Efficient Extraction of Cellulose Nanocrystals through Hydrochloric

Research Article pubs.acs.org/journal/ascecg

Efficient Extraction of Cellulose Nanocrystals through Hydrochloric Acid Hydrolysis Catalyzed by Inorganic Chlorides under Hydrothermal Conditions Miao Cheng, Zongyi Qin,* Yuanyu Chen, Shuo Hu, Zichu Ren, and Meifang Zhu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, and College of Material Science and Engineering, Donghua University, Shanghai 201620, China ABSTRACT: Four inorganic chlorides were introduced into hydrochloric acid hydrolysis to extract cellulose nanocrystals (CNCs) from microcrystalline celluloses (MCC) under hydrothermal conditions. The as-prepared CNCs were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT−IR), and thermogravimetric analysis (TGA). The role of inorganic chlorides including ferric chloride hexahydrate (FeCl3·6H2O), copper chloride dihydrate (CuCl2·2H2O), aluminum chloride (AlCl3), and manganese chloride tetrahydrate (MnCl2·4H2O) in the extraction and properties of high quality CNCs was determined. It is observed that the introduction of inorganic chlorides obviously enhanced the hydrolysis process through faster degradation of the disordered region of cellulose. Compared with those for pure hydrochloric acid hydrolysis, smaller diameter and a larger length to diameter ratio of CNCs could be obtained through saltcatalyzed hydrolysis, which could contribute to greater enhancement on the mechanical properties of polylactic acid (PLA) nanocomposite films. Moreover, it is found that the highest reinforcing effects for the PLA matrix as well as the best transparency among all the nanocomposites were achieved in the presence of ferric chlorides, benifiting from the largest length to diameter ratio and most white of the corresponding CNCs. These results show that the use of salt-catalyzed hydrolysis especially ferric chloride has a significant improvement in achieving the energy-efficient and cost-effective conversion of cellulose starting materials into high quality CNCs. KEYWORDS: Cellulose nanocrystals, Salt-catalyzed hydrolysis, Hydrochloric acid, Inorganic chloride, Hydrothermal conditions

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INTRODUCTION As the most abundant, green, and renewable resource in the world, cellulose is considered as an important natural polymer composed of D-glucopyranose units that are linked by β-1,4 glycosidic bonds.1−5 It is commonly considered that cellulose consists of disordered regions and crystalline regions. The mass fraction of the crystalline region in cellulose materials which is defined as cellulose crystallinity has great effects on the mechanical properties and degradation rate of cellulose.4,6,7 Recently, extraction and applications of cellulose nanocrystals (CNCs) have attracted great interest not only due to their unique physical and chemical properties but also inherent renewability and sustainability in addition to their abundance.1,3,4,8,9 CNCs are fine white powders with high crystallinity obtained from cellulose by the action of acids and the enzyme hydrolysis method, and show a lot of advantages such as low density (1.6 g/cm3), high reactivity, and biodegradability.2,4,9−12 The relatively important characteristics of CNCs are the nanoscale dimensions such as large length to diameter ratio, and their excellent mechanical properties including a Young’s modulus of 150 GPa and a tensile strength of 10 GPa.1,4,9,13,14 Moreover, CNCs have been © 2017 American Chemical Society

used in various industries such as organic reinforcing nanofillers in nanocomposites, drug delivery, enzyme immobilization, templates for mesoporous materials, support matrix for inorganic nanoparticle synthesis, and so on.1,2,4,8,9,15,16 CNCs are mainly obtained by mineral acids such as sulfuric acid (H2SO4) or hydrochloric acid (HCl) hydrolysis of cellulose starting materials, which could remove the disordered regions in the cellulose but leave the crystalline regions.2,10,12,16−19 However, strong H2SO4 has a poor selectivity for disordered regions, as some of the crystalline regions are also degraded, which would diminish the performance and the yield of CNCs.11,12,18,19 Meanwhile, CNCs extracted by H2SO4 hydrolysis can introduce a minor amount of sulfate groups which would catalyze the degradation of cellulose, restricting maximum processing temperatures, and thermal stabilities of their polymeric nanocomposites.3,12,18 Compared with H2SO4, HCl as a mild acid is also used to produce CNCs through acid hydrolysis of cellulose starting materials under ultrasonic Received: December 29, 2016 Revised: April 14, 2017 Published: April 26, 2017 4656

DOI: 10.1021/acssuschemeng.6b03194 ACS Sustainable Chem. Eng. 2017, 5, 4656−4664

ACS Sustainable Chemistry & Engineering

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irradiation, microwave irradiation, or hydrothermal conditions.11,12,19−21 Araki et al. obtained early CNCs by treating the kraft pulp with 4 M HCl at 80 °C for 225 min, but the yield was only 11%.19 Yu et al. extracted thermally stable CNCs with a high yield of 93% in 6 M aqueous solution of HCl under hydrothermal conditions at 110 °C for 3 h.12 Recently, Kontturi et al. produced CNCs from plant-based fibers in high yields of 97.4% through hydrolyses with 8.19 kPa HCl vapor for 4 h.20 More importantly, the use of HCl can avoid the decrease of the degradation temperature for CNCs obtained through H2SO4 hydrolysis, in which sulfated groups exist on the surface of CNCs.12 Note that some metal chlorides can effectively hydrolyze carbohydrates into useful chemicals and exhibit higher catalytic activity than inorganic acids which have been used to improve the decomposition rate of cellulose during the hydrolysis process.22 It has been reported that alkali and alkaline earth metal chlorides are not effective in the conversion of cellulose, whereas transition metal chlorides, especially CrCl3, FeCl3, MnCl2, CoCl2, NiCl2, ZnCl2, and CuCl2 and a group IIIA metal chloride (AlCl3), exhibit high catalytic activity and can be used as catalysts to accelerate the hydrolysis reactions of cellulose.23−28 Kamireddy et al. observed that metal chloride salts including FeCl3, CuCl2, and AlCl3 showed higher hydrolysis efficiency than the H2SO4 on pretreatment of corn stover, and these inorganic chlorides could act as Lewis acids and would aid in cleaving of the glycosidic linkages.22 Wiredu et al. further reported the effects of eight metal ions as co-catalysts on single step saccharification of corn stover catalyzed in acidic ionic liquids and found that all metal ions significantly enhanced the conversion of the biomass to fermentable sugars or saccharification.29 It is also noted that ferric ion enhancement of dilute acid pretreatment of biomass is a promising technology, which can enhance the release or conversion of sugars.30 Furthermore, these metal salts can be recovered as metal hydroxides after pretreatment by using a process called ultrafiltration, and these hydroxides can be treated with conjugated acids such as HCl to convert back to metal chlorides and reused in the process.31 In addition, the application of some pressure in the reactor also improves the efficiency of depolymerization.12,32 In order to reduce the energy consumption and efficiently control the degradation products, four inorganic chlorides including FeCl3, CuCl2, AlCl3, and MnCl2 were introduced into HCl hydrolysis of microcrystalline celluloses (MCC) at relatively low acid concentration in this work. Moreover, more attention should be paid to investigate the role of inorganic chlorides in the extraction and properties of CNCs, further providing insight into the selectivity of metal chlorides that could yield high quality CNCs with small diameter. It has been demonstrated that those smaller CNCs with larger length to diameter ratio can exhibit stronger reinforcing effects for polymeric matrixes.1,11,33 In addition, their nanoscale dimensions allow the production of composite films with excellent visible light transmittance. To evaluate the reinforcement effects of various CNCs prepared with various inorganic chlorides as catalysts on the polymeric matrix, poly(lactic acid) (PLA) would be chosen as a suitable sample due to its thermoplasticity, biodegradablity, ease of processing, transparency, and good chemical resistance against fats and oils.16,34 Moreover, green nanocomposites are being considered as the next generation materials.

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EXPERIMENTAL SECTION

Materials. Commercial microcrystalline cellulose (MCC) as a starting material was purchased from Shanghai Chemical Reagents (Shanghai, China) for producing CNCs. Hydrochloric acid (HCl), ferric chloride hexahydrate (FeCl3·6H2O), copper chloride dihydrate (CuCl2·2H2O), aluminum chloride (AlCl3) and manganese chloride tetrahydrate (MnCl2·4H2O) and chloroform were purchased from Guoyao Group Chemical Reagent CO., Ltd. PLA with the average molecular weight (Mw) of 100,000 was provided by Guanghua Weiye Biomaterials Company. All materials and reagents were used as received without further purification. Extraction of CNCs. CNCs were prepared through HCl hydrolysis of MCC under hydrothermal conditions as described elsewhere.12 Briefly, 1 g of MCC was hydrolyzed in the hydrothermal kettle at 110 °C for 3 h by using 60 mL of HCl solution with 0.1 M four different inorganic chlorides, respectively. Considering the addition of inorganic chlorides as catalysts, the concentration of HCl used in this work was reduced to 4 M instead of 6 M pure HCl used in previously reported work. For comparison, the CNCs were also prepared in 4 M pure HCl acidic medium under the similar conditions. After cooling to room temperature, the resultant suspension was centrifuged at 12,000 rpm for 15 min at 15 °C, washed with deionized water several times to remove the residual acid, and then dispersed under sonication (Kunshan Ultrasonic Instruments Co., Ltd., China; 50 W, 40 Hz) for 5 min for forming a stable aqueous suspension. Then, CNC powders were manufactured by the freeze-drying of the suspensions for 48 h. Preparation of PLA Nanocomposite Films. PLA/CNC nanocomposite films containing 5 wt % of CNCs were prepared by the solution casting method. First, the CNCs prepared without or with inorganic chlorides were dispersed into aqueous solution under the above-mentioned sonication conditions, and then, the concentration was adjusted to 100 mg/mL. To avoid the aggregation of the CNCs during nanocomposite preparation when the as-produced CNCs were dispersed into organic polymer solution again, a solvent exchange approach was applied. Briefly, CNCs were solvent-exchanged to acetone and then to chloroform by successive centrifuging (12,000 rpm at 15 °C for 15 min), and the obtained sediment was fully redispersed under ultrasonic conditions. The CNC suspension in chloroform was stored at 4 °C before use. Then, PLA (1 g) was dissolved in 10 mL of chloroform with vigorous stirring at room temperature until the pellets were fully dissolved. The mixed solutions were obtained by mixing a 5 wt % suspension of CNC in chloroform with PLA solution and then stirring for 4 h. The mixtures were then poured into Petri dishes with a diameter of 5 cm, and chloroform was allowed to evaporate at ambient temperature and pressure. The resulting films with a thickness of ca. 100 μm were peeled from the glass Petri dish and finally dried at 40 °C in a vacuum oven overnight. For comparison, the neat PLA film was also prepared under similar conditions. Characterization. Characterization of As-prepared CNCs. Particle size distribution and particle dimension of as-prepared CNCs were determined on a Zetasizer Nano ZS (Malvern Instruments, UK), providing multi-angle particle size analysis by dynamic light scattering (DLS). The measurements for 2.5 mg/mL CNC suspensions were conducted in triplicate for each sample, and each sample was scanned 12 times. The apparent particle size was then determined by averaging the diameters for each measurement. The morphologies were observed on a Hitachi S-4800 field emission scanning electron microscope (FE−SEM) and a JEM-2100 transmission electron microscopy (TEM). Chemical structures were characterized on a Fourier transform infrared (FT-IR) spectrophotometer (Nicolet 8700, USA) in the range of 400−4000 cm−1 at a resolution of 4 cm−1 with 64 scans. Prior to analysis, each sample was first ground with KBr and pressed into thin pellets. The crystal structures were characterized on a Philips PZ1200 X-ray diffractometer by using Cu Ka X-rays generated at a voltage of 40 kV and a current of 30 mA. The scattering angle range was 5−60° with a scan rate of 4°/ 4657


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Table 1. Apparent Particle Size and Distribution, Crystallinity Index (CrI), and Thermal Parameters for MCC and CNCs Extracted without or with Various Inorganic Chlorides sample MCC CNC no salt FeCl3 AlCl3 CuCl2 MnCl2

apparent size (nm)a

342 200 210 217 230

± ± ± ± ±

36 13 20 22 27

length (nm)b

245 168 190 208 221

± ± ± ± ±

24 25 17 23 21

width (nm)b

23 10 14 18 20

± ± ± ± ±

yield (%)

10 3 7 9 7

85 73 77 78 80

aspect ratio

10.6 16.8 13.6 11.6 11.1

CrI (%)c

T0 (°C)d

T5% (°C)d

Tmax (°C)d

83.5

342.7

326.8

358.7

89.4 92.3 91.8 91.5 91.0

336.6 320.8 319.7 322.0 330.2

322.0 295.6 296.1 308.2 310.9

356.5 340.3 341.8 343.7 349.3

a

Apparent size was calculated by a DLS particle size analyzer. bThe length and width were determined from FE−SEM images. cCrI was calculated from the WAXD patterns. dT0, T5%, and Tmax were calculated from the TGA curves.

Figure 1. SEM images of CNCs extracted without or with various inorganic chlorides. The insets give the photographs of corresponding CNC powders.

Figure 2. TEM images of CNCs extracted without or with various inorganic chlorides. Characterization of PLA/CNC Nanocomposite Films. Tensile properties of neat PLA and PLA/CNC nanocomposite films were tested on a Kexin WDW3020 electronic universal testing machine. The samples were 10 mm wide and 80 mm long, and the distance between the grips was 30 mm. The cross-head speed was maintained at 5 mm/min during testing. The average values of five individual samples are presented here. Differential scanning calorimetry (DSC) measurement of neat PLA and PLA/CNC nanocomposite films were performed on a TA Q20 system. To remove any thermal history, a sample of 5 mg was heated to 200 °C at a heating rate of 10 °C/min and maintained at this temperature for 5 min followed by cooling to room temperature at a rate of 10 °C/min. During subsequent test runs, the sample was reheated to 200 °C at a heating rate of 10 °C/ min, and test data were recorded. Optical properties were characterized on an ultraviolet−visible spectrophotometer (UV−vis,

min in steps of 0.02° at room temperature. The crystallinity index (CrI) of the samples was determined by Segal’s method:

CrI = [(I(200) − I(am))/I(200)] × 100%

(1)

where I(200) is the overall intensity of the peak at 2θ = 22−23°, and I(am) represents the intensity of the baseline at 2θ = 18−19°, where θ is the corresponding Bragg angle. Thermal stability was tested on a NETZSCH TG 209 F1 thermogravimetric analyzer (TGA) with a temperature range from 40 to 600 °C at a heating rate of 20 °C/min under a nitrogen flow of 20 mL/min. The degradation parameters including initial decomposition temperature (T0), the 5% weight loss temperature (T5%), and maximum decomposition temperature (Tmax) were calculated from the TGA curve. 4658


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Figure 3. Possible mechanism on the formation of CNCs through salt-catalyzed hydrolysis of MCC in the presence of various metal chlorides. Shimadzu UV-2550) in the wavelength region of 200 to 900 nm with a scan rate of 100 nm/min.

improving the mechanical properties of the polymeric matrix.1,11,33 In addition, relatively more white powders could be prepared in the presence of CuCl2 and FeCl3. Apparent size for CNCs calculated by a dynamic light scattering (DLS) particle size analyzer is also presented in Table 1. Although DLS is not a good way to measure CNC size due to the rod shape (DLS assumes spherical particles), it is always used to relatively compare relative particle size and size distribution. Apparent size of CNCs produced without salt or with MnCl2, CuCl2, AlCl3, and FeCl3 is 342 ± 36, 230 ± 27, 217 ± 22, 210 ± 20, and 200 ± 13 nm, respectively. These values were within the range of the previous reports, for example, Jiang et al. reported the apparent size of 329 ± 35 nm measured by DLS for HCl-hydrolyzed CNCs.17 Moreover, the change trend of apparent size measured by DLS was consistent with the results from SEM and TEM images for CNCs obtained under different reaction media. Mechanism of Catalytic Hydrolysis. It has been demonstrated that metal chlorides such as FeCl3, AlCl3, CuCl2, and MnCl2 exhibit higher catalytic activity than inorganic acids, more clearly, significant Lewis acid character emerging from their ability to attract electron pairs, which can result in the rapid degradation of cellulose.6,23−26,29 Metal chlorides could form hydrated complexes in aqueous solution and the coordination of the glycosidic oxygen of cellulose with metal cations. These metal cations acting as Lewis acid could help to breakdown the glycosidic linkages and then facilitate the cellulose hydrolysis process.32 Meanwhile, due to strong hydrogen bond accepting ability, −Cl groups can attack the cellulose hydroxyl atoms and combine with the hydrogen of the cellulose hydroxyl to form hydrogen bonds, implying that introducing −Cl groups into acid catalysts can further improve catalytic performance.22,28,36 Combined with these effects, the intra- and intermolecular hydrogen bonds of cellulose can be broken by introduction of metal chlorides as illustrated in Figure 3, and the opening of cellulose structure occurs especially in disordered regions and could accelerate acid permeating into the internal structure of MCC. Moreover, under hydrothermal conditions, the easy diffusion of metal cation and Cl− ions into the cellulose hydrogen bond network as well as the strong ability of Cl− ions to disrupt the massive hydrogen bond can be achieved; thus, the hydrolysis rate is greatly enhanced. The order of catalytic reactivity is strongly dependent on the type of metal ion, more clearly, the ability to attract electron pairs of these metal cations, which can be evaluated by comparing their pKa values (acid dissociation

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RESULTS AND DISCUSSION Morphology and Dimension. During the extraction of CNCs from cellulose starting materials through acid hydrolysis, the disordered regions of cellulose were disintegrated by hydrolytic cleavage of the glycosidic bond, whereas the highly ordered cellulose segments remained, with different degrees of crystallinity depending on the reaction conditions. Note that the conversion of cellulose into simple sugars could occur after long-time or high-temperature hydrolysis with strong acid. Therefore, three main parameters including acid concentration, temperature, and reaction time need to be optimized for the extraction of CNCs with high yield, large length to diameter ratio, high crystal quality, and low cost.10−12,17 It is found that the addition of metal chlorides obviously enhanced the hydrolysis process and that the concentration of HCl can be reduced from 6 to 4 M compared with that for pure HCl hydrolysis performance at 110 °C for 3 h under hydrothermal conditions.12 It is found that the yield of CNCs decreased in the presence of inorganic chlorides as listed in Table 1, while as shown in Figure 1, all of the CNCs exhibited rigid rod-shaped morphology and slight agglomeration due to strong hydrogen bonding between the nanocrystals.10,35 The average length of CNCs decreased from 245 ± 24 nm for without salt to 221 ± 21 nm for MnCl2, 208 ± 23 nm for CuCl2, 190 ± 17 nm for AlCl3, and 168 ± 25 nm as the shortest one for FeCl3; meanwhile, the average width also reduced from 23 ± 10 nm to 20 ± 7, 18 ± 9, 14 ± 7, and 10 ± 3 nm, respectively. The morphologies of the CNCs were further observed by TEM as displayed in Figure 2. Compared with that extracted by using pure HCl medium, the average length of CNCs decreased from 256 ± 30 nm to 222 ± 33 nm for MnCl2, 206 ± 16 nm for CuCl2, 178 ± 34 nm for AlCl3, and 152 ± 39 nm as the shortest one for FeCl3; meanwhile, the average width also reduced from 23 ± 5 nm to 19 ± 4, 17 ± 7, 14 ± 3, and 11 ± 3 nm, respectively. That is to say, CNCs with a smaller diameter and a larger length to diameter ratio could be obtained, indicating that metal chlorides could effectively facilitate the hydrolysis of the disordered region (even partially crystalline region) of MCC. The length to diameter ratio of CNCs is a crucial parameter which influences the reinforcing effect of the nanocrystals when CNCs are incorporated into a polymeric matrix. Generally, CNCs with large length to diameter ratio are believed to be more suitable as reinforcing nanofillers for 4659


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ACS Sustainable Chemistry & Engineering constant).22,32 The pKa value for Fe(III), Al(III), Cu(II), and Mn(II) cations is 2.2, 5.0, 6.5, and 10.6, respectively, implying that FeCl3 possesses strong catalytic activity to facilitate the hydrolysis process.22 Furthermore, the oxygen atoms of the glucose unit in cellulose located at C2, C3, and C6 positions in β-D-glucopyranose rings can be attacked by the metal cation due to its significant ability to attract electron pairs which can efficiently destroy the hydrogen bonds.22,37 In addition, the bond energies for C−C and C−O would decrease due to the increase in the bond angle and bond length induced by metallic ions, resulting in the formation of loose structure for cellulose.19 As a result, the hydrolysis process can take place at an inner component of MCC, leading to a great enhancement of cellulose hydrolysis. Chemical Composition. Figure 4 gives FT−IR spectra of MCC and CNCs extracted without or with various inorganic

CNCs still remained in the cellulose Iβ structure after the acid hydrolysis process. No significant difference in the infrared spectra could be found, implying no change in the chemical structure of cellulose whether metal chlorides were added or not. Crystalline Structure. Currently, the model of cellulose crystallinity in nature is highly debated. Although there is the underestimation of the contribution of the disordered region, the crystallinity index (CrI) has been widely used to account for cellulose crystallinity by characteristic X-ray diffraction patterns.4,5,7,38 All of the CNCs are of the cellulose Iβ structure similar to MCC, which can be further demonstrated by XRD patterns as shown in Figure 5. Five distinct characteristic peaks

Figure 5. XRD patterns of MCC and CNCs extracted without or with various inorganic chlorides.

could be observed at 2θ = 14.6°, 16.6°, 20.4°, 22.6°, and 33.8° for all CNCs, which were assigned to (11̅0), (110), (012), (200), and (040) crystallographic planes, respectively, and in accordance with the characteristic diffraction peaks of cellulose Iβ.2,6,11,17,23,28 This result indicates that the hydrolysis of MCC without or with inorganic chlorides did not disrupt all the crystal structure of cellulose starting materials but had a significant influence on the crystallinity of CNCs, being known as one of the most important factors determining their mechanical and thermal properties. The CrI values obtained by the peak height method were only comparable internally (for these samples) and should not be taken as absolute. However, this method was considered as a means of rapidly determining the relative crystallinity of various samples, and the results were summarized in Table 1. Compared with that of MCC (83.5%), the crystallinity of CNCs was improved especially in the presence of inorganic chlorides. More clearly, the CrI for the CNCs extracted in pure HCl medium was 89.4%. With the addition of metal chlorides, the CrI was further increased to 91.0%, 91.5%, 91.8%, and 92.3% for MnCl2, CuCl2, AlCl3, and FeCl3, respectively. It is believed that the ions could more easily penetrate into the disordered regions during saltcatalyzed hydrolysis of MCC and, as a result, promote the hydrolytic cleavage of glycosidic bonds greatly. More disordered regions should be decomposed, and high crystallinity of CNCs could be obtained.6,10,12 Furthermore, the highest CrI appeared in the presence of FeCl3 with the strongest catalytic activity among four metal chlorides. With the smallest diameter, the largest length to diameter ratio, and highest crystallinity, the CNCs extracted with FeCl3 could be expected to have a significant reinforcing effect on the mechanical properties of polymeric nanocomposites.1,33

Figure 4. FT−IR spectra of MCC and CNCs extracted without or with various inorganic chlorides.

chlorides. All samples exhibited similar characteristic bands in infrared spectra, which were usually divided into two regions: one region from 4,000 to 2,600 cm−1, and another “fingerprint” region assigned to various stretching vibrations from 1,800 to 400 cm−1. A strong band near 3,340 cm−1 was related to the stretching vibration of O−H groups and a characteristic band around 2,900 cm−1 to the stretching of asymmetric and symmetric C−H groups for the aliphatic moieties in polysaccharides. Because of the strong interaction between cellulose and water, an intense adsorption around 1,640 cm−1 originated from the adsorbed water could be found.6,10,12,16,28 Furthermore, the characteristic bands situated near 1,430 and 1,371 cm−1 were assigned to the −CH2 bending and the C−H bending, respectively. The bands around 1,164 and 1,059 cm−1 were attributed to the C−O−C stretching vibrations of glucose ring skeletal and pyranose, while the bands near 1,031 and 667 cm−1 to the stretching vibration of C−O and C−OH out of plane bending mode, respectively. In addition, the band near 895 cm−1 corresponded to the asymmetric out of plane ring stretching in cellulose caused by β-linkage and the disordered form in cellulose.6,10,12,37 Generally, the crystal structure for native cellulose can be defined as cellulose Iα or Iβ, and the O− H stretching and out of plane bending bands are used to distinguish these two crystal structures. It is clear that there is a sharp band near 1,430 cm−1 accompanied by the bands near 1,371, 1,342 and 1,319 cm−1, indicating the characteristics of cellulose I, and a shoulder appeared near 3,290 cm−1 for the −OH band and 710 cm−1 for out of plane bending, characteristic of cellulose Iβ.10,11,21 These results show that all 4660


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hydrolysis is 356.5 °C, and slightly lower than that for MCC (358.7 °C). On the contrary, Tmax is generally not higher than 250 °C for CNCs extracted by H2SO4 hydrolysis.12,18 With the addition of inorganic chlorides, the Tmax decreased further to 349.3, 343.7, 341.8, and 340.3 °C for MnCl2, CuCl2, AlCl3, and FeCl3, respectively. It is well-known that the thermal stability of CNCs was related to several factors including their dimension, crystallinity, and components, which in turn depended on extraction conditions.2,11,39 Generally, CNCs with high crystallinity would exhibit high thermal stability, but smaller dimension should cause the decrease of the degradation temperature. Moreover, for CNCs, high thermal conductivity is helpful for fast thermal transport in crystalline component. Although the CNCs extracted in the presence of inorganic chlorides became more thermally unstable compared with that when using a pure HCl reaction medium, the initial degradation temperature could still exceed 290 °C. As listed in Table 1, the T0 and T5% for the CNCs were 330.2 and 310.9, 322.0, and 308.2, 319.7, and 296.1, and 320.8 and 295.6 °C for MnCl2, CuCl2, AlCl3, and FeCl3, respectively. These findings indicate that CNCs extracted in the presence of metal chlorides especially FeCl3 are appropriate for use as reinforcing nanofillers for thermoplastic polymers composites since they resist thermal decomposition until temperatures of approximately 290 °C, especially at temperatures higher than 200 °C for biodegradable polymeric nanocomposite processing. Mechanical Properties. CNCs as organic nanofillers have been widely used as polymeric matrix reinforcement due to their excellent mechanical properties. It is found that the addition of inorganic chlorides during the preparation of CNCs had a great effect on the size and length to diameter ratio of CNCs, being known as an important parameter to determine the reinforcing effect of CNCs. To evaluate the relative reinforcing effects of the CNCs extracted without or with various inorganic chlorides, solvent-cast films of the PLA/CNC

Thermal Stability. Thermal stability is a crucial factor when CNCs are used as reinforcing nanofillers for preparing polymeric nanocomposites especially through melt processing followed by post-extrusion treatments at elevated temperatures. Figure 6 presents TGA curves for MCC and CNCs extracted

Figure 6. TGA curves of MCC and CNCs extracted without or with various inorganic chlorides.

without or with various inorganic chlorides, and the resulting thermal parameters are listed in Table 1. It is found that all samples exhibited a one-step degradation process, which could be divided into three stages: a small weight loss of 3% below 120 °C attributing to the removal of adsorbed water and low molecular weight compounds, followed by a plateau that was prolonged to the start of the main decomposition event. Then, a pronounced weight loss occurred in the range of 260−400 °C corresponding to the cleavage of glycosidic linkages of cellulose including several concurrent processes such as depolymerization, dehydration, and decomposition of glycosidic units, and finally carbonation occurred over 400 °C.10−12,28 As presented in Table 1, the Tmax for the CNCs extracted through pure HCl

Figure 7. Tensile strength and Young’s modulus (a) and elongation at break (b) of the neat PLA and nanocomposite films, Young’s modulus (E) as a function of the aspect ratio of CNCs (c), and thermal behavior of neat PLA and the nanocomposite films during the heating scan (d). 4661


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The increase in Tg could be attributed to the confinement of polymer chains induced by the intermolecular interactions between CNCs and polymeric matrix. Meanwhile, the melting temperatures (Tm) increased from 143.7 °C for neat PLA to 148.3 °C for the PLA/CNC nanocomposite film containing CNCs extracted by using neat HCl and further to 150.5, 149.6, 149.1, and 148.9 °C for CNCs extracted by using FeCl3, AlCl3, CuCl2, and MnCl2, respectively. The degree of crystallinity (Xc) for neat PLA and the nanocomposite films was calculated by

nanocomposites containing 5 wt % CNC content were prepared. The tensile properties of neat PLA and PLA/CNC nanocomposite films are shown in Figure 7a and b, and their maximum tensile strength (σmax), Young’s modulus (E), and the elongation at break (ε) are summarized in Table 2. It is Table 2. Mechanical Properties of Neat PLA and the Nanocomposite Films with CNCs Extracted without or with Various Inorganic Chlorides sample

σmax (MPa)

E (GPa)

ε (%)

PLA PLA/CNC No salt FeCl3 AlCl3 CuCl2 MnCl2

13.2 ± 0.4

0.36 ± 0.02

702 ± 6

18.8 34.0 29.2 26.3 24.9

± ± ± ± ±

0.8 1.0 0.8 0.3 0.9

0.52 0.69 0.63 0.54 0.53

± ± ± ± ±

0.02 0.02 0.03 0.01 0.01

667 520 552 603 630

± ± ± ± ±

the following equation: Xc(%) =

ΔHf − ΔHc ΔH 0f × Wm

× 100where ΔHf

and ΔHc are, respectively, the enthalpy of fusion and cold crystallization of the samples determined on the DSC, Wm the PLA matrix weight fraction in the composite sample, ΔH0f enthalpy of melting for a 100% crystalline PLA sample, taken as 93 J/g.41 It is found that Xc increased from 6.2% for neat PLA to 19.3% for the PLA/CNC nanocomposite film containing CNCs extracted by using neat HCl and further to 20.3%, 21.7%, 24.2%, and 30.5% for CNCs extracted by using MnCl2, CuCl2, AlCl3, and FeCl3, respectively. These results indicate that CNCs could act as a nucleating agent for PLA crystallization.15,34,40 Optical Property. The PLA/CNC nanocomposite films with the CNCs extracted without or with various inorganic chlorides were visually examined looking for qualitative differences in transparency and external appearance as shown in the photograph in Figure 8. There are several factors which

9 11 10 7 9

clear that the incorporation of CNCs in the PLA matrix can induce a significant enhancement on the mechanical properties of the nanocomposites. For the nanocomposite films containing the CNCs extracted by using neat HCl reaction medium, the maximum tensile strength of PLA increased from 13.2 for neat PLA films to 18.8 MPa and the modulus from 0.36 to 0.52 GPa, whereas the elongation at break (ductility) decreased from 702% to 667%. It has been reported that the nucleation capability and crystallization rate of neat PLA can be greatly enhanced by achieving homogeneous incorporation of CNCs in the polymeric matrix. More importantly, the mechanical properties and thermal stability of the nanocomposites can be greatly improved.1,15,33,34,40 The reinforcing effect of the CNCs depends not only on their nature, content, and state of dispersion within the polymeric matrix but also on the intermolecular interactions between CNCs and polymeric matrix.1,15,33,34,40 Significant differences on the reinforcing effect in this work can be mainly attributed to strong hydrogen bonding between CNCs and the PLA matrix. As displayed in Figure 7a and b, the stronger reinforcing effect could be achieved when the CNCs extracted through salt-catalyzed hydrolysis were added into the PLA matrix. It is also demonstrated that the length to diameter ratio of the CNCs is a crucial parameter that has a significant influence on the ability of the nanocrystals to enhance the mechanical properties of polymeric nanocomposites.1,33,34 Among these CNCs, the CNCs extracted by using FeCl3 provided the strongest reinforcing ability, and the tensile strength, Young’s modulus, and the elongation at break of the resulting nanocomposite film were 34.0 MPa, 0.69 GPa, and 520%, respectively. The Young’s modulus (E) as a function of the aspect ratio of the CNCs is shown in Figure 7c. It is found that CNCs with a larger aspect ratio had a greater effect of reinforcement on the properties of the nanocomposite. Thermal Properties. The thermal behavior of neat PLA and the nanocomposite films was investigated by DSC as shown in Figure 7d. It is clear that all samples experience glass transition, weak cold crystallization, and a melting process. The glass transition temperatures (Tg) increased from 59.3 °C for neat PLA to 62.3 °C for the PLA nanocomposite film containing CNCs extracted by using neat HCl, and Tg was further increased to 65.8, 64.1, 63.7, and 63.2 °C for the PLA nanocomposite film when CNCs extracted by using FeCl3, AlCl3, CuCl2, and MnCl2 were used as nanofillers, respectively.

Figure 8. UV−vis transmittance spectra of neat PLA and the nanocomposite films with CNCs extracted without or with various inorganic chlorides. The inset gives the photograph of neat PLA and the nanocomposite films.

influence the optical transmittance such as the wavelength of light, cellulose particle size, film thickness, cellulose fraction, index of refraction mismatch between cellulose and matrix (or porosity in neat structures), and surface roughness of the film. The CNCs applied in this work are in rod-like shape with an average width of 20 nm and an average length of 250 nm approximately, with less than half of any visible light wavelength; thus, good transparency could be obtained for all of the nanocomposite films. Compared with that for the CNCs extracted in pure HCl medium (78.4% at 660 nm), the transparencies for the nanocomposite films containing the CNCs in the presence of FeCl3 (80.9%) and CuCl2 (79.4%) were improved, benefiting from more white color for the CNCs as illustrated in Figure 8 whereas decreasing the light transmittance of the nanocomposite films for the CNCs by using AlCl3 (76.3%) and MnCl2 (75.4%). The absorption and transmission of light by polymeric materials with both high 4662


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strength and ductility are especially important in food packing.1,14 The introduction of inorganic nanoparticles such as carbon nanotubes into the polymeric matrix could overcome the drawback of PLA including its stiffness and brittleness, but the nanocomposite films became opaque. In this work, all PLA/ CNC nanocomposite films exhibited good transparency, making them more suitable for transparent packaging materials.

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CONCLUSION A simple and convenient approach was presented to efficiently control the degradation of cellulose for extracting CNCs at relatively low acid concentration, in which four inorganic chlorides including FeCl3, CuCl2, AlCl3, and MnCl2 were introduced into the HCl hydrolysis of MCC, respectively. It is found that these inorganic chlorides as Lewis acid exhibited excellent catalytic activity and obviously promoted the hydrolysis process. A possible mechanism was proposed for understanding the role of the inorganic chlorides played on the rapid salt-catalyzed hydrolysis of cellulose. Compared with that obtained in pure HCl reaction medium, the CNCs prepared by salt-catalyzed hydrolysis not only showed rod-like shape but also smaller diameter and larger length to diameter ratio, which would contribute to greater enhancement on the mechanical properties of PLA nanocomposite films. Especially in the presence of FeCl3, the CNCs exhibited the largest length to diameter ratio, and more importantly, the highest reinforcing effects for PLA matrix as well as the best transparency among all the nanocomposites could be achieved. These results show that the use of salt-catalyzed hydrolysis, especially FeCl3, is an energy-efficient and cost-effective way to convert cellulose starting materials into high quality CNCs, making the asprepared PLA/CNC nanocomposite films with high strength and ductility as well as good transparency more suitable for transparent packaging materials. Moreover, by combining the special high-pressure, high temperature reaction conditions provided by an inexpensive hydrothermal reaction kettle and the use of recyclable volatile HCl and chloride salts, high energy consumption and low yield as two major obstacles which need to be overcome for producing CNCs at the industrial scale can be significantly improved.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 21 67792855. E-mail: [email protected]. ORCID

Zongyi Qin: 0000-0002-6329-4005 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT1221). M.C. kindly acknowledges the support from the Innovation Research Funds for the Doctoral candidate of Donghua University (15D310606).

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