, including silica, hydrogels, resins, and plastics.16−19 These materials have potential applications as chemical sensors and optical filters.20,21 Therefore, understanding the self-assembly process of pure CNC dispersions will be essential for the design and development of CNC-based materials in the future. Much research has been directed toward controlling the helical pitch of CNCs, which can be accomplished through the use of additives or variation of temperature, humidity, magnetic field, electric field, substrate, and ionic strength.14,15,22−26 Dumanli et al. suggested that the self-assembly process in developing CNC films progresses through three stages: (a) phase separation; (b) gel vitrification; and (c) film formation.24 Tactoid formation upon phase separation is highly dependent on the CNC concentration. An increased anisotropic volume fraction exists above the critical CNC concentration (ca. 3 wt % CNC).27 At even higher CNC concentrations (ca. 8 wt % CNC), gel vitrification (step b) occurs and reduces the Received: November 14, 2017 Revised: December 14, 2017

A

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Figure 1. (A) Photograph and (B) scanning electron microscopy (SEM) image of a typical CNC film, which forms a left-handed helical structure. Scale bars represent (A) 1 cm and (B) 5 μm.

Figure 2. Schematic of evaporation experiments. (A) Covered evaporation method where the CNC suspensions were covered with a lid and slowly evaporated for 0−15 d. Lids were then removed, and the remaining suspension was evaporated under ambient conditions. (B) Diluted evaporation method where the CNC suspensions were diluted with various amounts of water and then evaporated under ambient conditions over 1−11 d. (C) Sealed evaporation method where the CNC suspensions were sealed with greased lids. The lids were removed after 0−15 d to completely evaporate under ambient conditions.

Figure 3. Optical spectroscopy of CNC films. (A) CD and (B) UV−vis spectra of the CNC films prepared by the covered evaporation method over 1−15 d. The insets are photographs of the CNC films with 1 cm scale bars. (C) CD and (D) UV−vis spectra of the CNC films prepared with varying water ratios to prolong evaporation times over 1−11 d.

B

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Figure 4. POM images of the center of the CNC films prepared by the varying drying conditions. (A) CNC suspension evaporated under ambient conditions in 1 d. (B) CNC suspension evaporated by covered CNC evaporation in 11 d. (C) CNC suspension diluted with water to prolong evaporation to 11 d. Scale bars represent 100 μm.

POM represent changes in the orientation for the domains and are qualitatively indicative of less order. The CNC suspensions that evaporated under ambient conditions in 1 d appear to have many small domains with different orientations (Figure 4A). The CNC films prepared by longer evaporation times, through covered CNC evaporation, are relatively more homogeneous when compared to the control film (Figure 4B). The domains extend undisturbed, in some cases for several millimeters, with fewer changes in color. Surprisingly, diluted CNC suspensions showed no improvement in order, even at longer evaporation times (Figure 4C). It should be noted that when CNC suspensions are diluted, they are below the critical CNC concentration (ca. 3 wt %) for tactoid formation. As a result, phase separation does not occur, and the CNC suspension remains isotropic until the critical concentration is reached. A closer look at the CNC films prepared through covered evaporation (Figure 2A) showed an increase in the order as suspensions had longer evaporation times from 1 to 15 d (Figure 5A−D). SEM also showed that longer evaporation

mobility of tactoids in the anisotropic phase, locking the structures in place.24 Finally, complete removal of water results in the formation of a free-standing film (step c). The threestage model is focused on the evolution of the helical pitch, but a complete mechanism for the self-assembly process remains unclear. We postulate that there is another major and important component alongside the helical pitch formation that consists of tactoid annealing. In this stage, mobile tactoids rearrange and fuse to reduce interfacial tension between the anisotropic and isotropic layers. Tactoid annealing likely occurs during an intermediate stage of the self-assembly process between phase separation and gel vitrification. In this work, we investigate the self-assembly process while studying the origin of the chiral nematic order and helical pitch in developing CNC films. We investigate the influence of evaporation at all stages of the self-assembly process. In doing so, we hope to provide a more nuanced understanding of the self-assembly process that will help in developing materials based on CNCs in the future.

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RESULTS AND DISCUSSION Effect of Evaporation on CNC Film Formation. We were interested in the influence of evaporation times on the self-assembly process for CNC film formation. To address this, CNC films were prepared through a covered CNC evaporation method, in which samples were prepared with controlled evaporation times of 0−15 days (Figure 2A). The evaporation rate was controlled by leaving the Petri dish loosely covered for a period of time, then allowing it to dry rapidly under ambient conditions after the prescribed amount of time. Both circular dichroism (CD) and UV−vis spectroscopy showed that longer evaporation times led to blue-shifted reflection wavelengths (Figure 3A,B) for the final films. Blue shifts in the CD spectra suggest that the chiral nematic helical pitch captured in the film decreases with longer evaporation times. In another study, CNC dispersions were diluted with water to prolong the evaporation times from 1 to 11 d (Figure 2B). Diluted CNC suspensions with longer evaporation times showed blue shifts in the reflection spectra (Figure 3C,D). The evaporation time of CNC suspensions represents a parameter to tune the optical properties of CNC films without using additives or changing the ionic strength. Despite both the evaporation methods (covered CNC and diluted CNC suspensions) having similar trends at longer evaporation times, polarized optical microscopy (POM) revealed significant differences in the overall film order (Figure 4). To avoid edge effects, polarized optical micrographs were recorded at the center of the CNC films. Perfectly ordered, monodomain CNC films would show a homogeneous color when observed by POM. Color changes in

Figure 5. POM images of the center of the CNC films prepared through the covered evaporation method over (A) 1, (B) 6, (C) 11, and (D) 15 d of evaporation. Corresponding cross-sectional SEM images of the CNC films prepared by evaporation over (E) 1 and (F) 15 d. Scale bars represent (A−D) 100 and (E,F) 5 μm.

times led to fewer defects (Figure 5E,F). The differences observed in POM and SEM, between covered evaporation and diluted CNC suspensions, suggest that tactoid annealing occurs only after phase separation and is critical in determining the homogeneity of the chiral nematic CNC films. Self-Assembly Model for CNC Film Formation. The current model for the self-assembly of the CNC films consists of three stages: (a) phase separation, (b) gel vitrification, and C

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Figure 6. Schematic representation of the self-assembly model for the CNC film formation starting from a CNC dispersion. Rods represent individual CNCs, and parallel rods represent a single plane within a tactoid. (A−C) Current model for the self-assembly process. (A) Phase separation is followed by (B) gel vitrification and (C) film formation.24 (D−F) Revised model for the self-assembly process. (A) Phase separation is followed by (D) tactoid annealing, (E) gel vitrification, and (F) film formation.

Figure 7. Evaporation profiles to investigate the tactoid annealing process. The blue “fast” evaporation profile shows the CNC suspensions evaporated under ambient conditions in ∼1 d. The red “slow” evaporation profile consists of an initial ambient evaporation period followed by restricted evaporation for ∼8 d and then ambient evaporation until film formation. Phase separation and tactoid annealing occur before gel vitrification, while gel vitrification is thought to occur after ∼60% of the CNC suspension has evaporated (ca. 8 wt %). Film formation occurs upon complete removal of water and is denoted by the dashed black line.

Figure 8. POM images of the center of the CNC films prepared by sealed evaporation for (A) 0, (B) 2, and (C) 5 d, followed by evaporation under ambient conditions in 1 d. (D) CD and (E) UV−vis spectra of the corresponding CNC films. Scale bars represent 100 μm.

(c) film formation.24,28 However, this model cannot completely explain the discrepancies in the chiral nematic order we observed in the films prepared through prolonged evaporation (Figure 4). We hypothesize that an intermediate stage called tactoid annealing, between phase separation and gel vitrification, may provide a more nuanced picture of the selfassembly process under certain criteria (Figure 6).

The CNC suspensions that evaporated under ambient conditions in ∼1 d showed many domains with various orientations (Figure 6A). The time between phase separation and gel vitrification in suspensions evaporated in 1 d is likely short (Figure 7). The CNC suspensions prepared this way may contain kinetically trapped tactoids that do not undergo significant tactoid annealing or rearrangement. However, it is D

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Figure 9. POM images of the center of the CNC films prepared by an initial evaporation step of 7 h, followed by sealed evaporation for (A) 0, (B) 2, and (C) 5 d. Lids were then removed, and suspensions were evaporated under ambient conditions in 1 d. (D) CD and (E) UV−vis spectra of the corresponding CNC films. Scale bars represent 100 μm.

Figure 10. (A) CD and (B) UV−vis spectra of the CNC films prepared by an initial evaporation period of 6−20 h, followed by sealed evaporation for 5 d. Lids were then removed, and suspensions evaporated under ambient conditions in 1 d. Two separate spectra were obtained for the film prepared with 12 h of initial evaporation, denoted by the solid and dashed blue curves. (C) Photograph of the CNC film prepared with 12 h of initial evaporation, where a red inner and blue-shifted outer region forms upon drying. Scale bar represents 1 cm.

sealed to prevent evaporation for 2−5 d followed by evaporation in 1 d under ambient conditions. However, there were no significant changes in the chiral nematic domains or optical properties, which can be attributed to the small volume fraction of tactoids formed during the phase separation (Figure 8). Although the complete evaporation time for these samples varied, the time after phase separation, where tactoid annealing can occur, was about the same for the samples. The anisotropic volume fraction formed upon phase separation may be important for both the overall film order

possible to extend the tactoid annealing stage by delaying the onset of gel vitrification. In doing so, we hypothesize that tactoids rearrange and fuse to reduce the overall interfacial tension between the isotropic and anisotropic phases (Figure 6D).5,29 The CNC films prepared this way have more order and fewer defects (Figure 5D,F). We further explored whether extending the duration of tactoid assembly by indefinitely suspending the onset of gel vitrification would produce CNC films with improved order. Isotropic CNC suspensions (i.e., before phase separation) were E

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evaporation under ambient conditions in ∼1 d. Ambient conditions were about 20−24 °C at around 50% relative humidity for most experiments. Covered CNC Evaporation. CNC suspensions (3.9 mL, 4.2 wt %) were cast in Petri dishes. Suspensions were immediately covered with a lid and left for 2−15 d to evaporate at ∼11 mg/h, except for a control film which was evaporated under ambient conditions in 1 d. Lids were then removed, and the remaining suspension was evaporated under ambient conditions at ∼134 mg/h. The samples were evaporated in a total time of 1, 3, 6, 11, and 15 d. Evaporation of Diluted CNC. CNC suspensions (3.9 mL, 4.2 wt %) were cast in Petri dishes. Suspensions were diluted with various amounts of water (0, 7.9, 11.8, or 15.6 mL), resulting in CNC concentrations of 4.2, 1.4, 1.1 or 0.8 wt %. The cast solutions were vigorously mixed and then evaporated under ambient conditions over 1, 6, 10, and 11 d. Sealed CNC Evaporation. CNC suspensions (3.9 mL, 4.2 wt %) were cast in Petri dishes, and lids were greased and placed on top of the Petri dishes after 0 or 7 h. The lids were removed after 0, 2, or 15 d to allow the water to evaporate under ambient conditions. Sealed CNC Evaporation for 5 d. CNC suspensions (3.9 mL, 4.2 wt %) were cast in Petri dishes. Lids were greased and placed on top of the Petri dishes after 0, 6, 9, 12, or 20 h. The lids were removed after 5 d to allow the water to evaporate under ambient conditions. Instrumentation. SEM was performed at the UBC BioImaging Facility on a Hitachi S4700 electron microscope (Hitachi Ltd.). Crosssections of the CNC films were prepared by cleanly breaking films between two glass slides and then mounting them on sample holders. Films were then sputter-coated with 5 nm of platinum/palladium (80:20, ρ = 19.52 g/cm3) before imaging. The SEM stage was tilted 90°, and cross-sectional images of the CNC films were obtained, typically at 5 kV accelerating voltage and 10 μA current. Light transmittance (% T) and CD spectroscopy of the solid CNC films were performed on a Cary 5000 UV−vis spectrophotometer (Agilent Technologies) and JASCO J-815 (JASCO Inc.), respectively, in the UBC Shared Instrument Facility. Films were mounted perpendicular to the incident light source, and spectra were collected over the visible spectrum (350−800 nm) for both UV−vis and CD spectroscopy. Optical properties were measured at θ = 0° incident light with air as the reference in each experiment. POM images were taken on an Olympus BX41 (Olympus Corp.) optical microscope. Solid films were imaged between two linear polarizers placed at 90° with respect to each other.

and helical pitch. To confirm this, a similar sealing experiment was performed, but the suspensions were sealed for 2−5 d, after an initial evaporation period of 7 h. The initial evaporation step increases the CNC concentration and promotes the formation of tactoids, thereby increasing the anisotropic volume fraction. As expected, an increase in the order was observed in the resulting films when suspensions were sealed for longer periods of time (Figure 9). This suggests that the anisotropic volume fraction, alongside the duration of the tactoid annealing stage, is critical in determining the domain homogeneity of the chiral nematic CNC films. We further investigated the dependence of the anisotropic volume fraction on the optical properties. The CNC films were prepared by an initial evaporation period of 6−20 h, followed by sealed evaporation for 5 d and finally evaporation under ambient conditions in 1 d. Initial evaporation times, up to 9 h, blue-shifted the reflection wavelengths (Figure 10A,B). We hypothesize that CNC suspensions given 20 h of initial evaporation pass the gel vitrification stage, which locks in the structure. Two distinct regions form at 12 h of the initial evaporation, an inner red region and an outer blue region (Figure 10C). We believe that we have captured the CNC suspension at a critical moment, just before gel vitrification. The blue shift in the reflection wavelength between the two regions, despite completely restricted evaporation, implies that tactoid annealing may influence the helical pitch formation up until gel vitrification locks in the structure.

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CONCLUSIONS The self-assembly process in developing CNC films was studied by controlling the evaporation time of CNC suspensions. We hypothesize that an intermediate stage, called tactoid annealing, provides a more adequate explanation of the self-assembly process. Tactoid annealing is thought to occur after phase separation and can influence the helical pitch and homogeneity of the chiral nematic domains until structural locking occurs at the onset of gel vitrification. This understanding is important to the development of films with the control over their structural color and for improving the optical quality of films.

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AUTHOR INFORMATION

Corresponding Author

EXPERIMENTAL SECTION

*E-mail: [email protected].

Materials. All compounds were used as received without any further purification. Aqueous suspensions of CNCs were provided by FPInnovations (CNC−H+, 4.2 wt %, pH 2.1). In brief, ultrapure aqueous CNC suspensions were prepared by dispersing spray-dried CNCs in deionized water, at a concentration of 2 wt %, by stirring the suspension overnight using a mechanical stirrer.30 The dispersed CNC suspension was then sonicated at 70% power for 30 min (in batches of 3 L) using an ultrasonicator Vibra-Cell VC750 (Sonics & Materials Inc.). The average energy input was ∼9000 J/g of CNCs. The suspension was then filtered, first using grade 4 Whatman filter paper, followed by grade 42 Whatman filter paper. The filtered CNC suspension was dialyzed against deionized water overnight. The dialyzed suspension was concentrated to the desired concentration using a rotary evaporator and then stored in the fridge at 4 °C until further use. The final pH and conductivity of the ultrapure CNC aqueous suspension were adjusted to the acidic form (CNC−H+, pH 2.1), using an ion-exchange resin (Dowex Marathon C hydrogen form, 23 to 27 mesh particle size, Sigma-Aldrich). Dimensions of CNC spindles were determined by transmission electron microscopy size distribution analysis to be 191 ± 80 nm and have an electrophoretic surface charge of −4.44. All CNC suspensions were sonicated for 1 h prior to use. The control film in each experiment involved casting 3.9 mL of CNC suspension in a 50 mm polystyrene Petri dish, followed by

ORCID

Mark J. MacLachlan: 0000-0002-3546-7132 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.T. thanks the University of British Columbia for a 4YF fellowship. M.J.M. thanks NSERC (Discovery, CREATE NanoMat) for support.

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ABBREVIATIONS CNC, cellulose nanocrystal; SEM, scanning electron microscopy; CD, circular dichroism; POM, polarized optical microscopy F

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(22) Yao, K.; Meng, Q.; Bulone, V.; Zhou, Q. Flexible and Responsive Chiral Nematic Cellulose Nanocrystal/Poly(ethylene Glycol) Composite Films with Uniform and Tunable Structural Color. Adv. Mater. 2017, 29, 1701323. (23) Pan, J.; Hamad, W.; Straus, S. K. Parameters Affecting the Chiral Nematic Phase of Nanocrystalline Cellulose Films. Macromolecules 2010, 43, 3851−3858. (24) Dumanli, A. G.; Kamita, G.; Landman, J.; van der Kooij, H.; Glover, B. J.; Baumberg, J. J.; Steiner, U.; Vignolini, S. Controlled, BioInspired Self-Assembly of Cellulose-Based Chiral Reflectors. Adv. Opt. Mater. 2014, 2, 646−650. (25) Nguyen, T.-D.; Hamad, W. Y.; MacLachlan, M. J. Tuning the Iridescence of Chiral Nematic Cellulose Nanocrystals and Mesoporous Silica Films by Substrate Variation. Chem. Commun. 2013, 49, 11296−11298. (26) Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Effects of Ionic Strength on the Isotropic−Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites. Langmuir 1996, 12, 2076−2082. (27) Schütz, C.; Agthe, M.; Fall, A. B.; Gordeyeva, K.; Guccini, V.; Salajková, M.; Plivelic, T. S.; Lagerwall, J. P. F.; Salazar-Alvarez, G.; Bergström, L. Rod Packing in Chiral Nematic Cellulose Nanocrystal Dispersions Studied by Small-Angle X-ray Scattering and Laser Diffraction. Langmuir 2015, 31, 6507−6513. (28) Mu, X.; Gray, D. G. Formation of Chiral Nematic Films from Cellulose Nanocrystal Suspensions Is a Two-Stage Process. Langmuir 2014, 30, 9256−9260. (29) Park, J. H.; Noh, J.; Schütz, C.; Salazar-Alvarez, G.; Scalia, G.; Bergström, L.; Lagerwall, J. P. F. Macroscopic Control of Helix Orientation in Films Dried from Cholesteric Liquid-Crystalline Cellulose Nanocrystal Suspensions. ChemPhysChem 2014, 15, 1477−1484. (30) Hamad, W. Y.; Hu, T. Q. Structure-Process-Yield Interrelations In Nanocrystalline Cellulose Extraction. Can. J. Chem. Eng. 2010, 88, 392−402.

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