Post-modification of Cellulose Nanocrystal Aerogels with Thiol–Ene

Article Cite This: Biomacromolecules 2019, 20, 2779−2785

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Post-modification of Cellulose Nanocrystal Aerogels with Thiol−Ene Click Chemistry Guus J. W. Aalbers,†,# Charlotte E. Boott,†,# Francesco D’Acierno,†,‡ Lev Lewis,† Joseph Ho,† Carl A. Michal,‡,† Wadood Y. Hamad,§ and Mark J. MacLachlan*,†,∥,⊥ †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada § FPInnovations, 2665 East Mall, Vancouver, British Columbia V6T 1Z4, Canada ∥ Stewart Blusson Quantum Matter Institute, University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada ⊥ WPI Nano Life Science Institute, Kanazawa University, Kanazawa 920-1192, Japan

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‡

S Supporting Information *

ABSTRACT: The functionalization of cellulose nanocrystal (CNC) aerogels was achieved through a two-step synthetic procedure. CNC aerogels were prepared under hydrothermal conditions, followed by solvent exchange and critical point drying. The CNC aerogels were functionalized with a methacrylate group and then underwent thiol−ene click chemistry to impart a range of functionalities onto the surface of the CNC aerogel. The use of the functionalized aerogels as oil absorbents was then investigated, with the most hydrophobic CNC aerogel, 1H,1H,2H,2H-perfluorodecanethiol-functionalized CNC aerogel, exhibiting the highest absorption of xylenes at 2.9 mL g−1.

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INTRODUCTION The preparation of materials from biorenewable sources is of current scientific interest. Cellulose is the most abundant natural polymer with an annual biosphere production of more than 9 × 1010 tonnes1 and it can be obtained from many different sources, including trees, plants, fungi, and bacteria. The structure of cellulose contains both crystalline and amorphous regions.2 Treatment of cellulose pulp with sulfuric acid selectively hydrolyzes the amorphous regions to produce cellulose nanocrystals (CNCs), which are spindle-like nanoparticles that are 5−20 nm in width and 100−300 nm in length.3−5 CNCs form stable colloidal suspensions in water due to the surface charge imparted by the half-ester sulfate groups. This surface charge is integral to the stability of the CNCs in solution as it provides an overall negative charge to the CNCs, which enables them to be dispersed in water due to electrostatic repulsion.6,7 Any modification to the CNC surface groups, such as reducing the surface charge, destabilizes the CNCs in suspension and favors attractive interactions that lead to hydrogel formation.8,9 This destabilization strategy has been successfully employed to prepare a range of CNC hydrogel materials.8−13 Nevertheless, this fine balance of the surface © 2019 American Chemical Society

charge and colloidal stability provides significant issues when it comes to the surface functionalization of CNCs in solution.14 Previous methods that have been employed to modify the surface functionalization of CNCs include oxidation, amidation, nucleophilic substitution, etherification, esterification, and silylation.6,15 Of these strategies, esterification has been most heavily studied because of the substantial reactivity of cellulose with acylating agents,16 and the ease in which these reactions can be confirmed by spectroscopic techniques (e.g., FT-IR). Specifically, Brand and co-workers have reported the reaction of vinyl esters with CNCs to introduce a range of different functional groups,17 while our group and Nielsen and coworkers have utilized acid anhydrides to modify the CNC surface functionalization.18,19 Aerogels are light-weight porous materials (usually ρ = 0.004−0.500 g cm−3)20,21 that were first synthesized by Kistler in the 1930s.22 Aerogels have been successfully employed in several applications such as thermal insulation,23,24 energy Received: April 17, 2019 Revised: June 9, 2019 Published: June 24, 2019 2779

DOI: 10.1021/acs.biomac.9b00533 Biomacromolecules 2019, 20, 2779−2785

Article

Biomacromolecules Scheme 1. Two-Step Functionalization Strategy for CNC Aerogels

conversion and storage,25 and oil absorption.26,27 Aerogels can be formed by the solvent exchange of hydrogels with ethanol followed by critical point drying. Although we reported the preparation of CNC aerogels via hydrothermal10 and solventinduced28 gelation methods, the functionalization of these CNC aerogel materials has not been described. A well-established functionalization procedure in polymer and materials chemistry is the thiol−ene click reaction.29,30 In this reaction, a thiol is added across a vinyl group to produce the corresponding thioether. Although it is less popular than the azide−alkyne click reaction due to potential side reactions arising from the reactivity of the resulting radicals,31 the thiol− ene click reaction can achieve functionalization in near quantitative yields in short reaction times. Herein, we report the two-step surface functionalization of CNC aerogels to introduce a variety of hydrophilic and hydrophobic groups. The use of these aerogels as possible oil absorption materials was then investigated using xylenes as a model system. The CNC aerogels functionalized with more hydrophobic thiols were able to absorb more xylenes per gram of aerogel than their hydrophilic thiol analogues.

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three times, and then the alcogel was dried with supercritical CO2 to yield aerogels (1). Characteristic gas adsorption data: surface area of 410 m2 g−1, pore volume of 1.20 cm3 g−1, and average pore size of 12 nm. Esterification of CNC Aerogels. Dry CNC aerogels (200 mg) were introduced into a specially designed piece of glassware (see Supporting Information for more details) to prevent the mechanical breakdown of the aerogels by the stir bar. Anhydrous pyridine (20 mL) was added under a nitrogen atmosphere. Methacrylic anhydride (4 mL) was added dropwise to the flask, and the reaction was heated at 80 °C for 5 h. The methacrylate-functionalized CNC aerogels (2) were then washed with hexanes multiple times and air-dried overnight. Characteristic IR peaks, ν̃ (cm−1): 3650−3000 (b, O−H stretch), 2903 (asymmetric C−H stretch), 1713 (s, α,β-unsaturated ester CO stretch), 1316 (C−O−H bend), 1106 (C−O−C bend), and 898 (C−O−C asymmetric stretch of the glycosidic linkage). Thiol−Ene “Click” (Radical Addition). Methacrylate-functionalized CNC aerogels (15 mg) and anhydrous DMF (2 mL) were added to a 20 mL glass vial. After 10 min of nitrogen bubbling, 1H,1H,2H,2H-perfluorodecanethiol (100 μL, 0.35 mmol) was added dropwise followed by 2,2-dimethoxy-2-phenylacetophenone (10 μL). The reaction was irradiated with UV-light for 3 h, and then the CNC aerogels were washed with hexanes three times and air-dried overnight. The same procedure was followed for all other thiols reported, with the exception of cysteamine to yield materials 3A−D. Characteristic IR peaks, ν̃ (cm−1): 3650−3000 (b, O−H stretch), 2903 (asymmetric C−H stretch), 1723 (s, fluorinated saturated ester CO stretch), 1316 (C−O−H bend), 1240 and 1205 (C−F stretches), 1106 (C−O−C bend), and 898 (C−O−C asymmetric stretch of the glycosidic linkage). EA data for aerogels: 41.73% C, 5.31% H, and 1.83% S [N was below limit of detection (LOD)]. Thiol−Ene “Click” (Michael Addition). CNC methacrylatefunctionalized CNC aerogels (15 mg), methanol (2 mL), and cysteamine (5 mg) were added to a 20 mL vial. The reaction mixture was stirred for 3 d, and the CNC aerogels were washed with hexanes three times and air-dried overnight to yield the desired product, 4. Characteristic IR peaks, ν̃ (cm−1): 3560−3050 (b, O−H stretch), 2899 (asymmetric C−H stretch), 1727 (s, saturated ester CO stretch), 1315 (C−O−H bend), 1161, 1106 (C−O−C bend), 1054, 1031, and 896 (C−O−C asymmetric stretch of the glycosidic linkage). EA data for aerogel: 1.35% N, 44.03% C, 6.19% H, and 3.49% S. Oil Absorption. Thiol-functionalized CNC aerogels were weighed and then submerged in 4 mL xylenes (isomer mixture) for 1 h and weighed again. The aerogels were then washed twice with hexanes and air-dried. This cycle was repeated for an additional four times. Characteristic oil absorption (mL g−1): 3A = 2.18 (3-mercaptopropionic acid) 3B = 2.38 (6-sulfonylhexan-1-ol), 3C = 2.78 (1dodecanethiol), 3D = 2.86 (1H,1H,2H,2H-perfluorodecanethiol), and 4 = 2.54 (cysteamine). Errors were calculated as the standard deviation of the absorption capacity over the 5 absorption cycles.

EXPERIMENTAL SECTION

General Experimental. CNC suspensions were obtained from FPInnovations (CNC, 4 wt %, pH = 2.2), and all other reagents were purchased from Sigma-Aldrich and used without further purification. Pyridine and N,N-dimethylformamide (DMF) were dried over molecular sieves before use. IR spectroscopy was run on a PerkinElmer FT-IR. Elemental analysis (EA) was performed by UBC’s Mass Spectrometry and Microanalytical Laboratory on a Fisons Instruments Elemental Analyzer EA 1108 (for the isocyanate functionalized CNC aerogels) or by UBC’s Department of Earth, Ocean and Atmospheric Sciences on an Elementar Americas’ Vario MICRO cube (for the thiol−ene functionalized CNC aerogels). Critical point drying with supercritical CO2 was done with a Tousimis Autosamdri 815B. Gas adsorption measurements were done on a Micromeritics ASAP 2020 sorption apparatus with nitrogen at 77 K. 1H−13C CP/MAS, 19F MAS, and 19 F−13C CP/MAS NMR experiments were performed on a Varian Unity Inova 400 spectrometer, using a Varian/Chemagnetics 3channel 4 mm T-3 MAS probe. Radio frequency (rf) field strengths were 60 kHz for 1H(19F)−13C CP and 80 kHz for 1H decoupling. All MAS experiments used a spinning rate of 5−6 kHz, and CP experiments used a contact time of 1 ms. Powder X-ray diffraction (PXRD) patterns were recorded at room temperature on a Bruker D8 ADVANCE diffractometer equipped with a Cu Kα-sealed tube X-ray source and a NaI scintillation detector. Samples were loaded on a flat sample holder and were analyzed from 5° to 90° 2θ, where all the relevant reflections are detected. Preparation of CNC Aerogels. A CNC suspension (10 mL, 4 wt %) was added to a hydrothermal bomb and heated at 120 °C for 20 h. The hydrogels were solvent-switched with ethanol by immersing the hydrogel in ethanol (100 mL) overnight after which the ethanol was removed and fresh ethanol was added. This procedure was repeated

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RESULTS AND DISCUSSION CNC aerogels (1) were prepared using a previously reported procedure.10 Briefly, the CNC aqueous suspension is hydrothermally treated to produce a CNC hydrogel, the water is slowly solvent-exchanged with ethanol, and the material is 2780

DOI: 10.1021/acs.biomac.9b00533 Biomacromolecules 2019, 20, 2779−2785

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Figure 1. (a) Normalized IR spectra of 1 and 2; unfunctionalized CNC aerogel (blue) and methacrylate-functionalized CNC aerogels (red). Peak 1 (red) = 1713 cm−1, peaks 2 = 1636 cm−1, and peak 3 = 1582 cm−1, (b) photograph of CNC aerogels, and (c) photograph of methacrylatefunctionalized CNC aerogels. Scale bars = (b) 2 cm, (c) 5 mm.

Figure 2. Normalized IR spectra of 3A−D, 3-mercaptopropionic acid-functionalized CNC aerogel (red), 6-sulfonylhexan-1-ol-functionalized CNC aerogel (black), 1-dodecanethiol-functionalized CNC aerogel (blue), and 1H,1H,2H,2H-perfluorodecanethiol-functionalized CNC aerogel (green). Peak 1 (green) = 1723 cm−1, peak 2 (blue) = 1719 cm−1, peak 3 (black) = 1728 cm−1, peak 4 (red) = 1716 cm−1, peaks at 5 = ∼1636 cm−1, peaks 6 = ∼1585 cm−1, peak 7 = 1240 cm−1, and peak 8 = 1205 cm−1.

thiol−ene “click” chemistry. Thiol−ene click chemistry is favorable for this system due to the facile synthesis and quantitative functionalization that can be achieved.29 Four different thiols and the photoinitiator, 2,2-dimethoxy-2phenylacetophenone, were used for the thioetherification of methacrylate-functionalized CNC (Scheme 1). The four thiols chosen ranged from the hydrophilic 3-mercaptopropionic acid to the hydrophobic 1H,1H,2H,2H-perfluorodecanethiol. All four thiols were successfully attached to the CNC aerogel as confirmed by a shift in the CO stretching frequency in the IR spectra for all materials (Figure 2). In all cases, the shift in the CO stretching peak was to a higher energy, which is consistent with the loss of the conjugated alkene upon reaction with the thiol and, therefore, thioether formation. In addition to the CO peak shift, for the 1H,1H,2H,2H-perfluorodecanethiol-functionalized CNC aerogel, peaks at 1240 and 1205 cm−1 (peaks 7 and 8) were present, which correspond to the C−F stretching modes. Successful incorporation of the thiol groups was further corroborated by EA (Table 1).

critical point-dried to produce a CNC aerogel. Nitrogen adsorption analysis showed that these aerogels have a surface area of 410 m2 g−1, pore volume of 1.20 cm3 g−1, and pore size of 12 nm. The surface hydroxyl groups of the CNC aerogels were first functionalized through an esterification reaction with methacrylic anhydride and pyridine (Scheme 1).19 This produced methacrylate-functionalized CNC aerogel (2), which was confirmed by IR analysis (Figure 1). The IR spectrum of the methacrylate-functionalized CNC aerogel, when compared to the unfunctionalized CNC starting material, contained two additional peaks. Peak 1, at 1713 cm−1, corresponded to the carbonyl peak of the methacrylate group. Peak 3, at 1582 cm−1, originated from the absorbed pyridine that was still present after washing with hexane.32 Peak 2 at 1636 cm−1, is present in both the starting material and product and is from the H−O− H bending of absorbed water. The methacrylate-functionalized CNC aerogels contained a terminal vinyl group, which we functionalized further using 2781

DOI: 10.1021/acs.biomac.9b00533 Biomacromolecules 2019, 20, 2779−2785

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can be observed in their 1H−13C CP/MAS NMR spectra between 30 and 40 ppm, depending on the local environment. However, the remarkably different molecular structure of the chains bonded to the thiol groups of the materials yields distinct line shapes in the saturated and unsaturated carbon regions (Figure 4). In addition, because of the high fluorine content of aerogel 3D, we investigated that sample by 19F NMR spectroscopy. A modified one-pulse 19F MAS experiment (Figure 5) revealed two intense bands: one at −125 ppm, assigned to CF2, and one at −80 ppm, assigned to CF3.40,41 Further confirmation of the presence of CF groups grafted on the surface of CNC aerogels was given by 19F−13C CP/MAS (Figure 5, inset), where the weak carbon signal is enhanced by the abundant fluorine rather than protons. A broad peak centered at 108 ppm confirms the existence of the carbon−fluorine bond and the absence of other atoms in the environment, such as oxygen, which would have resulted in distinct upfield peaks.41 The peaks assigned to CF2 and CF3 overlap, and they are not resolved individually. However, the CF3 peak appears as a downfield shoulder on the main peak. Using the weight percentage of C, N, and S present in the aerogels determined from EA, we tried to estimate the amount of surface functionalization for each aerogel. From EA, we were able to estimate the ratio of thiol-functionalized to unfunctionalized glucose units, which gave a range of 1:5 to 1:24, depending on the thiol. The bulkier dodecanethiol and 1H,1H,2H,2H-perfluorodecanethiol yielded a lower proportion of functionalized glucose units, 1:16 and 1:24, respectively, while the smaller thiols, cysteamine and mercaptopropionic acid both had ratios of 1:5. Assuming a cylindrical geometry without end caps for the CNC rods with a length of 122 nm and a diameter of 14 nm as previously reported,42 we calculate a surface functionalization of 3.7−0.8 groups nm−2 or 1100− 230 mmol kg−1, which is comparable to previously reported values for the surface functionalization of CNCs.43 There is a clear relationship between the steric bulk of the thiol and the level of surface functionalization; the larger thiols may shield some of the surface groups, preventing them from being functionalized. As well as functionalizing CNC aerogels using this two-step protocol, we also investigated the surface modification of nonporous CNC films. IR spectroscopy confirmed the

Table 1. EA Results of Thiol-Functionalized CNC Aerogels

a

no.

thiol

%N

%C

%S

3A 3B 3C 3D 4

3-mercaptopropionic acid 6-sulfonylhexan-1-ol 1-dodecanethiol 1H,1H,2H,2H-perfluorodecanethiol cysteaminea

0.23b 0.37b 0.39b 0.31b 1.36

42.33 44.78 45.31 41.76 42.35

2.86 1.58 0.68 0.82 3.08

Prepared under Michael addition conditions. pyridine.

b

From residual

We also prepared a cysteamine-functionalized CNC aerogel, which used a Michael addition reaction to insert the thioether (Figure 3a), instead of the photoinitiated radical thiol−ene reaction employed for the other four thiols reported. Once again, IR analysis of the reaction product indicated the successful incorporation of the cysteamine. This was further confirmed by EA (Table 1) with a sulfur weight percentage of 3.08% and a nitrogen weight percentage of 1.36%. To further confirm the successful functionalization, the aerogels were examined using solid-state 1H−13C CP/MAS NMR spectroscopy (Figure 4). All spectra displayed the typical line shape of the cellulose,33−37 with peaks in the region of 60−100 ppm, which are assigned to hydroxyl-bonded carbons of the polysaccharide structure.38 Signals from the functional groups incorporated in the CNC aerogels were less intense, as only the surface groups of the CNCs were functionalized. The presence of methacrylate groups on the surface of compound 2 is confirmed by the identification of features either in the region of saturated hydrocarbons (0−50 ppm), with a peak at 15 ppm corresponding to the methyl group and the region of unsaturated carbons (100−220 ppm), with a peak at 125 ppm for the vinyl group and one at 176 ppm for the carbonyl group.39 These bands are visible in the spectra of all subsequent materials, 3A−D and 4. All spectra are also characterized by a broad diffuse signal in the unsaturated hydrocarbon region caused by remaining traces of pyridine. The presence of the broad component as a sign of structural amorphization following the reaction has been ruled out because the crystallinity indices measured from the PXRD of the materials (Figure S1) have similar high values (Table S1). The thiol-functionalized CNC aerogels, 3A−D and 4, all contain methylene groups adjacent to the sulfur atoms, which

Figure 3. (a) Scheme for the Michael addition reaction of methacrylate-functionalized CNC aerogels with cysteamines. (b) Normalized IR spectra of 2 and 4; methacrylate-functionalized CNC aerogels (red), and cysteamine-functionalized CNC aerogels (purple). Peak 1 (red) = 1713 cm−1, peak 2 (purple) = 1727 cm−1, peaks 3 = 1636 cm−1, and peaks 4 = 1582 cm−1. 2782

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Figure 5. 19F MAS NMR spectrum of aerogel 4. Asterisks indicate spinning sidebands. (Inset) 19F−13C CP/MAS NMR spectrum of 4. Spectra have been normalized by their mass and the number of transients.

Therefore, we decided to focus on the 2-step thiol−ene approach as it enables the investigation of a much broader range of functional groups. Finally, inspired by recent work that has demonstrated that CNC-based aerogels exhibit great promise as oil absorbents,27,44,45 we investigated the use of the thiol-functionalized CNC aerogels for this application. Five different thiolfunctionalized CNC aerogels were used for the absorption of xylene, and five absorption cycles were performed (Figure 6). The average xylene absorptions were, 3A = 2.2 mL g−1, 3B = 2.4 mL g−1, 3C = 2.8 mL g−1, 3D = 2.9 mL g−1, and 4 = 2.5 mL g−1. Although the absorption values were within a 0.7 mL g−1 range, a general trend of the CNC aerogels functionalized with hydrophobic thiols, 3C and 3D, absorbing more xylene than the hydrophilic thiols, 3A, 3B, and 4, was observed. The average sorption of the cysteamine-functionalized CNC aerogel (4) was greater than the other hydrophilic CNC aerogels. This could be due to the fact that this CNC aerogel was prepared differently to the other functionalized CNC aerogels; this aerogel underwent thiol−ene “click” chemistry via Michael addition (using methanol as the solvent) instead of radical addition (using DMF as solvent). The different reaction conditions could have influenced the surface chemistry of the CNC aerogel and altered its absorption capacity. Furthermore, when the isocyanate-functionalized CNC aerogels were used in xylene uptake experiments, the highest absorption value obtained was ca. 1.8 mL g−1 (see Supporting Information for more details). These results, therefore, seem to indicate that this 2-step functionalization strategy enables a higher level of surface functionalization than the 1-step isocyanate process.

Figure 4. 1H−13C CP/MAS spectra of aerogels 2, 3A−D, and 4. Asterisks indicate spinning side bands. Spectra have been normalized by their mass and the number of transients. The fluoroalkyl chain of aerogel 3D is not observed, as expected, but its predicted position is indicated.

successful incorporation of both the methacrylate group and subsequent thioether formation for the CNC films. However, because of the lower surface area of these materials, the amount of sulfur and/or nitrogen present in the film was below the LOD for EA, leading us to focus on functionalizing the aerogels because of their increased surface area (see Supporting Information for more details). Furthermore, we investigated the direct functionalization of CNC aerogels with isocyanates. Despite successfully producing functionalized CNC aerogels, this 1-step strategy required different reaction conditions depending on the isocyanate being employed (see Supporting Information for more details). In addition, there are fewer commercially available isocyanates than thiols.

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CONCLUSIONS In summary, a two-step functionalization strategy to prepare CNC-based aerogels with new properties was reported. The strategy involved an esterification of the CNCs with methacrylic anhydride to install a methacrylate group, which can then be functionalized via thiol−ene “click” chemistry. The hydrothermally treated CNC aerogels were successfully functionalized with five different thiols, and this was confirmed 2783

DOI: 10.1021/acs.biomac.9b00533 Biomacromolecules 2019, 20, 2779−2785

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Figure 6. Xylene absorption with thioether-functionalized CNC aerogels 3A−D and 4: (a) 3-mercaptopropionic acid 3A, (b) 6-sulfonylhexan-1-ol, 3B, (c) 1-dodecanethiol, 3C, (d), 1H,1H,2H,2H-perfluorodecanethiol 3D, (e) cysteamine, 4, and (f) table of average absorption capacities. Errors = standard deviation.

by IR, EA, 1H−13C and 1H−19F CP/MAS NMR, and 19F MAS NMR spectroscopy. We then investigated the use of functionalized CNC aerogels as oil absorbents. The hydrophobicfunctionalized aerogels showed better xylene absorption than the hydrophilic-functionalized aerogels, with the 1H,1H,2H,2H-perfluorodecanethiol showing the highest amount of absorption at 2.9 mL g−1. However, despite these promising results, a significant decrease in the aerogel volume is observed after it has undergone the two-step functionalization process. Therefore, to increase the absorption capacity of these materials, we need to prepare more robust aerogels that are more resilient to pore collapse upon immersion in organic solvents.

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input from all co-authors. C.E.B., L.L., and M.J.M. devised the project. M.J.M. supervised the project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.J.W.A. thanks Fontys University, Netherlands for funding. C.E.B. thanks the Banting Postdoctoral Fellowships and Killam Postdoctoral Fellowships for funding. L.L. thanks UBC 4YF Fellowship for funding. M.J.M. thanks CFI (JELF) for funding. We also thank NSERC (Discovery Grants to C.A.M. and M.J.M.; CREATE NanoMat Grant to M.J.M.).

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ASSOCIATED CONTENT

(1) Pinkert, A.; Marsh, K. N.; Pang, S.; Staiger, M. P. Ionic Liquids and Their Interaction with Cellulose. Chem. Rev. 2009, 109, 6712− 6728. (2) Giese, M.; Blusch, L. K.; Khan, M. K.; MacLachlan, M. J. Functional Materials from Cellulose-Derived Liquid-Crystal Templates. Angew. Chem., Int. Ed. 2015, 54, 2888−2910. (3) Rånby, B. G.; Banderet, A.; Sillén, L. G. Aqueous Colloidal Solutions of Cellulose Micelles. Acta Chem. Scand. 1949, 3, 649−650. (4) Peng, B. L.; Dhar, N.; Liu, H. L.; Tam, K. C. Chemistry and Applications of Nanocrystalline Cellulose and Its Derivatives: A Nanotechnology Perspective. Can. J. Chem. Eng. 2011, 89, 1191− 1206. (5) Hamad, W. Y. Development and Properties of Nanocrystalline Cellulose. In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., Zhang, X., Pan, X. E., Eds.; American Chemical Society: Washington, DC, 2011; pp 301−321. (6) Habibi, Y. Key Advances in the Chemical Modification of Nanocelluloses. Chem. Soc. Rev. 2014, 43, 1519−1542. (7) Dong, X. M.; Revol, J.-F.; Gray, D. G. Effect of Microcrystallite Preparation Conditions on the Formation of Colloid Crystals of Cellulose. Cellulose 1998, 5, 19−32. (8) Chau, M.; Sriskandha, S. E.; Pichugin, D.; Thérien-Aubin, H.; Nykypanchuk, D.; Chauve, G.; Méthot, M.; Bouchard, J.; Gang, O.;

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biomac.9b00533.

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REFERENCES

Further information on the experimental setup and the attempted functionalization of CNC films (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lev Lewis: 0000-0002-5469-5456 Mark J. MacLachlan: 0000-0002-3546-7132 Author Contributions #

G.J.W.A. and C.E.B. contributed equally. G.J.W.A., C.E.B., J.H., and L.L. carried out the functionalization experiments. F.D. and C.A.M. performed the solid-state NMR experiments and analysis. C.E.B. and M.J.M. wrote the manuscript with 2784

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Biomacromolecules

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DOI: 10.1021/acs.biomac.9b00533 Biomacromolecules 2019, 20, 2779−2785