One-Pot Water-Based Hydrophobic Surface Modification of

Research Article pubs.acs.org/journal/ascecg

One-Pot Water-Based Hydrophobic Surface Modification of Cellulose Nanocrystals Using Plant Polyphenols Zhen Hu,† Richard M. Berry,‡ Robert Pelton,† and Emily D. Cranston*,† †

Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4L7, Canada CelluForce Inc., 625 Président-Kennedy Avenue, Montreal, QC H3A 1K2, Canada

‡

S Supporting Information *

ABSTRACT: An environmentally friendly procedure for the surface modification of cellulose nanocrystals (CNCs) in water is presented. Tannic acid (TA), a plant polyphenol, acts as the primer when mixed with CNCs in suspension, which are then reacted with decylamine (DA), the hydrophobe. Schiff base formation/Michael-type addition covalently attaches primary amines with long alkyl tails to CNC-TA, increasing the particle hydrophobicity (contact angle shift from 21 to 74°). After modification, the CNC-TA-DA particles in the water phase separate, allowing for easy collection of modified material. The dried product is readily redispersible in toluene and other organic solvents, as demonstrated by turbidity measurements, dynamic light scattering, optical microscopy, and liquid crystal self-assembly behavior. Electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, solidstate 13C NMR, and X-ray diffraction support the successful surface modification and indicate that CNC particle morphology is retained. The modified CNCs have a slightly decreased onset of thermal degradation (ca. 10 °C lower) compared with that of unmodified CNCs. We believe that this surface modification strategy presents a scalable, simple, and green approach to the production of hydrophobic biobased nanoparticles which may lend themselves as reinforcing agents in nonpolar polymer composites or stabilizers and rheological modifiers in nonaqueous liquid formulated products. KEYWORDS: cellulose nanocrystals, hydrophobicity, surface modification, plant polyphenols, tannic acid, dispersion

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INTRODUCTION Cellulose is the world’s most abundant natural, renewable, and biodegradable polymer and is a promising and low cost raw material for the preparation of various functional materials. Cellulose nanocrystals (CNCs), produced via the acid hydrolysis of cotton, wood pulp, or other cellulose sources, have many advantages such as excellent mechanical properties, nanoscale dimensions, and a high aspect ratio.1−3 Most commonly, sulfuric acid is used in the hydrolysis process, which grafts sulfate half ester groups on the CNC surfaces, imparting colloidal stability. In addition, the abundant surface hydroxyl groups provide opportunities for diverse chemical modification but also render CNCs hydrophilic. Though the hydrophilic nature of CNCs is attractive for water-based applications, this presents a challenge for their homogeneous dispersion in common nonpolar solvents and hydrophobic polymer matrixes.3,4 While many examples of hydrophobic surface modification of CNCs have been described in the literature, we present an entirely novel approach based on simple mixing of CNCs with the plant polyphenol tannic acid (TA), followed by mixing with decylamine (DA), all in water at room temperature. Past attempts to produce hydrophobic CNCs have included the adsorption of surfactants or polymers or covalent © 2017 American Chemical Society

modification of the particle surfaces as summarized in recent reviews.5,6 For example, quaternary ammonium salts bearing long alkyl tails have been used to modify CNCs based on ionic interactions between the sulfate half ester groups on the CNC and the ammonium group of the surfactant; these modified CNCs require extensive purification to remove excess surfactant, and such ionic bonds are not robust enough to withstand some processing techniques and media.7−11 Covalent modifications of CNCs generally include esterification (mostly acetylation, butyration, and palmitoylation), urethanization (also known as carbanylation), amidation, and silylation with contact angles reported in the range of 67 to 85° but generally lying on the lower end of these values.12−20 Furthermore, most of these modifications are carried out in organic solvents. To increase contact angle and compatibility with polymer matrixes, larger surface modifications such as attaching polymer chains via a grafting onto CNC approach have been demonstrated with polypropylene, polytetrahydrofuran, polycaprolactone, polyethylene glycol, etc. However, steric hindrance limits the graft densities achievable.15,21−25 The Received: February 9, 2017 Revised: April 15, 2017 Published: April 22, 2017 5018

DOI: 10.1021/acssuschemeng.7b00415 ACS Sustainable Chem. Eng. 2017, 5, 5018−5026

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ACS Sustainable Chemistry & Engineering

modification occurred in situ by reacting decylamine with the tannic acid coating on the CNC-TA particles through Schiff base formation and/or Michael-type addition. All steps were carried out in water at ambient conditions. Below, we describe the reaction protocol and straightforward separation process needed for the modification, alongside thorough characterization of the resultant CNC surface chemistry, particle morphology, and hydrophobicity. Our findings are relevant to extend the application scope of CNCs in nonaqueous formulations and nanocomposite manufacturing.

grafting from CNC approach was therefore introduced to provide higher graft densities. The catalytic ring-opening polymerization from CNCs, using the surface hydroxyl groups as initiating sites, is the most common route for the synthesis of CNCs with grafted polyesters such as the biobased polylactic acid.26−29 In addition, atom transfer radical polymerization has been extensively explored to synthesize well-defined polymers on CNC surfaces.30−34 These grafting from approaches have yielded highly compatible CNCs with contact angles in the range of 62 to 94°; however, the preattachment of initiator, polymerization reactions themselves, and the workup/separation are lengthy. To avoid the need for initiator attachment and these reactions which take place in organic solvents, we demonstrated free radical grafting of vinyl polymers from CNCs in water (using ceric initiation). However, the reaction produces very large amounts of homopolymer, making separation challenging, and the graft densities obtained were low, leading to CNC contact angles of only 35−47°.35,36 Despite vast efforts in CNC hydrophobic modification, many challenges remain that prohibit their large-scale implementation and commercial use. These methods generally suffer from one or more of the following problems: (1) the need for multiple steps (including solvent exchange and initiator attachment, etc.), (2) the use of large volumes of organic solvents in which CNCs are not colloidally stable, which leads to nonuniform products, (3) the use of expensive and nonsustainable reagents, (4) low yields of modified CNCs, and (5) changes in CNC morphology and degree of aggregation. A recent report from Yoo et al. successfully avoided a number of these issues by using bioderived lactic acid and fatty acids for the esterification of CNCs in water and thus improved the dispersion of modified CNCs in organic solvents such as acetone and toluene.37 However, their reaction required high temperature, vacuum, and toxic catalysts which do not fully meet “green” specifications, and the contact angle of the modified CNCs was not reported. Inspired by the catecholamine content of mussel adhesive proteins, in situ oxidative polymerization of catechol-based compounds such as 3,4-dihydroxy-L-phenylalanine (dopamine) at alkaline pH was recently discovered as a universal route for the deposition of polydopamine (pDA) coatings onto almost any type of surface.38 Recently, low-cost plant polyphenols such as tannic acid have been used as precursors for the formation of multifunctional coatings.39,40 These tannic acid coatings deposit under conditions similar to those of pDA due to their analogous structure to dopamine, yet are colorless and significantly less expensive than dopamine. A variety of secondary reactions using the tannic acid coatings as the base or “primer” layer led to multifunctional coatings on various substrates such as polycarbonate, polystyrene, tea bags, TiO2, and polytetrafluoroethylene.39 To the best of our knowledge, there are no reports showing the use of plant polyphenols for the surface modification of nanocellulose, although we previously demonstrated that the combination of tannic acid, methylcellulose, and CNCs leads to a dense shell capable of encapsulating oil through the drying of emulsions,41 and we filed a United States patent application for the modification route presented herein.42 The strategy described in this work offers a novel approach for developing hydrophobic nanomaterials in a green and costeffective manner. First, tannic acid-coated CNCs (CNC-TA) were prepared via oxidation and oligomerization of tannic acid on the surface of CNC nanoparticles. Next, the hydrophobic

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EXPERIMENTAL SECTION

Materials. Tannic acid (ACS reagent), decylamine (≥99.0%, GC), HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), and toluene were all purchased from Sigma-Aldrich. Tannic acid is a commercial from of tannin, and the structure is provided in Figure 1g; however, we note that this is a generalized formula. Realistically, our tannic acid is a mixture of polygalloyl glucoses or polygalloyl quinic acid esters with the number of galloyl moieties per molecule ranging from 2 to 12. Further purification or characterization of tannic acid was not undertaken in this work. CNCs were supplied by CelluForce Inc.

Figure 1. Photographs showing the appearance of CNCs at various steps in the one-pot water-based surface modification and their redispersion in toluene: (a) CNC dry powder, (b) CNCs in water (2 wt %), (c) CNC-TA in water (2 wt %), (d) CNC-TA-DA in water, (e) CNC-TA-DA dry powder, and (f) CNC-TA-DA in toluene (2 wt %). Chemical structure of (g) tannic acid and (h) decylamine. Water phase contains HEPES buffer such that the pH of all suspensions is 8. 5019


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ACS Sustainable Chemistry & Engineering as a spray dried powder in the sodium-form (i.e., H+ counterions on the sulfate half esters were exchanged for Na+ before drying). All water used was deionized and further purified with a Barnstead Nanopure Diamond system (Thermo Scientific). Preparation of CNC-TA-DA. The CNC-TA-DA nanoparticles were prepared via a two-step water-based process. In the first step, 2.0 g of CNC powder was homogeneously dispersed in 100 mL of purified water and sonicated for 2 min (Sonifier 450, Branson Ultrasonics at an intensity level of 3). HEPES (0.476 g) was dissolved in the CNC suspension, and pH was adjusted to 8.0 by addition of NaOH. To this suspension, 0.1 g of tannic acid was introduced and mechanically stirred at 200 rpm for 6 h at room temperature. (A portion of the CNC-TA suspension was removed and dialyzed against purified water, MW cutoff 14 000 Da, and freeze-dried for chemical analysis.) There is no need for purification between steps. In the second step, 4.0 g of decylamine was introduced to 100 mL of a 2 wt % CNC-TA suspension under mechanical shearing at 200 rpm. The reaction was stopped after 3 h, and the product (CNC-TA-DA) was concentrated by centrifugation at 1000 rpm for 10 min, and washed three times with purified water and three times with anhydrous ethanol. The CNC-TADA particles can also be separated from the liquid phase through straightforward filtering or even skimmed off the top of the water, as most of the hydrophobic particles float, but this may decrease the yield. The CNC-TA-DA particles were then freeze-dried or oven-dried at 80 °C and stored for subsequent chemical characterization. We note that modified CNCs could not be obtained if the order of the reactions was reversed: if TA and DA were first mixed, aggregates were formed which did not react further upon the addition of CNCs (data not shown). Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). The chemical composition of CNCs, CNC-TA, and CNC-TA-DA was analyzed with ATR-FTIR using an ATR accessory (Bruker Platinum ATR) with a Bruker Vertex 80 V spectrometer. The spectra were corrected to subtract the background signals and flatten the baseline. The band at 1060 cm−1 was chosen as the normalization wavenumber because this absorbance was attributed to ether groups and assumed to not participate in the reaction. X-ray Photoelectron Spectroscopy (XPS). CNCs, CNC-TA, and CNC-TA-DA dry powders were analyzed using a PHI Quantera II imaging and scanning X-ray photoelectron spectrometer (Physical Electronics Inc., MN) instrument. Sample surfaces were radiated with a source of monochromatized Al Kα (1486.7 eV) with a 45° takeoff angle, a 200 μm beam size, and a 50 W power. The chemical shifts were taken from the literature, and the spectra were corrected by setting the C−C contribution in the C 1s emission at 285.0 eV. The relative atomic concentrations were calculated from the photoelectron peak areas of high-resolution scans by using a Gaussian curve-fitting program. Data manipulation was performed using PHI MultiPak Version 9.4.0.7 software. Cross-Polarization, Magic-Angle Spinning, 13C Solid-State Nuclear Magnetic Resonance (CP-MAS 13C NMR). CNCs, CNCTA, and CNC-TA-DA dry powders were analyzed using a Bruker AVIII 850 NMR spectrometer operating at 20.0 T (214 MHz for carbon-13). The instrument was equipped with a 3.2 mm H/C−Si/N E-free CP/MAS probe configured with channels for 1H and 13C. Prior to addition of the dry powders, polytetrafluorethylene and a Teflon plug were added to the bottom of the rotor to position samples. Experimental parameters for CP-MAS 13C NMR experiments were as follows: 4000 scans, spinning rate of 15 kHz, contact time of 2 ms, acquisition time of 15 ms, and temperature of 278 K. High-power proton decoupling was carried out using a SPINAL64 sequence on all spectra. Transmission Electron Microscopy (TEM). A JEM-1200EX TEM (JEOL Ltd., Japan) was used to image individual CNCs, CNCTA, and CNC-TA-DA at an acceleration voltage of 80 kV. One drop of the particle suspension was placed onto a Formvar-coated 200-mesh TEM grid (Canemco Inc., Canada) and air-dried. CNC dimensions presented (cross section and length) were obtained from the analysis of a minimum of 100 particles. Confidence intervals presented are the standard deviation of the average particle dimensions measured.

X-ray diffraction (XRD). Measurements were made on disks prepared by pressing freeze-dried samples using a Bruker D8-Advance XRD diffractometer with Co Kα radiation at 35 kV and 45 mA. The XRD patterns were recorded in a 2θ range from 8 to 44° at a rate of 1°/min and a resolution of 0.04° at various temperatures. The XRD deconvolution was completed by using peak assignments based on standard powder diffraction patterns of cellulose I and II. The CNC crystallinity index (CI, %) was evaluated using eq 1:

⎛ Ac ⎞ CI (%) = ⎜ ⎟ × 100 ⎝ A a + Ac ⎠

(1)

where Ac represents the total crystalline area of deconvoluted XRD patterns and Aa represents the total amorphous area of a hump-like peak. Contact Angle Measurements. Uniform films for contact angle measurements were obtained by spin coating (Chemat Technology KW-4A, Northridge, CA) ∼1.0 wt % CNCs (in water), CNC-TA (in water), and CNC-TA-DA (in toluene) suspensions onto Si wafers at 3000 rpm for 30 s, followed by heat treatment for 12 h at 80 °C. A drop of purified water (5−10 μL) was added onto each surface using a Hamilton syringe at 25 °C. The image of the sessile drop was recorded, and the contact angle was measured using ImageJ software (advancing and receding contact angle measurements are described in the Supporting Information).

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RESULTS AND DISCUSSION Aqueous One-Pot Hydrophobic Modification of Cellulose Nanocrystals. CNCs used in this work were produced industrially from bleached Kraft pulp by CelluForce Inc.43 The CNCs are stable in aqueous suspension because their surface sulfate half ester groups impart electrostatic repulsion (sulfur content is 0.81 ± 0.03 g/100 g CNC, which is approximately 0.45 charges/nm2). CNCs were received as a spray dried powder in the neutral sodium form (Figure 1a) and were easily redispersed in water with sonication (Figure 1b).44 Mixing CNCs and tannic acid led to coated CNCs (CNC-TA), which maintained colloidal stability but with a slightly yellow discoloration (Figure 1c). Addition of decylamine to the TAcoated CNCs in the second step led to fast agglomeration of the particles and phase separation, implying increased hydrophobicity after the reaction (Figure 1d). The yellow agglomerated phase (CNC-TA-DA) was collected and freezedried or oven-dried, which produced the yellow powder shown in Figure 1e. The powder could be redispersed in toluene with mild sonication without any sign of agglomeration (Figure 1f). The chemical structure of the tannic acid primer, containing polydopamine-like catechol groups, is shown in Figure 1g, and decylamine, which imparts hydrophobicity, is shown in Figure 1h. The reaction mechanism is expected to follow those reported previously for plant polyphenols and polydopamines on other substrates.39 Although the exact coating mechanism is not fully understood, it is believed that the interactions between oxidized polyphenols and substrates result in the formation of new covalent bonds and/or other strong intermolecular interactions such as hydrogen bonding, metal chelation, and π−π interactions.38 In step 1, tannic acid is reacted with CNCs in suspension at pH 8 because under these conditions plant polyphenol oxidation and oligomerization is known to occur in the presence of available dissolved oxygen, leading to the formation of higher molecular weight species with decreased solubility.39 Presumably, the solubility decrease of tannic acid and its inherent affinity toward cellulose45 leads to surface deposition onto the CNC particles. This produces quinones 5020


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Information Table S2 and caveats about using DLS for rodshaped nanoparticles). The apparent size of CNC-TA-DA increases from 270 ± 40 nm in ethanol to 900 ± 100 nm in heptane, which suggests poorer dispersion of CNC-TA-DA particles in less-polar solvents. These numbers are larger than the apparent size for unmodified CNCs in water (∼80 nm),43 meaning that the CNC-TA-DA suspensions are slightly aggregated. This is unsurprising because, despite the compatibility of decylamine with organic solvents, we expect only minor steric stabilization from short alkyl tails, and no other mechanisms are in place to impart long-term colloidal stability. We also note that we cannot measure unmodified CNCs in organic solvents by DLS because they aggregate so severely, and the aggregates are so large that no data are obtained in the DLS size regime. As such, we conclude that initial dispersion of CNC-TA-DA is greatly improved compared to that of unmodified CNCs, which should aid in compounding and formulation processing. Though the turbidity data show that toluene and ethanol are the best solvents in which to disperse CNC-TA-DA particles, this may be more a result of refractive index similarities and optical effects, whereas the apparent size of the particles indicates that they can be similarly dispersed in all solvents tested in this work. The self-assembly behavior of CNCs in suspension before and after surface modification was also investigated to qualify the dispersion of particles: good nanoparticle dispersion is a prerequisite for liquid crystal self-assembly.7,9,12 Figure S4 shows that all samples were birefringent when observed by polarized optical microscopy. CNCs and CNC-TA in water exhibited the expected chiral nematic liquid crystal phase, implying stable CNC suspensions. CNC-TA-DA in toluene presented a crosshatch pattern characteristic of nematic ordering,46 which is similar to that in a previous report showing that hydrophobic CNCs in toluene form only nematic phases (without chirality).17 Nonetheless, this observation for CNC-TA-DA supports well-dispersed modified CNCs in toluene. Surface Chemistry after Modification. ATR-FTIR, XPS, and NMR spectroscopy were used to assess the chemical composition of CNCs, CNC-TA, and CNC-TA-DA. FTIR indicated that the two-step surface modification was successful (Figure 2). The unmodified CNCs exhibited IR peaks characteristic of cellulosic functional groups at 3000−3550 and 900−1100 cm−1, which correspond to −OH and −C−O− C vibrations, respectively.7,12,13,30 The spectrum of CNCs after tannic acid coating displayed some notable changes, including a

which, in the second step of the modification, react with primary amine groups via a Schiff-base reaction and/or Michael addition,38 as depicted in Scheme 1. Scheme 1. Suggested Chemical Reactive Pathway (Not Drawn to Scale) For the Surface Modification of CNCs with Tannic Acid (Top) Followed by Decylamine Addition (Bottom)a

a

The products of the amine reacting by a Schiff-base reaction and Michael addition are shown on the bottom left and right, respectively. The dotted line represents catechol binding to CNC through covalent and/or strong intermolecular interactions, as described for other substrates.

Evidence for the TA deposition onto the CNC surface and the need for oxygen for the reaction to occur, was provided by conductometric titrations (Supporting Information, Table S1) as well as the chemical characterization reported below. There is a significant reduction in accessible surface sulfate half ester groups following the reaction of CNCs with tannic acid that we attribute to the tannic acid coating prohibiting the titration of the CNC acid groups. Furthermore, if CNCs are reacted with tannic acid in the absence of oxygen (with N2 purge), no change in sulfate half ester content is measured, suggesting that no tannic acid coating formed on the CNCs. Dispersion of Modified CNCs in Nonpolar Solvents. Optical microscopy images of CNCs and CNC-TA-DA in toluene (Supporting Information, Figure S1) clearly demonstrate that CNC particles are better dispersed in toluene after the surface modification. Unmodified CNCs sediment quickly in toluene and show micrometer-sized aggregates by microscopy. The dispersibility of CNC-TA-DA in other solvents such as heptane, methyl ethyl ketone, chloroform, and ethanol was also tested, and it was found that toluene and ethanol gave the best suspensions based on visual inspection and suspension turbidity (Supporting Information, Figure S2). In addition, we could obtain strong “invertible” organogels in toluene with 3.0 wt % CNC-TA-DA (Supporting Information, Figure S3). This highlights the potential to greatly increase the viscosity of nonaqueous formulated products using CNC-TA-DA. Dynamic light scattering (DLS) measurements to assess CNC dispersion were performed on the same samples measured by turbidity (i.e., in Figure S2) by diluting the supernatant of CNC-TA-DA samples in various organic solvents to a concentration of 0.025 wt % (see Supporting

Figure 2. ATR-FTIR spectra of CNCs, CNC-TA, and CNC-TA-DA. 5021


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ACS Sustainable Chemistry & Engineering peak at 755 cm−1 from distortion vibrations of CC in the benzene rings and peaks at 1450 and 1560 cm−1 from stretching vibrations of the C−C aromatic groups.47 After decylamine addition, the presence of asymmetrical and symmetrical CH2 stretches from the C10 alkyl chain at 2850 and 2930 cm−1 (Supporting Information, Figure S5) suggests the attachment of decylamine.7,16,28,48 Furthermore, the FTIR spectrum for CNC-TA-DA exhibited a notable peak at 1490 cm−1, attributed to secondary N−H bending (indicating Michael addition occurred), and an increase in the peak at 1560 cm−1 that corresponds to N−H bending vibrations. XPS provided further support for the modification reaction: in the wide-scan spectra (Figure 3), the major components

CNCs, the measured ratio was 0.75; for CNC-TA, it was 0.83, and for CNC-TA-DA, it was 0.30. This follows the expected trend with decylamine addition where the O/C ratio drops significantly due to the addition of alkylamine chains with no oxygen. The discrepancy in O/C ratio for unmodified and CNC-TA is attributed to minor contamination. Interestingly, we can deduce that the tannic acid layer on the CNCs is fairly thin because if the XPS beam only interacted with tannic acid (with a given penetration depth of approximately 10 nm), then a much lower O/C ratio of 0.61 (O/C ratio for pure tannic acid) would be expected. Figure 4 shows the normalized CP-MAS 13C NMR spectra for unmodified and modified CNCs, where all spectra exhibited

Figure 4. Solid-state 13C NMR spectra of CNCs, CNC-TA, and CNCTA-DA. Figure 3. XPS wide-scan spectra of CNCs, CNC-TA, and CNC-TADA. Inset: the N 1s signal of CNC-TA-DA was resolved into two Gaussian component peaks: N1: 399.7 eV and N2: 400.5 eV.

chemical shifts characteristic of the D-anhydroglucose units (AGU) of cellulose. Unmodified CNCs displayed typical peaks corresponding to C1 (105.2 ppm), C4cryst (89.2 ppm), C4amorph (83.8 ppm), C6 cryst (65.4 ppm), and C6 amorph (62.7 ppm).16,17,30 NMR resonances of C2, C3, and C5 of the AGU backbone are in a similar chemical environment and showed close signals between 70 and 75 ppm. After modification with tannic acid, no noticeable new peaks appeared in the spectrum. The absence of peaks between 125 and 150 ppm that are characteristic of aromatic carbons suggest that the NMR signal is swamped by the large amount of bulk cellulose in the sample and implies that the tannic acid coating on CNCs is minimal.51 (Note that the NMR gives the total compositions, whereas XPS gives an exaggerated surface composition.) The addition of decylamine was more apparent than tannic acid by NMR; peaks in the δ 40 to 10 range are due to the hydrocarbons, specifically the secondary carbon −CH2− groups (δ 23.0, 26.0, 29.7, 34.3, and 40.5) and the primary carbon −CH3 groups (δ 14.3 and 15.7).16,17 These results further support the surface modification of CNCs with tannic acid and decylamine. In addition to the solid-state NMR results, DA, TA, and TADA were also analyzed using 1H NMR (in deuterated DMSO), as described in the Supporting Information, Figure S8. Peaks of aromatic protons from 6 to 8 ppm in TA 1H NMR spectrum are weakened, likely due to the grafting of DA molecules onto the TA chains. In addition, the peaks from 3 to 5 ppm in TADA spectrum are attributed to the protons on amines adjacent to the aromatic rings, suggesting the reaction between TADA.52 Lastly, the disappearance of phenol proton peaks from 9

before surface modification were carbon and oxygen with a trace of sulfur from the sulfate half ester groups, but after modification, a nitrogen peak (2.43%) appeared from the decylamine. The inset in Figure 3 shows that the nitrogen peak can be resolved into two components, N1 (399.7 eV) and N2 (400.5 eV), which are due to aromatic CN and aromatic C− N, respectively. These peaks indicate that both the Schiff-base reaction and Michael addition between tannic acid-coated CNCs and decylamine took place.49 Though the XPS peaks from tannic acid heavily overlap with the ones from aromatic C−N and CN, it is observed that there is an increase in the peak intensity at 288.3 and 286.8 eV (Supporting Information, Figure S6), which is attributed to aromatic C−N and CN, respectively. This supports the grafting of decylamine onto TA-coated CNC particles. Furthermore, DA and TA-DA reacted dry powders (without CNCs) were also analyzed, and the N 1s signals were resolved into Gaussian component peaks (Supporting Information, Figure S7). The appearance of aromatic CN and C−N peaks also indicate the reaction between TA and DA via Schiffbase reaction and Michael addition, respectively. We can draw some general conclusions from XPS atomic ratios despite the fact that XPS is notoriously difficult with cellulose samples due to the likelihood of contamination, which skews the signal that is nonlinear with penetration depth (and the high surface area of CNCs exacerbates this).28,43,50 The theoretical O/C ratio for pure cellulose is 0.83. For unmodified 5022


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ACS Sustainable Chemistry & Engineering to 10 ppm in the TA-DA spectrum suggests the oxidation of phenol groups to quinone groups. It should be noted that tannic acid was not oxidized beforehand in the NMR sample preparation (whereas it is in the reaction with CNCs), which implies that the addition of decylamine further induces the oxidation of tannic acid. CNC Morphology. TEM images of CNCs, CNC-TA, and CNC-TA-DA are shown in Figure 5, indicating that all CNCs

Figure 6. (a) XRD spectra of the unmodified CNCs and modified CNCs (CNC-TA and CNC-TA-DA) at 25 °C. (b) XRD spectra of CNC-TA-DA at different temperatures (25, 45, and 60 °C).

Though these 2θ peak values are relatively higher than the ones commonly reported in literature, we attribute this to the fact that we used Co radiation instead of Cu. Because d-spacing is radiation source-independent, the d-spacing values at these 2θ peaks were calculated to be 0.60, 0.53, 0.39, and 0.26 nm, respectively, which agrees well with the literature.3 Unmodified CNCs and CNC-TA gave similar XRD patterns, implying that the underlying particle crystallinity was unchanged by the thin tannic acid coating, whereas CNCTA-DA had a notably distinct XRD pattern (Figure 6a). XRD measurements of CNC-TA-DA at different temperatures (25, 45, and 60 °C) were performed (Figure 6b) and indicated that the XRD pattern is dominated by crystallized decyl chains at low temperature which disappear as the sample is heated. At 60 °C, the cellulose I peaks prevail with some minor broadening due to the surface coating, implying that the crystallized grafted moieties had melted. A similar side chain melting has also been reported on chitin whiskers functionalized with long chain alkyl groups.55 The unique XRD pattern was thus attributed to the ordered stacking of long alkyl chains and the formation of a new crystalline phase on top of the CNC particles. Interestingly, the XRD pattern for CNC-TA-DA implies that the surface modification with decylamine is fairly dense such that decyl chains are close enough to crystallize, and furthermore, the decyl chains on adjacent particles may interact, which could influence the self-assembly of CNC-TADA particles, as seen in the Supporting Information, Figure S4. We note that the alkyl tail length (and corresponding melting temperature) influences the modification reaction; dodecylamine and octadecylamine were also tested as the hydrophobe but led to enhanced crystallization and aggregation of modified CNCs in toluene and lower contact angles (data not shown). As expected, overall, the crystallinity index decreased for CNCs with increased surface modification from 81% for unmodified CNCs to 76% for CNC-TA and 61% for CNC-TA-DA (at 60 °C). Thermal stability of unmodified and modified CNCs was analyzed by thermogravimetric analysis (TGA), which showed minimal differences between samples (Supporting Information, Figure S9). CNC-TA-DA began to degrade at 257 °C, which is an 11 °C decrease in thermal stability from CNCs. This is most likely related to the low thermal stability of pure decylamine, which starts to degrade at 207 °C (Supporting Information, Figure S10). Overall, the minor changes in thermal stability, most apparent in Supporting Information Figure S9b, provide evidence for surface modification of CNCs without significant loss of stability, and furthermore, the one degradation peak for

Figure 5. TEM images of (a) unmodified CNCs dried on the TEM grid from water, (b) CNC-TA dried from water, and (c) CNC-TA-DA dried from toluene. No staining was used in the TEM sample preparation. Scale bars are 500 nm.

maintained their rod shape with an average length of 190 ± 70 nm regardless of the surface modification. The cross sections increased from 8 ± 3 nm for unmodified CNCs to 19 ± 5 nm for CNC-TA and 23 ± 6 nm for CNC-TA-DA. Previous work has shown that thin coatings of tannic acid spontaneously deposited onto gold substrates increase in thickness with incubation time.39 We believe that the enhanced contrast dark coating on the surface of CNC-TA and CNC-TA-DA nanoparticles seen by TEM comes from the aromatic groups in tannic acid. Both unmodified CNCs and modified CNC-TA in aqueous suspension showed well-dispersed individual nanoparticles without large agglomerates. Modified hydrophobic CNC-TA-DA nanoparticles in toluene also appeared unaggregated at the micrometer scale. The molecular diameter of tannic acid is theoretically predicted to be 3 nm,53 implying that only 24% of the total available CNC surface area would be coated if all of the tannic acid used reacted with CNCs to form a monolayer. Interestingly, this roughly aligns with the 28% reduction of accessible surface sulfate half ester groups on the CNCs after reaction with tannic acid that was measured by conductometric titration (Supporting Information, Table S1). However, because the CNC cross section increased by 11 nm after tannic acid addition, the coating is more likely nonuniform, with some areas uncoated and others (ca. 12% of the surface) coated with two layers of oligomerized tannic acid molecules stacked on top of each other. This is supported by the variation in width seen by TEM, which is accentuated after the addition of decylamine. There is approximately 20 times more decylamine added than there are catechol groups, implying that unreacted decylamine remains in the water phase. If we assume that all of the reactive sites on CNC-TA are functionalized with decylamine as shown in Scheme 1, the final ratio of components would be 0.05 tannic acid:0.1 decylamine:1 CNC by mass, which is approximately 0.05 tannic acid and 1 decylamine per nm2 of CNC surface area. CNC Crystallinity. XRD was used to study the effect of surface modification on the CNC crystal structure (Figure 6). The unmodified CNCs showed typical 2θ peaks at 17.3, 19.4, 26.2, and 40.5° that correspond to the crystal planes of 101, 101̅, 002, and 040 in the crystal structure of cellulose I.12,54 5023


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ACS Sustainable Chemistry & Engineering CNC-TA and CNC-TA-DA imply “hybrid materials” as opposed to blends. Contact Angle. Model CNC surfaces were prepared by spin coating, and sessile drop water contact angle measurements were conducted. Figure 7 shows that the hydrophobicity

versatile, and other polyphenols and alkylamines/alkylthiols may be used. The water-based, room-temperature reaction is ideal for both colloidal stability and reproducible surface modification of CNCs and is environmentally friendly and costeffective. CNCs are emerging biobased nanoparticles that will ideally serve as a valuable renewable resource for producing novel solvent-formulated products such as paints, adhesives, household products, food, and cosmetics as well as enhanced sustainable nanocomposites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00415. Optical micrographs of CNCs and CNC-TA-DA in toluene, photographs and the corresponding absorbance at 550 nm of CNC-TA-DA suspended in various organic solvents, photograph of toluene suspension of CNC-TADA at 3 wt %, polarized optical microscopy images of unmodified and modified CNC liquid crystal textures, ATR-FTIR spectra of TA and DA, TGA and DTG analysis for CNC, CNC-TA, CNC-TA-DA, and pure TA and DA (PDF)

Figure 7. Water contact angles (measured after 30 s) for CNCs, CNCTA, and CNC-TA-DA films produced by spin coating.

of the CNCs increased from 21 to 74° after the two-step modification reaction, which correlates with the easy dispersion of CNC-TA-DA in toluene. A slight decrease in contact angle to 19° was observed after CNCs were coated with tannic acid, which suggests that the tannic acid itself does not make CNCs hydrophobic and agrees with past studies that have used tannic acid to reduce the contact angle of hydrophobic substrates (polypropylene and Teflon).39,56 While sessile drop water contact angle measurements are not the most robust measurement of surface wettability, we note that our values agree with the literature for smooth CNC surfaces measured through more in-depth contact angle methods.57 We furthermore explored the difference between advancing and receding contact angles (i.e., contact angle hysteresis) as described in the Supporting Information and specifically in Figure S11. The advancing and receding contact angle measurements support the contact angle of 74° for CNCTA-DA, but we note that the hysteresis measured was 16°, implying that heterogeneity in both structure (due to film preparation) and chemical composition (due to the “patchy” coating on CNCs observed by TEM) may be significant. Nonetheless, all measurements support the notion that CNCTA-DA particles are less hydrophilic than unmodified CNCs. Compared to other hydrophobic CNCs that can be redispersed in toluene, as reported in the literature, our contact angle value for CNC-TA-DA is similar.28 To the best of our knowledge, the highest contact angle value reported is 94°, in which high density grafting of hydrophobic polystyrene drastically increased the hydrophobicity of CNCs.30 However, our reaction is easier than polymer grafting and gives particles with limited aggregation or changes in morphology. This work presents a straightforward route to produce CNC particles (CNC-TA-DA) with a greatly increased contact angle and the ability to be redispersed in organic solvents. FTIR, XPS, NMR, TEM, and XRD corroborate the successful surface modification and imply a thin tannic acid coating but a dense decylamine outer functionalization. The tendency for modified CNCs to form liquid crystal phases and the dispersion measured by turbidity and DLS imply a uniform and sufficient surface hydrophobization for CNC use in nonaqueous systems. We believe that this modification strategy is scalable and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhen Hu: 0000-0003-2060-7056 Robert Pelton: 0000-0002-8006-0745 Emily D. Cranston: 0000-0003-4210-9787 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge CelluForce Inc. for supplying the CNCs used in this study. The authors thank X. Dong for ATR-FTIR technical support and Dr. B. Berno for 13C solid state NMR analysis. V. Jarvis is acknowledged for sample analysis with XRD. This work was carried out using instruments in McMaster’s Biointerfaces Institute and the Canadian Centre for Electron Microscopy. The authors thank NSERC for funding this work through an Engage Grant. R.P. holds the Canada Research Chair in Interfacial Technologies.

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REFERENCES

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