One-Pot Preparation of Carboxylated Cellulose Nanocrystals and

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One-pot Preparation of Carboxylated Cellulose Nanocrystals and Their Liquid Crystalline Behaviors Lijuan Zhou, Na Li, Jie Shu, Yunxiao Liu, Kuntao Wang, Xiang Cui, Yuan Yuan, Beibei Ding, Yong Geng, Zhaolu Wang, Yongxin Duan, and Jianming Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02926 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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ACS Sustainable Chemistry & Engineering

One-pot Preparation of Carboxylated Cellulose Nanocrystals and Their Liquid Crystalline Behaviors Lijuan Zhou†, Na Li†, Jie Shu‡, Yunxiao Liu†, Kuntao Wang†, Xiang Cui†, Yuan Yuan†, BeiBei Ding†, Yong Geng†, Zhaolu Wang†, Yongxin Duan†, Jianming Zhang*†

† Key Laboratory of Rubber-Plastics, Ministry of Education/ Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China.

‡ Analysis and Testing Center, Soochow University, Renai Road 199, 215123 Suzhou, China

* To whom all correspondence should be addressed. Fax: +86-532-84022791 E-mail: [email protected] (J. Z.)

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ABSTRACT: Carboxylated cellulose nanocrystals (CNCs-COOH) have attracted great

attention for their potential applications in reinforcing polymer materials and surface modification. Herein, we developed a low-cost approach to prepare CNCs-COOH from pulp with high yield at mild reaction conditions (50 °C, 1 wt.% sulfuric acid medium) using potassium permanganate (KMnO4) and oxalic acid (OA, H2C2O4) as the oxidizing and reducing agents, respectively. The oxidant dosage in this strategy is much lower than that in a conventional Tempo method and the yield of CNCs-COOH can reach as high as 68.0%, with a carboxylate content of 1.58 mmol/g. In this reaction system, the presence of the OA can complex with Mn3+ to form [Mn(C2O42-)]+ and prevent the Mn3+ from being reduced to Mn2+, leading to the strong oxidizing capacity of the reaction system maintained for a longer time Atomic force microscopy (AFM) analysis showed that rod-like CNCs were obtained with an average size of 10 ~ 22 nm in diameter and 150 ~ 300 nm in length. The crystal structure of as-prepared CNCs-COOH was nearly unchanged and the crystallinity was 89.2% based on WAXD analysis. Of particular interest, CNCs-COOH suspension with high concentration (>6 wt.%) also exhibited the same intrigue chiral nematic liquid crystalline self-assembly behaviors as sulfate CNCs prepared by traditional H2SO4 hydrolysis method. This study provides an efficient and cost-effective way to fabricate CNCs-COOH, leading to great potential applications in constructing advanced functional material.

KEYWORDS: Cellulose nanocrystals, Potassium permanganate/oxalic acid redox system, Surface carboxyl groups, Chiral nematic


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INTRODUCTION Cellulose nanocrystals (CNCs), extracted from natural cellulose based materials (wood, cotton etc.), have emerged as a new class of nanomaterials for polymer reinforcement and nanocomposite formulation due to excellent mechanical properties (modulus of 100 ~ 140 GPa), low density (1.6 g cm-3) and environmental sustainability.1-2 Till now, sulfate acid hydrolysis and TEMPO-mediated oxidation are the most widely used methods to prepare water-dispersible CNCs, in which the negative-charged groups, OSO3- and COO-, are introduced to form stable aqueous suspensions due to electrostatic repulsion, respectively.3-8 However, for sulfate acid hydrolysis method, it need too much sulfuric acid and it is easy to cause acid pollution to the environment; Besides that, the cost of recycling sulfuric acid is very high9. Comparing to the sulfated CNCs, surfaces of carboxylated CNCs (CNCs-COOH) are easier to be modified, such as taking part in covalent crosslinking and grafting reactions,10 although sulfate acid hydrolysis method has the minimum reaction time till now (t < 1 h). Moreover, CNCs-COOH show higher thermal stability7. Therefore, much effort has been devoting to the preparation and application of CNCs-COOH.10-13

Currently, TEMPO mediated oxidation proposed by Nooy et al.14 is the most popular method to synthesize CNCs-COOH due to the highly selective oxidation of C6 primary hydroxyl groups of the polysaccharides to carboxyl ones, and the content of carboxyl group can reach as high as 1.7 mmol/g.6 However, TEMPO-mediated oxidation still has some intrinsic limitations, for example, (i) the pH values should be maintained at 10 ~ 11 during the whole TEMPO-mediated oxidation process;3 (ii) toxic TEMPO reagents would lead to environmental issues;14 (iii) the TEMPO-oxidized cellulose requires the post-treatment with



NaBH4 to reduce C6-aldehydes and C2/C3-ketones present in the oxidized cellulose to hydroxy

groups.6 Besides the TEMPO-mediated oxidation, John et al. reported a one-step

method to prepare CNCs-COOH by ammonium persulfate (APS) oxidation.10 Nevertheless it is noted that the amount of APS used in the reaction is too high (228.01 g APS needed in 10 g pulp). Recently, Kang et al. reported the catalyst assisted-H2O2 oxidation green method to prepare CNCs-COOH without using harsh chemicals or organic solvents.15 However, this method needs the pH of the reaction mixture be maintained in the range of 1-2 by 1 M HCl addition, leading to the preparation process is relatively rigorous. Moreover, the yield of CNCs-COOH by this approach is much lower (about 20%).

Thus, it remains challenge to

develop a facile and cost-effective method to facilitate the fabrication of CNCs-COOH for readily large-scale production and practical applications.

Compared to other oxidants, potassium permanganate (KMnO4), as a green and industrial oxidant,16 is preferred for its attractive characteristics of relatively low cost, easy handling, effectiveness, comparative stability over a wide pH range.17 Inspired by classic Hummers method18-19 for preparing water dispersible graphite oxide, we used KMnO4 in dilute sulfuric acid (H2SO4) medium (1 wt.%) to produce CNCs-COOH in the present study. It was found that high yield CNCs-COOH could be obtained in short time (8 h) by KMnO4 oxidation at mild reaction condition (50 °C) with the introduce of the oxalic acid (OA, H2C2O4) as a reducing agent. The present work demonstrated a rapid and efficient route for the production of CNCs-COOH from pulp in a large-scale with the lowest cost thus far reported. The resulted CNCs-COOH were characterized by various techniques, and their properties were compared with those obtained by conventional methods. Furthermore, the


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effect of concentration on liquid-crystalline self-assembled behavior of thus prepared CNCs-COOH suspension was also investigated systematically since cholesteric liquid crystal is a promising candidate for advanced functional materials. EXPERIMENTAL SECTION Materials. Cotton pulp with cellulose polymerization degree (DP) of 700 was supplied by Silver Hawk Co. Ltd. (Gaomi, China). Potassium permanganate (99%), oxalic acid (99%), sulfuric acid (H2SO4, 98 wt. %), hydrogen peroxide (H2O2) and sodium hydroxide (NaOH, 96%) with analytical grade were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Double-distilled water was used for all experiments. Preparation of carboxylated CNCs (CNCs-COOH). The pulp was pretreated following the method reported in our previous study.20 Briefly, the pulp was milled and soaked by 4 wt. % NaOH solution for 24 h, followed by washed with distilled water to neutral pH; and then the treated pulp was dried at 60 oC in a vacuum oven. After that, 5 g pretreated pulp, 200 mL of deionized water with sulfuric acid (1 wt.%), 10 g potassium permanganate (KMnO4) and 5 g oxalic acid (OA) were added to the flask successively and reacted at 50 oC for designed time (t = 4, 6, 8, 10 h) with vigorous stirring (600 rpm). The reaction was stopped by adding hydrogen peroxide (H2O2) to remove excess KMnO4 at the end of oxidation reaction. The resulting mixture was processed with aeration to remove manganese ions (Mn2+)21 and then washed repeatedly with deionized water until the supernatant turned turbid , which is an indication of the presence and release of the CNCs 20. Finally, an ivory-white CNCs suspension was dialyzed using a dialysis tube with MWCO 8000 ~ 14000 to remove the acid and soluble carbohydrates and the solid content of the



obtained CNCs suspension was around 1.0% (w/w), measured gravimetrically. Characterization of CNCs-COOH. Morphology and dimensions. AFM image was acquired using a Multimode V (VEECO) under contact mode. Before test, dilute CNCs (0.1 mg/mL) suspension was spin-coated on freshly cleaved mica at 3000 rpm for 1 min. Chemical and physical structures. The Fourier transform infrared spectroscopy (FT-IR) spectra were collected from 4000 to 400 cm-1 for 64 scans at a resolution of 4 cm-1 using a Bruker VERTEX 70 spectrometer. The SSNMR

13

C cross polarization (CP) experiments were conducted on a Bruker

Avance III HD 400 spectrometer equipped with a DVT H/X 3.2 mm magic anglespinning (MAS) probe. Spectra were accumulated at a MAS frequency of 10 kHz, by using CP contacting time of 1.5 ms, recycle delay of 5 s and 4600 scans with TPPM-15 high power 1H decoupling sequence during acquisition. All the spectra were referenced with respect to tetramethyl silane (TMS), using adamantane (13C, δ = 38.484 ppm) as a secondary reference. The zeta (ζ) potentials of the CNCs were measured on a Malvern Nano ZS90 light scattering instrument at a concentration of 0.01 mg mL-1 with a Zetasizer analyzer. Deionized water was used as a dispersant. The ζ potential was calculated with the Smoluchowski equation. The carboxyl content of the obtained CNC samples (CNCs-COOH) was determined via potentiometric titration.11 In brief, 20 mL of CNCs-COOH suspensions containing 0.15 g of solid content were sonicated for 10 min to obtain a well-dispersed suspension. Then, 0.1 M NaOH was added to the mixture to set the pH in the range of 11 ~ 12. The CNCs-COOH


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suspensions were then titrated with 0. 1 M HCl using a PHS-3E meter (Leici, China) and the carboxylic acid contents were determined from the resulting conductivity curves. Typical titration curves exhibit a discontinuity assigned to the presence of a weak acid (i.e. carboxylic acid groups introduced during the KMnO4-mediated oxidation reaction). The carboxyl content (mmol/g) of the sample was calculated based on the following equations: [COOH] = (V2-V1)×CHCl /mCNCs,

1)

where Vi is the volume of HCl (mL), CHCl is the exact HCl concentration (mol L-1) and mCNCs is the dry weight of the sample (g). The degree of crystallinity of CNCs-COOH was examined by X-ray diffraction (XRD). XRD patterns for the dried CNCs-COOH powder were collected at room temperature on a Rigaku UltimaIV diffractometer with Cu Kα radiation (λ = 0.154 nm), the diffraction signals were recorded in the range of 2θ = 5 ~ 40° with a step interval of 0.02° and a scanning rate of 5° min-1. The crystallinity index of CNCs-COOH was calculated using the following equation:22

C I (%) =

I 002 − I am × 100 I 002

2)

where I002 is the maximum intensity of the peak corresponding to plane having the miller indices 002 and Iam is the minimal intensity of diffraction of the amorphous phase at 2θ = 18o. Thermal stability analysis. The thermal stability of CNCs-COOH was determined using TA Instruments Q500 thermogravimetric analyzer under a nitrogen atmosphere. The temperature was set from 40 to 600 °C with a heating rate of 20 °C min-1. Polarized Light Photographs and Pitch Measurement. An aliquot of each suspension was ﬁlled into rectangular cross-section glass capillary having an optical path length of 1 mm



(VitroCom Inc., NJ) and kept at room temperature for 5 days. To take the digital picture (Nikon D300s digital camera) for identifying the phase separation, the glass capillaries with CNCs-COOH suspension inside were placed between two polarizing films in a dark box. Photomicrographs were taken using a Leica DM2500P optical microscope equipped with a CCD camera and crossed-polarizers, the chiral nematic pitch was measured directly from the spacing of the fingerprint texture in the images, where the distance between the lines is equivalent to the half of the full pitch. The chiral nematic pitch of the liquid crystalline phase was determined for each sample at a variety of CNC concentrations. RESULTS AND DISCUSSION Preparation and reaction mechanism.


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Scheme 1. (a) Illustration of the preparation process of CNCs-COOH by KMnO4 method and (b) Mechanism for the formation of strong oxidizing capacity and explain the role of OA. As illustrated in Scheme 1a, one-pot hydrolysis reaction was started off when the pretreated pulp, KMnO4, dilute H2SO4 (1 wt.%) and AO were added to the flask successively at 50℃. After a period of time, the slurry of oxidized pulp was obtained. Then the slurry was converted to suspension by subsequent ultrasonic and centrifuge treatment. At the beginning, we only used KMnO4 as an oxidant in dilute H2SO4 medium to prepare CNCs. In this system, MnO4− can be reduced to generate soluble Mn3+, both MnO4− and Mn3+ are reactive species, which can oxidize the amorphous region of cellulose at very high rates.17, 23-24 However, we found that this system needs prolonged reaction time (t > 30 h, see Table S1), as Mn3+ is easily reduced to Mn2+,25-27 resulting in the difficulty in preparing Mn3+ effectively. Therefore, we introduced oxalic acid (OA) in KMnO4/H2SO4 system to complex with Mn3+ to form [Mn(C2O42-)]+, which not only has strong oxidation activity, but also can prevent the Mn3+ from being reduced to Mn2+. Therefore, the addition of OA maintained the strong oxidizing capacity of this redox system for a longer time. By doing so, the reaction time was greatly reduced (less than 10 h). The CNCs-COOH as depicted in the carton of Figure 1a were



obtained due to the strong oxidizing capability of this redox system. The reaction mechanism of KMnO4/ OA redox system in acid medium is shown in Scheme 1b. The role of OA is two-fold. Firstly, the Mn(VII) of permanganate ion (MnO4-) can be reduced by OA fast to produce highly active Mn3+, and the in situ formation of Mn2+ and MnO2 provides a second positive feedback loop in the reaction as it also can be reduced by OA to produce Mn3+.25-27 All these Mn3+can oxidize amorphous region of cellulose at very high rates. Secondly, OA can complex with Mn3+ to form [Mn(C2O42-)]+, which still has strong oxidizing capacity,25-27 so that the activity of Mn3+ can be maintained. To our knowledge, KMnO4/ OA oxidation in dilute acid medium has never been recognized previously in the preparation of CNCs. Effect of reaction time on final product. Reaction time is one of the most important parameters to control the hydrolysis of pulp. Overlong reaction time can make the pulp decompose to small molecules completely, and short reaction time will result in undispersible aggregates with less functional groups. Therefore, it is necessary to find out the optimal reaction time. In this study, pulp was subjected to 4 ~ 10 h oxidation treatment using m(KMnO4) : m(OA) = 2 in H2SO4 medium (1 wt.%). The resulted CNCs under various reaction time was named as CNC-X, where X is the reaction time.


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Figure 1. (a) XRD profile and (b) TGA traces of pulp and the CNCs prepared via KMnO4/OA oxidation process under different reaction time.

Crystallinity: As shown in Figure 1a, the CNCs prepared all give the typical X-ray diffraction pattern of cellulose I, with the main diffraction peaks located around 2θ = 14.8, 16.4 and 22.7◦, normally assigned to the (1-10), (110), and (200) diffraction planes, respectively.22 This suggests that KMnO4/OA mediated oxidation of pulp does not change the polymorphism of cellulose I in the CNCs. The crystallinity index (CI) of CNCs was then estimated using an integral method based on the ratio of the areas of crystalline region to total scattered intensity,22 the results of which are summarized in Table 1. In general, the CI of CNCs was noticeably higher than that of its parental counterparts (CI of the pulp is 76.8%), and the crystallinity range observed in this study is even higher than the earlier reported values for CNCs obtained via acid hydrolysis,28 and the crystallinity increases with the increasing of reaction time.

Thermogravimetric analysis. Figure 1b gives the thermograms of pulp and CNCs. The onset degradation temperature (Tonset) for pulp was 292.6 °C, whereas a slight decrease in



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Tonset was observed for the CNC10. This slight decrease can be attributed to the increase in the surface area. In comparison, CNCs obtained from sulfuric acid hydrolysis have lower thermal stability (≈150 ~ 200 °C) due to the presence of sulfate groups.28 It is noted that the CNCs obtained via KMnO4/OA oxidation can endure the processing temperature of many thermoplastic polymers.29 Besides that, it clearly shows that the carbon residue (see Table 1) of the CNCs increases as the reaction time increasing. As the crystallinity of the CNCs is gradually increased when the reaction time was prolonged, and the higher crystallinity makes the heat transfer relatively slow, so that the heat resistance of CNC10 is better.

Table 1. Crystallinity index (CI) and Thermal Parameters and for the Carboxylated CNCs Prepared under Different Reaction Time. Samples

CI

Tonset/oC

Tmax/ oC

Residual at 600 oC (wt.%)

(%) Pulp

76.8

292.6

371.4

12.9

CNC4

86.7

266.2

333.5

20.6

CNC6

88.2

268.9

333.8

23.6

CNC8

88.5

269.4

347.8

24.4

CNC10

89.2

280.1

363.8

25.8

FT-IR spectra. Figure 2 shows the FT-IR spectra of the CNCs extracted under different reaction times. It is found that all the extracted CNCs treated with KMnO4/OA exhibited the same chemical structure. The very broad band around 3400 cm-1 is assigned to the stretching vibrations of –OH groups. The contribution near 2900 cm-1 is due to the stretching vibrations of C–H bonds. The absorption at 1640 cm-1 is related to the adsorbed water due to the presence of abundant hydrophilic hydroxide radical in the cellulose. The characteristic bands


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at 1167 and 897 cm-1 are corresponded to C–O–C bending and symmetric stretching at the β(1-4) glycosidic linkage, and at 1060 and 1113 cm-1 to C–O–C stretching of pyranose and glucose ring skeletal vibration, respectively.12 By comparing the spectrum profiles with the reported ones, it is indicated that all the samples are primarily in the form of cellulose I. These spectra confirm that the chemical structure of the extracted cellulose remains the same after treatment with KMnO4/OA, indicating no significant changes of the conformation of the cellulose structure for all reaction time. Besides that, the FT-IR spectra also shows a vibration located at 1728 cm−1, indicating the presence of carboxylic acid groups.22 The content of carboxyl groups of the CNCs (shown in Table 2) was determined using a potentiometric titration method to fall in the range of 0.87 ~ 1.58 mmol/g. CNC8 has a higher carboxyl group content, and the content of carboxyl groups can reach as high as 1.58 mmol/g.

Figure 2. FT-IR spectra of pulp and CNCs prepared via KMnO4/OA oxidation process under



different reaction time.

Solid State NMR. To confirm the existence of carboxyl groups on the surface of CNCs, solid state NMR 13C cross polarization experiments were conducted. The spectra are shown in the Figure 3. By comparing the spectra of the CNC8 to the pulp, a signal at 174.8 ppm was probed for CNC8. Based on the chemical shift and references, this resonance is assigned to the carboxyl groups.10, 30 Meanwhile, the crystalline contribution at 65 ppm remains constant (as shown in Figure 3 inset A). Thus, the onset of this carboxyl contribution arises at the unique expense of the amorphous C6 contribution.30 The oxidation of C2 and C3 secondary alcohols is known to induce the cleavage of the glucopyranose ring, resulting in detrimental lowering of the crystallinity.30 Here, the crystallinity of CNCs prepared in this method increases with prolonging the reaction time (see Table 2). Therefore, the hydroxyl group of C2 and C3 secondary alcohols were not oxidized. Any significant changes in the respective chemical shifts of the CP-MAS 13C-NMR signals confirmed the intact crystalline structure of CNCs prepared in KMnO4/OA system. This observation strongly indicates that only the primary hydroxyl groups are oxidized in the insoluble part, and in the disordered regions of the sample. Both FT-IR and NMR data suggested that the oxidation occurred preferentially at the C6 primary alcohol of cellulose.


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Figure 3. Solid-state NMR spectrum of CNCs prepared with 8 h reaction time and original pulp. Inset (A) shows the enlarged view between 110 ~ 50 ppm; Inset (B) shows the structural formula of the AGU in cellulose with numbered carbons. Table 2. Yield, carboxyl content, dimensions, and crystallinity index (CI) of the CNCs prepared under different reaction time. Reaction time (h)

CNC4

4

48.0

299.8 ±18.2

21.5±3.0

Carboxyl content (mmol/g) 0.52±0.05

CNC6

6

56.4

258.3 ±19.7

18.5±2.4

0.81±0.08

CNC8

8

68.0

210.3 ±18.7

15.9±2.9

1.58±0.06

CNC10

10

62.8

158.5 ±20.3

10.6±2.5

1.13±0.07

Samples

Yield (%)

Dimensions(nm) length

diameter

In summary, as shown in Table 2, the diameter and length of the thus-obtained CNCs ACS Paragon Plus Environment


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determined by AFM are 10 ~ 22 nm and 150 ~ 300 nm, respectively, and CNC10 shows a smaller average particle length, as well as the diameters, narrower polydispersity, attributed to the longer hydrolysis time; CNC8 has the highest yield; and the yield can reach 68.0% as measured using a gravimetric method. However, longer reaction time (t = 10 h) introduces more oxidation on the amorphous part,31 which explains why CNC10 has lower yield. Thus, the reaction for 8 h was chosen for KMnO4/OA oxidation at 50 oC in acidic aqueous solution.

Advantages of KMnO4 method compared with Tempo and APS method for preparing CNCs-COOH.

Figure 4. Comparison of various ways of preparing CNCs-COOH: (a) AFM Morphologies; (b) Dosage of oxidant and yield; (c) Zeta potential and reaction time. Different CNCs-COOH preparation processes, started from the same pulp material, were used to produce CNCs in an attempt to compare the performance (see Figure 4).


The APS

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and tempo-mediated oxidation methods are shown in Supporting Information (SI).

From

the AFM images shown in Figure 4a, the sizes of the cellulose nanocrystals prepared by APS and tempo closely match those reported previously. Here, CNCs-COOH prepared from KMnO4/OA mediated oxidation show the typical rod-like morphology with an average size of 10 ~ 22 nm in diameter and 150 ~ 300 nm in length. As shown in Figure 4b and 4c, the CNCs-COOH yield of KMnO4/OA mediated oxidation was 68.0%, which is higher than those of APS treatment and tempo-mediated oxidation methods, although the pulp was treated with the lowest dosage of oxidant and the least time in this work. In this redox system, both MnO4− and the generated Mn3+ can oxidize amorphous region of cellulose at very high rates.23 More active species make the rapid oxidation of cellulose (see Scheme 1b), so that the reaction time of KMnO4/OA mediated oxidation is the shortest but the yield is the highest. As potassium permanganate is a low cost and green industrial oxidant, the preparation of CNCs-COOH in such a manner is expected to facilitate the scaling up of the preparation. As shown in Figure 4c, the absolute magnitude of zeta potential of KMnO4-CNC suspensions is the highest, which indicates the highest degree of oxidation on the surface of KMnO4-CNC.

Isotropic to Chiral nematic Transition. CNCs (above certain aspect ratio) suspension would form liquid crystal phase when the concentration reaches the critical value.32 Due to the chiral nature of the cellulose, the liquid crystalline phase generated is normally cholesteric liquid crystal (ChLC), which is also called chiral nematic.11 Birefringence is an important symbol whether liquid crystals are formed, which is normally the most significant optical property difference between anisotropic and isotropic materials.33 The critical concentration of the CNCs-COOH in forming cholesteric phase could be different depending on the



preparation methods. For the CNCs-COOH prepared by TEMPO-mediated oxidation method, 4.1 wt.% of the CNCs-COOH suspension can form anisotropic phase with clear optical birefringence, and fingerprint texture can be observed when the concentration is 9.0 wt.%;32 For the CNCs-COOH prepared by APS oxidation method, 5 wt.% suspension of acid-form CNCs-COOH can form anisotropic phase with clear optical birefringence, and the tactoids can coalesce and form a fingerprint pattern characteristic of a chiral nematic phase after 2 weeks.11 In this study, CNCs-COOH can also generate ChLC phases. As yield of CNC8 is the highest, so this sample was selected for the optical characterization. Birefringence of the CNCs-COOH suspension was imaged by first keeping at room temperature for 5 days before placed between crossed polarizers. Different concentrations of CNC8 suspensions were prepared. The images of CNC8 suspensions with the concentrations of 2.0 wt.%, 3.0 wt.%,4.0 wt.%, 5.0 wt.% , 6 wt.%, 7.0 wt.%, 8.0 wt.% , 9.0 wt.% and 10.0 wt.% sandwiched between crossed polarizers are shown in Figure 5a.

Figure 5. (a) Images of liquid crystals formation for CNC8 suspension under different


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concentration (Arrows indicating directions of the polarizer (P) and analyzer (A). Samples were sandwiched between polarizer (P) and analyzer (A); The scale of glass capillary is 10 mm in width); (b) The critical concentration of phase separation for CNC8 suspension; (c) Tactoids formation from aqueous CNC8 suspensions at the concentration of 7.0 wt.% ~ 10.0 wt.%. As shown in Figure 5a and 5b, the CNC8 suspensions are homogenously isotropic when the concentration is below 5 wt.%, but they separate into two phases with an upper isotropic phase and an anisotropic bottom one in the course of quiescent standing when the concentration reaches 7.0 wt. %. Thus, it could be concluded that the critical concentration of cholesteric liquid crystal formation for KMnO4-CNC8 suspension is around 7.0 wt.%. Considering that the structure of the chirality nematic phase could be arrested in its solid film, chirality of the cholesteric phase of KMnO4-CNC8 suspension determined by characterizing its CNC iridescent films through UV−vis spectrophotometer and circular dichroism (CD) spectroscopy (the preparation and characterization of CNC iridescent films are shown in Supporting Information, SI), , and the results show that the cholesteric phase is left handed (see Figure S1) ,which is consistent with the general observation for the chiral self-assembly behavior of CNC.34 Besides, the lower anisotropic part of tactoids35 could also be observed obviously when the CNC8 suspension concentration was increased to 7.0 wt. % ~ 10.0 wt.% (see Figure 5c). As shown in Figure 5c, as the concentration of CNC suspensions is increased, bigger tactoids start to appear. Fingerprint texture was also observed in pure CNC8 suspension at the concentration of 7.0 wt.% ~ 10.0 wt.% and the helical pitch decreased as the concentration increased; and the helical pitches in the range of 5.3 ~ 8 µm were observed



in this study. CONCLUSIONS In summary, a low-cost and high-yield way to prepare CNCs-COOH from pulp was successfully developed by using KMnO4 as a reagent for oxidation at mild reaction conditions (50 oC, 1 wt.% sulfuric acid medium). Interestingly, it is found that the addition of H2C2O4 in the oxidation system is helpful for reducing the usage amount of KMnO4 as well as the reaction time. The diameter and length of CNCs-COOH prepared are 10 ~ 22 nm and 150 ~ 300 nm, respectively. Comparing to conventional CNCs-COOH prepared from wood celluloses by Tempo or APS method, KMnO4-CNC uses less chemicals with higher yield (68.0%). It may be advantageous for obtaining CNCs-COOH in a large-scale at the industrial level for applications such as reinforcements in polymer composites. Furthermore, the self-assembled behavior of the liquid crystals from as-prepared CNCs-COOH demonstrated great potential in constructing advanced functional materials.

ASSOCIATED CONTENT Supporting Information The relationship between the yield and dimensions of the prepared cellulose nanocrystal and the reaction time by using potassium permanganate as an oxidation in mild acid condition, CNCs-COOH preparation processes by APS and Tempo-mediated oxidation method.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Fax: +86 532 84022791; Tel: +86 532 84022604;


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Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the financial support from Natural Science Foundation of Shandong Province, China (ZR2016EMB10) and the National Natural Science Foundation of China (51573082, 21774068, 21604047, 21673148, 21303111 and 51403095)

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For Table of Contents Use Only

Carboxylated cellulose Nnanocrystals prepared by potassium permanganate/oxalic acid redox system and their AFM Morphologies and liquid crystalline behaviors.


ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32


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One-Pot Preparation of Carboxylated Cellulose Nanocrystals and

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