Modified ammonium persulfate oxidations for efficient preparation of carboxylated cellulose nanocrystals

Journal Pre-proof Modified Ammonium Persulfate Oxidations for Efficient Preparation of Carboxylated Cellulose Nanocrystals Yunxiao Liu, Lehuan Liu, Kuntao Wang, Hui Zhang, Yuan Yuan, HongXiu Wei, Xin Wang, Yongxin Duan, Lijuan Zhou, Jianming Zhang

PII:

S0144-8617(19)31240-8

DOI:

https://doi.org/10.1016/j.carbpol.2019.115572

Reference:

CARP 115572

To appear in:

Carbohydrate Polymers

Received Date:

28 August 2019

Revised Date:

26 October 2019

Accepted Date:

5 November 2019

Please cite this article as: Liu Y, Liu L, Wang K, Zhang H, Yuan Y, Wei H, Wang X, Duan Y, Zhou L, Zhang J, Modified Ammonium Persulfate Oxidations for Efficient Preparation of Carboxylated Cellulose Nanocrystals, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115572

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Modified Ammonium Persulfate Oxidations for Efficient Preparation of Carboxylated Cellulose Nanocrystals Yunxiao Liu†, Lehuan Liu†, Kuntao Wang†, Hui Zhang†, Yuan Yuan†, HongXiu Wei†, Xin Wang†, Yongxin Duan†, Lijuan Zhou*†‡, Jianming Zhang†

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† Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial

Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao

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266042, China.

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‡ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu

To whom all correspondence should be addressed.

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*

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610065, China

Fax: +86-532-84022791

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E-mail: [email protected] (L. Z.)

Graphical Abstract

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Highlights

A high yield of CNCs-COOH was achieved based on the modified APS method.

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Activation of APS and ultrasonic assisted disintegration were simultaneous introduced.

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An efficient modification strategy for preparing CNCs-COOH is proposed.

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ABSTRACT

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Ammonium persulfate (APS) was able to produce carboxylated cellulose nanocrystals

(CNCs-COOH) directly from the raw materials of cellulose. However, the industrial production of CNCs-COOH by this method is obstructed by the lower preparation efficiency. Herein, by the activation via N,N,N’,N’-tetramethylethylenediamine (TMEDA) and ultrasonic assisted disintegration, modified APS method to extract CNCs-COOH from pulp was presented. A high 2

yield (up to 62.5%), low usage of APS (8.5 g APS per gram pulp) and less reaction time (6 h) was achieved. The as-prepared CNCs-COOH exhibited high carboxyl group content (1.45 mmol/g) and high crystallinity index of 93%. The reaction mechanism has been studied, and the results show that with the addition of TMEDA, S2O82–can be converted to free radicals and hydrogen peroxide more quickly. Our studies suggested that modified APS method may be a suitable and economic alternative for the preparation of CNCs-COOH. Keywords: Carboxylated cellulose nanocrystals; Activation of ammonium persulfate;

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N,N,N’,N’-tetramethylethylenediamine (TMEDA); Ultrasound

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Keywords: Carboxylated cellulose nanocrystals; Activation of ammonium persulfate; N,N,N',N'-tetramethylethylenediamine (TMEDA); Ultrasound

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1. Introduction

Carboxylated cellulose nanocrystals (CNCs-COOH) have a reactive surface of carboxyl groups that facilitate grafting chemical species to achieve different surface properties (Grishkewich, Mohammed, Tang, & Tam, 2017; Trache, Hussin, Haafiz, & Thakur, 2017), thus expanding their application potential. Moreover, the carboxylate group can interact in

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numerous ways with diverse chemical species, such as through hydrogen bonding (Akhlaghi, Berry, & Tam, 2013), electrostatic interactions (Lombardo, & Thielemans, 2018), coordination interactions (Janicki, Mondry, & Starynowicz, 2016). Accordingly, CNCs-COOH can provide

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active sites for the template-based preparation of nanoparticles or hybrid nanocomposites (Yonatan et al., 2018; Parize et al., 2017), surface modification (Cao, Fan, Huang, & Chen,

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2019) as well as adsorbent/flocculants (Song et al., 2019). In addition, CNCs-COOH show

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excellent thermal stability (Zhou et al., 2018). Therefore, the preparation and application of CNCs-COOH have attracted much attention in recent years.

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To date, CNCs-COOH have been successfully fabricated via various methods, such as 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation (Saito, & Isogai,

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2004), periodate-chlorite oxidation (Suopajärvi et al., 2013), hydrochloric/nitric acid

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hydrolysis (Miao, Qin, Chen, Liu, & Ren, 2017), citric/hydrochloric acid hydrolysis (Yu, Chen, Wang, & Yao, 2015), and APS oxidation (Leung et al., 2011). Among them, TEMPO-mediated oxidation is the most used method. To obtain CNCs-COOH by TEMPO method, either hydrolysis with an acid or mechanical treatment is required after the oxidation step (Batmaz et al., 2014). It is therefore a lengthy process with the use of harsh chemicals (Batmaz et al., 2014; 4

Saito, & Isogai, 2004). Of note, APS oxidation method has two unique advantages: 1) The preparation process is much simpler, as APS can produce CNCs-COOH directly from the raw materials of cellulose by simultaneously removing lignin, hemicellulose, and other plant contents (Leung et al., 2011; Oun, & Rhim, 2018); 2) The post-processing is much simpler. Unlike the KMnO4 oxidation method reported by Zhou et al (Zhou et al., 2018), which need to remove the Mn2+ by aeration, this method just need to wash the suspension by centrifugation

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with deionized water (Jiang, Wu, Han, & Zhang, 2017). Besides that, CNCs-COOH produced through APS treatment show higher charge densities (Mascheroni et al., 2016). Unfortunately, the time-consuming procedures (16 – 24 h) and requirement of high amounts of APS (228.01

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g APS needed in 10 g pulp) make the cost of this method prohibitive for large-scale application

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(Leung et al., 2011). Nevertheless, such an easily scalable approach could offer a commercially viable method for extracting CNCs-COOH. Therefore, it is necessary to further modify this

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approach. However, few efforts have been devoted to this goal to date.

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It is well known that when S2O82− is chemically or thermally stimulated, SO4•− (S2O82− + initiator → SO4•− + SO4•− or SO42-) is easily produced (Kamagate, Amin Assadi, Kone,

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Coulibaly, & Hanna, 2018). Several chemical initiators have been exploited to activate S2O82−, such as alkaline pH (Furman, Teel, & Watts, 2010), transition metals (Naim, & Ghauch, 2016),

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and organic amines (Feng, Guo, & Qiu, 1988). Among these chemical initiators, N,N,N’,N’tetramethylethylenediamine (TMEDA) as a redox initiator, can form a redox-initiating system with APS, serving as an electron donor to catalyze the decomposition of S2O82– to generate SO4•−, hydroxyl radical (HO•) and bisulfate radical (HSO4•) within a relatively short time (Dan et al., 2013; Feng, Guo, & Qiu, 1988). Since all of these free radicals possess much higher 5

redox potentials than their parent compounds (Eo (APS) = 2.01 V, Eo (SO4•−) = 2.60 V; Eo (HO•) = 2.70 V) (Kamagate, Amin Assadi, Kone, Coulibaly, & Hanna, 2018; Oh, & Shin, 2014), they can achieve the rapid decomposition of a wide range of organic compounds. Therefore, the goal of the present study was to determine the influence of TMEDA in generation of free radicals under thermal for activating APS in the preparation of CNCs-COOH. We further introduced an ultrasonic smash step during the reaction process to increase the

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specific surface area of the partially oxidized pulp to increase the reaction sites and simultaneously improve the preparation efficiency. This study could offer a rapid and efficient

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route for the production of CNCs-COOH from pulp via APS oxidation at the lowest cost (less APS usage and shorter reaction time) thus far reported.

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2. Experimental section

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2.1. Materials.

Cotton pulp with cellulose polymerization degree (DP) of 600 was supplied by Silver

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Hawk Co. Ltd. (Gaomi, China), the content of cellulose in cotton pulp is 96.2%. Ammonium

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persulfate (APS, 99%), N,N,N',N'-Tetramethylethylenediamine (TMEDA, 99%), potassium iodide (KI, 99.5%), sodium bicarbonate (NaHCO3, 99.7%), sodium hydroxide (NaOH, 96%),

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hydrochloric acid (HCl, 36.5%) with analytical grade were purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used for all experiments. 2.2. Preparation of CNCs-COOH using TMEDA as redox initiator to form active system couples with APS.

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Some parameters, such as initial mass ratio of APS and TMEDA (see supplementary data, Table S1), mass ratio of APS and pulp (see supplementary data, Table S2), and reaction temperature were evaluated to find the optimal conditions for the efficient preparation of CNCs-COOH. 5 g cotton pulp were soaked in 200 mL deionized water for 24 h. After that, the designed quantity of APS and TMEDA were added to the flask successively and reacted at setting temperature (T = 60 oC − 80 oC) for 4 h with vigorous stirring firstly, then 1mL of the

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partially oxidized pulp suspension was taken out for studying the effect of smashing on the oxidation degree with polarizing optical microscope (POM), the rest of the partially oxidized pulp suspension was ultrasonically smashed using an ultrasonic cell disrupter (20 kHz, Scientz-

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JY92-IIDN) for 10 min, followed by reacting for another 2 h, respectively. Subsequently, the

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suspension was washed by centrifugation with deionized water until the supernatant turned turbid, and the yield of the obtained CNCs-COOH suspension was measured gravimetrically.

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Supernatant of the reaction system was taken out every 10 min for the first hour and then every

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one hour to determine the fate of S2O82− by Raman spectroscopy (Lam et al., 2013) and UV techniques (Liang, Huang, Mohanty, & Kurakalva, 2008). Dialyzed the CNCs-COOH

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suspension using a dialysis tube with MWCO 8000 − 14000 to remove the soluble carbohydrates and the solid content of the obtained pure CNCs-COOH suspension. The

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preparation of CNCs-COOH through thermally activated of APS were taken with the similar process.

2.3. Characterization of CNCs-COOH. The morphology and dimensions were observed by atomic force microscope (AFM). A 7

drop of CNCs-COOH suspension (0.1 mg/mL) was spin-coated on freshly cleaved mica at 3000 rpm for 1 min. AFM image was acquired using a Multimode V (VEECO) under contact mode. The Fourier transform infrared spectroscopy (FT-IR) spectra were obtained on a Bruker VERTEX 70 spectrometer. The spectra were recorded from 4000 to 400 cm-1 for 64 scans at a

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resolution of 4 cm-1. Raman spectra were collected on a Horiba/Jobin Yvon laser Raman analyzer LabRAM HR 800 (Horiba/Jobin Yvon, Longjumeau, France) equipped with a diode 784.8 nm laser,

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operating at 1000 μm. The freeze-dried powder of supernatant was pressed into a pellet with the help of tablet press, and five replicate spectra were obtained from each sample.

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The carboxyl group contents of the obtained CNCs-COOH samples were determined via

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conductometric titrations. In brief, 20 mL of CNCs-COOH suspensions containing 0.15 g of solid content were sonicated to get a well-dispersed suspension. Then, 0.01 M NaOH was

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added at a rate of 0.1 mL/min to set the pH in the range of 11 − 12. The CNCs-COOH suspensions were then titrated with 0.01 M HCl using a PHS-3E meter and the carboxyl

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contents were determined from the resulting conductivity curves (Zhou et al., 2018). This

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procedure was performed in triplicate. The zeta potentials of the CNCs-COOH were determined with a Malvern Nano ZS90 light

scattering instrument. 2.5 mL CNCs-COOH suspensions at a concentration of 0.01 mg mL-1 were carried out in triplicate at 25 °C. The crystal structures and degree of crystallinity of CNCs-COOH were investigated 8

using X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.154 nm) the range of scatter in angle (2θ) was from 5° to 40° with a step interval of 0.02° and a scanning rate of 5° min-1. The crystallinity index (CI) of CNCs-COOH was calculated using the Segal method (Luzi et al., 2019). The thermal stability of CNCs-COOH were carried out using TA Instruments Q500 thermogravimetric analyzer under a nitrogen atmosphere. The temperature was set from 40 °C

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to 600 °C with a heating rate of 20 °C min-1 to record the TGA analysis curves.

The UV-vis absorption spectra were carried out in the range of 200 to 800 nm to evaluate

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the residual amount of APS. The calibration curve of APS concentration was conducted by the methods reported by Liang et al. (Liang, Huang, Mohanty, & Kurakalva, 2008), and the

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experiments were conducted in duplicate and data presented in all figures are mean values.

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3. Results and discussions

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3.1. Comparison of conventional and modified APS oxidation processes

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Fig. 1. Illustration of the conventional and modified APS oxidation mechanism and oxidation process. Leung and coworkers reported that CNCs-COOH could be prepared directly from a variety of cellulosic biomass via APS oxidation hydrolysis (Leung et al., 2011). However, the amount of APS required with this method was prohibitively high and the reaction time was too long, indicating the weak hydrolysis ability of APS oxidation at 60°C, the conventional APS

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oxidation mechanism and oxidation process were shown in Fig.1a and c. As shown in Fig. 1a, the conventional APS oxidation only produced one type of free radical. TMEDA as a redox

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initiator, it can form a redox-initiating system with APS to produce two more types of free radicals more quickly. As shown in Fig. 1b, in addition to SO4•− and H2O2, several more types

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of strong oxidizing free radicals (HO•, HSO4•) were also produced using our modified approach

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(the activation mechanism of the TMEDA/S2O82− system is shown in Fig. 2a) (Feng, Guo, & Qiu, 1988). Moreover, the generated HO• can disintegrate the pulp fiber faster with its much

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higher redox potentials (Eo = 2.70 V) (Kamagate, Amin Assadi, Kone, Coulibaly, & Hanna, 2018). As shown in Fig. 1d, introduction of the ultrasonic smashing step to the modified

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oxidation process can decrease the length of the fibers while increasing the surface area (see

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Fig. 2b), thereby increase the number of possible reaction sites to further improve the preparation efficiency.

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Fig. 2. (a) Scheme of the activation mechanism of the TMEDA/persulfate system; (b) Process of smashing the partially oxidized pulp.

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To demonstrate the influence of TMEDA in generating of free radicals under thermal for

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activating APS, we selected TMEDA-based activation system at 75 °C and thermal activation alone at 80 °C to prepare CNCs-COOH for comparison (the prepared CNCs-COOH were

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designated CNC/TM-75 and CNC/80, respectively), along with ultrasonically smashing the

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partially oxidized pulp. The characteristics of these CNCs-COOH obtained through this modified method were then compared to those obtained using the conventional method

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(designated CNC/Ref.APS).

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Fig. 3. Comparison of various ways of preparing CNCs-COOH: (a) ratio oxidant/pulp and yield; (b) zeta potential and reaction time. As shown in Fig. 3, the yield of CNC/Ref.APS only reached 36% at 60°C for 36 h, and the absolute zeta potential value was relatively low (Zhou et al., 2018), which is mainly due to the long half-life of APS at 60 °C (t1/2 = 38.5 h) (Wacławek et al., 2017), resulting in the low effective concentrations of the free radicals and H2O2, that are essential for breaking down

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amorphous cellulose (Koshani, van de Ven, & Madadlou, 2018). By contrast, with our modified APS oxidation method, we used less amounts of APS with a shorter reaction time, while

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obtained a relatively higher yield and absolute zeta potential value of the generated CNCsCOOH (CNC/80 and CNC/TM-75). In addition, the yield and absolute zeta potential values of

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CNC/TM-75 were both higher than those of CNC/80. This improvement is mainly attributed

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to the thermal activation with TMEDA, resulting in production of more diverse strong oxidizing free radicals (HO•, HSO4•) (Kretlow, Klouda, & Mikos, 2007) and H2O2 to breaking

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down the cellulose chain in the amorphous region rapidly. These results indicated that the modified APS oxidation method results in stronger oxidative and greater efficiency for

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preparation of CNCs-COOH.

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3.2. Effect of activation methods on the final product For the sake of clarity and convenience, we here designate the CNCs-COOH produced

with thermal activation (i.e., varying temperatures) only and under thermal activation with TMEDA as CNCT and CNCTM-T, respectively. Table 1. Carboxyl content and zeta potential yield for the CNCs-COOH prepared under various 12

conditions with fixed reaction time (6 h). APS /g

TEMED /g

Temperature /°C

Zeta potential/mV

Yield /%

60

Carboxyl content (mmol/g) 0.35±0.08

CNC60

42.5

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-26.55±0.5

40.56

CNC65

42.5

-

65

0.42±0.06

-28.67±0.6

44.94

CNC70

42.5

-

70

0.48±0.02

-29.02±0.4

45.60

CNC75

42.5

-

75

0.58±0.03

-30.71±0.8

48.70

CNC80

42.5

-

80

0.72±0.04

-32.77±0.7

49.81

CNCTM-60

42.5

1.7

60

0.85±0.05

-35.72±0.5

48.00

CNCTM-65

42.5

1.7

65

0.92±0.04

-39.37±0.3

49.92

CNCTM-70

42.5

1.7

70

1.28±0.03

-41.61±0.5

55.00

CNCTM-75

42.5

1.7

75

1.38±0.05

-49.99±0.4

62.50

CNCTM-80

42.5

1.7

80

1.45±0.06

-52.23±0.3

50.76

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Samples

As shown in Table 1, for the CNCs-COOH produced with thermal activation system

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(CNCT) only, when the reaction temperature increased from 60 °C to 80 °C, the carboxyl

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content of the prepared CNCs-COOH increased from 0.35 to 0.72 mmol/g, and the absolute zeta potential increased from 26.55 to 32.77 mV, respectively. Under the same temperature

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increase, for the thermal and TMEDA synergistically activated system (CNCTM-T), the carboxyl content increased from 0.85 to 1.45 mmol/g, and the absolute zeta potential increased from

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35.72 to 52.23 mV, respectively. These effects can be attributed to the generation of more active radical species in the thermal and TMEDA synergistic activation system, and the oxidization

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of more primary hydroxyl groups to carboxyl groups in the insoluble part (Leung et al., 2011; Zhou et al., 2018). This further indicates that the modified APS oxidation method has stronger oxidization ability than the conventional APS method. Moreover, the repulsive forces between the charged CNCs-COOH were sufficient to prevent the aggregation of CNCs-COOH (Miao, Qin, Chen, Liu, & Ren, 2017), thus the suspension can remain stable and exhibit good 13

dispersion stability. The Fig.S2 shown photographs of CNCTM-75 and CNC80 suspensions after six months, the results indicate that they still exhibit good dispersion stability. As shown in Fig. 4a, for the CNCs-COOH produced with thermal activation system only, the yield of the CNCs-COOH was 40.6% at 60 °C; when the reaction temperature was increased to 80 °C, the yield of CNCs-COOH increased to 49.8%. This result indicated that the preparation efficiency of CNCs-COOH depended on the reaction temperature to some extent,

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because increasing the reaction temperature can provide more energy to break down the O-O bonds for producing SO4•− and H2O2 more quickly (Chen, Bruell, Marley, & Sperry, 2003; Liu

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et al., 2018). For the thermal and TMEDA synergistically activated system, the yield of CNCsCOOH increased from 48.0% to 62.5% when the reaction temperature increased from 60°C to

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80°C, and the maximal yield was obtained at 75°C. This result indicated that TMEDA as a

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redox initiator, can reduce the decomposition activation energy of APS to some extent (Murthy, Mohan, Rao, Raju, & Sreeramulu, 2005), enabling APS to decompose and produce more free

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radicals and H2O2 at the lower temperature more quickly. Based on these results, CNC80 and

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CNCTM-75 were selected for further comparison and characterization.

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Fig. 4. (a) Yield of CNCs-COOH obtained under different activation system: Thermal, with only thermal activation, and TMEDA+Thermal, with thermal and TMEDA synergistic

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activation. (b) and (c) X-ray diffraction profile, (d) Thermogravimetric analysisi traces, and (e)

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FT-IR spectra of cotton pulp and the CNCs-COOH prepared via the APS oxidation process under different reaction conditions.

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The crystallinity of CNCs is an important factor for controlling the rigidity and thermal stability. Therefore, the changes of the crystalline structure and crystallinity were evaluated

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from comparison of XRD spectra. As shown in Fig. 4b- c, the XRD patterns of CNCs-COOH

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clearly showed the characteristic diffraction peaks of cellulose I crystal structure, such as diffraction peaks at 2θ = 15.1°, 16.5°, and 22.6°, corresponding to the (1-10), (110), and (200) crystallographic planes, respectively (Park, Choi, Oh, & Hwang, 2019). These results suggest that the modified APS hydrolysis system did not alter the original crystal integrity of cellulose. Moreover, the CI of all obtained CNCs-COOH increased from 77.1% for pulp to over 90% 15

after hydrolysis with the modified APS hydrolysis system, which indicated that the amorphous region of pulp undergoes oxidized to form CNCs-COOH. The high CI reflect the fast hydrolysis of disordered regions of the pulp. As shown in Fig. 4d, the thermogravimetric analysis showed that the Tonset values of all as-prepared CNCs-COOH were lower than that of the original pulp, which is most likely due to the increased amount of carboxyl groups on the surface of CNCs, leading to the formation

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of unstable anhydroglucuronate units that would decompose at an early stage (Li et al., 2015; Sharma, & Varma, 2014). Moreover, the Tmax values of the as-prepared CNCs-COOH were

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slightly lower than that of pulp, which may be attributed to the increase in the surface area (Zhou et al., 2018). Of note, the CNCs obtained via APS oxidation could endure the typical

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processing temperatures of many thermoplastic polymers. Further, as shown in Fig. 4c, the

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residual at 600°C wt.% of CNCTM-75 was lower than that of CNC80 despite its higher CI, which may be ascribed to its smaller size (as shown in Fig. 5), corresponding to a larger specific

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surface area that leads to the relatively faster heat transfer. Fig. 4e shows the FT-IR spectra of pulp, CNC80, and CNCTM-75. All characteristic peaks

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of pulp were observed in the spectra of CNCs-COOH samples, including the O-H stretching

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vibration around 3427 cm−1, C-H stretching vibration at 2896 cm−1, C-O-C bending and symmetric stretching at the β(1−4) glycosidic linkage at 1162 cm−1 and 897 cm−1, respectively, and C-O-C stretching of pyranose and glucose ring skeletal vibrations at 1056 cm−1and 1116 cm−1, respectively. The absorption peak at 1640 cm−1 is related to the adsorbed water due to the presence of abundant hydrophilic hydroxide radicals in cellulose (Luzi et al., 2019; Yang 16

et al., 2014). This result indicated that the cellulose structure of the CNCs-COOH was preserved. In addition, a new carbonyl peak at 1728 cm−1 was observed, conforming the existence of carboxyl groups (-COOH) on the surface of the prepared CNCs (Cheng et al., 2013), due to the carboxylation of cellulose from the oxidative degradation of APS. Moreover, the carboxyl peak strength of CNCTE-75 was significantly greater than that of CNC80, which is in line with the difference in the carboxyl content and zeta potential values of the prepared

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CNCs-COOH. Overall, these results indicated that the incorporation of TMEDA can decrease the decomposition activation energy of APS to facilitate breakdown of the O-O bond and more

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easily generate free radicals with strong oxidative ability.

AFM showed that both CNC80 and CNCTM-75 have a rod-like morphology with lengths of

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120 – 370 nm and 80 – 350 nm (size distribution of CNC80 and CNCTM-75 were shown in Fig.S3),

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and diameter of 4 – 12 nm and 3 – 12 nm, respectively (Fig. 5a and b). The smaller size of CNCTM-75 than CNC80 reflects the strong oxidative reaction, which suggested that a higher

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concentration of free radicals was produced in the thermal and TMEDA synergistic activation system, even at lower temperature. This was a very important finding, since the uniformity and

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small diameter of such nanocrystals are critical for their intended applications as nanofillers or

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for drug delivery.

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Fig. 5. Comparison of morphologies of CNCs-COOH prepared under thermal activation with or without TMEDA observed by AFM. AFM height images and height profiles of (a, c) CNC80 and (b, d) CNCTM-75. Height profiles correspond to the location of write dotted lines in the AFM

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3.3. Effect of smashing on the oxidation degree

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height images.

Fig. 6. Comparison of POM images of partially oxidized cotton pulp obtained under different activation conditions for 4 h before (A - C) and after (a - c) ultrasonic smashing. (A, a) Cotton pulp oxidized under 60 oC; (B, b) Cotton pulp oxidized under 80 oC without TMEDA; (C, c) Cotton pulp oxidized under 75 oC with TMEDA. 18

As shown in Fig. 6A, the length of the fibers oxidized at 60 °C for 4 h was the longest, whereas the length of the partially oxidized fibers obtained via thermal activation with TMEDA at 75 °C was the shortest (as shown in Fig. 6C). This difference is mainly due to the concentrations of free radicals and H2O2 which are low at 60 °C (Bashar, Zhu, Yamamoto, & Mitsuishi, 2019), resulting in a lower oxidation degree of cotton pulp; When the reaction temperature increased to 80 °C, more energy can be provided to breakdown the O-O bond in

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APS, so that the free radicals and H2O2 contents increased (Chen, Bruell, Marley, & Sperry, 2003), therefore, the partially oxidized cotton fibers is shorter (as shown in Fig. 6B); For pulp oxidized under 75 °C with TMEDA system, as TMEDA can activate APS to form more diverse

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and strong oxidizing free radicals and H2O2 rapidly (Feng, Guo, & Qiu, 1988), which in turn

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co-hydrolyzed the 1,4-β bonds of the cellulose chain in the amorphous region, resulting in rapid hydrolysis of the cotton pulp. After the pulp was oxidized by APS for 4 h, the swollen and

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partially oxidized pulp was smashed by an ultrasonic cell disrupter. As shown in Fig. 6a-c, the

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length of the partially oxidized cotton fibers obviously decreased; thus, after a further 2 h reaction, the CNCs-COOH with high yield were obtained. Compared with conventional APS

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oxidation, up to 48% of the pulp was converted to CNCs-COOH via APS hydrolysis with the assistance of ultrasonication, even with a lower amount of APS and reduced reaction time, as

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listed in Table 1. This result indicated that the ultrasonically smashed processes greatly improved the preparation efficiency of CNCs-COOH. 3.4. Effect of temperature and TMEDA on the decomposition of APS

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Fig. 7. (a) Thermal activation by increasing the reaction temperature from 60 oC to 80 oC only

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and (b) with TMEDA for 1 h characterized by Raman spectra; (c) UV absorption spectra of APS and calibration curve of the APS concentration; (d) Comparisons of residual amounts of

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APS at 60 oC activation with and without TMEDA.

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To further clarify the degradation process of the pulp during APS treatment under the different activation conditions, the small-molecular-weight degradation products in the

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supernatants were analyzed by Raman spectroscopy. As shown in Fig. 7a, APS exhibited three major Raman peaks: 211 cm−1 (δm: deformational vibration of the S-O-O-S bridge), 835 cm−1 (νm: symmetric stretching vibration of the S-O-O-S bridge), and 1074 cm−1 (νs: symmetric stretching vibration of the S-O bond of the SO3− group of the persulfate anion) (Lam et al., 2013). 20

Upon thermal treatment, cleavage of the weakest bond of APS, the peroxide bond, is expected to generate two SO4− radical ions (S2O82− + heat → 2SO4•−). As shown in Fig. 7a, when the pulps were incubated in the APS solution under thermal activation with an increase from 60 oC to 80 oC for 1 h, the Raman peak of the supernatant assigned to the O-O bond of the persulfate was almost identical to that obtained with an increase from 60 oC to 65 oC, with the characteristic peaks at 211 cm−1, 835 cm−1, and 1074 cm−1 maintained, and no new peaks

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emerged; Remarkably, when the reaction temperature increased to 70 oC and above, the characteristic peak at 211 cm−1 began to diminish, and those at 835 cm−1 and 1074 cm−1 disappeared, indicating that the S−O−O−S bridge and S−O bond of the SO3− group of the

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persulfate cleaved to give two SO4•− radical anions (Lam et al., 2013). The most striking

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observation from the Raman spectra is the peaks at 880 cm-1, 1040 cm-1, and 1021 cm-1, which are ascribed to the asymmetric stretching vibration of HSO4-, symmetric stretching vibration

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of HSO4- ions, and the ν1(SO42−) band, respectively (Lam et al., 2013); For the TMEDA and thermal synergistic activation system, the characteristic peaks at 211 cm−1, 835 cm−1, and 1074

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cm−1 began to diminish after 1 h, even at 60 oC, and these three peaks completely disappeared

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at 70 oC (Fig. 7b). Except for the peaks at 880 cm-1, 1040 cm-1, and 1021 cm-1, which are ascribed to the asymmetric stretching vibration of HSO4-, symmetric stretching vibration of

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HSO4- ions and the ν1 (SO42−) band, respectively, an additional Raman band at 976 cm-1 was also displayed, which could be assigned as the ν1 (SO42−) band of ammonium sulfate, as an inorganic byproducts of the reaction (Lam et al., 2013). These results further support that TMEDA can accelerate the decomposition of APS to produce free radicals more quickly, thus greatly improving the preparation efficiency of CNCs-COOH. 21

To further clarify the fate of APS under different activation conditions, UV spectrophotometric determination of persulfate was used to evaluate the residual amount of APS in the different activation systems. Fig. 7c shows the absorption spectra and calibration curve of absorbance of persulfate/NaHCO3/KI solutions; the calibration curve of absorbance versus the concentration of APS at 352 nm was used to convert the absorption values of APS to its corresponding concentration (Liang, Huang, Mohanty, & Kurakalva, 2008). Under

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hydrolysis at 60 oC, the residual amount of APS remaining in the TMEDA-assisted APS oxidation system reduced more quickly than that without TMEDA (the residual amount of APS in the different activation system were compared systematically and shown in Fig. S1),

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validating that TMEDA is indeed an effective activator for APS to form free radicals and H2O2

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rapidly.

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4. Conclusions

A highly efficient modified APS oxidation method was developed for the preparation of

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CNCs-COOH, based on thermal activation with the synergistic effects of TMEDA and ultrasonic-assisted disintegration. The optimal reaction conditions for CNCs-COOH

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production was 75 °C with TMEDA for 6 h, with a pulp:APS:TMEDA mass ratio of 1:8.5:0.34.

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Under these conditions, TMEDA could accelerate the decomposition of APS to produce strong oxidizing free radicals more quickly. The POM results indicated that ultrasonically smashing the partially oxidized pulp facilitated the fibrillation of the fibers, to further improve the preparation efficiency. The rod-like CNCs-COOH could be produced with the highest reported yield (up to 62.5%), along with high crystallinity (92.1%), a high carboxyl group content (up 22

to 1.45 mmol/g, corresponding zeta potential of -52 mV), with a length of 80 – 350 nm and diameter of 3 – 12 nm. This modified APS oxidation process can offer an alternative method for CNCs-COOH preparation. Supplementary data The effect of initial mass ratio of APS/TMEDA and APS/pulp on the yield of CNCs-

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COOH, photographs of CNCs-COOH suspensions dispersion stability after six months, size distribution of CNC80 and CNCTM-75, comparisons of residual amount of APS in the different activation system.

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Acknowledgements

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The authors acknowledge the National Natural Science Foundation of China (51573082, 21774068, and 21604047) and Financially Supported by the Opening Project of State Key

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Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme2019-4-

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