Research Article pubs.acs.org/journal/ascecg
Highly Hydrophobic Thermally Stable Liquid Crystalline Cellulosic Nanomaterials Eldho Abraham,† Yuval Nevo,† Rikard Slattegard,‡ Noam Attias,† Sigal Sharon,† Shaul Lapidot,‡ and 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, Rehovot 76100, Israel ‡ Melodea Ltd, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel S Supporting Information *
ABSTRACT: Highly hydrophobic cellulosic nanomaterials were prepared via iodine-catalyzed butyrate esterification of cellulose nanocrystals (CNC). The structure and properties of butyrated cellulose nanocrystals (Bu-CNC) were investigated via advanced spectroscopic, morphological, optical, thermal, contact angle, and coating analyses. Bu-CNC retained cellulose crystallinity, was hydrophobic with a static contact angle of 81.54° and displayed 18.5% enhancement in its thermal stability. Moreover, Bu-CNC possessed a solid multilamellar cellulose II structure and showed liquid crystalline behavior over a wide range of temperatures. Bu-CNC formed transparent flexible films upon drying and was easily dispersible in ethanol and acetone. As a thermally stable hydrophobic liquid crystalline biobased material, Bu-CNC presents a new class of nanomaterial, which potentially suits various industrial and medical applications. KEYWORDS: CNC, Butyration, Hydrophobicity, Birefringence, Cellulose liquid crystals
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INTRODUCTION Cellulose is the most abundant biopolymer on earth and has traditionally been used for thousands of years in many applications. The most complex form of cellulose in nature is in the plant cell wall, where it appears as a composite with other polysaccharides, such as hemicellulose, pectin and lignin.1 These polymer composites, ordered in unique architectures, support high load transfer when cells are subjected to mechanical stress.2 The crystalline form of cellulose is the principal load-bearing component and hence, is a promising potential renewable material for various applications in the current environmental scenario.3 Aqueous suspensions of cellulose nanocrystals (CNC) were prepared by H2SO4-driven (64%) hydrolysis of the plant biomass. CNC is renewable and biodegradable, with potential applications in the field of cellulose nanotechnology, features a high surface area and reactive surface groups. It exhibits interesting characteristics of birefringence and chiral nematic properties.4 One of the major drawbacks of CNC is its highly hydrophilic nature, which limits its applications. There are many techniques © 2016 American Chemical Society
in the literature for inducing CNC hydrophobicity, but most of them are multistep and inefficient or fail to secure strong covalent bonding between the hydroxyls of CNC and modified surface groups.5,6 Numerous approaches involve esterification of CNC and most of these reactions affect cellulose chains localized at the crystal surface only, and are thus limited to the very few peripheral hydroxyl groups.3 Hence, the degree of substitution (DS) of CNC modifications reported in the literature fall within the range of 0.01 and 1.7 Esterification reactions of CNC with organic fatty acid chlorides of different sizes, have also been reported, but the maximum DS was less than 0.5.8 Esterification of cellulose via a solvent-free, gas-phase treatment, was both limited to microfibrils, and left the inner hydroxyl groups of cellulosic clusters unmodified.9 Simultaneous cellulose hydrolysis and hydroxyl group acetylation in a one-step acid-catalyzed reaction was established as a green Received: October 23, 2015 Revised: December 29, 2015 Published: January 4, 2016 1338
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic presentation of CNC butyration. Source of the cellulose (a), waste paper macrofibril cellulose pulp (b), transmission electron microscopy (TEM) of CNC (c), structure of cellulose (d), esterification mechanism (e), centrifuged pellets before and after butyration (f), centrifuged pellet of Bu-CNC after washing (g) and Bu-CNC smectic liquid crystals (h). CNC was solvent exchanged from aqueous to Bu2O (Figure 1f) and heated to a temperature of 105−110 °C for 1 h (0.02 g/mL CNC in Bu2O), before the catalyst (0.0012 g/mL I2 in Bu2O) was added. The flasks were maintained for 30 min in an oil bath. The reaction was then terminated by adding acetone to the brown-colored reaction mixture. Figure 1e outlines the mechanism of the iodine-catalyzed butyrate esterification of CNC by butyric anhydride. The modified CNC samples were washed with acetone, to remove iodine traces and unreacted anhydride, if any, until a clear transparent jelly mass of butyrated CNC (Bu-CNC) in acetone was obtained (Figure 1g). The usual mass yield of the Bu-CNC was 68% of the starting mass of CNC. The initial step of the esterification 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 butyric acid to generate the ester.17 Cellulose can be regarded as a polyalcohol, and iodine as a Lewis acid catalyst. Catalyst iodine activates one of the carbonyl carbons of Bu2O, which in turn, protonates the nearby hydroxyls of cellulose and eventually leads to butyric esterification of CNC. Bu-CNC can disperse in acetone and ethanol and forms a transparent viscous gel-like material (Figure 1g) with liquid multilamellar structures. Bu-CNC in acetone forms clear transparent films when subjected to controlled drying at a constant temperature (40 °C). Characterization Methods. Determination of sulfate ester content in the prepared CNC was performed by conductometric titration. CNC in an aqueous suspension was combined with NaCl and titrated against dilute NaOH.18The size of the elementary particles was determined by atomic force microscopy (AFM) and TEM. AFM analysis were conducted in the tapping mode (Scanning Probe Microscope, Dimension 3100 Nanoscope V from Veeco/Bruker, Santa Barbara CA, USA) and with a confocal laser scanning microscope (CLSM; Leica TCS SP5, Confocal microscope from Leica Microsystems, Mannheim, Germany). For TEM, drops (5 μL) were deposited on glow-discharged carbon-coated copper grids and blotted 1 min thereafter. The grids were negatively stained with 2% uranylacetate (incubation time 30 s) and observed with a Tecnai-12 (Philips) TEM operated at 120 kV. The specimens were observed under lowdose conditions. Images were digitally captured with a TVIPSF224 CCD camera. Thin films of CNC and Bu-CNC were prepared by controlled drying of their respective suspensions (2 wt %) in glass Petri dishes at a constant oven temperature (40 °C). FTIR analysis of
alternative to commonly employed reactions. 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 DS of 0.91.10 Hence, to date, most of the applied CNC modification methods designed to form a stable hydrophobic cellulosic nanomaterial, are inadequate and unsatisfactory.11 In this study, we present an esterification method of CNC via butyric anhydride (Bu2O) with iodine as a catalyst, to yield a highly hydrophobic liquid crystalline cellulosic nanomaterial. Bu2O is an acid anhydride that disperses CNC, and functions both as a solvent and reactant for efficient esterification of CNC. Esterification of CNC by butyration, without a cosolvent, is appreciable, because solvents lower the rate of the reaction and promote the solubility of the modified cellulosic crystals and destroy the crystalline structure of the CNC, thereby compromising its mechanical and spectral properties. The prepared Bu-CNC is a promising nanofiller candidate in both conventional and optically active polymer nanocomposites12 because it possesses exceptional hydrophobicity along with liquid crystalline properties at polymer processing temperatures (25−200 °C). Bu-CNC can also be integrated in hightemperature transparent hydrophobic coatings,13 birefringent thin-film polarizers,14 high temperature biosensors and bioimaging.15 The hydrophobic CNC films form nanostructured interlayer networks, which can hold nonionized drugs that normally would not associate with unmodified CNC, and is a promising biobased nanomaterial in drug delivery applications.16
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EXPERIMENTAL SECTION
Materials and Methods. CNC was prepared by H2SO4-driven (64%) hydrolysis of waste paper pulp, which was made from bamboo fibers (Figure 1a).3 The butyric anhydride, acetone and catalyst iodine (Sigma-Aldrich) were of analytical grade and used without further purification. 1339
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
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Figure 2. AFM (a) and TEM (b,c) of Bu-CNC.
Figure 3. FTIR spectra of CNC and Bu-CNC, magnification of −OH stretching (a) and the 1800−900 cm−1 spectral region (b). contact angles of sessile drops of water on the films were measured at room temperature with an OCA20 automated and software-controlled Video-Based Contact Angle Meter (Data Physics Instruments GmbH, Filderstadt, Germany). The contact angle acquisition was performed using a static and dynamic sessile method during the first 60s after deposition. Thermogravimetric analysis of the films was carried out using a TGA 7 (PerkinElmer, USA) analyzer, under nitrogen atmosphere. The weight of the film samples varied from 6 to 8 mg, scanning range was 50−500 °C, and the heating rate was 10 °C/min. Microstructural analysis of the Bu-CNC films was performed using a scanning electron microscope (SEM) (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).
CNC and Bu-CNC was performed using a Thermo Scientific Nicolet 6700 spectrometer in the absorption mode (100 scans at a nominal 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 CNC and Bu-CNC. Solidstate 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. 13C CPMAS experiments were carried out at a spinning rate of 8 kHz, using a 2.5 ms 1H 90, 2 K data points, 720 scans and a 2 ms ramped-cp period. Proton decoupling using the SPINAL composite pulse sequence at a field of 100 kHz, was used during acquisition and a 3 s recycle delay between acquisitions. Chemical shifts were given with respect to adamantane (38.55, 29.497 ppm). Xray diffraction (XRD) analysis of CNC and Bu-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°. The peaks and separation of the specific peaks of cellulose I and II were analyzed by Originlab 7.5 data analysis and graphics software. The diffraction peaks were fitted with pseudovoigt peak functions of Lorentzian components. 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. Polarized optical microscopy (POM) analyses were conducted on 4 wt % aqueous suspension of CNC, 2 wt % acetone suspension and dry films of CNC and Bu-CNC (200 μmthick), which were prepared by evaporating the solvent in a Petri dish with the help of light shearing force of nitrogen gas. Images were captured with a Nikon DS Camera control unit DS-U2 on a Nikon Polarizing Microscope Eclipse LV100POL. The samples for contact angle measurements were prepared by oven drying of CNC (4 wt % in water) and Bu-CNC (2 wt % in acetone) dispersions to yield a 200 μm-thick film. Dispersions were poured into Petri dishes and dried at a constant temperature (40 °C) for 48 h. The measurements were performed by depositing 5 μL of water droplets on the surface of the films, and angles obtained were recorded with a CCD camera. The
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RESULTS AND DISCUSSION
Structural Morphology of Dispersed Bu-CNC. Hydrolysis of the waste paper pulp resulted in water-dispersed CNC, with a sulfur content of 0.34 mmol sulfate half-ester per 1 g cellulose. Figure 2a shows an AFM image of Bu-CNC, containing modified cellulosic nanocrystals of 400−1500 nm in length and 10−40 nm in width. Bu-CNC crystals were relatively uniform in size, with an aspect ratio varying from 20 to 60. Figure 2b,c shows TEM images of Bu-CNC crystals, which resemble the physical appearance of the crystals seen in the AFM images. AFM revealed that Bu-CNC was individualized but they are bundled rod-like nanocrystals, as clearly evident from the TEM images. The bundling of the Bu-CNC can be due to intramolecular hydrogen bonds of the modified nanocrystals, or the remnants of a small amount of unmodified paracrystalline cellulose molecules that hold the rods together. Bu-CNC had nonuniform crystalline dimensions, due to the bulky pendant substitution, unlike the perfect crystalline nature of unmodified CNC (Figure 1c). 1340
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interactions (e.g., hydrogen bonding) of the COC group. COC group is a proton acceptor and can form hydrogen bonds with adjacent proton donors such as hydroxyl groups.22 The perturbed COC stretching subsequently caused the downshift of the CH2 rocking peak at 1316 cm−1 and was due to the antiparallel arrangement of cellulose II.21 The hydroxyl density of Bu-CNC, yielding a moderate peak at 3400 cm−1, is evidence of unmodified inner crystalline domains in CNC clusters.23,24 Thus, it is evident from the substantial lowering of hydroxyl absorption (3326 and 1636 cm−1) and the marginal increment in the ester CO stretching (1737 cm−1) of Bu-CNC, that butyration of CNC was not limited in its peripheral hydroxyls, but to the inner crystalline domains of individual cellulose crystals. Figure 4 shows the normalized CP-MAS 13C NMR spectra of CNC and Bu-CNC. In the 13C NMR spectrum of CNC, the
Nanocrystals of CNC arranged in their lattice layers and were bonded to one another by intermolecular hydrogen bonds between the hydroxyls of carbon atoms C6 and C3 in the crystal lattice of cellulose Iβ allomorph.3 They were arranged in parallel, forming a chiral nematic phase and crystallite strands, enabled by secondary valence hydrogen bonds of hydroxyl groups.3−5 AFM and TEM analysis of Bu-CNC demonstrated the crystalline structure of modified cellulose crystallites, despite reduced density of the secondary hydrogen bonds of hydroxyl groups, which is predominant in CNC. The insertion of long bulky butyric groups, replacing peripheral hydroxyl groups, introduced a steric effect and increased the distance between the interlayers of individual crystals of Bu-CNC. The crystalline arrangement of CNC was changed by esterification with butyric groups and new CNC crystals, Bu-CNCs, were formed. Extent of Esterification and Crystallinity. Butyric esterification induced changes in the crystalline structure of native cellulose I of CNC (Figure 3), along with changes in absorption bands of CNC, due to ester substitution. The intramolecular hydrogen bonds in CNC between C2-OH and O-6 and between C3-OH and O-5, and the intermolecular hydrogen bonds between C6-OH and O-3′ in cellulose I allomorph, are manifested at 3420, 3350, and 3270 cm−1, respectively, as a combined broad peak at 3326 cm−1, and by valence vibration of H-bonded hydroxyl groups at 3570−3450 cm−1 (Figure 3a). In contrast, the FTIR spectra of Bu-CNC exhibited a notable reduction in the intensity of the hydroxyl groups (3326 cm−1) and evidence of the introduction of the ester (−COO−) group (high intense peak at 1737 cm−1) in the structure of CNC (Figure 3b). This O−H stretching peak was shifted to a higher wavenumber (3326 → 3473 cm−1) after esterification, which can be ascribed to the strong intramolecular hydrogen bond between C2-OH and the O-6 ester of cellulose II allomorph. The conversion of cellulose I to cellulose II upon esterification is thoroughly discussed in the literature.18−21The intramolecular hydrogen bond between C2OH and O-6 ester was due to the antiparallel arrangement of cellulose II. The C6 methyl hydroxyl group is more susceptible to esterification than C2 and C3 hydroxyls. Hence, most of the intermolecular hydrogen bonds between C6-OH and O-3′ were diminished due to the esterification of the methoxyl groups, which reduced intermolecular interactions. Intensity of the cellulose I absorbance peaks was decreased by esterification, whereas a new band appeared at 3211 cm−1, relating to an intermolecular hydrogen bond between C2-OH and O-2′ ester and/or an intermolecular hydrogen bond of unmodified C6-OH and O-2′ ester in Bu-CNC18 (Figure 3a). The marked intensity of the carbonyl peak at 1737 cm−1 in the Bu-CNC spectrum demonstrated its high DS (Figure 3b). The peak at 1636 cm−1, corresponding to hydroxyl bending, was lower for Bu-CNC because of this ester substitution. The 1420 cm−1 band was characteristic of cellulose II,21 whereas the 1450 cm−1 contained a deformation vibration, often called a scissoring mode, of the CH2 group on C6, likely to be contributed by the enhanced hydrogen bond strength of stable cellulose II components of Bu-CNC. The CH bending bands (1370 cm−1) were widened and CH2 rocking vibration at C6 (1316 cm−1) decreased due to esterification. The COC stretching peak of cyclic oxygen (1111 cm−1) was diminished in the absorbance spectrum of Bu-CNC, due to the strong constraints of butyric groups. The intensity of this peak was influenced by the crystallinity and the intermolecular
Figure 4. Solid-state 13C NMR of CNC and Bu-CNC.
typical signals of cellulose were assigned to C1 (105 ppm), C4cryst (89 ppm), C4amorph (83.7 ppm), C2/C3/C5 (75.3/73.1/ 71.6 ppm, respectively), C6cryst (65.5 ppm), and C6amorph (63.3 ppm). The spectrum exhibited a pattern of a relatively high degree of crystallinity, with cellulose I exhibiting clear doublets for the resonances at the C1, C4, and C6 positions, as described in the literature.9 The shoulder signals, at around 63.3 ppm, and the small broad signal, at around 83.7 ppm, arise from the C6 and C4 carbons in the amorphous region, respectively. The characteristic second peak of the C4 carbon atom (C4cryst) showed both an upfield side (89 ppm) and the first peak (C4amorph) in the downfield side (83.7 ppm), which proves that CNCs were principally arranged in cellulose Iβ form. After esterification, several new resonances appeared on the spectrum, in addition to these six cellulosic carbon signals. The signal at 172 ppm can be unambiguously assigned to the resonance of the carbonyl peak of the butyl ester, whereas the prominent peak at 36 ppm can be assigned to the methylene next to the carbonyl, at 19 ppm to the methylene near the terminal methyl and at 13.8 ppm to the terminal methyl carbons of the substituted butyric groups. The relatively high intensity of these four prominent resonances confirms that the cellulose hydroxyls were well esterified with butyric groups. The decrease in the intensity of the signals from the crystalline and amorphous C4 and C6, and the overlapping signals from C2, 1341
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
Research Article
ACS Sustainable Chemistry & Engineering C3, and C5 in the Bu-CNC spectrum indicated a change in the structural arrangement of cellulose crystals. 13 C NMR spectra provided some additional information regarding DS of esterification, crystallinity, and crystal orientation. Crystalline and disordered components of cellulose I and II were detected in solid-state 13C NMR spectra as downfield and upfield signals for the C4 or C6 carbons.25 A clear difference was observed between the upfield signal position of C6, i.e., 65.5 ppm for cellulose I in CNC, and the corresponding broadened 13C chemical shift of C6 of Bu-CNC, i.e., 63.3 ppm for cellulose II.26 Both signals merged to yield a broad signal, which proved the more or less equal presence of cellulose I and II in Bu-CNC. The partial conversion of cellulose I to II upon butyric esterification, discussed in regards to the FTIR spectra, is much clearer from 13C NMR. The C2, C3, and C5 carbon atoms of Bu-CNC were antiparallel in cellulose II, with an environment similar to that of cellulose I, and gave a single peak at 72.7 ppm, instead of the split signal seen in CNC. The signal at 101 and 108.3 ppm in the Bu-CNC spectrum is ascribed to highly crystalline cellulose II, which is assigned to the α-type anomeric C1 carbon of the reducing end of the glucose residue in the cellulose oligomers. The crystallinity index (CI) of CNC and Bu-CNC was calculated using the peak separation method of Newman,27 by dividing the area of the crystalline peak by the total area assigned to the C4 peaks. The CI of CNC and of Bu-CNC was 78% and 44%, respectively. But this is the contribution based on C4 carbon only; Bu-CNC possesses additional side chain crystalline arrangements of substituted butyrate groups. The ester contribution to the C1 resonance was expressed at 105 ppm, where a broad and intense peak was observed after the butyration, as well as in the C6 area, where the Bu-CNC signals also became broad due to successful replacement of methyl hydroxyl groups by a corresponding C6 ester. The C2,3,5 regions 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.28 Taken together, a progressive decrease of the native cellulose I signals was observed, alongside a persistent increase in the butyration of CNC, which is in line with previous studies.25 Normalization of the NMR spectra27,28 revealed that Bu-CNC possesses a DS of 2.1, a determination supported by CHNS elemental analysis of CNC and Bu-CNC (Figure S1, ESI). The XRD pattern of CNC contained three 2θ peaks at 14.98°, 22.21°, and 34.7°, (Figure 5), which can be attributed to the cellulose Iβ structure.9 The XRD profile of Bu-CNC contained broader and similar crystalline peaks, along with additional peaks at 3.2°, 6.5°, 12.1°, 16.2°, and 19.2°. The XRD confirmed that both CNC and Bu-CNC were crystalline in nature and that the Bu-CNC possesses additional side chain crystalline arrangements. The intensity of the 22.2° peak of BuCNC was lower and broader than the corresponding CNC peak, indicative of partial decrystallization of CNC interlayers, resulting from the introduction of bulky butyric pendants on the CNC structure. Peaks at 3.2° and 6.5° in the Bu-CNC arose from the additional side chain crystalline arrangements of the grafted butyric groups. The position of these peaks, as reported by Heritage et al., depends on the size of the substituent of the hydroxyl groups of cellulose.30 The peaks were attributed to the lateral spaces between the crystallites of CNC, generated by the functionalization of the glucopyranose rings, which favors an
Figure 5. XRD spectra of CNC and Bu-CNC.
increase in the interlayer distance of individual Bu-CNC crystals. Another important finding from the XRD pattern of Bu-CNC was the partial crystalline transformation of the cellulose I allomorph of unmodified CNC to cellulose II, which is discussed in the FTIR and NMR analyses. The peaks observed at 12.1°, 16.2°, and 19.2° in the Bu-CNC spectrum, correspond to cellulose II allomorphic crystallographic planes.31 At the same time, cellulose I peaks of 14.98°, 22.37°, and 34.1° were also observed. The peak separation of cellulose I and cellulose II in CNC and Bu-CNC was also analyzed (Figure S2, ESI). Thus, it is evident that Bu-CNC is a mixture of cellulose I and II, with additional crystalline orientation along the crystallographic (C) axis. Some segments of Bu-CNC (including unmodified domains) retained their cellulose I identity, while other segments were converted to cellulose II, which is a more thermodynamically stable allomorph. Fundamentally, CNC forms a chiral nematic crystalline phase above a specific concentration (>3 wt %).3,4 Esterification first occurs in the peripheral regions of the crystal clusters followed by inner crystalline domains. As a result, some of the hydrogenbonded CNC crystal clusters opened, disrupting their closely packed arrangement. The unmodified domains of Bu-CNC crystals retained the A-type allomorph cellulose I nature of CNC, which gave peaks of 2θ at 14.98° and 22.37°. The bulky substituted cellulose nanocrystals32−34 and hydrophobic liquid crystals35 formed smectic order. Thus, the XRD results are in accordance with AFM, TEM, FTIR and 13C NMR findings. The main crystal structure is affected by the constraints of butyric groups attached to its surface, while good surface crystallization and degree of modification are achieved. Hydrophobicity. Hydrophobicity of Bu-CNC was characterized by analyzing dispersibility in organic solvents, mung bean seeds coating and water contact angle measurements. The dispersibility of CNC and Bu-CNC with 2 wt % in water, alcohol, and acetone was analyzed with the help of sonication (Figure 6a). CNC formed a clear transparent suspension in water, but its hydrophilicity and agglomeration was clear in acetone and ethanol, in which it formed a turbid dispersion. Interestingly, Bu-CNC formed a turbid flocculent precipitate in water and finally formed a dry, white, spongy material that floated over the water surface (Figure 6a). The white spongy Bu-CNC precipitated in water (Figure 6c) and displayed a more hydrophobic nature than the smooth film (Figure 6e) made by drying an acetone dispersion of Bu-CNC. The hydrophobicity of Bu-CNC was also evident from its dispersion in acetone and ethanol, in which it formed a transparent 1342
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
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Thermal Degradation Behavior. Characterization of the thermal degradation of CNC and Bu-CNC, via thermogravimetric analysis (TGA), and of their differential thermal derivatives (DTG), showed significant differences in their decomposition profiles (Figure 7). Onset of its first
Figure 7. TGA and DTG analysis of CNC and Bu-CNC. Figure 6. Dispersibility of CNC and Bu-CNC in water, acetone and ethanol solvents (2 wt %). (a) Water swelling analysis of CNC- and Bu-CNC-coated mung bean seeds over time (b), highly hydrophobic behavior of water-precipitated spongy Bu-CNC (c), transparency and smoothness of dried Bu-CNC film (d), and water drop on the top of Bu-CNC film (e).
decomposition of CNC was apparent at 255.7 °C, where the crystalline regions begin to undergo destruction leading to decrystallization followed by depolymerization. The second event began at 274 °C, corresponding to the complete destruction of the crystalline region, while the cellulose decomposed into a monomer of D-glucopyranose, which was further decomposed to lower molecular weight gaseous products and char residue. The sulfate ester present in the CNC plays a catalyst role, shifting the threshold for thermal decomposition to a lower temperature and facilitating higher char residue (35%).36 Bu-CNC decomposition took place within the temperature range of 300−400 °C. Bu-CNC has a significantly higher (18.5%) first thermal decomposition temperature, when compared with its pristine unmodified CNC. The higher thermal stability of Bu-CNC was solely from the modification and depended on the crystallinity, structure and DS of butyrate esterification.36 Bu-CNC displayed two major pyrolysis steps, first at 302 °C, followed by a major decomposition at 347 °C. Side chain crystallization of butyrate esters with high DS and the contribution from thermally stable cellulose II components plays major role in the overall enhancement of thermal stability of Bu-CNC.36 Although there were fewer intermolecular hydrogen bonds after esterification, the intramolecular hydrogen bonds of crystalline glucopyranose rings of Bu-CNC are stronger. DTG of Bu-CNC clearly showed two differential peaks in the cellulose degradation area, confirming the presence of two cellulose components with different thermal stabilities. Thermodynamically, cellulose II is more stable than cellulose I, which might also explain the two significantly different peaks, one with lower (318.5 °C) and another with higher (370.2 °C) DTG degradation profiles for Bu-CNC. The existence of cellulose II in Bu-CNC, which was confirmed by FTIR, XRD, and NMR analyses, is well supported by the DTG of modified CNC. The sudden weight loss observed in Bu-CNC at temperatures beyond 370 °C is due to increased formation of gaseous products by the ester-free radicals. The 18.5% increment in the thermal stability of modified CNC is advantageous over many nanocomposite and coating applications since the processing and use temperature of this biobased
suspension. Bu-CNC in ethanol was fully transparent and was supported by UV−vis analysis (Figure S3, ESI). Hydrophobic transparent coating analysis were done on thoroughly dried mung bean seeds immersed in water for 30 h (Figure 6b). The analysis was conducted at room temperature, with reference samples of uncoated and CNC-coated beans. The uncoated and CNC-coated samples started to swell soon after immersion in water and were significantly swollen within approximately 8 h. In both cases, clear development of sprouts was observed within 12 h. Beans coated with CNC demonstrated increased sprout growth in the beginning, when compared to uncoated beans, because the water-swelled CNC coating acted as a suitable dormitory, which promoted the germination of beans. In sharp contrast, Bu-CNC-coated beans were resistant to water uptake and no sprouting was observed. Bu-CNC bears potential as a transparent waterproof biobased material. Bu-CNC films prepared by evaporating the solvent (acetone), were transparent and flexible (Figure 6d). Transparency provides further evidence of the size of the individual Bu-CNC crystals that was below the wavelength of visible light (10−40 nm), as discussed in the AFM, TEM, and UV−vis analyses of CNC and Bu-CNC (Figure S3, ESI). Water contact angle on the transparent smooth film of CNC showed static and dynamic values of 31.71° and 46.23°, respectively, which decreased by 15−20° within a few seconds of putting the water ball on the surface of the film and finally dropped to 0°, due to its hydrophilicity. After butyl grafting, these static and dynamic sessile contact angles of Bu-CNC films increased to 81.54 and 91.78°, respectively, indicating the exceptional hydrophobicity of Bu-CNC. It is noteworthy that water droplets were rapidly adsorbed on the surface of the CNC, while the initial shape of the drops remained unchanged with time in the case of BuCNC (Figure 6e). The hydrophobicity analyses confirmed that the entire outer surface of Bu-CNC clusters protected the unmodified inner core crystals from absorbing traces of water. 1343
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
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Figure 8. POM images of CNC (a−c): 4% suspension at 25 °C (a), shear force-dried film at 25 °C (b), and the film at 200 °C heating (c); and of Bu-CNC (d−f): 2% acetone suspension at 25 °C (d), shear force-dried film at 25 °C (e), and the film at 200 °C (f).
nanomaterial could be extendable to up to 300 °C, in additional to its hydrophobic credits.29 Liquid Crystalline Behavior. Polarized optical microscopy (POM) was used to characterize the orientation of CNC and Bu-CNC crystallites under shear force and various temperatures (Figure 8). Figure 8a−c shows the POM images of CNC and Figure 8d−f shows the POM images of Bu-CNC. All the samples were found to be more or less birefringent between crossed polarizers. We assume that Bu-CNC might possess a smectic liquid crystalline arrangement, as several reports32−35,37 have suggested that ester-modified CNC adopts the smectic order rather than the chiral nematic order of unmodified CNC.4,37 The 4 wt % CNC at 25 °C showed different phases with isotropic and anisotropic separation (Figure 8a). Alignment of these phases with the direction of the shear force was clearly evident from the POM images of the dried film of CNC (Figure 8b). Thermal treatment of the film for 120s at 200 °C caused destruction of the unmodified cellulose crystalline domains due to the lower thermal stability of CNC (Figure 8c). POM images of 2 wt % acetone suspension of Bu-CNC (Figure 8d) at 25 °C showed a characteristic pattern of liquid multilamellar crystalline domains37 with a relatively long pitch of smectic arrangement because of the butyric pendants. The homogeneous dispersion of Bu-CNCs, which were actually liquid multilamellar structures, evaporated in the Petri dish under light shearing force of nitrogen gas, yielded ordered birefringence patterns at 25 °C (Figure 8e). As evaporation proceeded, the liquid multilamellar structure of Bu-CNC became a solid multilamellar film and organized into a continuous order of smectic crystals in the direction of shearing force (Figure 8e). Each solid lamella was in the submicrometer range, and its birefringence was explainable by this light interference of polarized microscopy.37 Heating of the Bu-CNC films at 200 °C for 120s, followed by POM analysis, induced a characteristic anisotropic pattern of cholesteric liquid crystals with a relatively long pitch of smectic directors (Figure 8f). Bu-CNC became a disordered lamellar crystalline melt at higher temperatures, as has been seen by others.37−39 POM results demonstrated that Bu-CNC possesses a significant degree of liquid crystalline birefringence over a wide range of temperatures. HR-SEM Analysis of the Bu-CNC Films. According to Onsager’s theory about colloidal particles and liquid crystalline
materials,40 formation of the liquid crystalline phase of the rodlike particles is attributed to the interparticular repulsive forces, and their corresponding liquid crystal textures strongly depend on the anisotropy of the particles. HR-SEM imaging of the cross sections provided insights about the structural arrangement of Bu-CNC layers in the dry film. In the case of CNC, the cellulosic layers were arranged in close proximity to one another, yielding a chiral nematic crystalline arrangement (Figure S4, ESI). In contrast, greater interlayer spacing was observed in Bu-CNC films (Figure 9a,b), with an average interlayer distance of 70 nm, which supports the formation of multilamellar structure with smectic order.
Figure 9. HR-SEM images of the cross section of the Bu-CNC film at different magnifications (a,b), smectic orientation of Bu-CNC crystals (c), and structure of Bu-CNC (d).
The formation of a liquid crystalline arrangement in Bu-CNC was clearly evident from the HR-SEM of the cross section of their dried thin films (Figure 10). Bu-CNC dispersed in acetone contained modified cellulosic rods (whiskers), most of which were ∼10 nm wide and ∼600 nm long (Figure 2) that neither precipitated nor flocculated. The rods possessed a charge density because of the polar ester pendants and unmodified hydroxyls and formed an isotropic phase in acetone suspension, in which the particles experience steric and electrostatic repulsion. During film formation, concentration 1344
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
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ACS Sustainable Chemistry & Engineering
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Elemental analysis and XRD peak separation of the CNC and Bu-CNC, UV−vis spectrum of the CNC and BuCNC films and dispersions which reveal the extent of modification and hydrophobicity of Bu-CNC are discussed. HR-SEM analysis of the cross sections of CNC films are also compared with its modified counterparts (PDF).
AUTHOR INFORMATION
Corresponding Author
Figure 10. HR-SEM of the cross sections of a Bu-CNC film (a,b), with a periodic layer-to-layer isotropic and anisotropic arrangement.
*Prof. Oded Shoseyov. Tel: +972-8-9489084. Fax: +972-89462283. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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of the suspension increased due to evaporation of the solvent acetone, leading to a phase separation and the sample was divided into an isotropic phase on top and an anisotropic phase at the bottom (Figure 10).39 In the latter phase, the proximity of the rods caused them to self-orient along a vector director, resulting in the formation of a smectic liquid crystalline arrangement (Figure 9c).37 When the suspension reached a critical concentration, Bu-CNC rods formed a smectic ordered phase, which displayed characteristics typical of a cholesteric liquid crystal, i.e., rods organized in layers, with a director axis that varied from layer to layer in a periodic fashion (Figure 10). Upon full drying of the suspension, the rods remained in this arrangement (Figure 9b). As a result, Bu-CNC films display interesting optical properties of liquid crystalline birefringence.39 This thermally stable, optically active hydrophobic cellulosic nanomaterial is suitable for various applications, including thinfilm polarizers14 and biosensors.15 The Bu-CNC films with nanostructured networks can hold nonionised drugs that normally would not associate with unmodified CNC and is a promising bionanomaterial for drug delivery applications.16
ACKNOWLEDGMENTS We acknowledge the financial support of Melodea Ltd, MINERVA program and the Israel National Nanotechnology Initiative (INNI).
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CONCLUSIONS Highly hydrophobic, thermally stable cellulosic nanomaterials with liquid crystalline behavior were prepared by the esterification of CNC, using Bu2O as a solvent-cum-reactant, and iodine as a catalyst. AFM, TEM, FTIR, NMR, and XRD analyses revealed a high degree of esterification with a new order of crystalline arrangement in the modified CNC. These observations provided evidence of the partial transformation of native cellulose I to cellulose II upon butyration. Bu-CNC is a highly hydrophobic cellulosic nanomaterial with enhanced thermal stability, as was shown by contact angle, coating, and thermal analyses. POM and HR-SEM analyses demonstrated smectic liquid crystalline behavior with multilamellar arrangement in the submicrometer range. Thus, ester-modified cellulosic nanomaterial is a new class of biobased material which can potentially suit various industrial and medical applications. Specifically, Bu-CNC can be of value in high temperature transparent hydrophobic coating, as an optically active nanocomposite in fillers, in birefringent thin-film polarizers,14 in high temperature biosensors and bioimaging,15 and in nonionised drug delivery platforms.16
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01363. 1345
DOI: 10.1021/acssuschemeng.5b01363 ACS Sustainable Chem. Eng. 2016, 4, 1338−1346
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