Stiffened Nanocomposite Hydrogels by Using Modified Cellulose

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Stiffened nanocomposite hydrogels by using modified cellulose nanofibers via plug flow reactor method Xianpeng Yang, Subir K. Biswas, Hiroyuki Yano, and Kentaro Abe ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01322 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 1, 2019

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ACS Sustainable Chemistry & Engineering

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Title:

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Stiffened nanocomposite hydrogels by using modified cellulose nanofibers via plug flow reactor

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method

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Authors:

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Xianpeng Yang†*, Subir K. Biswas†, Hiroyuki Yano†, Kentaro Abe† *

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†Affiliation: Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji,

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Kyoto, 611-0011, Japan

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*Corresponding author:

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Xianpeng Yang: [email protected];

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Kentaro Abe: [email protected]

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Abstract

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Cellulose nanofibers (CNFs), hydrophilic natural nanomaterials, are widely used to reinforce the

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stiffness of hydrogels, which have wet properties similar to those of human tissues. Interactions

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between the nanofibers and polymer matrix play an important role in reinforcing hydrogels. Here,

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we used a CNF cake as a reactor to modify the surface of CNFs, which was based on the principle

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of a plug flow reactor (PFR), excepting to enhance interfacial interactions. As a result, the degree

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of substitution of maleic acid monoester-modified CNFs reached 0.14 at room temperature within

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25 min. The crystallinity and morphology of modified CNFs were well maintained. The modified

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CNFs were used to form composites with poly(acrylamide-co-acrylic acid) networks, followed

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by ionic cross-linking. The elastic modulus of the nanocomposite hydrogels increased

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considerably from 10.27 ± 0.48 MPa to 24.67 ± 0.53 MPa after the CNF modification process.

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This PFR method shows great potential for preparing a range of CNF-based functional

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nanocomposite materials.

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Key words: hydrogels, cellulose nanofibers, stiffness, modification, plug flow, engineering

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approach


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Introduction

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Hydrogels are wet and soft materials, comprising hydrophilic polymer networks, which hold large

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amounts of water.1 Hydrogels have attracted considerable research attention in recent years owing

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to their similar wet properties to human tissues.2-3 However, the majority of synthesized hydrogels

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are softer than human tissues, such as skin, cartilage, and ligament.3-4 Cellulose nanofibers (CNFs)

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are hydrophilic natural materials, which are extremely stiff and strong and are thus widely used

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to reinforce hydrogels.5-6 The addition of a small amount of CNFs markedly improves the stiffness

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of hydrogels; however, the stiffness remains much lower than that of human tissues for three

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reasons.7 First, the stiffness of CNF networks in wet condition is lost because of disengagement

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of interfibrillar hydrogen bonds. Second, it is difficult to increase the CNF content owing to the

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high viscosity. Third, physical interactions between CNFs and the polymer matrix are weak under

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wet conditions. We recently reported on stiff hydrogels with a high CNF content formed by

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filtering a CNF suspension to form a wet cake.8-9 Modification of the surface of CNFs represents

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another way to introduce covalent or non-covalent bonds between nanofibers and polymer matrix

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to give stiffer hydrogels.5-6

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Although CNFs, which are made up cellulose, are amenable to a range of modifications,10-11 it

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remains difficult to develop a simple and efficient method to modify the surface of CNFs.

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Modification processes typically require the CNFs to be present at low concentrations, which

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limits the reaction efficiency and increases solvent consumption. Furthermore, pre-treatment and

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post-treatment by solvent-exchange are required, making the modification process tedious and

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environmentally unfriendly.10 Additionally, under severe reaction conditions, the crystallinity

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and/or morphology of CNFs might change.10 On the other hand, the difficulties of CNF

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modification can be addressed through engineering approaches rather than chemistry alone. For

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example, chemical modification is performed before nanofabrication to reduce energy

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consumption.12-13 New groups have also been induced during nanofabrication by ball milling or



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with the use of nanocellulose sheets.14-15

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Here, we report a new CNF modification method, inspired by the fluid in a plug flow reactor

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(PFR). The fluid in the PFR is not mixed in the axial direction and thus no back-mixing occurs.

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Similarly, filtering a CNF suspension results in a wet CNF cake where nanochannels form with a

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large aspect ratio.16 Fluid moves through the nanochannels under the force of a vacuum may show

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negligible back-mixing phenomenon. The water in the CNF cake can be readily replaced by a

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mixture of solvent and reactant. At the same time, the reaction between the CNFs and reactant

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mixture occurs and the surface of CNFs is modified. We examined the reaction of CNFs and

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maleic anhydride (MA) using a CNF cake as reactor.17 Notably, Fe3+-cross-linked

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poly(acrylamide-co-acrylic acid) [poly(AM-co-AA)] is widely used to prepare tough

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hydrogels.18-19 We used the maleic acid monoester modified CNFs (MACNFs) to form composites

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with poly(AM-co-AA), followed by ionic cross-linking. Here, the MACNFs, containing double

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bonds and carboxyl groups, were grafted through poly(AM-co-AA) chains and ionically cross-

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linked with AA units, which improved interactions between the nanofibers and matrix. We

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performed tensile testing to compare the mechanical properties of the nanocomposite hydrogels

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reinforced by modified CNFs having different degree of substitution (DS).

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Experimental section

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All the chemicals were laboratory grade and used as received. The CNFs were prepared according

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to our previously reported method.9, 20 The MACNF cake was prepared using the PFR method. In

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a typical procedure (Entry 4, Table 1), a total of 200 mL of CNF suspension (0.1 wt%) was

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vacuum filtered to form a wet CNF cake with a water content of approximately 90 wt%. A mixed

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solution, containing 1.16 g of MA, 1.6 mL of triethylamine (TEA), and 30 mL of

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dimethylacetamide (DMAc), was filtered through the cake. Then, 10 mL of ethanol and 50 mL

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of water were filtered through the cake in turn to obtain the purified MACNF cake. In the whole

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process, the degree of vacuum, produced by a circulating water aspirator, was kept constant at 2.5 4 ACS Paragon Plus Environment

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kPa.

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The resulting MACNF cake was immersed in a mixture containing the monomers (AM and AA),

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the initiator (ammonium persulphate, APS), and the cross-linker (N,N’-methylenebisacrylamide,

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MBA). The concentration of AM and AA was fixed at 2 and 0.3 mol/L corresponding to the

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volume of water. The moles of APS and MBA were fixed to be 0.001 times the monomer content.

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After immersion at 4 °C for 24 h, the cake was fixed between two glass slides and sealed with a

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silica cushion. The polymerization was performed at 80 °C for 12 h to form the poly(AM-co-AA)

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networks. The polymerized cake was immersed in FeCl3 aqueous solution (0.06 M) for 24 h to

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ionically cross-link the poly(AM-co-AA) network. Finally, the ionically cross-linked hydrogel

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was immersed in a large amount of water to remove excess FeCl3 and reach an equilibrium state,

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resulting in MACNF hydrogels. The composition of the MACNF hydrogels was tested by weight

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method. The MACNF hydrogels were named as DSn according to the DS of the used nanofibers.

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For example, DS0.063 means that the MACNF hydrogel was prepared from the MACNFs with a

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DS of 0.063. DS0 means that the hydrogel was prepared from the CNFs without modification.

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The DS of MACNFs was measured by a titration method.21 Simply, a MACNF suspension (20 g,

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0.05 wt%) was neutralized with NaOH aqueous (0.02 M), which was repeated twice. Fourier

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transform infrared (FTIR) spectra were recorded on a Spectrum Two spectrometer (PerkinElmer

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Inc., Waltham, MA) in the range of 400–4000 cm−1. X-ray diffraction (XRD) measurements were

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performed with CuKα radiation (40 kV and 300 mA) from 5° to 40° in reflection mode.9 The

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SEM images were obtained at an acceleration voltage of 1.5 kV. Samples were dried at room

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temperature overnight and at 110 °C for 10 min, and then coated with platinum by sputtering for

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90 s.9 For thermogravimetric analysis (TGA), dried samples of 5-mg weight were heated under a

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nitrogen atmosphere from 110 °C to 600 °C at a heating rate of 10 °C min−1.9 The tensile testing

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was performed at a stretch velocity 20 mm·min−1 using dumbbell-shaped samples (width: 2 mm,

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length: 35 mm, gauge length: 20 mm). Five specimens were tested for each sample.9



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Results and discussions

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Modification of CNFs via PFR method

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First, a CNF suspension (200 ml, 0.1 wt%) was filtered to form a CNF cake where there exist

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nanochannels (Fig. 1a).16 The nanochannels had a large aspect ratio because the thickness of the

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CNF cake was about 0.5 mm, much larger than the diameter of nanochannels (tens of nanometers).

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Therefore, the flow in the nanochannels under the force of vacuum might show negligible back-

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mixing phenomenon (Fig. 1b). Then, a reactant mixture containing MA, TEA as the activator/H+

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scavenger,17 and DMAc as the solvent was passed through the CNF cake. It was assumed that the

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water in the CNF cake was replaced by the reactant mixture gradually ( I to III, Fig. 1c). At same

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time, the reaction between the surface of CNFs and MA occurred (Fig. 1d). After the reaction, the

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cake was washed by passing ethanol and water through the cake. The translucent CNF cake

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became much more transparent after modification (Fig. 1a), owing to the negative charge of the

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maleic acid monoester.21 Fig. 1e illustrates how the concentration of nanofibers changed in

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preparation of MACNF cake by using the PFR method and the conventional solvent exchange

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(SE) method. For PFR method, pre-treatment process was not required and the concentration of

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nanofibers almost kept constant after filtering a CNF suspension. In addition, the reaction was

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performed at high concentration and the purification process was also simplified. In contrast, the

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SE method was more tedious and less effective.


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Fig. 1. PFR inspired modification of CNFs. (a) Illustration of the modification process: CNF suspension

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was vacuum filtered and the reactant solution was passed through the wet CNF cake. (b) Model of PFR: no

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back-mixing occurs in the axial direction. (c) Fluid and reactions in the CNF cake. Orange dashed arrows

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show the path of the reactant. Blue dashed arrows show ethanol. I to III: water was gradually replaced by

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the reactant mixture; III to V: the reactant mixture was gradually replaced by ethanol. (d) Reaction between

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CNFs and MA. (e) Concentration of nanofibers during different steps. Step I: pre-treatment. Several times

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of solvent exchange are required for SE method. Step Ⅱ: reaction. Step III: purification. Several times of

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solvent exchange are required for SE method. Step Ⅳ: preparation of MACNF cake. SE method was

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described according to reported process.21

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Fig. 2a shows the normalized FTIR spectra of CNFs and MACNFs, confirming the successful

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modification. Absorption peaks of the maleic acid monoester group attached to CNFs appeared

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at 1722, 1637, 1578 and 1423 cm−1. We attributed the peak at 1722 cm−1 to stretching vibrations

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of the C=O groups in the ester linkage and the peaks at 1578 and 1423 cm−1 to carboxylate groups

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with a conjugated form.13, 21 We assigned the peak at 1637 cm−1 to characteristic C=C stretching

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vibrations. Within 25 min at room temperature, the DS reached 0.14 (Table 1, No. 4), indicating

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that the modification was effective. The DS was controlled by changing the volume and/or

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composition of reactant solution (Table 1). The volume of solvent had a more pronounced effect



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on the DS than did the concentration of MA, indicating that residence time was the controlling

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factor. A lower vacuum force also prolongs the residence time, which may result in a higher DS

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for the same reactant mixture.

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Table 1. Composition of reactant mixtures and corresponding DS of MACNFs. Reactant mixture Entry

DS of MACNFs MA/g

TEA/mL

DMAc/mL

1

0.29

0.4

15

0.045

2

0.29

0.4

20

0.054

3

1.16

1.6

20

0.073

4

1.16

1.6

30

0.14

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The XRD spectra in Fig. 2b show that both the CNFs and MACNFs had cellulose I crystal

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structures. Unlike CNF modification processes performed under more severe conditions, no

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broadening of the CNF peaks was observed in our case.21 The crystallinity index (CrI), calculated

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by the Segal method,22 increased from 63.2% to 68.4% as the DS of MACNFs increased from 0

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to 0.17 (Fig. 2b). Furthermore, the MACNFs and CNFs shared a 15–40-nm diameter and a length

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of several micrometers (Fig. 3). The MACNFs showed the more distinct appearance than that of

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CNFs. We suggest that the changes of crystallinity and appearance occurred through partial

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degradation of amorphous regions. The mild conditions had minor effects on both the crystallinity

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and morphology. In terms of thermal stability, as the DS increased to 0.17, the initial

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decomposition temperature (i.e., the 5%-degradation temperature) of the MACNFs decreased to

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229.8 °C, which was lower than that of the CNFs (257.5 °C). This decrease might be attributed

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to the larger surface areas and the presence of less stable carboxyl groups in the MACNFs.10, 13

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To illustrate the versatility of current method, we performed modifications with various other

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kinds of groups. The current method is applicable for different types of modifications in both

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organic solvents and aqueous conditions. We will build mathematical models of CNF cake-based 8 ACS Paragon Plus Environment

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reactor and report in another paper. We expect that other nanocelluloses, including cellulose

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nanocrystals and bacterial cellulose might be modified based on this wet cake reactor principle. (a)

1722

1578

(b)

1423

200

CNFs MACNFs

1800

1700

1600

1500

Intensity

Absorbance

1637

1400

CNFs MACNFs

CrI=68.4% CrI=63.2%

4000

3200

2400 1600 Wavelength (cm-1)

800

(c)100

10

15

20 25 2 [deg.]

30

35

(d)1.5

CNFs MACNFs

60 40 20

40

CNFs MACNFs

1.2

80

0 100

158

5

Deriv. (%/oC)

Mass remaining (%)

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


0.9 0.6 0.3 0.0

200

300

400

500

600

Temperature (oC)

200

300

400

500

600

Temperature (oC)

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Fig. 2. (a). FT-IR spectra of CNFs and MACNFs. (b). XRD spectra of CNFs and MACNFs.

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Thermogravimetric analysis of CNFs and MACNFs: integral curves (c); differential curves (d).

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Fig. 3. SEM images of CNFs (a) and MACNFs (b).

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Stiffened nanocomposite hydrogels

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We applied our MACNF cake to prepare nanocomposite hydrogels, which we expected to

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improve the stiffness of the resulting composite. As shown in Fig. 4a, after modification and

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purification, the MACNF cake was immersed in monomer solution containing 2 mol/L of AM,



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0.3 mol/L of AA and a small amount of covalent cross-linker and initiator. Poly(AM-co-AA)

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networks were formed after polymerization (Fig. 4a-2). MACNFs could be grafted through

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poly(AM-co-AA) chains, through double bonds on the maleic acid ester groups. Then, the

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carboxyl groups on the acrylic acid groups of poly(AM-co-AA) and the surface of MACNFs were

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further cross-linked by Fe3+. Finally, the resulting hydrogels were immersed in water until

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equilibrium to form MACNF hydrogels (Fig. 4a-3). Three samples of MACNF hydrogels were

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prepared by using the nanofibers with different DS, as shown in Table 2. DS0.063 and DS0 had

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almost the same compositions while DS0.17 showed a little decrease in water content. When

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slightly modified MACNFs (DS = 0.063) were used, the elastic modulus increased from 10.27 to

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17.53 MPa and the fracture stress increased from 6.08 to 7.32 MPa, compared with that of the

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DS0 hydrogels (Fig. 4b-c). When the DS of the MACNFs reached 0.17, the stiffness and strength

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increased to 24.67 and 7.68 MPa, respectively. Considering that the samples possessed very

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similar compositions, the differences of the interactions between the nanofibers and matrix might

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be the main reason for the increasement of elastic modus and fracture strength. For DS0 sample,

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the layered CNFs were firmly surrounded by tough polymer matrix which might enable the

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orientation of nanofibers. As a result, the elongation was relatively high. However, the main

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interactions between CNFs and matrix were hydrogen bonds and entanglements, which were

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relatively weak. For DS0.063 and DS0.17 samples, covalent and ionic bonds were introduced

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between nanofibers and matrix. The additional cross-linking enhanced the friction force between

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nanofibers and matrix, resulting in increased modulus and strength. The decreased elongation of

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MACNF hydrogels probably arose from the rigidity of additional ionic cross-linking,23 which

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restrained the orientation of nanofibers.

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We previously reported that a high CNF content, formed by filtering a CNF suspension to form a

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wet cake, facilitates the improvement of stiffness.8-9 Here, we conclude that generating strong

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interactions between the nanofibers and polymer matrix is also essential for preparing stiff

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hydrogels. Although the importance of interfacial interactions has received considerable 10 ACS Paragon Plus Environment

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attention,24-25 extremely stiff hydrogels have rarely been achieved with CNF reinforcement, as

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analyzed in the Introduction section. In the next work, we will study on the role of covalent and

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ionic bonds separately by different modification of CNFs. In addition, the use of a wet nanofiber

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cake as a reactor represents a simple and versatile method for modifying CNFs with various kinds

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of groups. We predict that our PFR method will be applicable to other CNF-based functional

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materials, such nanopapers and aerogels.26-27

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Fig. 4. (a) Preparation and structure of MACNF hydrogels. Tensile properties: (b) stress-strain curves; (c)

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fracture stress and elastic moduli of different samples.

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Table 2. Composition of MACNF hydrogels MACNF hydrogels

Water, %

Nanofibers, %

Polymer/Fe3+, %

DS0

73.1%

6.2%

20.7%

DS0.063

72.9%

6.5%

20.6%



DS0.17

71.7%

6.8%

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21.5%

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Conclusion

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In summary, we used a CNF cake as a reactor to modify CNFs. The reaction between CNFs and

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MA indicated that this novel modification method was simple but effective. We used the modified

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CNF cake to prepare nanocomposite hydrogels with reinforced stiffness. The increased stiffness

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resulted from reinforcing interactions between the nanofibers and polymer matrix. We propose

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that this PFR method is effective for nanocomposite hydrogels and might also be applicable to

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other CNF-based functional materials.

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References

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Synopsis: A sustainable material, cellulose nanofiber, was quickly modified with a simple

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procedure and less solvent.


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Stiffened Nanocomposite Hydrogels by Using Modified Cellulose

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