Impact of Monovalent Cations on the Rheology of Cellulose Nanofibrils

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Impact of Monovalent Cations on the Rheology of Cellulose Nanofibrils

Rahmini

¹

, Soyoung Juhn

²

, Hyun-A Seong

^3‡

, Soo-Jeong Shin

^4†

Received October 10, 2019; Received in revised form November 7, 2019; Accepted March 2, 2020

ABSTRACT

In this study, we investigated the rheological properties of 1.5% cellulose nanofibril (CNF) with various concentrations of monovalent ions using a rheometer at 25℃. Monovalent ions (Na

⁺

, K

⁺

, and Li

⁺

) were used for the ionic crosslinking of CNFs. The shear viscosity, storage moduli (G′), and loss moduli (G″) of 1.5% CNFs with various added cations were measured at concentrations of 500, 1,000, 5,000, and 10,000 ppm. The presence of cat- ions in CNFs significantly induced strong hydrogels: As the cation concentration in- creased from 500 to 10,000 ppm, the viscosity, storage moduli, and loss moduli increased as well. All samples exhibited shear-thinning behavior with a monotonically decreased viscosity and an increased shear rate. Regarding hydrogel strength, we found that Li

⁺

>Na

⁺

>K

⁺

because of the different radii of the cations and interfibrillar bridging on the hydrogels. We concluded that the rheological properties of CNFs with various concentra- tions of monovalent ions differ depending on the ionic radius and concentration of each cation. Further research is proposed to determine the maximum strengths of hydrogels with added cations.

Keywords: Ccellulose nanofibrils, rheology, storage modulus, loss modulus, monovalent cation, ionic cross-linking

Printed in Korea http://dx.doi.org/10.7584/JKTAPPI.2020.04.52.2.12

1 Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea, Student 2 Nature Costech Inc., Cheongju, 28578, Republic of Korea, Director for R&D

3 Department of Biochemistry, Chungbuk National University, Cheongju, 28644, Republic of Korea, Professor

4 Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea, Professor

† Corresponding Author: E-mail: [email protected] (Address: Department of Wood and Paper Science, Chungbuk National University, Cheongju, 28644, Republic of Korea)

‡ Co-corresponding Author: E-mail: [email protected] (Address: Department of Biochemistry, Chungbuk National Uni- versity, Cheongju, 28644, Republic of Korea)

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1. Introduction

Cellulose is a natural polymer and is thus a bio- degradable, biocompatible, and renewable material that can be synthesized from various natural sources, such as plants, algae, bacteria, fungi, and marine animals. Nanocellulose, which is known as a cellulose nanofibril (CNF)—also called micro- fibrillated cellulose, cellulose nanocrystal, and bacterial cellulose—is an interesting nanomaterial derived from native cellulose. It has unique biolog- ical, physical, and chemical properties and thus has many potential applications in various com- mercial products and services, such as hydrogels, food additives, cosmetics, tissue engineering, composites (coatings, films, and foams), biomedi- cals, drug delivery, coating materials, rheology modification, and other functional materials.

^1-6)

A CNF is nanostructure cellulose with a diameter of less than 100 nm and a length of 0.76 µm to ≥4 µm;

the sizes of the fibrils vary depending on pre- treatment, plant sources, and the fibrillation pro- cess.

⁷⁾

There are several ways to produce CNFs through a high mechanical delamination process using grinding, high-pressure homogenization, micro-fluidization, and a conventional high-speed blender.

^8,9)

However, mechanical fibrillation requires high energy consumption. Therefore, chemical pre-treatments, such as tempo-mediated oxidation,

¹⁰⁾

carboxymethylation,

¹¹⁾

oxidative sul- fonation,

¹²⁾

and quaternization

¹³⁾

are required to help reduce the energy consumed during the fibril- lation process. By subjecting CNFs to chemical pre-treatment, the cohesion between microfibrils due to hydrogen bonding can be reduced by intro- ducing charge groups that generate electrostatic repulsion between fibrils.

¹⁴⁾

The rheological properties of CNF suspensions have been previously studied.

^15-20)

For example, CNF suspensions exhibit high viscosity with shear-thinning behavior. Moreover, increasing

CNF suspensions to between 0.3% and 2.6% in turn increases the storage moduli and loss moduli.

²¹⁾

Furthermore, an investigation on the impact of divalent and trivalent cations on carboxylated CNFs shows that the valency of the metal cations and the strength with which they bind to carbox- ylate play an important role in the storage moduli of hydrogels.

²²⁾

It is worth noting that the addition of cations into the cellulose suspension signifi- cantly affects hydrogel strength.

In this study, the rheological properties (shear viscosity, strain amplitude, and frequency) of CNFs with a monovalent salt addition were examined.

Only a limited number of publications have focused on the rheology of CNFs with added monovalent salts, specifically 0-100 mM sodium chloride (NaCl).

^21,23)

To the author’s knowledge, the rheology of CNFs reacted with monovalent salts NaCl, potas- sium chloride (KCl), and lithium chloride (LiCl), particularly in concentrations of 500-10,000 ppm, has not been reported.

2. Materials and Methods

2.1 Materials

The material used for CNF preparation was dried kraft pulp obtained from M. Company. Chemicals were purchased from various suppliers: Ethanol (EtOH) from Samchun Pure Chemical Co., Ltd., Korea; sodium hydroxide (NaOH) from OCI Com- pany Ltd., Korea; monochloroacetic acid (MCA) from Denak Co., Ltd., Japan; NaCl from Junsei Chemical Co., Ltd., Japan; anhydrous LiCl from Samchun Pure Chemical Co., Ltd., Korea; and KCl from Duksan Pharmaceutical Co., Ltd., Korea.

Distilled water was obtained from a water purifi-

cation system in the Department of Wood and

Paper Science at the Chungbuk National Univer-

sity.

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2.2 Preparation of CNF

Dried kraft pulp (100 g) was soaked in ethanol containing NaOH (14.11 g) and stored at room tem- perature for 1 h. MCA (28.9 g) was dissolved in EtOH corresponding to 0.6 mmol/g cellulose then added slowly to the mixture; the reaction was con- ducted at 75℃ for 2 h. To obtain cellulose fibers, the reaction mixture was then washed and filtered several times with distilled water until neutral. The cellulose fiber was then mixed with distilled water to create 2% w/w CNF.

CNF was prepared from the fiber suspension by being passed through a GEA Homogenizer proces- sor (Panda PLUS 2000, GEA, Italy). First, the fiber suspension at a concentration of 2% w/w was ground in two passes. Then, the fiber suspension was homogenized in two passes at a pressure of 600-800 bar. To obtain 1.5% CNF for further experiments, 2% CNF was diluted. Overall proce- dure for experiment was summarized in Fig. 1.

2.3 Preparation of CNF gels

CNF hydrogels were prepared by adding monova- lent salt solutions of NaCl, KCl, and LiCl to the CNF suspension to obtain concentrations of Na

⁺

, K

⁺

, and Li

⁺

ranging from 500 to 10,000 ppm.

Approximately 2% CNF was diluted with a metal chloride solution to reach 1.5% CNF, and the mix- tures were homogenized by shaking. The descrip- tions and compositions of the samples are listed in Table 1.

2.4 Rheological characterization

Rheological measurements were conducted using the rheometer (MCR 102, Anton Paar, Austria) with a parallel plate (25 mm diameter; 1 mm gap).

All the measurements were conducted at 25℃.

Detailed descriptions of rheological measurements can be found in other studies.

¹⁹⁾

The rheometer was used to measure the viscosity, amplitude sweep, and frequency sweep of the samples. The apparent viscosity was measured by decreasing the shear

Table 1. Descriptions and compositions of cations added to the gel Cations Cation charge number Cation radius

Cation concentration (mg/L)

10,000 ppm 5,000 ppm 1,000 ppm 500 ppm

Na

⁺

1

⁺

1.0 25,000 12,500 2,500 1,300

K

⁺

1

⁺

1.4 19,000 9,500 1,900 1,000

Li

⁺

1

⁺

0.6 48,000 24,000 4,800 2,400

Ionic radii were taken from the article of Luo et al.

²⁴⁾

Fig. 1. CNF preparation process.

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Fig. 2. Viscosity as a function of shear rate of 1.5% CNF with Na

⁺

at various concentrations.

rate from 100 to 1 s

^-1

. Amplitude sweep was per- formed to determine the linear viscoelastic (LVE) region at a frequency of 10 rad/s (shear strain ranging from 0.01% to 100%). Following this experiment, the frequency sweep was performed at 0.5% strain at 0.1-100 rad/s.

3. Results and Discussion

CNFs gels can be obtained with different compo- sitions of treatments using cations, as described in Table 1. Table 1 also presents the charge numbers and cation radii of the added salts. The rheological properties were an important parameter to assess the application of the CNF product. As such, the rheological properties (in particular, the shear rate and apparent viscosity) of CNFs with added cation solutions were examined.

The variation of the viscosities and shear rates of 1.5% CNFs with cations are summarized in Figs. 2-4.

These figures show that the addition of cations increased the viscosity of the hydrogel. The appar- ent viscosities of CNFs with added salt were higher than those of CNFs without added salt.

Fig. 2 demonstrates the effect of various concen- trations of added sodium on viscosity. As is evident, the viscosity increased significantly as the concen- tration of sodium increased up to 10,000 ppm.

Similarly, increasing the concentration of NaCl resulted in an increased viscosity.

²⁵⁾

The sodium in the MFC suspension influenced the viscosity because the fibril-fibril interaction associated with hydrogen bonds increased.

¹⁶⁾

However, the opposite effect was observed when the concentration of NaCl was increased (NaCl>1 mM, ≈23 ppm): The viscosity decreased significantly.

²¹⁾

In this study, the addition of high concentrations of sodium (1,000 to 10,000 ppm) prevented the gel form net- work from coalescing well; thus, the gel structures became stiffer. These conditions caused the water release from the plate during flow behavior mea- surement as the shear rate increased. This is evi- dence that the sodium concentration significantly affected gel strength. Furthermore, Tandjawa et al., who investigated the rheological properties of MFC suspensions, proposed increasing the viscosity by increasing the cellulose concentration.

²⁵⁾

Viscosity as a function of shear rate of 1.5% CNF

with K

⁺

at various concentrations is shown in Fig. 3.

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The potassium concentration had an effect similar to that of sodium: Increasing the potassium concen- tration resulted in increased viscosity. The viscosity increased from 34.4 Pa·s at a low concentration (500 ppm) to 189.2 Pa·s at a high concentration (10,000 ppm). The gel strength of the CNF clearly increased because of the addition of sodium and potassium. However, samples containing potas- sium showed lower gel strength than those con- taining sodium.

The apparent viscosity of 1.5% CNF with various concentrations of Li

⁺

is presented in Fig. 4. The viscosity of all samples gradually increased as the lithium concentration increased. Compared with the other samples, samples containing Li

⁺

had higher viscosities regardless of concentration. This indicates that the gel strength of CNF with lithium is much higher than that with sodium or potassium (Li

⁺

>Na

⁺

>K

⁺

). This is maybe due to the denser aggregation of surfactant molecules in the mem- Fig. 4. Viscosity as a function of shear rate of 1.5% CNF with Li

⁺

at

various concentrations.

Fig. 3. Viscosity as a function of shear rate of 1.5% CNF with K

⁺

at

various concentrations.

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branes. It was reported that increasing the salt concentration induced stronger gelation, which thus increases viscosity.

²⁶⁾

All samples exhibited low shear-thinning behavior with a monotonically decreased viscosity and an increased shear rate, which is similar to the results reported by Li et al.

²⁷⁾

The results in this study indicated that the viscos- ity of the CNFs with added cation is significantly dependent on shear rate, cation addition, and cat- ion concentration.

Amplitude sweep was performed to determine the LVE region of the hydrogel and evaluate gel strength. Gel strength was evaluated by comparing the values of the storage and loss moduli as a function of strain amplitude. The amplitude mea- surements of CNFs with various concentrations of cations are shown in Figs. 5-7. The storage and loss moduli as functions of strain amplitude for 1.5% CNFs with Na

⁺

at various concentrations are presented in Fig. 5. The storage and loss moduli of

CNFs increased from 1,505.8 Pa to 6,497.1 Pa as concentrations of sodium ions increased from 500 to 10,000 ppm, respectively. The amount of cation used as a cross-linker has a significant effect in that it increases the elastic moduli, which means a stronger fibrous network is formed.

²⁸⁾

Similar behavior was reported by Maestri et al.

²³⁾

: The storage modulus of the carboxylated CNC increased when the concentration of NaCl increased from 10 to 100 mM. Contrary to this study, Naderi et al.

²¹⁾

reported an increase in the storage modulus when the concentration of NaCl decreased from 0.1 to 10 mM.

Fig. 6 illustrates the effect of various concentra- tions of potassium on gels. Similar to the effect of sodium, increasing the potassium concentration increased the storage and loss moduli, indicating that the strength of the gels increases as the con- centration increases. As shown in Fig. 7, the stor- age and loss moduli of CNFs with added lithium follow trends identical to those of CNFs with

Fig. 5. Storage and loss moduli as functions of strain amplitude at a frequency of 10 rad/s for

1.5% CNF with Na

⁺

at concentrations of (a) 500, (b) 1,000, (c) 5,000, and (d) 10,000 ppm.

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Fig. 7. Storage and loss moduli as functions of strain amplitude at a frequency of 10 rad/s for 1.5% CNF with Li

⁺

at concentrations of (a) 500, (b) 1,000, (c) 5,000, and (d) 10,000 ppm.

Fig. 6. Storage and loss moduli as functions of strain amplitude at a frequency of 10 rad/s for

1.5% CNF with K

⁺

at concentrations of (a) 500, (b) 1,000, (c) 5,000, and (d) 10,000 ppm.

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sodium and potassium ions. Variations of the elas- tic modulus of 1.5% CNF as a function of cation concentration also revealed decreased strength at low concentrations and increased strength at high concentrations.

Oscillatory measurement was used to determine the LVE region and viscoelastic behavior. It also provides information regarding hydrogel charac- teristics and the degree of crosslinking.

²⁶⁾

The G′

value at a lower frequency (0.1 rad/s) was used to compare the degree of cross-linked of different samples. We investigated the storage and loss moduli as functions of the angular frequency of CNFs with different cation concentrations, as shown in Table 2. Table 2 demonstrates that the increase of the ionic radii of the cations in CNF gels led to a decrease in the G′ and G″

(Na

⁺

<K

⁺

<Li

⁺

). This trend is different from that of CNC gels with added cations, in which the G′ and G″ increased as the ionic radii of cations increased.

²⁹⁾

The higher the G′ value, the higher the degree of crosslinking, which will cause the

material to stiffen. Furthermore, the ratio of G′

was much higher than that of G″, indicating the former’s solid-like structure, which means the CNFs with added cations have increased elasticity.

The storage modulus as a function of different cation concentrations is presented in Fig. 8. In all samples, the storage modulus increased as the cation concentration increased, indicating that the gel is stronger at higher concentrations. Assuming that cations induce fibril aggregation and inter- fibrillar bridging at higher concentrations, the network stiffens.

³⁰⁾

We noticed that there were dif- ferences between the CNF gels after the addition of Li

⁺

, Na

⁺

, and K

⁺

. The suspension without cation was weaker compared with those with added cat- ions. Furthermore, the storage moduli of the CNF gels follow the order of Li

⁺

>Na

⁺

>K

⁺

, indicating that Li

⁺

increases gel strength. The difference in ionic radii among Li

⁺

, Na

⁺

, and K

⁺

ions signifi- cantly affects hydrogel strength. Regarding the specific differences among Li

⁺

, Na

⁺

, and K

⁺

, our results demonstrate that, overall, low hydrogel

Table 2. Rheological properties of 1.5% CNFs at different cations

Cations Conc. ω G′ G″ Ratio

[ppm] [rad/s] [Pa] [Pa] [G′:G″]

No salt 0 0.1 205.6 21.1 10:1

Li

⁺

10,000 0.1 8,957.3 1,087.9 8:1

5,000 0.1 5,109.8 554.6 9:1

1,000 0.1 2,591.6 236.8 11:1

500 0.1 1,679.9 146.0 12:1

Na

⁺

10,000 0.1 4,971.4 673.5 7:1

5,000 0.1 3,429.0 347.3 10:1

1,000 0.1 1,021.0 85.2 12:1

500 0.1 769.8 67.9 11:1

K

⁺

10,000 0.1 4,146.7 408.6 10:1

5,000 0.1 1,656.8 137.1 12:1

1,000 0.1 749.0 62.2 12:1

500 0.1 373.4 31.2 12:1

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strength occurs when cations with larger radii, such as K

⁺

and Na

⁺

, are added compared with the addition of Li

⁺

, which has a smaller radius. Lith- ium has a very hard cation and thus lower in polarizability than sodium and potassium. How- ever, soft cation, such as potassium has high polarizability, less ionic interaction, and more covalent in nature.

³¹⁾

4. Conclusions

In this study, we examined the rheological prop- erties of monovalent ions (LiCl, NaCl, and KCl) in 1.5% CNF at concentrations of 500, 1,000, 5,000, and 10,000 ppm. We found that cation type and concentration have a significant impact on the rhe- ology of CNF gels. Adding Li

⁺

, Na

⁺

, and K

⁺

at con- centrations of 500, 1,000, 5,000, and 10,000 ppm gradually increased the viscosity, storage moduli, and loss moduli, which indicate an increased hydrogel strength. Regarding hydrogel strength, we found that Li

⁺

>Na

⁺

>K

⁺

because of different

cation radii and interfibrillar bridging on the hydrogel. Further research is necessary to deter- mine the maximum strengths of hydrogels with added cations.

Acknowledgement

This research was supported by the National Research Foundation of Korea government (NRF) funded by Ministry of Education (NRF-2019 R1I1A3A01058481).

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