Stability of soluble dialdehyde cellulose and the formation of hollow

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Stability of soluble dialdehyde cellulose and the formation of hollow microspheres: optimization and characterization Guihua Yan, Xiuqiang Zhang, mengzhu li, Xiaoyu Zhao, Xianhai Zeng, Yong Sun, Xing Tang, Tingzhou Lei, and Lu Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04825 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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ACS Sustainable Chemistry & Engineering

Stability of soluble dialdehyde cellulose and the formation of hollow microspheres: optimization and characterization

Guihua Yan†, ‡, Xiuqiang Zhang‡§, Mengzhu Li†, ‡, Xiaoyu Zhao†, ‡, Xianhai Zeng*†, ‡, Yong Sun†, ‡, Xing Tang†, ‡, Tingzhou Lei‡§, Lu Lin*†, ‡

†

College of Energy, Xiamen University, Xiang’an South Road, Xiamen, 361102, P.

R. China ‡ Fujian

Engineering and Research Center of Clean and High-valued Technologies for

Biomass; Xiamen Key Laboratory of Clean and High-valued Applications of Biomass, ‡§

Xiamen University, Xiang’an South Road, Xiamen 361102, P. R. China

Henan Key Laboratory of Biomass Energy, Huayuan Road 29, Zhengzhou, Henan

450008, P. R. China * Corresponding author. Tel./Fax: +86-592-2880701; Email: [email protected] (Xianhai Zeng), [email protected] (Lu Lin).

Abstract The poor solubility of raw cellulose greatly limits its applications. Herein, the periodate oxidation of cellulose was conducted to obtain dialdehyde cellulose (DAC), which is a kind of soluble and figurable cellulosic derivatives with great potential applications as a functional material. The oxidation of cellulose included a process of exfoliating the cellulose from its original source, and the particle size was reduced after a prolonged oxidation. The dissolved DAC, solubilized in water by heating, showed an obvious Tyndall phenomenon with a strengthening trend with aging, and the molecular weight decreased sharply at the beginning of aging and then tended to be stable after 3 months. A large change in the freeze-dried DAC solutions with

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different oxidation degrees, ranging from crystal string beading to hollow microspheres, was explicated by layer peeling during oxidation, while the solution was at a steady state after aging. The special structure of DAC, such as the existence of aldehydes with low molecular weight in solution-based processes and the formation of regular hollow spheres after freeze-drying, enabled it to be used in cosmetics, food, wound dressings and drug carriers. Key words: Dialdehyde cellulose, Soluble, Stability, Hollow microspheres, aging, oxidation degree Introduction As a kind of natural polymeric material, cellulose has many advantages, such as its

low

cost

and

wide

availability,

renewability,

biodegradability,

and

biocompatibility, which have garnered sharply increased interest in its potential for the replacement of fossil resource-based materials.1-3 However, the fibrils from various biomass materials (bamboo pulp, wood pulp, cotton linters, tunicates, etc.) exist in the form of aggregation structures and their applications are strictly limited due to the large number of hydroxyl groups in the crystalline regions, which forms a large hydrogen bond network and restricts its solubility in water or common solvents.4,5 The modification of cellulose, as a major direction for further materials preparation, emerged quickly.6-9 Among

the

various

chemical

modification

pathways

(sulfonation,

TEMPO-mediated oxidation, silylation, etc.), periodate oxidation is a simple one that can selectively break the chemical bonds between C2 and C3 in the anhydroglucose units (AGU) of the cellulose and form two aldehyde structures that are called dialdehyde cellulose (DAC) (Scheme 1).10,11 This treatment introduces active groups at the cellulose surface and further convert into many other groups for derivatization.12,13 However, the long reaction time severely limits its application and produces some byproducts according to previous studies.14,15 The destruction mode during the oxidation and the stability of DAC solution during aging is still unclear.


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Therefore, it is necessary to make the periodate oxidation faster and controllable. It has been reported that DAC is soluble in water16,17, which offers a good premise for further applications, such as reinforcing hydrogels,18 assembling transparent cellulose films,19 or grafting copolymers.20,21 However, the specific existence form of DAC was still unclear. Another problem directly related to the activity of aldehyde groups is the stability of DAC in solution. According to Kim et al. (2004),17 the content of the aldehyde groups in solution decreased significantly after 3 weeks, while Lukas et al. (2017) reported that the molecular weight of DAC increased significantly after 14 days of aging and showed a decreasing trend after 28 days.16 Similarly, Sirvio et al. advised the use of a fresh DAC solution due to the significant abatement of the aldehyde content after 2 weeks.18 The structural changes and solution stability after aging for longer periods have not been reported yet. In this study, a mechanical method combined with sodium periodate oxidation was applied to assemble freeze-dried DAC with regular hollow spheres after aging. The composition as well as the morphology in a dry state and long-term stability of the aqueous solution at different oxidation stages were studied. In addition to the soluble DAC, the insoluble DAC powder and the supernatant of DAC solution were carefully analyzed. This study provides new insights into the preparation of DAC which represents the basis for potential applications in the future. Experimental Section Materials Sodium periodate (AR, ≥99.5%), hydroxylamine hydrochloride (AR, ≥98.5%), sodium acetate trihydrate (AR, ≥99.0%) and glacial acetic acid were obtained from Aladdin, China. The other chemicals, i.e. CaCl2 and NaOH were of reagent grade (Wako Chemicals, Tokyo). Deionized water was used throughout the work. Preparation of the cellulose microcrystals (CMCs) from bamboo pulp The bamboo pulp, obtained by homemaking it in the lab,22 was soaked in



deionized water and mechanically stirred for 2 h. Then the pulp was treated by fibrillation using circulation feeding with an ultrafine grinder (CM2000/10, Shanghai Yiken Machinery Co., Ltd., China). The suspension was homogenized by a high-pressure homogenizer (D-3L, PHD Technology Co., Ltd., USA) more than 20 times and concentrated to obtain a stable cellulose suspension (0.55 wt%). Preparation of DAC by periodate oxidation Sodium periodate, CMCs (1.5, molar ratio of oxidant to anhydroglucose unit (AGU)) and 100 mL deionized water were stirred at room temperature (approximately 25 °C) in the dark for 6-72 h.23 At the end of the reaction, the suspension was repeatedly centrifuged and dispersed into deionized water until the conductivity of the supernatant was close to that of deionized water. The resulting samples are referred to as DAC-n, with n being the periodate oxidation time. The reactions were performed in the same manner as conducted in section 2.2 at different temperatures and proper amounts of metal chloride, such as CaCl2. Preparation of the soluble DAC 1.0 g DAC obtained in section 2.3 was added into 30 mL deionized water and gently stirred at 100 °C in an oil bath. After that, solid-liquid separation was performed by a supercentrifuge at 11000 rpm for 10 min at 5 °C and the solid residue was obtained. The solubility of DAC was calculated based on the weight of the solid residue after freeze-drying. Determination of the aldehyde content According to the ammoniation of hydroxylamine hydrochloride and aldehyde groups, the aldehyde content was calculated from the nitrogen content of the product based on one mole of aldehyde and one mole of NH2OH·HCl (Scheme 1b). DAC (0.1 g) and NH2OH·HCl (0.5 g) were added to a conical flask containing 100 mL of a 0.1 M acetic acid buffer (pH=4.5) and stirred for 24 h. After that, the solution was centrifuged, washed, and freeze-dried accordingly. The aldehyde content was


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calculated by determining the nitrogen content of DAC using an element analyzer (Vario EL Ⅲ, Elementar Analysen Syetem GmbH, Germany).24

Scheme 1. (a) Periodate oxidation of cellulose; (b) Reaction between DAC and NH2OH•HCl Characterization The Tyndall test The dispersion of DAC in water showed an obvious Tyndall effect. In this study, the Tyndall effect was tested with a common laser indicator (Deli 2802, China), which emits red light at 650 nm.25-27 Before measurement, the supernatant of the DAC must remain static for more than 1 h to reach a stable state. Gel permeation chromatography (GPC) GPC analysis was performed on freshly diluted samples of soluble DAC (dilution factor of 10) using a BreezeTM HPLC system equipped with a Waters 1525 ELS detector. The column used was an Ultrahydrogel™ Linear (300 mm×7.8 mm id×2). An ammonium carbonate water solution (0.05 M) was used as the mobile phase. The column temperature was 30 °C and the flow rate of the mobile phase was 0.8 mL/min. Glucan with Mw in a span of 180-200000 g/mol was used as the external standard. Atomic force microscope (AFM) The surface topography and roughness of DAC were studied using an AFM



(Bruker MULTIMODE8, USA). The DAC suspension and soluble DAC were coated on a silicon chip with a size of 5 mm × 5 mm and then fully naturally dried. The images were captured by the tapping mode. The probe in the scanning range of 5 μm × 5 μm had an optical vibration frequency of approximately 200-400 kHz and the scan rate was 1.5 Hz. The scan lines and scan points were set to 256. Field emission scanning electron microscope (FE-SEM) An FE-SEM (Supra 55 Sapphire, Germany) was used to observe the surface morphology of DAC samples. The freeze-dried sample was coated on the SEM stage and sprayed with gold for 15 s. The analysis signal adopted the mode of SE2, and the acceleration voltage was 15 kV. 13C

solid NMR The

13C

NMR spectra were obtained by solid-state NMR techniques (Bruker

AVANCE III HD 400 M, USA). The spinning speed was 12 kHz. The standard conditions of the scan were to record at least 2000 times when the contact time and the cycle delay were 5 ms and 2 s, respectively. Sodium chloride was used to fill the two ends of the column, accompanied by a longer test time while the sample was of a small quantity. Thermogravimetric analysis (TGA) The thermal stability of the freeze-dried samples was measured by an STA 449 F5. Approximately 6 mg of sample material was placed in a platinum pan and heated at a uniform heating rate of 20 °C/min from 30 to 700 °C and the test was conducted under a N2 flow of 40 mL/min.


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Results and discussion Periodate oxidation of CMCs at different temperatures

100

12

80

10

60

8 y=0.14+0.15x

6

40 4 20 0

2 0

Aldehyde content (mmol/g)

14

Recovery (%)

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 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 10 20 30 40 50 60 70 80 90 100 110

Oxidation time (h) Figure 1. The recovery and aldehyde content of DAC produced by periodate oxidation at different oxidation times at room temperature. One mol of periodate was combined with one mol AGU to produce the corresponding dialdehyde derivatives, as shown in Scheme 1b. In this work, we used excess oxidant (i.e., 1.5 molar equivalent per AGU) to provide the basic conditions for the sufficient oxidation of cellulose at room temperature. Compared with the related periodate oxidation reported in the literature, a much longer oxidation time was used to explore the relationship between the time and aldehyde content.16 The aldehyde content was determined by titration, and the recovery of DAC was calculated by the solid content after filtration.17 As shown in Figure 1, when the oxidation time was increased from 0 to 72 h, the relationship between the oxidation time and aldehyde content was approximately a positive correlation (linear relation: y=0.14+0.15x) and reached a maximum of 12.1 mmol/g at about 72 h (0.17 mmol/g per hour). The corresponding recovery of DAC showed a decreasing trend and reached zero, indicating that the CMCs was fully oxidized at this point. This behavior may have been due to the high heterogeneity of



the reaction mode, resulting in the separation of unreacted cellulose crystals. When the reaction time was further increased to 96 h, the aldehyde content remained stable. Oxidation trials were also conducted at different temperatures other than room temperature, and the results were represented in Figure 2. It showed that the aldehyde content in DAC could be elevated considerably by raising temperature. The oxidation for 26 h at 45 °C reached completion, which was nearly three times shorter than that at room temperature. The oxidation rate of the CMCs at 45 °C (linear relation: y=0.74+0.42x) was obviously higher than that at room temperature (linear relation: y=0.14+0.15x). The aldehyde content could also be greatly improved by increasing the temperature (Figure 2a), and the recovery was reduced to zero much more sharply (Figure 2b). The complete oxidation was achieved in 20 h at 55 °C or 10 h at 65 °C, which was significantly faster than was achieved at room temperature (72 h). The results indicated that raising the temperature was beneficial to periodate oxidation.

Figure 2. The aldehyde content (a) and recovery (b) of DAC produced by periodate oxidation at different temperatures. Figure 2 also showed that when the temperature was above 75 °C, the aldehyde content rapidly increased up to almost half of the theoretical value and remained basically unchanged, and the corresponding recovery decreased rapidly.Furthermore, we found that the CMC suspension presented a pale-yellow clear state with brownish solids precipitating in the upper part, which may indicate the formation of I2 when the temperature was too high and the time was prolonged (≥3 h at 75 °C or 1 h at 85 °C).


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Using acetone as the solvent, the pure iodine (I2) and the precipitated brown substance were determined by spectrophotometry. The λmax values were both 363.5 nm, which proved that the brown substances precipitated at the reactor mouth were iodine (I2). One thing that could not be ignored was that even when heated, there was no brown precipitate for a short time after the beginning of the reaction, but only when the reaction had occurred for a period of time. Therefore, we think that the observation of a brownish color indicated that the appearance of aldehyde and metal salts might have catalyzed the decomposition of periodate.24,28 This was mainly due to a serious simultaneous periodate decomposition at high temperature (>75 °C), and side-reactions taking place with no change in the aldehyde content when the periodate ion was reduced to I2. The result indicated that the appropriate increase in the reaction temperature could greatly shorten the reaction time, and 65 °C was selected as the best reaction temperature for the further exploration experiments. Periodate oxidation of CMCs assisted by calcium chloride The stable network structure formed by the hydroxyl groups made it difficult for cellulose to be dissolved in water and other organic solvents. LiCl, a water-soluble alkali metal salt that can dissolve cellulose, can destroy the intermolecular hydrogen bond between the oxygen atom at the C3 site in the cellulose molecule and the hydroxyl hydrogen atom at the C6 site in another cellulose molecule, resulting in the improved efficiency of periodate oxidation.29 Similar to the Li+ ion, the Ca2+ ion has been shown to interact with cellulose and dissolve cellulose in the form of Ca(SCN)2·3H2O.24 Combined with this point we would like to know whether CaCl2 had a similar effect as LiCl in the oxidation process to improve the efficiency. Herein, we tested the effect of the concentration of CaCl2 on the oxidation efficiency at room temperature for 6 h. The results presented in Figure 3a indicated that CaCl2 could slightly improve the oxidation efficiency of CMCs at room temperature, and a ratio of 12 was chosen for further study.



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Figure 3. The recovery and aldehyde content of DAC produced by adding (a) different concentrations of CaCl2 at room temperature for 6 h and (b) a certain amount (12 mmol) of CaCl2 at 65 °C for various oxidation times. CaCl2-assisted periodate oxidation at 65 °C was conducted and the results are shown in Figure 3b. Excitingly, a sharp rise in the aldehyde content was achieved. Moreover, a much shorter reaction time (nearly 10 times shorter than that at room temperature and 2 times shorter than that at 65 °C) was needed with CaCl2-assisted oxidation than that without CaCl2. CaCl2 could obviously enhance the oxidation efficiency in this study, and even the promotion effect was stronger than that of LiCl.24 The oxidation efficiency was further improved by adding CaCl2 as a catalyst that destroyed the hydrogen bonds between the cellulose molecules. The reaction time to achieve the same aldehyde content was sharply reduced by raising the reaction temperature and using CaCl2 as a catalyst. The results showed that the periodate oxidation was made more efficient and more environmentally friendly. Morphology Table

1.

Soluble

DAC

distribution

with

GPC

analysis

(LogMW

=

1.36e+001-5.47e-001T^1, R2=0.9984) MW Mn MW Mn MW Mn MW Mn PDI PDI PDI PDI 4 4 4 4 Recovery (×104) (×104) (×10 ) (×10 ) (×10 ) (×10 ) (×104) (×104) Conditions (%) Fresh After 1 month After 3 months After 6 months 6 h RTa

95.6

2.84

1.60 1.78 1.91

0.30 3.03 0.22


0.09 2.44 0.21

0.09 2.33

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12 h RT

91.3

2.37

1.30 1.83 1.64

0.54 3.01 0.28

0.14 2.00 0.30

0.14 2.14

36 h RT

57.9

1.99

1.00 1.99 1.25

0.41 3.05 0.32

0.13 2.46 0.28

0.14 2.00

72 h RT

0

1.31

0.62 2.11 0.82

0.27 3.04 0.30

0.15 2.00 0.29

0.15 1.93

0

1.44

0.65 2.22 0.74

0.25 2.96 0.23

0.12 1.92 0.23

0.12 1.91

0

1.47

0.70 2.10 0.78

0.25 3.12 0.31

0.14 2.21 0.31

0.14 2.21

10 h (65 °C) 6 h CaCl2 (65 °C)b a

Oxidized at room temperature for 6 h, corresponding to DAC-6 in the text.

b

Oxidized at 65 °C for 6 h assisted by CaCl2. The long-term stability of soluble DAC was examined in terms of the chain

length (Table 1) and aldehyde content (Table 2). Table 1 showed the weight-average molecular weight (MW) and polydispersity index (PDI) of soluble DAC by heating at 100 °C for 2 h. MW, a parameter that was more able to represent the overall level of DAC, showed a considerable decrease when completely oxidized in the fresh group (MW = 1.31×104 g/mol; 53% lower than that of DAC-6, MW=2.84×104 g/mol). However, the fresh solutions with full oxidation had similar values of MW, which indicated that the degree of oxidation directly affected the kinetics of DAC degradation. Aging the fresh solutions at room temperature, the MW had a significant decrease, and the values were very close to each other after 3 months (MW=0.2×104-0.3×104 g/mol). The fresh solutions were unstable, and self-degradation occurred until a steady state was reached. Note that the tendency of MW to decrease sharply and then to maintain stability contradicted the results of the study of Luka et al. (2017) and Kim et al. (2004), where the Mw was found decreasing until complete degradation after aging long enough. The average molecular weight (Mn) decreased from 1.6×104 g/mol (fresh solution of DAC-6) to approximately 0.65×104 g/mol (fresh solutions obtained from complete oxidation) and subsequently dropped to 0.1×104 g/mol (aging for 3 months). This indicated that the decreased value of Mn can be correlated with the formation of low-molecular fragments, and the DAC chains maintained stability by splitting into small molecular fragments. The values of PDI



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(MW/Mn) first increased and then decreased as a result of the interaction between the formation of the low molecular fragments and the recombination of molecules. Table 2. The aldehyde content of soluble DAC

Conditions

Aldehyde content (mmol/g)c

Recovery (%)

Fresh

After 1 month

After 3 months

After 6 months

6 h RTa

95.6

0.83

0.78

0.55

0.52

12 h RT

91.3

2.31

2.15

1.77

1.71

36 h RT

57.9

5.19

4.68

3.97

3.95

72 h RT

0

11.12

8.37

4.41

4.36

10 h (65 °C)

0

11.09

8.24

4.33

4.31

6 h CaCl2 (65 °C )b

0

11.37

8.48

4.46

4.40

a,b

c

Same as Table 1.

The theoretical value for dialdehyde cellulose is 12.5 mmol/g. The detailed results on the aldehyde content of soluble DAC were given in Table

2. Compared with fresh solution, the aldehyde content of aqueous solution decreased significantly with the prolongation of the aging time. After 3 months the values were very close to those after 6 months. These changes could be ascribed to chain scission due to hydrolysis. To summarize, although the aldehyde group was easy to recombine into a macromolecular hemiacetal, the degradation/fragmentation of the molecular chains had a stable equilibrium point, causing MW to remain unchanged after 3 months. To explain the formation of these new bonds, the literature on the periodate oxidation of cellulose was reviewed, and it was known that the production of carbonyl groups was inevitable in near-primary and secondary alcohols, resulting in rapid recombination into hemiacetals. The main combinations were shown in Figure 4.


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Figure 4. Schematic diagrams of intra-chain hemiacetal recombination, the newly formed hemiacetal moieties are colored in green. In addition to the formation of hemiacetals in these intra-chains (Figure 4), inter-chain connections recombination could theoretically exist, but they were very difficult. It was mainly related to the inter-chain distances produced by crystal lattice and could only be achieved if the core of the microfibril crystal was intact. When the fresh soluble DAC samples were irradiated by a laser, a bright pathway was produced, which was identified as an obvious Tyndall effect.27 Interestingly, the phenomenon gradually waned, but it did not disappear with the increase in the cellulose oxidation degree (Figure 5A-D). This result indicated that the supernatant of DAC-6 dissolved in water was not truly "dissolved" but existed in water in a “suspended” or “aggregation” state. The same result was obtained when the cellulose was completely oxidized under different oxidation conditions (Figure 5D-F).



Figure 5. Optical photos of the fresh and the aged for 6 months samples of the as-prepared soluble (A/a) DAC-6, (B/b) DAC-12, (C/c) DAC-36, (D/d) DAC-72, (E/e) DAC-10 (65 °C) and (F/f) DAC-6-CaCl2 (65 °C). Notably, there was no apparent stratification in the solution samples and all showed a strong Tyndall phenomenon when aged for 6 months at room temperature (Figure 5a-f). This might be the occurrence of fragmentation of molecular chains in the process,30 resulting in a sharp decrease in the molecular weight after 6 months, which was consistent with the GPC test results.


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Figure 6. SEM images of the insoluble DAC and its freeze-dried aqueous solutions, (a, b) DAC-6; (c, d) DAC-36; (e, f) DAC-72. The surface morphology of the stored samples was observed by an SEM (Figure 6). The as-prepared DAC-6 (Figure 6a) appeared as dead branch shape with a length ranging from several microns to several hundred microns and without a fixed form. The surface structure of DAC-6 became loose but still retained the original irregular shape of CMCs, and its freeze-dried aqueous solution presented a thick withered leaf shape (Figure 6b). With further oxidation, the surface structure became looser (Figure 6c), and the large amorphous chains were visible with many spherical particles on the surface that were marked by ovals (Figure 6d). The structure of the completely oxidized CMCs became messy stripes with many small spherical particles under the numerous disorderly dead branches (Figure 6e), and its aqueous solution presented thin winged flakes embellished with crystal particles (Figure 6f). These phenomena indicated that the process of cellulose chain peeling occurred during the oxidation.



Figure 7. SEM images of the freeze-dried DAC after aging for 6 months at room temperature, (a) DAC-6, (b) DAC-36, and (c) DAC-72 and (d) the corresponding magnification diagram of DAC-72 , (e) DAC-10 (65 °C), and (f) DAC-6-CaCl2 (65 °C). However, when the soluble DAC samples aged for 6 months at room temperature, hollow microspheres were formed after freeze-drying as was seen in the SEM images. The difference was that with the increase in the oxidation degree, the existence form of the circular spheres gradually changed from a beaded crystal string (Figure 7a-b) and gradually transformed completely into hollow microspheres (Figure 7c), ranging in diameter from tens to hundreds of nanometers (Figure 7d). The results showed that the treated cellulose had great agglomeration potential and transformed to its most stable form, which was a hollow shape. The samples of DAC-10 (65 °C) and DAC-6-CaCl2 (65 °C) were the product of the complete oxidation of cellulose, similar to DAC-72, except that the paths to complete oxidation were different. They could


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also produce hollow microspheres similar to DAC-72, and the corresponding SEM images after aging for 6 months at room temperature were added as Figure 7e and f. Most spheres were well dispersed with uneven diameters between tens and hundreds of nanometers, indicating that the aging aqueous solution evolved into nanosized microspheres after freeze-drying. Further related study is ongoing in subsequent experiments.

Figure 8. AFM micrographs of (a) CMCs, (b) the soluble fraction of as-prepared DAC-6, (c) DAC-36, (d) DAC-72, (e) the soluble DAC-72 aged for 6 months and (f) the surface roughness of the materials. As shown in Figure 8, it could be clearly observed that the AFM images of the untreated cellulose exhibited a double shadow phenomenon, which might be due to the existence of an amorphous region with a loose structure resulting in the adhesion of the sample to the probe (Figure 8a). After oxidation, cracks began to appear on the surface of the rod-like structures, which tended to split into smaller particles but did not greatly change in size (Figure 8b). With further oxidization, the agglomerated cellulose was dispersed into small particles, and the size was obviously reduced (Figure 8c-d). This confirmed the previous conjecture about the layer peeling of the cellulose chains during the oxidation process, which was helpful to analyze the mechanism of oxidation. The parameters representing surface roughness, such as the



root-mean-square (RMS) and average roughness (Ra), were presented in Figure 8f. The Ra of DAC with different oxidation degrees gradually decreased from 14.37 to 2.45 nm. After storage for 6 months, the shape of DAC-72 became more regular and uniform, which was similar to the conclusion reached by the SEM analysis and indirectly explained why the Tyndall effect had been gradually enhanced. Chemical structure studies

Figure 9.

13C

solid-state NMR spectra of (a) CMCs, (b) DAC-6, (c) DAC-36, (d)

supernatant of DAC-6 dissolved in water, (e) DAC-72, and (f) soluble DAC-72 after aging for 6 months, (g) DAC-10 (65 °C), and (h) DAC-6-CaCl2 (65 °C). The

13C

solid-state NMR spectra of CMCs and DAC samples revealed more

specific information on the

molecular structural changes (Figure 9). The CMCs

presented typical resonance peaks between 60 and 110 ppm, which was identical to the results of previous studies.31,32 Compared with the CMCs (Figure 9a), the striking feature resulting of the DAC samples (Figure 9b-e) was the gradual disappearance of the resonance band corresponding to C1 carbons of the native cellulose at 105 ppm and the appearance of new resonances between 90 and 105 ppm with the increase in the oxidation degree. However, it cannot be ignored that no signal was detected in the carbonyl region (170-210 ppm) in DAC, while the periodate oxidation selectively broke the chemical bonds between C2 and C3 in an AGU of the cellulose and formed


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two corresponding carbonyl groups. In addition, the vanishing spectrum was replaced by new resonances located in the 55-63, 66-68, 78-85 and 92-102 ppm regions, ultimately overwhelming the original resonances. This phenomenon was attributed to the progressive loss of crystallinity and the recombination of the carbonyl groups with alcohol hydroxyl at the C6 site.33 The spectrogram of the supernatant of DAC-6 was basically the same as that of DAC-72, indicating that they had been transformed by hemiacetal crosslinking (Figure 9d and e).14 It could be inferred that the reaction may have occurred at the internal chains when the oxidation was not sufficient. While the oxidized was completed, the crystal structure of the cellulose was destroyed and resulted in chain recombination. There was no significant change in the spectrum of the complete oxidation of cellulose (Figure 9e-f). It was therefore the oxidation time after raising the temperature and adding CaCl2 as a catalyst that efficiently shortened the path to complete oxidation.

Figure 10. Thermogravimetric analysis of CMCs and DAC samples, (a) TG and (b) DTG curves. The TG curves of CMCs and DAC samples were performed (Figure 10a). The oxidizing reaction significantly changed the thermal degradation of DAC compared to that of the pure CMCs.13 The first region (T≈100 °C) corresponded to the loss of the unbound water. The onset of rapid mass loss at approximately 220 °C for DAC-6 was shifted to lower temperatures (about 150 °C) for DAC-72. This behavior could be attributed to a reduced molecular weight of the DAC. The third region (300



°C

Stability of soluble dialdehyde cellulose and the formation of hollow

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