Highly Modified Cellulose Nanocrystals and Formation of Epoxy

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Highly Modified Cellulose Nanocrystals and Formation of Epoxy-CNC Nanocomposites Eldho Abraham, Doron Kam, Yuval Nevo, Rikard Slattegard, Amit Rivkin, Shaul Lapidot, and Oded Shoseyov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09852 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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ACS Applied Materials & Interfaces

Highly Modified Cellulose Nanocrystals and Formation of Epoxy-CNC Nanocomposites Eldho Abraham*, Doron Kam*, Yuval Nevo*, Rikard Slattegard‡, Amit Rivkin*, Shaul Lapidot‡, Oded Shoseyov*† *

R.H. Smith Institute of Plant Sciences and Genetics and The Harvey M. Krueger Family Center for

Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Israel ‡

Melodea Ltd, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel

Abstract: This work presents an environmentally friendly, iodine-catalysed chemical modification method to generate highly hydrophobic, optically active cellulose nanocrystals (CNC). The high degree of ester substitution (DS=2.18), hydrophobicity, crystalline behaviour and optical activity of the generated acetylated CNC (Ac-CNC) were quantified by TEM, FTIR, solid

13

C NMR, contact

angle, XRD and POM analyses. Ac-CNC possessing substantial enhancement in thermal stability (16.8%) and forms thin films with interlayer distance of 50-150 nm, presenting cavities suitable for entrapping nano and micro particles. Generated Ac-CNC proved as an effective reinforcing agent in hydrophobic polymer matrices for fabricating high performance nanocomposites. When integrated at a very low weight percentage (0.5%) in an epoxy matrix, Ac-CNC provided for a 73% increase in tensile strength and a 98% increase in modulus, demonstrating its remarkable reinforcing potential and effective stress transfer behaviour. The method of modification and the unique properties of the modified CNC (hydrophobicity, crystallinity, reinforcing ability and optical activity) render them a novel bionanomaterial for a range of multipurpose applications.

Keywords: hydrophobic CNC, esterification, birefringence, epoxy, nanocomposite

INTRODUCTION Nanocrystalline cellulose (CNC) obtained from natural bioresources has evolved into a fascinating building block option for the design of new biomaterials in nanotechnology.1,2 Derived from an abundant and renewable biopolymer, it is drawing a tremendous degree of scientific attention, especially in the green composite industry, due to the sustainability megatrend which is still on the rise. CNC is one of the major bio-nanomaterial as reinforcing 1 ACS Paragon Plus Environment


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filler in the polymeric nanocomposite industry, where annual global market size is projected to reach $17.2 billion by 2025. This growing interest is due to the unsurpassed quintessential physical and chemical properties of CNCs. The growing demand for such bio-based nanomaterial-filled composites is expected to propel demand for high performance products in the near future. In addition to serving as a central component of polymeric nanocomposites,3,4 CNCs use has been extended to packaging and food additives5, photonic devices6, drug delivery platforms,7,8 and supercapacitor applications.9 Yet, owing to their hydrophilicity and low thermal stability, their utilization is restricted to applications involving hydrophilic or polar media, which limits their exploitation. Therefore, there has been great interest in utilizing their hydroxyl surface chemistry to modify properties by introducing new functionality. A plethora of reports describe a variety of cellulose modification techniques10,11, yet most are ineffective due to inadequate CNC dispersion in the reaction media, inability of the suitable reagents and the catalysts to protonate the hydroxyls of cellulose . Simultaneous isolation of CNC and acetylation of its hydroxyl groups in a single-step, acid-catalysed reaction was performed as a simple and green alternative to commonly reported modification methods. However, to favor ester formation, either an excess of one of the reagents, high catalyst concentrations, removal of water or a combination of these measures, was necessary and yielded a maximum degree of substitution (DS) of 0.91.12 Most modification approaches leave the inner crystal surfaces undispersed, the hydroxyl groups intact and most of the cellulosic clusters unmodified. Hence, the reported modification methods developed to achieve stable hydrophobic cellulosic nanomaterial, are inadequate and unsatisfactory, with the DS falling within the range 0.01 and 1.13,14 It is reported in the literature about the modification of cellulose and wood materials with acid anhydride where iodine as catalyst.15,16 Here, we present the simple, solvent-free esterification approach, which provides for effective acetylation and dramatic modification of the cellulose nano crystals with a DS exceeding 2.1. The method is advantageous over the alternative approaches, in its reaction media where solvent act as the reactant with iodine as catalyst, high substitution efficiency, as well as in the hydrophobic, crystalline, optical and thermal properties of the resulting cellulosic nanomaterial.17 We compared the outstanding performance of the resulting

Ac-CNC

nanomaterial

with

its

unmodified

counterpart.

Ac-CNC

formed

nanostructured interlayer networks and films, which can hold nonionized drugs that would normally not associate with unmodified CNC, rendering them suitable for integration in bio2 ACS Paragon Plus Environment

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based nanomaterial in drug delivery applications.18 Additional potential applications for Ac-CNC include birefringent thin-film polarizers19 and high temperature transparent hydrophobic coating.20 The present study also aimed to evaluate the reinforcing effect of the derived AcCNCs on polymeric matrices for nanocomposite application. Epoxy resin, a class of polymeric materials with extensive applications, was selected as the model polymer matrix for the fabrication of nanocomposites.21 There are reports of use of unmodified CNC modified CNC

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and resilin-

as reinforcement for making elastic epoxy nanocomposites, yet these attempts

failed to provide effective reinforcement of the matrix due to incompatibility between the filler and matrix. Ac-CNC is expected to provide a combination of high stiffness and strength to the epoxy resin, together with optical properties, most desirable in many industrial and aerospace applications.

EXPERIMENTAL Materials and Methods. Aqueous dispersion of neutral CNC was prepared from cellulosic waste materials (Figure 1a) by the reported sulfuric acid (64%) hydrolysis method.24 Acetic anhydride and iodine (catalyst) were of analytical grade (Sigma Aldrich) and used without further purification. Commercial epoxy matrix, EPON 828 and Epikure 3140 (Momentive Specialty Chemicals Inc., Israel) were used as resin and hardener, 23 respectively, for preparation of the Ac-CNC reinforced nanocomposites. Sulfuric Acid Hydrolysis: Recycled cellulosic waste material (Figure 1a) obtained from wood pulp industry was purchased from Melodea Ltd. Israel. Recycled cellulosic raw material contains 9.8% hemicellulose and 0.9% lignin. Raw material was hydrolyzed to CNC using sulphuric acid (64% by mass) at 50 oC and 3 hrs reaction time. Esterification of CNC and film formation. Solvent-exchange from acetone to Ac2O of an aqueous neutralised CNC solution was performed by successive centrifugation. CNC in Ac2O (0.02 g/ml CNC in Ac2O) was heated to a temperature of 100-105oC for 1h, after which, the catalyst (0.001 g/ml I2 in Ac2O) was added. The reaction chamber was maintained at this temperature for an additional 25 min and the functionalization reaction was terminated by adding acetone to the brown-coloured reactant. Transmission electron microscopic (TEM) images of the unmodified CNCs are shown in Figure 1b. The iodine-catalysed acetylation of CNC is summarised in Figure 1c where Ac2O acting together as a solvent and reactant. The initial step of 3 ACS Paragon Plus Environment


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the acetylation involves a nucleophilic attack on the acyl carbon of the anhydride molecule by a lone pair of the alcoholic hydroxyl group, followed by subsequent loss of acetic acid to generate the ester.25 The catalyst (iodine, a Lewis acid catalyst) then activates one of the carbonyl carbons of Ac2O, which, in turn, protonates the nearby hydroxyls of cellulose (polyalcohol) and eventually esterifies CNC. The mechanism of acetyl ester substitution on the hydroxyls of CNC with a high DS is shown in Figure 1d. The modified CNC samples were washed with acetone/ethanol to remove traces of iodine and unreacted Ac2O, until a clear, transparent dispersion of acetylated CNC (Ac-CNC) in acetone/ethanol was obtained. The typical mass yield of the Ac-CNC was 60% of the initial CNC mass. The TEM of the Ac-CNC (Figure 1e) and the schematic presentation of acetate-esterified CNC crystals (Ac-CNC) are shown in Figure 1f. The centrifuged samples from the reaction chamber with specific time interval were also shown (Figure S1).

Figure 1. (a) Source of CNC. (b) TEM of CNC. (c) Modification of CNC with high degree of acetate esters. (d) Mechanism of CNC esterification by iodine-catalyzed reaction involving nucleophilic substitution. (e) TEM of Ac-CNC. (f) Schematic presentation of Ac-CNC crystals with protruded ester pendants

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Dry films of CNC and Ac-CNC were prepared by controlled drying of their respective suspensions (2 wt.% in water and acetone, respectively) in glass Petri dishes at a constant oven temperature (50 oC). For the polarised optical microscopy (POM) analysis, a few drops of the CNC and Ac-CNC suspensions were placed on a microscopic glass slide and allowed to dry it at constant oven temperature. POM images were captured before and after of drying the suspension. The samples for contact angle measurements were prepared by oven drying CNC and Ac-CNC dispersions on microscopic slides, yielding ∼200 µm-thick films. The dispersibility of Ac-CNC in various solvents, including water, ethanol, acetone, toluene, ethyl acetate, DMSO and DMF were also analysed, by mixing with 2 wt.% CNC and Ac-CNC in respective solvents. Preparation of epoxy/Ac-CNC nanocomposites. Epoxy nanocomposites were prepared by reinforcing Ac-CNC in epoxy (EPON 828) resin, with Epikure 3140 as a hardener. Ac-CNC dispersed in acetone (1 wt.% Ac-CNC) was sonicated with epoxy for 15s before curing with Epikure (Figure S2). We have compared the dispersion of unmodified CNC in EPON 828 matrix with aqueous and acetone dispersion of the former (Figure S3). Subsequently, the acetone was gradually removed through heating and degassing the mixture on a magnetic hot plate, with stirring. After the Ac-CNC/epoxy mixture had cooled to room temperature, nanocomposite samples were generated by mixing Epikure 3140 hardener with EPON 828, at a weight ratio of 1:0.75, stirring at room temperature for 15 min and then casting into aluminium molds. The samples were then degassed for 15 min, under low vacuum. Curing was performed at room temperature for ∼24 h, followed by an overnight post-cure at ∼80oC. Characterization methods. The size of the individual crystals of CNCs, before and after modification, was determined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). For TEM, drops (5 µL) were deposited on glow-discharged, carbon-coated copper grids and blotted 1 min thereafter. The specimens were observed under low-dose conditions. The grids were negatively stained with 2% uranyl-acetate (incubation time 30 s) and observed with a Tecnai-12 (Philips) and operated at 120 kV. Images were digitally captured with a TVIPSF224 CCD camera. AFM analyses were conducted in the tapping mode (Scanning Probe Microscope, Dimension 3100 Nanoscope V from Veeco/Bruker, Santa Barbara CA, USA) and confocal laser scanning microscopy (CLSM; Leica TCS SP5, Confocal microscope from Leica Microsystems, Mannheim, Germany). FTIR analysis of CNC and Ac-CNC was performed using a Thermo Scientific Nicolet 6700 spectrometer in the absorption mode (100 scans at a nominal 5 ACS Paragon Plus Environment


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resolution of 4 cm−1). The FTIR sample was prepared with KBr powder (IR grade, Aldrich) as the background, by mixing with an approximate 2% weight of the powdered samples. Solid-state NMR experiments of the soxhlet-purified samples were performed with a Bruker Advance III 500 MHz narrow-bore spectrometer, using a 4 nm double-resonance magic angle spinning (MAS) probe.

13

C CPMAS experiments were carried out at a spinning rate of 8 KHz, using the

following settings: 2.5ms 1H 90, 2 K data points, 720 scans and a 2 ms ramped-cp period. Proton decoupling, using the SPINAL composite pulse sequence, in a field of 100 KHz, was applied during acquisition; with a 3 s recycle delay between acquisitions. Chemical shifts were calculated with respect to adamantane (38.55, 29.497 ppm). The spectra were normalized by positioning the peak at δ = 75 ppm, attributed to the C2, C3, and C5 carbons in the crystalline cellulose. X-ray diffraction (XRD) analysis of CNC and Ac-CNC films was conducted with a D8 Discover X-ray Diffractometer (Bruker AXS Inc., Madison, MI, USA), using a Cu Kα (40 kV/35 mA) source, between 2θ = 0° and 50°, with an angle step size 2θ = 0.02°. Spectral intensities of light passing through the cuvette and dry film were recorded by a UV-vis spectrophotometer (model 8453 Agilent, Palo Alto, CA). Weight percentages of organic elements (CHNS) in the samples were determined through a PerkinElmer Elementar CHNS analyzer. POM images were captured with a Nikon Polarizing Microscope Eclipse LV100POL equipped with a Nikon DS-U2 camera control unit. The contact angle measurements were performed by depositing 5 µL water droplets on the surface of the films, which were then photographed with a CCD camera. The measurements were performed at room temperature with an OCA20 automated and softwarecontrolled Video-Based Contact Angle Meter (Data Physics Instruments GmbH, Filderstadt, Germany), using both static and dynamic sessile methods during the first 60s after deposition. Thermogravimetric analysis (TGA) of the films was carried out using a TGA 7 (Perkin Elmer, USA) analyser, under nitrogen atmosphere. The weight of the film samples varied from 6-8 mg, scanning range was 50-500°C, and the heating rate was 10°C/min. Microstructural analysis of the Ac-CNC films was performed using a scanning electron microscope (HR-SEM, FEI QUANTA200 (FEI, Hillsboro, OR, USA). Prior to SEM imaging, the sample surfaces were coated with a thin layer of gold, using a BAL-TEC SCD 005 sputter coater (Leica, Wetzlar, Germany). Characterization of Ac-CNC/epoxy nanocomposites. FTIR analysis of the films of nanocomposites was performed using a Thermo Scientific Nicolet 6700 spectrometer in the 6 ACS Paragon Plus Environment

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absorption mode (100 scans at a nominal resolution of 4 cm−1). The mechanical properties of the casted nanocomposite dog-bone samples (5 mm wide and 0.2 mm thick) were tested using an Instron 5944 equipped with a 5000 N load cell. The gap between the clamps was 30 mm and the test speed was 1mm/min. The morphology and the degree of dispersion of Ac-CNC in epoxy matrix was analysed using an SEM (HR-SEM, FEI QUANTA200 (FEI, Hillsboro, OR, USA), after a thin layer of Pt/Pd was sputter-coated on the sample.

RESULTS AND DISCUSSION Morphology and dispersibility. TEM analysis of unmodified CNC (Figure 2a) showed individual 200 ±50 nm-long and 10 ±5 nm-wide CNC crystals, demonstrating perfect crystalline nature and possessing an average aspect ratio of 20. Ac-CNCs showed similar morphologies, mostly of the same length but wider (>30 ±10 nm) than unmodified CNC nanocrystals, owing to the ester substitution on cellulose crystals (Figure 2b-c). Introduction of the bulky side chain of an acetate ester in place of the hydroxyl functional groups of CNC, increased the size of the individual crystals, which was proportional to the degree of substitution. Moreover, Ac-CNCs tended to bundle as rod-like nanocrystals and had non-uniform crystalline dimensions, due to bulky pendant substitution (Figure 2c). Bundling of Ac-CNC may be the result of intramolecular hydrogen bonds between the modified nanocrystals, or the remnants of undispersed CNC crystals holding the cellulose rods together. AFM images (Figure 3a,b) of Ac-CNC resembled the TEM images, showing 200 ±50 nm-long and 20±10 nm-wide nanocrystals. In both AFM and TEM analyses, Ac-CNC appeared as individualized, relatively uniform structures, with an aspect ratio ranging between 10 and 30. The cross sectional AFM measurements of the Ac-CNC single crystals proves that they possess an average diameter of 20 nm and a surface roughness value of 10 nm (Figure 3c). The profoundly protruding ester pendants of highly esterified Ac-CNC crystals play a major role in the applicative potential of this nanomaterial (Figure 1f).



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Figure 2. TEM image of CNC (a) and Ac-CNC (b,c)

Figure 3. AFM images of Ac-CNC (a,b) and cross sectional surface roughness of Ac-CNC single crystals (c) The dispersibility of CNC and Ac-CNC (2 wt.%) was analysed in a series of solvents of different polarities and dielectric constants (Figure S4a,b). Unmodified CNC formed a white flocculent or precipitate with all the organic solvents tested. On addition to water, CNC formed a clear homogenous transparent gel solution, whereas Ac-CNC immediately precipitated and formed a clear white, floating jelly mass. Individual hydrophobic Ac-CNC crystals pulled each other apart in the aqueous environment, resulting in this turbid white precipitate. Due to replacement of hydroxyl groups in cellulose by hydrophobic esters, Ac-CNC was well-dispersed in most of the tested organic solvents, with no sedimentation observed over time. Ac-CNC formed a transparent solution in ethanol, acetone and ethyl acetate, where some degree of opacity was observed at AcCNC concentrations exceeding 1.5%. Clear transparent solutions of Ac-CNC were formed in both toluene and DMSO. When added to DMF, Ac-CNC formed a white flocculent and finally settled as a precipitate.


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Figure 4. HR-SEM images of cross-sections of dry films of CNC (a-c) and Ac-CNC (d-f), at different magnifications

SEM images of the cross sections of dry films showed that modified and unmodified cellulose nanocrystals exhibit a self-assembled arrangement in its dry form. Simple evaporation of the CNC and Ac-CNC suspensions leading to the self-assembly of cellulosic crystals and forms solid films that holds specific structural organization (Figure 4). The closely packed layer-by-layer arrangement was due to the self-assembly properties of liquid crystalline materials and antiparallel crystalline arrangement of the cellulose Iβ structure.26 SEM images of the crosssections were further reveals the mesostructure of CNC and Ac-CNC crystals with long range order. At moderate magnification, CNC and Ac-CNC indicates that the layers remain parallel to each other. But at high magnifications, Ac-CNC gives a pitch length and the preferential orientation of crystals in a specified direction (Figure 4f). The low magnification (50 µm) images of CNC and Ac-CNC proves that they are organized into a mesoscopic layers, where the CNCs are aligned parallel to each other, and rotate helically through the stack. This is an indication of the formation of liquid crystalline arrangement which is further confirmed by the polarised optical microscopic analysis. Ac-CNC forms highly flexible transparent films (Movie S1) upon drying the suspensions. AcCNC films demonstrated enhanced interlayer distances (Figure 4d-f), resulting from both the presence of bulky pendants of acetate esters in each cellulose crystal and reduced intermolecular 9 ACS Paragon Plus Environment


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interactions due to the replacement of hydroxyl groups of cellulose. The average interlayer distance between individual Ac-CNC layers was 50-150 nm, presenting cavities suitable for entrapping nano and micro particles (Figure S5). The hydrophobic transparent properties of the films, together with the presence of nanostructural interlayer cavities, render the Ac-CNC films a versatile biomaterial for many applications.27 Contact angle analysis of dry Ac-CNC films demonstrated static and dynamic sessile contact angles of 76.54 and 88.78°, respectively, indicating the exceptional hydrophobicity of Ac-CNC. The surface roughness of CNC and AcCNC films were calculated from the AFM measurement and gives 2.1 nm and 2.3 nm respectively. It is noteworthy that the initial shape of the drops on the surface of Ac-CNC films remained unchanged with time (Figure S6) contrary to the hydrophilic behaviour of the CNC films.17 Time dependence of the water contact angle on CNC and Ac-CNC surfaces were confirmed the hydrophobicity of Ac-CNC (Figure S7).

Figure 5. XRD analysis of CNC (black) and Ac-CNC (red) Structure and chemical analysis. The chemical structure of Ac-CNC was determined by solid 13

C CP-MAS NMR, FTIR, XRD and CHNS analyses, and confirmed the evidences of a high

degree of CNC modification alongside maintained basic cellulose I crystalline behaviour. The Xray diffraction patterns of CNC and Ac-CNC reflected the effect of chemical modification on the crystalline integrity of nanocrystals (Figure 5). CNC displayed characteristic cellulose I diffraction peaks of 2θ angles at 14.92o and 22.31o. This cellulose I structure of the CNC material 10 ACS Paragon Plus Environment

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was preserved in Ac-CNC (after the iodine catalysed esterification). It is noteworthy that the respective XRD peaks are broader and less intense than their corresponding peaks in the CNC counterpart14. Moreover, the absence of any crystalline transformation during the acetylation is also evident from the observed peaks of Ac-CNC. The peaks of Ac-CNC were contributed from the lateral spaces between the crystallites of modified CNC, generated by the acetylation of the glucopyranose rings which favors an increase in the interlayer distance of individual crystals. Thus decreased intensity and broadness of the peak of Ac-CNC at 22.31o was indicative of partial decrystallisation of CNC interlayers, resulting from the introduction of bulky acetate ester pendants on the structure of CNC. The cellulose I peaks of Ac-CNC was more evident from the Gaussian peak splitting analysis (Figure S8). Ac-CNC showed decreased cellulose I peak intensities due to the introduction of the bulky pendants in between the individual crystals. Overall crystallinity of Ac-CNC was contributed from the added crystalline arrangements of grafted ester pendants in additional to the cellulose I structure of the basic crystals. The additional arrangement of acetate esters are highlighted on 2θ angle at 3.44o and 8.4o. The position of the additional XRD peaks depends on the size of the substituents of the hydroxyl groups of cellulose.28 The crystallinity index (Ic) of CNC and Ac-CNC was calculated using the Newman peak separation method,29 by dividing the area of the crystalline peak by the total peak area assigned. Crystallinity index (Ic) of CNC (78.9%) and Ac-CNC (42.1%), further demonstrated the comparative crystalline integrity of the unmodified and modified cellulose nanocrystals. It would be beneficiary for many structural applications of Ac-CNC to preserve the crystalline nature of CNC, after high degree of esterification on the cellulose structure. For example, to act as a rigid stress-transfer reinforcing agent in nanocomposite materials, the crystalline structure of Ac-CNC should be preserved. In addition, morphological and crystalline integrity of the cellulose nanoparticles was maintained during esterification, in contrast to a previous report.11 Taken together, the TEM and XRD findings confirmed the fact that the acetyl ester substitution on the hydroxyls of CNC, in the iodine-catalysed reaction, did not disrupt the main cellulose structure.



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Figure 6. Solid 13C NMR (a) and FTIR (b) of CNC and Ac-CNC. The structure of Ac-CNC shown in the insets. When CNCs were modified with acid anhydride, using iodine as a catalyst, a high DS, manifested by the peak intensities of the acetate esters in solid

13

C NMR and FTIR spectra of

Ac-CNC was observed. The chemical shifts for the unmodified CNC can be assigned to C1 (105 ppm), C4 crystalline (89 ppm), C4 amorphous (84 ppm), C2/C3/C5 (72 and 75 ppm), C6 crystalline (65 ppm), and C6 amorphous (63 ppm) (Figure 7a). This basic cellulose structure of CNC was preserved after iodine catalysed high ester substitution (as discussed in XRD) which is obvious from the 13C NMR spectra of Ac-CNC. Moreover, additional prominent chemical shifts of carbonyl and methyl groups of acetyl esters at 172 ppm and 21 ppm, reflecting carbonyl and methyl groups of acetyl ester, respectively, were noted on the Ac-CNC spectrum. The split signal of C1 at 102 ppm and 105 ppm in the Ac-CNC spectrum is assigned to the α-type anomeric C1 carbon of the reducing end of the glucose residue in the cellulose I oligomers. In contrast to the split signal seen in CNC, the C2, C3 and C5 carbon atoms of Ac-CNC yielded a single peak at 73 ppm, due to the similar chemical shift environment of the ester-modified hydroxyls. A DS of 2.18 was calculated from the normalised

13

C NMR spectra for Ac-CNC, and was strongly

supported by CHNS elemental analysis (Figure S9). It is expected for a higher surface functionalisation of each CNC crystal (DSsurf) than in its inner core. We reported the overall DS of Ac-CNC, that thoroughly purified by soxhlet extraction and then cryo-grounded before solid mass 13C NMR and CHNS analyses.


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The change in CNC crystallinity following acetylation was also apparent from the diminished C4 crystalline (89 ppm) and C6 crystalline (65 ppm) peaks. The ester contribution to the C1 resonance was expressed at 105 ppm, where a broad, split and less intense peak was observed after the acetylation, as well as in the C6 area, where the Ac-CNC signals also became broad due to successful replacement of methyl hydroxyl groups by a corresponding C6 ester. The C2,3,5 region displayed a clearer evolution, illustrated by a steep enhancement in the intensity of the combined broad peak. Concomitantly, the C4 area underwent several modifications, including a decrease in the C4cryst and an increase in the C4amorph components.30 Taken together, a decrease of the native cellulose I signals was observed with the acetylation of CNC.27 FTIR spectra of CNC featured distinct hydroxyl peaks and enol formation of free hydroxyls, resulting from moisture absorption. The notable reduction in the intensity of the hydroxyl groups (3311 cm-1) and the introduction of ester (-COO-) groups (1736 cm-1) in the Ac-CNC were clearly evident in the FTIR spectrum (Figure 6b). The absorbance peak at 3311 cm-1 was ascribed to the stretching vibrations of three hydroxyl groups of cellulose, and at 1736 cm-1, to the stretching vibration of carbonyl esters (-COO-) of acetate functional groups. It is also interesting to note that the peak corresponding to intramolecular hydrogen bonding (1640 cm-1) arising from the interaction between substituted esters of same Ac-CNC crystals, as discussed in TEM analysis, was of considerable contribution to the Ac-CNC spectrum. The –CH bending (1352 cm-1) and C-O stretching (1220 cm-1 and 1026 cm-1) peaks were also prominent in AcCNC spectra. In summary, solid 13C NMR and FTIR findings demonstrated that iodine catalysed esterification of the surface of cellulose nanocrystals yielded substantially prominent acetate ester-substituted cellulose nanocrystals. Taken together, the NMR, FTIR and XRD analyses indicated that ester modification of CNCs resulted in good surface coverage without significantly compromising the crystallinity of the cellulose structure.



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Figure 7. POM images of (a) a CNC suspension, (b) dry CNC film, (c, d) Ac-CNC acetone suspension, (e, f) dry Ac-CNC film Optical properties. Optical properties of CNC and Ac-CNC were compared by POM analysis of their respective suspensions and dried films (Figure 7). Figure 7a shows the POM-viewed image of an aqueous 4% CNC suspension, which appeared as a grainy texture typical of dispersed chiral nematic phases of CNC crystals.30 The optical response of the liquid crystals involved in the reflection of variant colors from a continuously varying refractive index, due to the rotation of the birefringent CNC rods. Upon drying, this ordered phase starts to separate from the isotropic phase31 and the evaporation of CNC suspension led to the formation of oriented nanocrystals in the dried films as discussed in the SEM analysis of the cross-sections. Thus the liquid crystalline behaviour with well-defined arrangement of chiral nematic order in micro molecular level was much more obvious in the dried films of CNC32 (Figure 7b). As discussed in SEM analysis, it is interesting to note that the iodine catalysed acetate esterification preserved the optical activities of birefringence domains which is clear from the POM analysis on the acetone dispersion (4%) of Ac-CNC with formation of similar liquid crystalline phases (Figure 7c,d). Figure 7c and 7e are the cross polarised images of the suspension and dry film of Ac-CNC respectively which gives the intensity of the liquid crystalline domains. Figure 7d and 7f are the respective color images which gives the orientation and the direction of the crystals in the liquid crystalline phase. The preferential mean orientation of CNC and Ac-CNC crystals can be given by the particular color of the birefringent domains (Figure 7d,f).33 Liquid crystalline phases seen in Ac-CNC are from the interference generated by the birefringence of the modified cellulose crystal clusters. During drying, the rearrangements of liquid crystalline Ac-CNC phase into a regular order were not predominant. We presumed that the birefringence domains of Ac-CNC are nothing but a cholesteric liquid crystalline arrangement34 and the long range crystal 14 ACS Paragon Plus Environment

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anisotropy was absent in the dry films of Ac-CNC (Figure 7e, f). Birefringence of cellulose crystals largely depended on thickness and the mean refractive index gave a more complex color pattern if the concentration of the Ac-CNC increased. In conclusion, Ac-CNC crystals possessed cholesteric liquid crystalline domains rather than a chiral nematic arrangement typical of CNC samples. The birefringent domains in Ac-CNC films suggests that the modified cellulose crystals were dispersed in organic solvents as individual crystallites and aggregated into cholesteric clusters at high concentration without forming a long range crystal anisotropy which is predominant in CNC films.

Figure 8. TGA and DTG of CNC and Ac-CNC Thermal degradation analysis. Thermal analyses of CNC and Ac-CNC and the corresponding derivatives of the weight loss curves revealed different degradation profiles (Figure 8). The thermal degradation of CNC consisted of concurrent cellulose degradation processes, such as dehydration, depolymerisation, and decomposition of glycosyl units, followed by the formation of a charred residue. The initially small weight loss of unmodified CNC in the temperature range of 100-150oC, can be attributed to the evaporation of physically bound moisture due to the hydrophilicity of CNC. The main peaks corresponding to the thermal degradation of CNC were observed between 240-280oC, and a small wide dip was observed between 310-450oC. CNC lost nearly 35% of its mass in the 150–280◦C range, followed by another 35% mass loss at temperatures between 300-500°C, leaving significantly high (nearly 30%) residue at 500°C. 15 ACS Paragon Plus Environment


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Traces of the sulfate ion present in the CNC samples shifted its thermal decomposition to lower temperatures. Thermal analysis of Ac-CNC displayed a wide range of single degradation peaks in the 290-400oC range, higher thermal stability than CNC with a decomposition onset at 298°C, as opposed to 255°C for CNC (16.8 % enhancement in thermal stability). The traces of sulfate ions removed during ester substitution (Figure S9), alongside the long pendant alkyl groups, prevented early thermal degradation of the Ac-CNC cellulose structure. The additional oxygen content in Ac-CNC, introduced by the acetate esters, enhanced thermal burning at temperatures above 400o (thermal analysis done in N2), yielding noticeably less char residue than unmodified CNC. Taken together, the crystalline hydrophobic behaviour and increased thermal stability could be made this material usable for industrial applications, including as a reinforcing agent in nanocomposites, providing a 16% extension of processing and use temperature of the product material.

Figure 9. Tensile stress-strain properties of Ac-CNC-reinforced epoxy nanocomposites

Epoxy/Ac-CNC nanocomposites: Epoxy nanocomposites, generated by reinforcing the prepared EPON (828) resin with Ac-CNC, using Epikure (3140) as a curing agent, were fabricated to demonstrate the industrial application of Ac-CNC as a filler material in polymer nanocomposites. The reinforcing ability of Ac-CNC with epoxy resin was measured by analysing the Ac-CNC/matrix interface and tensile mechanical properties of the resultant nanocomposites. Dispersion of unmodified CNC in a polymeric system is very difficult due to its 16 ACS Paragon Plus Environment

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high hydrophilicity (Figure S4a).35 However, Ac-CNC easily compounded with and dispersed within epoxy-like matrices in acetone solution (Figure S2). Ac-CNC content in the resin was systematically increased from 0.01 to 1 wt. %, to verify the extensive effect of reinforcement. Tensile mechanical analysis of dog bone-shaped epoxy/Ac-CNC nanocomposite samples (Figure S10) revealed that Ac-CNC had a remarkable reinforcing effect on the tensile strength and modulus of epoxy nanocomposites (Figure 9). The ultimate tensile strength and modulus clearly increased with increasing Ac-CNC content. The high strength and modulus of composites was due to the combination effect of nanodispersion of Ac-CNs in the pre-cured soft EPON matrix, high stress transfer capacity of crystalline Ac-CNC and intimate interaction between highly hydrophobic Ac-CNC crystals and the cross-linked epoxy network. However, as Ac-CNC concentrations rose >1%, the nanocomposite samples exhibited decreased strain elongation, mechanical strength and modulus, due to lack of sufficient wetting and stress transfer between the epoxy matrix and Ac-CNC.36 Above 1 % of Ac-CNC, filler-filler interactions were increased, rendering them resistant to strain elongation of the nanocomposites. The tensile strength and modulus of neat epoxy resin was 26.15±1 MPa and 836±12 MPa, respectively. Introduction of 0.5 wt.% Ac-CNC into the epoxy matrix led to a two-fold enhancement of these mechanical properties of the nanocomposites to 45.22±1.2 MPa and 1674 ±24 MPa, respectively. The orientation of Ac-CNCs against the tensile pulling direction (Figure 10b-d), driven by its strong filler-matrix interaction, contributed to the dramatic enhancement in the mechanical strength and modulus of the product. When assessing the overall mechanical properties of the studied nanocomposites (Table S1), it is obvious that Ac-CNCs displayed a strong reinforcing effect and may have even induced a synergistic effect on the mechanical performance of the Ac-CNC particles along with the strong matrix-filler interaction. In summary, tensile tests revealed that the Ac-CNC-reinforced nanocomposites possess both higher tensile strength and modulus, together with competent strain-to failure properties compared to neat epoxy resin and to various conventional epoxy nanocomposites reinforced with cellulose nanocrystals35, carbon nanotubes37 or nanoclays.38 The mixing of Ac-CNC in the EPON matrix before adding Epikure, facilitated the homogenous nano dispersion of the former in the epoxy resin (Figure S2). Comparison of the SEM images of the fractured surfaces of the neat epoxy matrix (Figure 10a) versus of the 0.5% Ac-CNC/epoxy nanocomposites (Figure 10b) clearly showed that Ac-CNC crystals/clusters were individually 17 ACS Paragon Plus Environment


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distributed in the matrix (Figure 10c), with no phase separation between the crystals and epoxy resin (Figure 10d). SEM micrographs of these horizontal parts of the tensile fracture surfaces proved that the interface between the filler and matrix was continuous and the Ac-CNC crystals were in a fitting attachment with the epoxy matrix, probably though the hydrophobic ester pendants and amino groups of cured epoxy resin.

Figure 10. SEM images of the fractured surfaces of (a) neat epoxy matrix, (b, c) 0.5 % Ac-CNCreinforced epoxy nanocomposites, at different magnifications, (d) Ac-CNC crystals (arrows) in epoxy matrix without phase separation. (e) 3500-2000 cm-1 FTIR absorption peak area and (f) proposed interaction between epoxy and Ac-CNC during curing and nanocomposite formation.

FTIR analysis of the cured epoxy/Ac-CNC nanocomposites (Figure S11) provided information about the possible interaction between Ac-CNC and the epoxy matrix, whose amine groups form strong hydrogen bonds with the carbonyl oxygen of acetate esters. The FTIR-detected intermolecular hydrogen bonds between carbonyl carbon and polymer amides have been extensively discussed in the literature.39,40 The small peak observed at 1720 cm-1 in nanocomposite analyses was ascribed to the Ac-CNC esters. The enhanced broadened and intense peak at 1640 cm-1 originated from the interaction between the carbonyl oxygen of the 18 ACS Paragon Plus Environment

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acetate esters and the amine group of the cured epoxy.39,40 It has been suggested that the O-H stretching bands of polymer composites at 3400-3440 cm-1 are unbound, while those at 32803340 cm-1 are related to hydroxyls that are hydrogen bonded to amino groups. There was a marked enhancement in the intensity of the stretching absorption of this peak at 3300 cm-1 for the epoxy/Ac-CNC nanocomposites (Figure 10e).

In addition, the obvious shift of the stretching

absorption band of hydrogen bonded amino group from 3400 cm-1 to the lower wavelength at 3280 cm-1 in the case of epoxy/Ac-CNC nanocomposite samples, confirmed the strong fillermatrix interaction between the reinforced Ac-CNC and epoxy resin. The absorption bands of neat epoxy at 3400 cm-1 were assigned to non-hydrogen-bonded N-H, and the corresponding shifted broad band of Ac-CNC-reinforced epoxy nanocomposite at 3300-3330 cm-1 was assigned to internally hydrogen bonded N-H. The reinforcement of Ac-CNC crystals in between the polymer molecule was also clear from the marked enhancement in the C-H stretching bands of nanocomposites at 2920 cm-1. Enhanced intensity of C-H stretching bands at 2920cm-1 of the nanocomposites contributed from the individually dispersed Ac-CNC crystals in the epoxy matrix. Strong bonding interaction between of the cured epoxy and Ac-CNC crystals resulted in an epoxy nanocomposite with enhanced mechanical strength. The tensile mechanical properties of the nanocomposites further demonstrated formation of hard epoxy microdomains that were bound together and specifically associated via strong interactions between the epoxy and AcCNC crystals. Taken together, dispersibility analyses, tensile testing, SEM of the fracture surface and the FTIR analysis of the epoxy/Ac-CNC nanocomposites, suggest an interaction mechanism between epoxy amino group and carbonyl oxygen from Ac-CNC crystals and cured epoxy (Figure 10f).

CONCLUSIONS Highly esterified CNC materials were prepared via an iodine-catalyzed, single-step, solvent-free esterification reaction. Structure and chemical analyses of the modified CNC were performed using advanced characterisation techniques. TEM and AFM demonstrated the surface modifications on the individual CNC crystallites. FTIR and normalised NMR spectra quantified the replacement of hydroxyls of cellulose with an acetate ester, at a DS of 2.18. Dispersibility and contact angle measurements confirmed the hydrophobicity of Ac-CNC. XRD spectra confirmed the cellulose I crystalline structure of Ac-CNC, with a new order of crystallites contributed by the substituted ester pendants. Optically active birefringence cholesteric domains 19 ACS Paragon Plus Environment


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of the Ac-CNC crystals in solution and dry form were confirmed by SEM and POM. TGA analyses proved the substantial increase (16.8%) in the thermal stability of Ac-CNC. Ac-CNC forms thin films with average interlayer distance of 50-150 nm, presenting cavities suitable for entrapping nano and micro particles which render this biomaterial as a hydrophobic drug carrier18 in medical applications. The outstanding reinforcing effect of low concentrations of AcCNC in epoxy matrix (73% increase in tensile strength and 98% increase in modulus) demonstrated its advanced reinforcing capability with effective stress transfer properties in high performance polymeric nanocomposites. In conclusion, the adopted esterification method and functionalised bio-based nanomaterial is suitable for a range of industrial and medical applications.

ASSOCIATED CONTENT Supporting Information Physical appearance of CNC during ester modification, dispersion of CNC and Ac-CNC in epoxy, dispersibility analysis with different solvents, curcumin encapsulation and dry film analysis, contact angle measurements of Ac-CNC films, XRD peak splitting of CNC and AcCNC, CHNS elemental analysis, dog-bone nanocomposite samples, FTIR and mechanical properties of epoxy/Ac-CNC nanocomposites are presented. Movie S1, Flexibility of the Ac-CNC films (AVI)

AUTHOR INFORMATION †

Corresponding author: Prof. Oded Shoseyov; R.H. Smith Institute of Plant Sciences and

Genetics, The Hebrew University of Jerusalem, Israel, Rehovot. Tel: 972-8-9489084 Fax: 972-89462283, email: [email protected]

ACKNOWLEDGEMENTS We acknowledge the financial support of Melodea Ltd, MINERVA Center Bio-Hybrid Complex Systems, PBC fellowship and Israel National Nanotechnology Initiative (INNI).

REFERENCES


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Optically active hydrophobic cellulose nanocrystals 251x113mm (72 x 72 DPI)

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Highly Modified Cellulose Nanocrystals and Formation of Epoxy

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