Concurrent Cellulose Hydrolysis and Esterification to Prepare a

Research Article pubs.acs.org/journal/ascecg

Concurrent Cellulose Hydrolysis and Esterification to Prepare a Surface-Modified Cellulose Nanocrystal Decorated with Carboxylic Acid Moieties Stephen Spinella,†,‡ Anthony Maiorana,† Qian Qian,† Nathan J. Dawson,§ Victoria Hepworth,† Scott A. McCallum,† Manoj Ganesh,‡ Kenneth D. Singer,§ and Richard A. Gross*,†

ACS Sustainable Chem. Eng. 2016.4:1538-1550. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/16/19. For personal use only.

†

Department of Chemistry and Biology, Rensselaer Polytechnic Institute (RPI), 4005B BioTechnology Building, 110 Eighth Street, Troy, New York 12180, United States ‡ Department of Chemical and Bimolecular Engineering, NYU Polytechnic School of Engineering, Six Metrotech Center, Brooklyn, New York 11201, United States § Department of Physics, Case Western Reserve University, 2076 Adelbert Road, Cleveland, Ohio 44106, United States S Supporting Information *

ABSTRACT: Cellulose nanocrystals (CNCs) were modified with natural diand tricarboxylic acids using two concurrent acid-catalyzed reactions including hydrolysis of amorphous cellulose segments and Fischer esterification, resulting in the introduction of free carboxylic acid functionality onto CNC surfaces. CNC esterification was characterized by Fourier transform infrared spectroscopy, 13C solid state magic-angle spinning (MAS), and conductometric titration experiments. Average degree of substitution values for malonate, malate, and citrate CNCs are 0.16, 0.22, and 0.18, respectively. Despite differences in organic acid pKa, optimal HCl cocatalyst concentrations were similar for malonic, malic, and citric acids. After isolation of modified CNCs, residual cellulose coproducts were identified that are similar to microcrystalline cellulose based on SEM and XRD analysis. As proof of concept, recycling experiments were carried to increase the yield of citrate CNCs. The byproduct was then recycled by subsequent citric acid/HCl treatments that resulted in 55% total yield of citrate CNCs. The crystallinity, morphology, and substitution of citrate CNCs from recycled cellulose coproduct is similar to modified citrate CNCs formed in the first reaction cycle. Thermal stability of all modified CNCs under air and nitrogen resulted in T10% and T50% values above 256 and 323 °C, respectively. Thus, they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. Nanocomposites of poly(vinyl alcohol) with modified CNCs (1 wt % malonate, malate, citrate, and unmodified CNCs) were prepared. An increase in the thermal decomposition temperature by almost 40 °C was obtained for PVOH-citrate-modified CNC nanocomposites. KEYWORDS: Cellulose nanocrystals, Fischer esterification, Cellulose hydrolysis, Carboxylic acid, Nanocomposite, Poly(vinyl alcohol)

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INTRODUCTION

have stimulated the investigation of these materials for reinforcement agents in hydrogels7 and hydrophobic polymers,8 components in tissue engineering materials9 and drug delivery systems,10 supports for enzyme immobilization,11 and as building blocks for electronic materials.12 Despite vast progress in CNC modification and scalability,13 many challenges remain that prohibit realization of their largescale production and commercial use. Acid hydrolysis of cellulose is usually performed with strong mineral acids (e.g., sulfuric and hydrochloric acid), and the resulting CNCs are inherently hydrophilic due to the large number of hydroxyl groups present on CNC surfaces. Drying unmodified CNCs results in irreversible agglomeration due to strong hydrogen

Cellulose nanocrystals (CNCs) are of great interest due to their unique physical and chemical properties, intrinsic abundance in nature, renewability, and sustainability.1,2 Controlled acid hydrolysis of macroscopic cellulose effectively removes amorphous domains thereby liberating highly crystalline rodlike CNCs.2 Because CNCs are derived from the most abundant polymer on earth, developing efficient routes for their production and modification can lead to low-cost nanocrystals that (i) are biodegradable, (ii) have a high axial elastic modulus (110−180 GPa),3 (iii) low density (approximately 100 g/m3), and (iv) possess highly reactive surfaces due to the presence of hydroxyl groups.4 The aspect ratio (length/width) of uniaxial CNCs can further be controlled by selecting the appropriate cellulose source. Aspect ratios range from 10 to 30 for CNCs extracted from cotton5 to 70 for CNCs extracted from tunicates.6 The unique structure and morphology of CNCs © 2015 American Chemical Society

Received: November 13, 2015 Revised: December 23, 2015 Published: December 28, 2015 1538

DOI: 10.1021/acssuschemeng.5b01489 ACS Sustainable Chem. Eng. 2016, 4, 1538−1550

Research Article

ACS Sustainable Chemistry & Engineering bonds.14 Cellulose hydrolysis with sulfuric acid results in functionalization of CNCs with sulfate ester moieties that form stable colloidal solutions.15 However, these sulfate bonds have poor chemical stability and are cleaved under mild alkaline conditions or at high temperatures.2 The modification of CNC surfaces has been extensively explored. The following are examples of modification methods currently used, all of which suffer from one or more of the following problems: (i) the need for multistep processes, (ii) use of toxic reagents, (iii) use of large solvent volumes, and (iv) reaction conditions that lead to low modified CNC volumetric productivity. For example, many papers report passing aqueous suspensions of CNCs through a series of solvent exchanges so they can reside in a nonpolar solvent to accommodate reactions that require nonaqueous conditions.13,16 Silylation of CNC surfaces has been performed in citrate buffer to introduce amino and methacrylate groups onto CNC surfaces. 17 Oxidation of C6 hydroxyl groups at CNC surfaces to carboxylic acids with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is conducted in low solid loading suspensions of unmodified CNCs and requires a primary oxidant, such as sodium hypochlorite.18 Carboxylic acid groups obtained by TEMPO oxidation have been used for subsequent modifications, such as grafting of amine-terminated polymers (e.g., PEG).19 Furthermore, CNC surfaces have been decorated with double and triple bonds, ATRP initiators, and mercapto groups by two step processes, such as CNC synthesis by acid hydrolysis followed by Fischer esterification.20,21 Our research team reported that cellulose can be hydrolyzed and modified in a single step by a one-pot procedure that combines concurrent cellulose acid hydrolysis and organic acidcatalyzed Fischer esterification, resulting in surface-modified CNCs.22,23 For example, surface modification of purified cotton cellulose was performed using a catalytic quantity of HCl and an organic acid (e.g., acetic, butyric, or lactic acid) that both serves as the reaction solvent and reagent for esterification reactions.23,24 The organic acid used can be selected to introduce the desired functionality, hydrophilicity, or hydrophocity to the modified CNCs. For instance, CNCs were modified with lactate groups that facilitated the dispersion of corresponding lactate-CNCs within a polylactide matrix by direct melt compounding.23 Despite the different organic acids and functional groups that have been used in the concurrent cellulose acid hydrolysis and Fischer esterification process, the relationship of organic acid pKa on cellulose hydrolysis has been largely neglected. Modifying CNCs with the acetic acid derivatives (i.e., phenylacetic acid and 2-bromopropanoic acid) with pKa values of 4.31 and 4.0, respectively, required a two-step process due to high organic acid pKa values.25 Additionally, it was shown that CNC acid hydrolysis conducted in the presence of only high pKa organic acids required up to 10 days at 100 °C.20,26 In fact, the majority of concurrent cellulose acid hydrolysis and Fischer esterification reactions have been conducted with organic acids with pKa values ≥4.0, thus necessitating the addition of an HCl catalyst or a two-pot process.23,24 It is well-known that total proton concentration is critical for cellulose hydrolysis.27 Strong mineral acids (e.g., sulfuric and hydrochloric acid) dissociate completely in water to H+ and A−. Consequently, these acids have been the focus of most work on acid-catalyzed cellulose hydrolysis. For example, acid hydrolysis of cellulose using sulfuric acid at 70 °C was studied by Zhu et al., who showed that acid concentration was the most critical

factor for CNC formation. Furthermore, a H2SO4 concentration of 58% resulted in CNCs in superior yield with rodlike morphologies and high crystallinities.28−30 A 20 wt % solution of formic acid has similar cellulose hydrolysis reactivity to 0.5 wt % sulfuric acid at 180 °C under elevated pressure.31 Hou et al. showed that formic acid can be used for the one-step acid hydrolysis of cellulose/Fischer esterification to produce formate surface-modified CNCs.32 However, a high proton concentration consisting of 90% formic acid and 3 M HCl was used at 90 °C for 3 h to prepare formate-modified CNCs, and the relative contributions of formic acid and HCl to the reaction were not investigated. Organic dicarboxylic acids have been used to hydrolyze cellulose at high temperatures (>200 °C) under pressure.27,33 Oxalic acid (pKa = 1.25) and maleic acid (pKa = 1.9) showed little cellulose degradation after reactions conducted with αcellulose performed at 105 °C for 3 h.33 Most of the abovementioned studies were performed with the goal of producing glucose and other biofuels and not producing modified CNCs, and thus the effect of dicarboxylic acid on the overall morphology and crystallinity is unknown. To date, no work has addressed the opportunity to modify cellulose nanocrystals via concurrent acid hydrolysis/Fischer esterification with diacids to introduce carboxcylic acid functionality on CNC surfaces. In the present work, green, solvent-less, one-pot concurrent acid hydrolysis/Fischer esterification reactions were investigated using biobased di- and trifunctional organic acids (citric, malonic, or malic). The success of these reactions would provide a simple route to decorate CNC surfaces with acid functionalities. A key focus of this study was to determine the effects of reaction variables (e.g., HCl concentration and organic acid pKa) on modified CNC yield, CNC morphology, and surface substitution degree. We hypothesized that lower pKa di- and trifunctional organic acids would possess an enhanced ability to hydrolyze cellulose and, consequently, would require less of the HCl cocatalyst for concurrent acid hydrolysis/Fischer esterification reactions. Furthermore, the influence of HCl cocatalyst concentration on the corresponding modified CNC yield and crystallinity was investigated. A coproduct of the reaction, which is similar to microcrystalline cellulose, was recovered and reused for subsequent reaction cycles to increase cellulose conversion to modified CNCs. The substitution degree of modified CNCs (malonate, malate, and citrate CNCs) was characterized by FTIR and solid state 13C NMR experiments. Furthermore, nanocomposites of poly(vinyl alcohol) (PVOH) and citrate-modified CNCs were prepared and improvements in PVOH’s thermal properties were determined.

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EXPERIMENTAL SECTION

Materials. Ramie cellulose was purchased from the Woolery (http://www.woolery.com/) and purified according to previous reports.34 Citric acid (ACS reagent, >99.5%), malic acid (>99%), malonic acid (>99%), poly(vinyl alcohol) (PVOH, Mw = 90 kg/mol, >99% hydrolyzed), and HCl (ACS reagent grade, 37% HCl) were purchased from Sigma-Aldrich and used as received. Preparation of HCl-CNCs, Citrate, Malonate, or Malate Cellulose Nanocrystals. The method for preparation of HClCNCs from ramie cellulose described below followed a literature method used to prepare the CNCs from cotton linters.24 Ramie fibers were purified by two successive washing cycles with 4% NaOH solutions that resulted in removal of residual lignin and hemicellulose that equaled 3% of the initial ramie fiber weight. In all preparations of 1539

DOI: 10.1021/acssuschemeng.5b01489 ACS Sustainable Chem. Eng. 2016, 4, 1538−1550

Research Article

ACS Sustainable Chemistry & Engineering HCl and organic acid-modified CNCs, the concentration of ramie was fixed at 4 wt %. The organic acids were dissolved in water by heating (between 80 and 90 °C) to prepare 80 wt % aqueous solutions of citric, malonic, and malic acids. To prepare HCl CNCs, 13 g of ramie cellulose pieces (2 cm × 2 cm) were mixed with 310 g of 0.4 M HCl and soaked overnight at room temperature (20 °C). For organic acidmodified CNCs and HCl CNCs, ramie pieces (2 cm × 2 cm) were soaked overnight at room temperature (20 °C) in 80 wt % concentrated aqueous solutions of citric, malonic, and malic acid. The reaction mixtures (HCl, organic acid, and ramie pieces) were transferred to 1 L round-bottom flasks, which were maintained with magnetic stirring under reflux (oil bath temperature 140 °C) for 3 h. Organic acid solutions were refluxed at 120 °C, whereas HCl solutions were refluxed at 100 °C. Subsequently, reaction suspensions were mechanically agitated in a Waring laboratory blender for 5 min. Then, the reaction mixture was diluted one-fold by addition of DI water, and the modified and HCl CNCs were isolated by repeated centrifugation (15 cycles) at 14,000 rpm for 8−10 min with replacement of the initially clear supernatant with an equal volume of DI water. Once the pH reached approximately 5, the supernatant remained turbid after centrifugation, indicating the presence of CNCs. After collection of the first turbid supernatant, the pellet was resuspended with a shear homogenizer at 25,000 rpm for 1 min and again centrifuged to obtain additional CNCs in the form of a turbid supernatant. The ten turbid supernatants (i.e., CNC aqueous suspensions) were recovered, freezedried, and analyzed. CNC yields were obtained gravimetrically, taking the initial weight as that of purified ramie and the final weight as the product on a cellulose basis (i.e., not taking into account the increase in molecular weight due to modification) by determining the recovered CNC amount and subtracting the weight due to esterification by the organic acid (from degree of substitution values). Characterization. Thermal Gravimetric Analysis (TGA). The thermal stability of CNCs and PVOH nanocomposites were analyzed using a Thermal Analysis Q50 with an alumina pan. All samples were approximately 10 mg. CNC powder was compacted at the bottom of alumina pans. For PVOH/CNC nanocomposites, small pieces of film (∼2 μm in thickness) were cut and weighed (∼10 mg). For all experiments, heating was carried out at 20 °C/min under nitrogen or air from room temperature to 800 °C. Fourier Transform Infrared Spectroscopy (FTIR). Attenuated total reflectance FTIR were performed using a PerkinElmer Spectrum One with a ZnSe Plate 45° HATR sampling accessory. Spectra were recorded from 450 to 4000 cm−1. 13 C Cross-Polarization, Magic-Angle Spinning, Solid-State Nuclear Magnetic Resonance (13C CP MAS ssNMR). 13C CP MAS ssNMR experiments were performed following R.E. Taylor’s method.35,36 Spectra were obtained using a 600 MHz 89 mm widebore Bruker Advance III spectrometer equipped with a 4 mm HXY solid-state MAS probe configured with channels for 1H and 13C. Prior to addition of predried samples, polytetrafluoroethylene (“10 mg”) and a Teflon plug were added to the bottom of the rotor to position samples. The rotor was weighed before and after sample addition. Experimental parameters for 13C CP MAS ssNMR experiments were as follows: 7000 scans, spinning rate of 11.3 kHz, acquisition time of 0.02 s, and temperature of 278 K. CNC crystallinity indexes were calculated by dividing the area under the C4 crystalline portion by the total area of C4 resonances from residual amorphous and crystalline domains.37 Transmission Electron Microscopy (TEM). A Zeiss Libra 200EF transmission electron microscope (TEM) was used to take bright field images of the CNCs. We used 400 mesh copper grids with carbon (3− 4 nm) and Formvar (25−50 nm) thin film coatings purchased from Electron Microscopy Sciences. TEM grids were prepared by placing them in a Novascan PSDP-UV8T UV-ozone system for 30 min at room temperature. A suspension of CNCs in deionized water was prepared at a concentration of 0.4 mg/mL. The suspension was sonicated in a bath for 2 h at room temperature and then set aside for another hour where any remaining aggregates were allowed to form a precipitate at the bottom of the vial. A drop of the aqueous suspension of CNCs and

two individual drops of 2% solutions of uranyl acetate in deionized water were placed on Parafilm. The UV-ozone-treated grids were floated on the drop with suspended CNCs for 60 s. The grid was removed from the drop and dried by placing filter paper at the edge of the grid. The grid was washed by floating it on the first drop of uranyl acetate solution for 5 s followed by a drying step with filter paper. Then, the grid was stained by floating it on the second drop of uranyl acetate solution for 60 s and dried with filter paper. The grids were then placed in the TEM for imaging with a CCD camera. Scanning Electron Microscopy (SEM). Experiments were performed on modified cellulose with 3 nm platinum sputter coating using a Versa3D SEM operating at 20 kV. Wide-Angle X-ray Spectroscopy. Wide-angle X-ray Spectroscopy (WAXS) patterns were obtained using a Panalytical X’Pert Pro X-ray diffractometer with Cu Kα radiation (0.154 nm) at 45 kV and 40 mA. The scan rate was 10°/min. The diffraction angle was measured from 5° to 60°. The crystallinity index (IC) was calculated according to the equation IC = 1 − (I1/I2), where I1 is the intensity at 2θ = 18.8° and I2 is the intensity when 2θ = 22.8°.38 Conductometric Titration. Modified CNCs were dispersed in DI water at 2 mg/mL at room temperature for 30 min using a tip sonicator fitted with a QSonica Q125 tip probe. Subsequently, the pH was adjusted to 10 with 0.1 M NaOH. The resulting solution was titrated with 0.1 M HCl, and the change in voltage was monitored using a hand-held conductivity meter (VWR product # 89094-958). Average degree of substitution values were calculated according to ATSM D1439.39 This method involves dispersing modified CNCs in a 0.1 M NaOH solution and titrating with a 0.1 M HCl solution. Degrees of substitution were calculated using the following equation: G = 0.162A/(1 − XA) where A = (BC − DE)/F with B = mL of NAOH solution added, C = E = 0.1 (molarities of HCl and NaOH), F = amount of modified CNCs used in g, 162 = the molar mass of the AGU unit, and X = increase in molecular mass for the modified CNC (i.e., X = 58 for malonate-, 118 for malate-, and 176 for citratemodified CNCs. Values obtained for citrate CNCs were divided by two due to the fact that each citrate unit introduces two free carboxylic acids. Poly(vinyl alcohol)/CNC Preparations. PVOH (Mw = 90 kg/mol, >99% hydrolyzed) was dissolved in DI water (10% by wt PVOH) by stirring at 90 °C for 1 h. CNCs (2 mg/mL) were dispersed in the PVOH DI water solution by sonication performed for 20 min at room temperature or until the solution was transparent. Sonication was carried out using a sonicator fitted with a QSonica Q125 tip probe. Nanocomposites were then prepared by solution casting 10 wt % solutions of PVOH containing 1 wt % suspended CNCs. Samples were dried under ambient conditions for 48 h before being transferred to a vacuum oven for water removal at 60 °C and 10-1 mbar for 1 week Karl Fischer Titration of Poly(vinyl alcohol) Films. Karl Fischer titration experiments were performed using a Denver instrument (model 725) Karl Fischer titrator. All poly(vinyl alcohol) films were dried in a vacuum oven ( citrate CNCs. Corresponding values for T50% are 366, 365, 350, and 345 °C, respectively. The amount of residual char at 600 °C for HCl, malonate, malate, and citrate CNCs is 8, 13, 16, and 20%, respectively. Hence, it follows that increasing the T50% for diand triacid-modified CNCs results in correspondingly lower char formation. Increased char amounts is likely due to CNC functionalizations that lead to a relatively larger number of cross-linking events at elevated temperatures. Because the exclusion of air during melt processing is generally not practical, the effect of CNC modification on thermal stability in air was also determined, and the corresponding TGA thermograms are displayed in Figure S3. Values for T10% and T50% are listed in Table S8. The thermal stability of modified CNCs, determined from the peak of the DTG (Figure S3, right panel), are as follow: HCl CNCs ≈ malonate CNCs > malate CNCs ≈ citrate CNCs. Corresponding values of T50% are 335, 334, 323, and 324 °C, respectively. The general trend of thermal stabilities as a function of CNC modification chemistry is almost identical for analyses conducted under N2 (g) and air. Values of T10% in air for HCl-, malonate-, malate-, and citrate-modified CNCs are 311, 273, 305, and 267 °C, respectively. Higher T10% values for malate-modified CNCs may be due to the presence of malate oligomers attached to CNC surfaces.70 Similarly, the potential formation of lactate oligomers for lactate-modified CNCs is believed to increase their thermal stability relative to modification by acetic acid.23 The T10% values under N2 (g) relative to air differ by +6, +53, +21, and −49 °C for HCl, malonate, citrate, and malonate CNCs, respectively. Unlike TGA performed under nitrogen, cellulose tends to combust at high temperatures.71 Consequently, under air, there is little char remaining after heating to 600 °C.72 From the above, all modified CNCs prepared herein have T10% and T50% values under air and nitrogen that are greater than 256 and 323 °C, respectively. Hence, the relatively high thermal stability of modified CNCs indicates they can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition to produce polymer CNC nanocomposites. Repeated Hydrolysis Cycles on Recovered Cellulose. Reactions performed to prepare modified CNCs also resulted in a nonhydrolyzed cellulose coproduct that was separated by centrifugation from modified CNCs suspended in the supernatant. On the basis of SEM and WAXS analysis (see Figures S4 and S5), recovered cellulose is comprised of crystalline particles (crystallinity index of ∼70%) that are similar to MCC.

Furthermore, it is well-known that CNCs can be isolated from MCC using controlled acid hydrolysis.73 This led us to explore whether we could convert residual cellulose to modified CNCs to improve the overall yield of citrate CNCs. Hydrolysis reactions on recovered cellulose were conducted using 80% citric acid solutions with 0.05 M HCl for 1 h, and the results from three hydrolysis cycles are listed in Table 1. The reaction Table 1. Yield of HCl-CNCs and Acid-Modified CNCsa,b organic acid citric acid citric acid citric acid

CNC yield (%)b

yield (%) nonsolubilized byproductc

yield (%) of soluble coproductsd

first

20 ± 5.2

72 ± 2

7 ± 3.7

second

25 ± 3.0

68.7 ± 2.0

third

10 ± 6.1

83 ± 5.1

hydrolysis cycle

6.3 ± 3.0 6.7 ± 23.2

a

Values reported are the mean from three independent experiments and the corresponding standard deviation. bDetermined gravimetrically from isolated, modified CNCs after freeze-drying. cDetermined gravimetrically from isolated nonsolubilized material. dPresumed to consist of glucose and corresponding oligomers. The % yield is determined from the following equation: (100 − [% yield of CNCs + nonsolubilized cellulose]).

time of 1 h was selected because longer times (e.g., 3 h) had deleterious effects on CNC crystallinity without leading to improved yields. For example, WAXS showed that 2 h reactions gave lower crystallinity (by ∼20%) CNCs. Hence, hydrolysis on recovered cellulose from the first and second reaction cycles led to yields of 25 and 10%, respectively. Consequently, the overall yield obtained over 3 reaction cycles is 55%. However, this increase in overall yield comes at the cost of expending additional reagents and energy-input. Future studies are planned to circumvent the use of additional reagents by recovering and reusing those from the previous reaction cycle. Figure 7 shows TEM images of CNCs recovered from three reaction cycles. For each cycle, citrate CNCs have nearly identical average lengths and widths (approximately 240 nm × 10 nm). Consequently, there is no significant difference in the overall morphology of CNCs recovered from the first, second, and third hydrolysis cycles. Analysis of the crystallinity index from WAXS performed on citrate CNCs isolated from the first, second, and third hydrolysis cycles (Figure S6 and Table S9) give crystallinity index values of 80, 76, and 75%, respectively. Furthermore, FTIR and CP-MAS NMR experiments per1545

DOI: 10.1021/acssuschemeng.5b01489 ACS Sustainable Chem. Eng. 2016, 4, 1538−1550

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ACS Sustainable Chemistry & Engineering

Figure 7. TEM of citrate-modified CNCs from rehydrolysis experiments: (a) first, (b) second, and (3) third hydrolysis. For the first hydrolysis, the reaction was conducted for 3 h at 140 °C with 0.05 M HCl and 80% organic acid. The second and third reaction cycles were performed under identical conditions as the first except that the reaction time was reduced to 1 h. Scale bar = 100 nm.

Figure 8. Thermogravimetric analysis of poly(vinyl alcohol)/CNC nanocomposites under N2.

nanocomposites were prepared by casting films from aqueous solutions containing 10% by wt PVOH solution and 1% by wt with respect to PVOH of HCl, malonate, malate, and citrate CNCs. Subsequently, films were first dried under ambient conditions and then in vacuo (60 °C, 10−1 mbar, 1 week). Karl Fischer titrations were performed to ascertain the effect of modified CNCs on the amount of residual water present in films.77 PVOH films both with and without modified CNC after drying have less than 0.1% residual water. Thermal stability of PVOH nanocomposites was studied by TGA under N2 (g) (Figure 8). Values of T10% and T50% listed in Table S10 show that PVOH thermal stability is dependent on the di- or triacid that was used for CNC modification. Neat PVOH has T10% and T50% values of 288 and 346 °C, respectively. In comparison, T10% and T50% values for PVOH blended with 1% by wt citrate, malate, HCl, and malonate CNCs are 306, 299, 289, and 283 °C, respectively, and 385, 360, 353, and 345 °C, respectively. Hence, the largest improvement in thermal stability (18 and 39 °C for T10% and T50%, respectively) was achieved by incorporating citrate CNCs in PVOH. PVOH degradation is known to occur by a multistep process that includes elimination and dehydration reactions. Subsequent degradation phenomena occur by radical and cyclization reactions.74 Study of DTG plots (Figure 8, right panel) reveal

formed on citrate CNCs from each reaction cycle show that each has similar degrees of modification. Poly(vinyl alcohol)/Acid-Modified CNC Nanocomposites. Malonate, malate, and citrate CNCs after sonication form stable aqueous colloidal suspensions (2 mg/mL) with no sedimentation after 7 days. In contrast, HCl CNCs precipitate over hours (data not shown). Increased colloidal stability is attributed to repulsion of negatively charged carboxylic acid functionalities present on CNC surfaces. One potential application of stable aqueous colloidal CNC solutions is their incorporation in water-soluble polymers to improve their physicomechanical properties. Poly(vinyl alcohol) (PVOH) is an industrially relevant water-soluble polymer. Despite PVOH’s numerous advantages, its low thermal stability results in degradation prior to melting. This hinders PVOH melt processing and applications at high temperatures.74 One potential route to enable PVOH melt processing is by the addition of plasticizers such as polyols75 and urea.76 Although these plastizers effectively depress the melting point, they can leach from the processed material over time, causing changes in material properties and problems for use in certain applications, such as for food contact. Thus, improving PVOH heat stability will allow PVOH to be used in melt blending applications while potentially reducing the amount of plasticizer. In an attempt to circumvent the low thermal stability of PVOH, PVOH/CNC 1546

DOI: 10.1021/acssuschemeng.5b01489 ACS Sustainable Chem. Eng. 2016, 4, 1538−1550

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ACS Sustainable Chemistry & Engineering

on the overall modified CNC yield, as malic, malonic, and citric acids all had optimal yields between 0.025 and 0.1 M HCl. All modified CNCs were shown to be highly crystalline with little effect on overall CNC morphology. That is, the TEM of modified and unmodified CNCs had similar lengths, widths, and shape factors. WAXS studies showed that modification had little effect on CNC crystallinity, as unmodified and modified CNCs had similar crystallinity indexes. Thus, modifying CNCs with biobased poly-organic acids proved to be an efficient way to introduce carboxylic acid functionality onto CNC surfaces. This provides strong evidence that CNC modifications primarily occur on CNC surfaces. Structural analysis by FTIR and 13C solid state CP-MAS NMR studies is consistent with the expected structures of modified CNCs. Quantitative direct detection carbon NMR experiments gave average degree of substitution (d.s.) values of malonate, citrate, and malate CNCs of 0.16, 0.18, and 0.22, respectively. Conductometric titration showed that the acid value of modified CNCs can be tuned with acid values ranging from 1108 ± 94, 1617 ± 170, and 1884 ± 1224 mmol COOH/mg cellulose for malate, malonate, and citrate CNCs, respectively. Yields of citrate-modified CNCs were increased to 55% for citrate CNCs by reusing the residual cellulose byproduct. CNCs isolated from rehydrolysis cycles have similar degrees of modification and crystallinity as that obtained from the first cycle. Analysis of modified CNC thermal stability under air and nitrogen showed that T10% and T50% values are above 256 and 323 °C, respectively. These results demonstrate that the modified CNCs prepared herein can be used for melt-processing operations performed at moderately high temperatures without thermal decomposition. One potential application of CNCs modified with free acids is the fabrication of nanocomposites with water-soluble polymers. For this purpose, nanocomposites were prepared by solution casting from water PVOH with 1% CNC loading. With 1% citrate and malate CNCs, the T50% of PVOH increased by 14 and 49 °C relative to neat PVOH films. Incorporation of modified CNCs in PVOH films lead to increases in PVOH crystallinity of 34% for neat PVOH to 45 and 48% for malate and citrate CNC nanocomposites. Although we have not quantitatively assessed the extent that the 12 principles of Green Chemistry79 were followed by the one-pot CNC esterification with organic acids bearing two or three acid moieties, the following provides a qualitative discussion. The CNCs prepared herein use readily renewable feedstocks, toxic reagents are not used or generated, which circumvents the need for solvent exchange that has often been used to pass aqueous CNC suspensions from water to nonpolar solvents for ester bond forming reactions (see the Introduction for examples and references). Furthermore, the modified CNCs are expected to be fully degradable when disposed in a bioactive environment. Although we have improved the conversion of cellulose to CNCs by recycling of recovered cellulose, this approach increases energy and chemical utilization. For improving the sustainability of this multicycle process, it is critically important that future work address the development of a practical method for recovery and reuse of the large excess of organic acid that is not used in the first reaction cycle. Finally, future studies are underway with a series of watersoluble biosourced polymers to gain insight into effects of modified CNC structure on nanocomposite mechanical properties.

a shift in the degradation steps for PVOH by incorporation of 1% citrate CNCs. We hypothesize that the additional carboxyl group of CNC citrate ester moieties and the resulting enhanced hydrogen bonding with PVOH hydroxyl groups is responsible for improving PVOH thermal stability. Also, incorporation of rigid CNCs in PVOH can restrict diffusion in the matrix, which can further contribute to increasing PVOH thermal stability. Differential scanning calorimetry (DSC) was performed on as-cast dried PVOH/CNC nanocomposites and the DSC thermograms are shown in Figure 9. Values of the peak melting

Figure 9. DSC of poly(vinyl alcohol)/CNC (PVOH/CNC) nanocomposites with 1% modified CNCs under N2 flow (second scan) at a heating rate of 10 °C/min from 0 to 250 °C.

transition (Tm), enthalpy of melting (ΔH1, J/g), and % crystallinity are listed in Table S11. The presence of modified and unmodified CNCs has little effect on PVOH Tm (variation of up to 4 °C). The fact that the Tm did not decrease by addition of the modified CNCs indicates they do not plasticize PVOH. By incorporating 1% malate- and citrate-modified CNCs into PVOH, calculated % crystallinity from experimentally determined ΔH1 values increased to 45 and 48%, respectively, relative to 34% for neat PVOH.40 Hence, it appears that malate and citrate CNCs act as nucleating agents for PVOH. Similar effects have been observed by Jorfi et al. who showed that sulfonated CNCs act as nucleating agents for PVOH, albeit with slight higher concentrations of CNCs (4% loading w/w).78 However, unlike Jorfi et al., who showed a significant increase in PVOHs Tg, little difference was observed in nanocomposites with modified CNCs in this study. These results show that fine-tuning the surface chemistry of modified CNCs can have a profound effect on crystallization phenomena of corresponding nanocomposites.

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CONCLUSIONS CNCs were modified with pendant malonate, malate, and citrate groups by a one-pot, solvent-less, concurrent acidcatalyzed cellulose hydrolysis and Fischer esterification reaction. Modified CNCs were compared to CNCs produced by HCl hydrolysis (i.e., produced in the absence of organic acid). We hypothesized lower pKa acids (i.e., malic vs citric acid) would require different amounts of HCl cocatalyst for optimal CNC yield. However, little effect of acid pKa was found 1547

DOI: 10.1021/acssuschemeng.5b01489 ACS Sustainable Chem. Eng. 2016, 4, 1538−1550

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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.5b01489. Quantitative solid-state NMR spectra, conductometric titration, TGA thermograms for neat CNCs and poly(vinyl alcohol)/CNC nanocomposites, SEM of unhydrolyzed cellulose byproducts and WAXS of CNCs from residual cellulose particles, and tables including literature values for organic acid pKa values, tabulated modified CNC yields, CNC crystallinity indexes, CNC acid values from titration, and CNC thermal stability under nitrogen and air (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 518-276-3734. Fax: 518-2763405. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for funding received from the National Science Foundation Partnerships for International Research and Education (PIRE) Program (Award #1243313).

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REFERENCES

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