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Microcrystalline cellulose surface-modified with acrylamide for reinforcement of hydrogels Changzhuang Bai, Xuejiao Huang, Fei Xie, and Xiaopeng Xiong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02781 • Publication Date (Web): 11 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Microcrystalline cellulose surface-modified with acrylamide for reinforcement of hydrogels Changzhuang Bai, Xuejiao Huang, Fei Xie, Xiaopeng Xiong*

Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China * Corresponding author at: Department of Materials Science and Engineering, College of Materials, Xiamen University, 422 SiMing South Road, SiMing District, Xiamen 361005, China. Tel.: 0086 592 2189358; Fax: 00865922183937. E-mail address: [email protected] (X.P. Xiong)


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Abstract In this work, microcrystalline cellulose particles (MCC, 25 µm) were modified with acrylamide under microwave irradiation. The functional groups or polyacrylamide (PAM) chains modified on the surface of the particles facilitated their even dispersion in water. So, the acrylamide-modified MCC (AM-MCC) particles were then dispersed in aqueous solution of monomer of N,N-dimethylacrylamide (DMAA), which was later polymerized to prepare poly(N,N-dimethylacrylamide) (PDMAA) based composite hydrogel. The microstructure of the composite hydrogel was carefully studied with Fourier transform infrared spectroscopy and scanning electron microscope. The mechanical properties of the composite hydrogel were investigated through the compressive and the tensile tests. Our results indicate great improvement in mechanical properties upon addition of the AM-MCC particles for the composite hydrogel. The analyses suggest that the evenly distributed AM-MCC particles can act as physical cross-linker to connect the neighboring polymer chains to strengthen the composite hydrogel, based on their strong interaction including hydrogen bonding and chain entanglements. However, over loading (more than 5.6 wt%) of the AM-MCC would lead to aggregation of the AM-MCC particles to destroy the uniform microstructure of the composite hydrogel, resulting in reduction of reinforcement effect. The reinforcement effect the AM-MCC has also been evidenced by PAM based composite hydrogels, which were prepared in a similar way. Moreover, the recoverability, the cyclic and the swelling behaviors have been measured to indicate potential applications of the composite hydrogels. Therefore, our work provides an inexpensive but active filler of AM-MCC, which can be well dispersed in water media and is suitable to reinforce hydrogels. Keywords: Microcrystalline cellulose; surface modification; composite hydrogel; mechanical property; reinforcement



Introduction Hydrogel, a type of soft material of polymer networks that can take up large amount of water, has attracted more and more attention because of the scientific importance and the wide applications especially in biomedical field [1]. However, relatively weak mechanical strength has largely limited the applications of pure polymer hydrogels [1-3]. Therefore, improving the mechanical strength of hydrogel becomes one of the focuses in this field. Preparation of composite hydrogel is one of the most efficient ways to obtain strong hydrogel [4-6]. Through introducing fillers with high modulus such as fibers and nano/micron particles, into polymer networks, composite hydrogel can then be obtained. If the fillers could be evenly dispersed, they would dissipate the subjected energy to significantly strengthen the hydrogel [1, 3, 7-10]. For examples, cellulose nanocrystals have been evenly dispersed in quaternized cellulose solution via their oppositely charged ionic interaction to prepare markedly reinforced injectable composite hydrogel [8], and poly(N-isopropyl acrylamide) chains were covalently grafted onto clay nanoparticles to obtain mechanically much improved nanocomposite gel [9]. It has been pointed out that both the filler-filler and the filler-polymer interaction [8], as well as with the improved cross-linking points on the surface of the filler [9, 10], will contribute to the reinforcement of hydrogel. Rigid fillers can even be fabricated into geometric frames [11] or skeleton [12] to prepare extremely tough hydrogels, where the fillers of glass fiber or low-melting-point alloys and their interfacial bonding with the polymer matrices play synergetic effects on the improvement in strength. Cellulose, the sustainable, biodegradable and the most abundant natural polymer, has long been used to reinforce polymer materials [13]. However, even dispersion of cellulose in hydrogel matrix is still challenging [14]. Lately, the rod-like shape cellulose nanocrystals (CNCs) have attracted extensive attention for reinforcement of polymer materials, because the lateral dimension in the range of a few nanometres and the length ranging from 50 nm to 300 nm endow them with high modulus [15, 16]. Based on the abundant functional groups on the surface and the reduced size, CNCs can be feasibly embedded in polymer networks to prepare tough hydrogels [15-18]. In spite of the fascinating properties and excellent potentials, the production of CNCs with low cost is still challenging [19], limiting their application. However, another


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form of cellulose, i.e. microcrystalline cellulose (MCC) with particle size ranging from micrometers to hundreds of micrometers, has been successfully industrialized for decades [20]. The MCC exhibits the advantages such as the high mechanical strength, the easily accessible and the relatively low cost [20], so that it can serve as a good candidate of reinforcement agent. However, the large particle size of MCC may restrict its homogeneous distribution in polymer networks. Reducing the size of MCC will be highly cost [19], so surface modification of MCC particles to facilitate their even distribution is of key importance to maximize their reinforcement effect [21, 22]. In our previous work [22], MCC particles were surface-modified with urea to be homogeneously dispersed in chitosan matrix, and the toughness of the obtained composite material reached over 6 times that of pure chitosan. In this work, MCC was mixed with acrylamide (AM) and then irradiated under microwave for the surface-modification. The acrylamide-modified MCC (AM-MCC) would provide polyacrylamide (PAM) chains and functional groups on the particles surface, so that its dispersion in water could be feasibly improved. After that, monomers of N,N-dimethylacrylamide (DMAA) dissolved in water were polymerized in the presence of the dispersed AM-MCC particles to prepare composite hydrogels. The poly(N,N-dimethylacrylamide) (PDMAA) chains in the networks would interact with the PAM chains or interact with the functional groups on the surface of the AM-MCC particles. Based on the filler-polymer interaction [8] and the multi-interactions of polymers chains [9, 23], greatly reinforced composite hydrogels would thus be prepared. The mechanical properties of the composite hydrogels have been carefully measured and analyzed to suggest the reinforcement effect. Another kind of the AM-MCC reinforced composite hydrogel (PAM based hydrogel) was prepared in a similar way to further evidence the above discussion. Moreover, the recoverability in large strain amplitude and the swelling behaviors of the composite hydrogels were measured to indicate their potentials.

Experimental Materials Monomer of N,N-dimethylacrylamide (DMAA) was purchased from Macklin Biochemical Co., Ltd (Shanghai, China). Microcrystalline cellulose (MCC) with particle diameter labeled 25 µm, N,N,N',N'-tetramethylethylenediamine (TEMED,



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catalyzer) and N,N'-methylenebis(acrylamide) (NMBA, cross-linker) were purchased from

Aladdin

Industrial

Corporation

(Shanghai,

China).

Analytical

grade

potassiumpersulfate (KPS, initiator ), sodium bisulfite (SBS, reductant), acrylamide (AM, monomer) and all other reagents were purchased from Sinopharm Chemical Reagent Corporation (Shanghai, China) and were used without further purification. Deionized water was used throughout. Modification of MCC particles Acrylamide was firstly dissolved in water to prepare a 30 wt% solution. Then, MCC was added to be soaked in the solution with stirring for over 24h at room temperature, where the mass ratio of MCC to AM was 1:4. After drying in air at 60°C, the mixture was irradiated for 3 min with a power of 800W in a MM823LA-NS microwave oven (Midea, Fushan, China). After that, the reaction product was stirred in water for over 30 min and then centrifuged at 4000 rpm for 10 min. After repeating the washing process for three times, the precipitant was dried in air at 60 °C and then ground to obtain a powdery material of acrylamide-modified MCC (AM-MCC). Preparation of the composite hydrogels The preparation is schematically demonstrated in Figure 1. Twenty five grams DMAA and 0.25g initiator of KPS were dissolved in water to prepare 100 mL solution, and then desired amount of the above obtained AM-MCC was dispersed in the solution and was stirred for 2h. After that, nitrogen gas was bubbled in the mixture for 15 min to remove dissolved oxygen, and 0.2 mL TEMED was added and stirred for 10min. Next, the mixture was carefully injected into a Teflon mold, avoiding any air-bubbles in the solution. After 24h reaction at room temperature, the monomer of DMAA was polymerized through the free radical polymerization [23, 24] to form the poly(N,N-dimethylacrylamide) (PDMAA) based hydrogel, which was then repeatedly immersed into water to remove low molecular weight components completely. Different composite hydrogels were successfully prepared with various amounts of AM-MCC loadings. The obtained composite hydrogels are coded as PDMAA-Gx, where x represents the added amount of AM-MCC (g). The detailed formulations are listed in Table 1. The AM-MCC particles were mixed with 1 mL 1 w/v% KPS and 1 mL 0.6 w/v% SBS at 7 °C, and then nitrogen gas was bubbled for 15 min. After that, 2.0g AM and 2.5 mL 0.5 w/v% NMBA were added, and the mixture was diluted with water to 10 mL under stirring. After another 5 min of nitrogen gas bubbling, the mixture was poured ACS Paragon Plus Environment

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into glass tube with 10 mm diameter. After standing for 24h at room temperature, the AM monomers in the mixture were polymerized [25, 26] to prepare a series of polyacrylamide (PAM) based composite hydrogels. The compositions of the AM-MCC were 0, 1, 2, 3, 4, 5, 6, 7 and 8 mass percentages (w/w%) with respect to the whole solid mass of the PAM based composite hydrogels.

Figure 1 Schematic preparation of the PDMAA based composite hydrogel (a) and photograph (b) of the PDMAA-G0 (left) and the PDMAA-G1.5 (right) hydrogels on a letter paper. Table 1 Formulations of the PDMAA-based composite hydrogels. Hydrogel

AM-MCC

DMAA(g) KPS(g) TEMED(mL)



Code

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Characterizations The nitrogen content of the obtained AM-MCC was analyzed by a VarioEL III Element Analyzer (Elementary Analysen System GmbH, Germany). Fourier transform infrared (FTIR) spectroscopies of the samples were recorded with a Nicolet Avatar 360 instrument (Nicolet, Madison, WI, USA). The specimens were freeze-dried and ground into powder to mix with KBr to produce disks for the measurements. The hydrogel specimens were frozen in liquid nitrogen and snapped immediately, and then freeze-dried. The fractured specimens were then coated with a thin layer of gold (about 2 nm) to observe their microstructures by using a TM300 scanning electron microscope (SEM, Hitachi, Japan). Measurements A hydrogel sample was immersed into deionized water for desired time intervals. Then it was taken out to remove the surface water with filter paper and weighted. The swelling ratio (SR, %) was calculated according to SR=(mt/m0)×100% (%)

(1)

where mt is the mass of the hydrogel after t time (min) of absorbing water, and m0 is the initial mass of the dried hydrogel before swelling. All of the mechanical tests for the hydrogels were performed on an electronic universal testing machine (AGS-X, Shimadzu, Japan) at room temperature. The specimens were cut into cylindrical shape with 25 mm diameter and 10 mm thickness for the compression tests, where the compressing speed was 30 mm/min and the maximum compressive strain was set to be lower than 80% to guarantee safety of the equipment. For the tensile tests, the specimens were cut into rectangle with 40 mm in length, 8 mm in width and 5 mm in thickness. The tensile peed was 60 mm/min


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except stated otherwise. The recoverability tests for the PDMAA-G1.5 composite hydrogel was carried out through a loading-unloading process [26].The specimen was firstly stretched to an elongation of 1000% under a 60 mm/min tensile speed, then unloaded from the equipment to rest for different time intervals, and then was stretched again. The stress-strain curves for the tensile tests were recorded to evaluate the recoverability.

Results and discussion Modification of MCC with acrylamide Figure 2 shows the FTIR spectra of the unmodified MCC and the surface-modified MCC (AM-MCC). The peaks at around 897 and 1161 cm-1 in the FT-IR spectrum of the unmodified MCC are assigned to the β(1→4) linked D-glucose units of cellulose, and the C-O-C functional groups are found at 1033 and 1056 cm-1, and the O–H bending vibration of absorbed water is found to peak at 1639 cm-1, while the –OH stretching vibration band is located at around 3350 cm−1 [22]. The spectrum of PAM exhibits two typical bands of amide respectively peaked at 1648 cm-1 and 1617 cm-1. In comparison with those of the unmodified MCC and the PAM, the bands peaked at 1677 cm-1 and 1619 cm-1 for the AM-MCC are attributed to C=O and N-H stretching of amide of the grafted AM molecules [27], respectively. The above analysis suggests that free radicals on the molecular backbone of the MCC might be generated under microwave irradiation [28], which provides heat with high efficiency. The free radicals would then initiate the polymerization of the neighboring AM molecules [28-30]. However, the exact mechanism is under investigation, and the optimum condition for the modification of MCC will be reported shortly. The N element of the obtained AM-MCC (WN, %) was averaged from three independent determination to be 8.1±0.5%, and the content of AM (WAM, %) grafted onto MCC can be calculated by Equation (2) WAM=(WN/0.1971)×100%

(2)

where the numerical value 0.1971 is the nitrogen content in molecule AM. The grafting rate (GR, %) has been calculated through GR=[WAM/(1- WAM)]×100%

(3)

to be 70.0%. Those results suggest successful modification of the MCC particles with acrylamide.



897

MCC

Transmittance (%)

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1639

AM-MCC

1161 897

3349 1619 1677 1161

3349

PAM

3422

PDMAA-G0 3445

1648 1617

1630 1497 1356

PDMAA-G1.5 3466

4000

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3500

3000

2500

1357 1642 1495 2000 1500 1000

Wavenumber ( cm ) -1

500

Figure 2 FT-IR spectra of the unmodified MCC, the AM-MCC, the hydrogels of PAM, PDMAA and the composite hydrogel of PDMAA-G1.5.

PDMAA based hydrogels It is worth mentioning that the modified MCC particles, i.e. the AM-MCC, was able to be suspended in the aqueous solution of DMAA for about 1 week without obvious sedimentation, while the un-modified MCC particles would precipitate markedly within 24h. Therefore, polymerization of the monomers in the solution has been initiated in the presence of the AM-MCC particles to prepare the composite hydrogel. As displayed in Figure 1, stable composite hydrogel has been prepared. Because of the light scattering and/or reflecting at the AM-MCC particles, the composite hydrogel is not as transparent as the PDMAA hydrogel. Figure 2 also includes the FTIR spectra of the PDMAA hydrogel and a representative composite hydrogel of PDMAA-G1.5. The spectrum of PDMAA exhibits the typical band peaked at 1630 cm-1 of the arbonyl of amide, and the deformation vibration of -CH3 peaked at 1497 cm-1 and 1356 cm-1 [31]. In comparison, the spectrum of the composite hydrogel PDMAA-G1.5 exhibits combination of those of the AM-MCC and the PDMAA hydrogel (PDMAA-G0). However, the bands for C=O and N-H of amide groups have been shifted or overlapped. Meanwhile, the peak for -OH and-NH groups shifted to 3466 cm-1 for the composite hydrogel, which generally implies strong interactions between the components [22, 32, 33]. This means that new


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intermolecular hydrogen bonding between the AM-MCC and the PDMAA chains might have been formed. Figure 3 shows the SEM images of the PDMAA based hydrogels. Though all of the hydrogels display typical network of interconnected porous structures, the pore size increases with increasing the amount of the AM-MCC particles, and the wall of the network matrix becomes thicker. The larger pores but much rougher and more compact walls of the porous network structure are attributed to the PDMAA chains gathering around the AM-MCC particles because of their strong interaction [9, 25], which will be discussed in more detail in the next parts. When the added amount of AM-MCC is below 5.6%, the wall is smooth. Tiny particles with the size of micrometers (Figure 3d) can be observed on the wall of the composite hydrogel of PDMAA-G1.5 (5.6% of the AM-MCC content), as indicated by the arrows in the images. They are more obvious when more AM-MCC particles are load in the composite hydrogel (Figure 3e and f). Those particles are considered as the AM-MCC particles. The SEM observation suggests that proper amount of the AM-MCC particles can be evenly dispersed in the composite hydrogel, while some of them would be separated out when overloaded.



Figure 3 SEM images of the hydrogels: PDMAA-G0 (a), PDMAA-G0.5 (b),PDMAA-G1.0 (c),PDMAA-G1.5 (d),PDMAA-G2.0 (e) and PDMAA-G2.5 (f). The arrows indicate the AM-MCC particles. Figure 4 shows the compressive stress-strain curves of the PDMAA based hydrogels. None of the hydrogels has broken even when the compressive strain reached up to 75%. It can be seen that all of the hydrogels exhibit smooth compressive stress-strain curves, while the stress increase markedly upon addition of the AM-MCC particles. The compressive moduli (E, kPa) of the hydrogels were evaluated from the linear parts of the compressive strain-stress curves within the range of ε= 20% -40%. The


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insert in Figure 4 displays the dependences of the compressive stress at 75% compressive strain (σε=75%) and the compressive modulus on the AM-MCC content. It is interesting to find that the tendencies are similar: both of the σε=75% and the E increase with the loading of the AM-MCC, and then decrease when AM-MCC is over 5.6% (PDMAA-G1.5). The PDMAA-G1.5 displays the highest σε=75% and E of 38.7±0.6 kPa and 8.5 ±0.4 kPa, and are respectively 1.8 and 2.2 times those of the pure PDMAA hydrogel (σε=75%=21.5±1.3 kPa and E=3.9±0.2 kPa).

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σε=75%

PDMAA-G2.5 PDMAA-G2.0 PDMAA-G1.5 PDMAA-G1.0 PDMAA-G0.5 PDMAA-G0

E

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ε (%) Figure 4 Compressive stress-strain curves of the PDMAA based hydrogels. Inserts are the dependences of the compressive strength at 75% compressive strain and the compressive modulus on the AM-MCC content for the hydrogels, and the error bars represent one standard deviation of at least three replicates. On the other hand, the hydrogels display very high stretchability upon uniaxially drawing. Figure 5a shows the representative tensile stress-strain curves for the hydrogels. It is obvious that the stress-strain curves of the composite gels are above that of the PDMAA hydrogel, meaning that they exhibit much higher tensile strength. Meanwhile, the Young's moduli (E, kPa) of the hydrogels have been evaluated from the linear parts of the strain-stress curves within the range of ε= 10% -40% and the fracture energies (U, kJ/m3) were calculated by integrating the area under the stress-strain curves U = ∫σdε


(4)


Thus obtained E and U, together with the tensile strength (σb, kPa) and the breaking elongation (εb, %) for the hydrogels have been plotted versus the content of AM-MCC. As shown in Figure 5b, it is very interesting to find that the dependences of the σb, E and U on the AM-MCC content have similar tendencies: They increase with the AM-MCC loading at first, and reach the highest when AM-MCC is 5.6% (PDMAA-G1.5), and then decrease with further increasing the AM-MCC content. Those variations are similar as those of the compression strengths too. Meanwhile, it is found that the breaking elongation decreases monotonously with the rising of the AM-MCC content. The variations of the mechanical properties could be understood that the modified MCC particles have strong interaction with the PDMAA gel matrix, as indicated by the FTIR results in the above part. When the AM-MCC content is lower than 5.6%, the particles could be evenly dispersed to act as “physical cross-linkers” [22], and dissipate the subjected energy to improve strengths of the composite hydrogel but to reduce the stretchability. When the AM-MCC content is 5.6%, the σb, E and U of the composite hydrogel of PDMAA-G1.5 reach the highest of 55.4±1.5 kPa, 8.5±0.2 kPa and 11.3±0.3 kJ/m3, and are respectively 2.2, 7.1 and 1.9 times those of the pure PDMAA gel. It is worth mentioning that the mechanical properties of PDMAA-G1.5 are comparable with those of the hydrogels reinforced by CNC [34], suggesting the promising potential of AM-MCC as reinforcement. Further increasing the AM-MCC content would lead to obviously aggregating of the particles, which is indicated in Figure 3. However, the aggregating of the AM-MCC particles has not formed rigid frameworks, which have been reported to drastically improve strength of hydrogels [3, 11, 12, 26, 33, 35-37]. Instead, the aggregation of the AM-MCC particles in the present work would have destroyed the uniform microstructures of the composite hydrogels, resulting in reduction of the mechanical properties. The above results clearly evidence the reinforcement of the PDMAA hydrogel by proper amount of AM-MCC particles.


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AM-MCC content (%) Figure 5 Stress-strain curves for the PDMAA based hydrogels with different AM-MCC contents under a 60 mm/min tensile speed (a), and the dependences of the tensile strengths on the AM-MCC content (b), where the error bars represent one standard deviation of at least three replicates. In order to know more detail of the reinforcement of the PDMAA hydrogel by the AM-MCC particles, the PDMAA-G1.5 was stretched under different tensile speeds. Figure 6 displays the dependences of the tensile strength and of the breaking elongation on the tensile speed. It is observed that the breaking elongation increases monotonously, while the tensile strength increases with rising tensile speed under ACS Paragon Plus Environment


relatively low tensile speeds (below 120 mm/min), but decreases under high tensile speeds (over 120 mm/min). The results suggest that new entanglements of the polymer chains in the composite hydrogel could have been established upon stretching under a relatively low tensile speed. The new entanglements should be based on the strong interaction between the PDMAA chains and the AM-MCC particles, which has been surface modified with AM molecules to possibly have PAM chains in different lengths. The interaction could be the hydrogen bonding between the PDMAA chains and the PAM chains on the AM-MCC particles, and also their entanglements. This interaction could be refreshed continuously [3, 8, 37] when the tensile speed is not very high (below 120 mm/min in the present study), so the stretching energy could be dissipated to improve the strength of the composite hydrogel. In contrast, the re-arrangement of the chains to form new entanglements could not take place in time under high tensile speed (over 120 mm/min), and as a result the tensile strength of the composite hydrogel decreased. Those results suggest the physical but not chemical cross-linking of the PDMAA chains by the AM-MCC particles [3, 8, 23].

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ε b ( %)

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σ b (kPa)

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

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Tensile speed (mm/min) Figure 6 Dependences of the tensile strength (σb) and the breaking elongation (εb) on the tensile speed for the PDMAA-G1.5 composite hydrogel. Error bars represent one standard deviation of at least three replicates. As discussed above, the AM-MCC particles could act as physical cross-linker to enhance the entanglements of the polymer chains in the matrix to strengthen the


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composite hydrogel, and were beneficial to the re-arrangements of the chains under proper tensile speed. Meanwhile, the results suggest recovery ability of the composite hydrogels under relatively low tensile speed [26]. As illustrated in Figure 7, a specimen of the composite hydrogel of PDMAA-G1.5 was firstly stretched under 60 mm/min to 1000% elongation, which is far below the breaking elongation. After unload from the tensile test equipment and rest for 5 min, the same specimen was stretched to1000% strain again. The process was repeated for the same specimen after various resting intervals, and the stress-strain curves have been recorded. The obvious is the similarity of the stress-strain curves for all of those tensile tests. All of the stress-strain curves at the initial stage (ε ˂ ~5%) are almost identical, and the curves for those after resting are above that of the first stretching in most of the following strain range (ε˃ ~5%). This could be attributed to the orienting of the polymer chains and the AM-MCC particles during the previous stretchings. The energy applied to stretching is also calculated according to Equation (4) by integrating the area under the stress-strain curve to represent its toughness. Compared with the first stretching, the toughness for the second increased slightly after resting for 5 min (13.9% improvement), suggesting fast re-arrangement of the polymer chains. After the second stretching and the subsequently resting for another 15 min, the toughness increased about 4.6% for the third stretching. The improvements in toughness for the fourth and the fifth stretchings are 8.6% and 8.9%, respectively. The very close toughness of the fourth and the fifth stretchings mean that new networks with relatively high stability could have been formed after the former stretchings. As discussed above, the re-arrangements and the re-organization of the chains would be easy and fast, so the improvement in toughness of the new networks with higher stability would be resulted from the AM-MCC particles, which have strong interactions and entanglements with neighboring PDMAA chains. The analysis suggests concentrating of cross-linking points on the surface of the AM-MCC filler [9], even though the cross-linking points were not covalently bonded. This is similar as the literature report, where polymer networks were formed in swollen chitosan microparticles to obtain tough hydrogels [23, 36]. Therefore, our results indicate the excellent recoverability in large strain amplitude (1000%), and potential cyclic applications of the composite hydrogel.



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initial rest 5 min rest 15 min rest 30 min rest 45 min

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Toughness ( kJ/m 3)

σ (kPa)

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

Page 18 of 36

10

15 10 5 0 0

0 0

200

400

10

20

30

Rest time (min)

600

800

40

50

1000

ε (%) Figure 7 Recovery behaviors after different resting times for a same PDMAA-G1.5 composite hydrogel specimen, which has been stretched under a 60 mm/min tensile speed to 1000% strain. Insert shows the toughness change after different rest times. Figure 8 shows the swelling behaviors of the hydrogels. It has been observed that the hydrogels can absorb water very quickly at the initial stage, and the swelling ratios are over 200% within 2h. Then, the hydrogels gradually reach equilibrium swelling in about 7 days, and all the equilibrium swelling ratios are greater than 2600%. It is noted that the equilibrium swelling ratio decreases with increasing the AM-MCC content in the composite hydrogel. It is known that the swelling of hydrogel is largely determined by its cross-linking density, and higher cross-linking density would normally lead to lower equilibrium swelling ratio [38]. According to preparation conditions as listed in Table 1, all of the hydrogels should have similar cross-linking density because of the identical monomer and cross-linking agent concentrations. However, our results indicate the increasing of the cross-linking density with increasing the AM-MCC content in the composite hydrogel. This means once again that the AM-MCC particles have acted as “cross-linker” in the composite hydrogel [3, 26], and more “cross-linker” of the AM-MCC particles would certainly lead to higher cross-linking density, which is consistent with the previous analyses.


Page 19 of 36

PDMAA-G0 PDMAA-G0.5 PDMAA-G1.0 PDMAA-G1.5 PDMAA-G2.0 PDMAA-G2.5

4000

3000

4000

2000

3000

2000

1000 1000

Equilibrium SR (%)

SR (%)

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


AM-MCC content (%)

0 0

3

6

9

0 0

50

100

150

200

Time (h) Figure 8 Swelling behaviors of the PDMAA based composite hydrogels. Insert is the dependence of the equilibrium swelling ratio on the AM-MCC content in the composite hydrogel. Error bars represent one standard deviation of five replicates.

PAM based hydrogels The PAM based hydrogels were too prepared in the presence of the AM-MCC particles. Figure 9 shows the dependence the compressive strength at 80% compressive strain on the AM-MCC content for the PAM based composite hydrogels. Similarly, the σε=80% increases with the AM-MCC content and reaches the highest (541.1±4.3 kPa) to be 3.0 times that of the pure PAM hydrogel (181.2±6.5 kPa) when the AM-MCC content is 4.0%, exhibiting excellent reinforcement performance compared with that using CNCs as reinforcement [18, 25]; further increasing the AM-MCC content will result in decreasing of the compressive strength of the composite hydrogel. Likewise, the strengthening of the PAM hydrogels could also be attributed to the cross-linking effect of the surface modified AM-MCC particles.



600 500 400

σ (kPa)

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

Page 20 of 36

300 200 100 0 0

2

4

6

8

AM-MCC content (%) Figure 9 Dependence of the compressive strength at 80% compressive strain on the AM-MCC content for the PAM based composite hydrogels. Error bars represent one standard deviation of at least three replicates.

CONCLUSIONS Microcrystalline cellulose particles have been modified with acrylamide on the surface under microwave irradiation to have functional groups such as amine and PAM chains with different lengths, which facilitates their dispersion in aqueous solutions of the monomers such as DMAA and AM. After polymerization of the monomers, PDMAA and PAM based composite hydrogels have been successfully prepared, respectively. The results of the mechanical measurements clearly show that the composite hydrogels have been greatly reinforced upon addition of the AM-MCC particles. The mechanical properties including the compressive stress, compressive modulus, the tensile strength, the Young's modulus and the toughness increase with increasing the AM-MCC amount, and reach the highest when the AM-MCC content is 5.6% for the PDMAA based hydrogel, and 4.0% for the PAM based hydrogel. The analyses of the microstructures of the composite hydrogels suggest that the AM-MCC microparticles with proper amounts can be evenly distributed in the matrices of the hydrogels. The evenly distributed AM-MCC particles can act as physical cross-linker to connect the neighboring polymer chains to strengthen the composite hydrogels, based on their strong interaction including the hydrogen bonding and the chain


Page 21 of 36 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


entanglements. However, over loading the AM-MCC particles in the hydrogels will destroy the uniform microstructures of the composite hydrogels, resulting in reduction of the mechanical properties. As a result of the cross-linking effect of the AM-MCC particles, the composite hydrogel exhibits excellent recoverability in large strain amplitude and potential cyclic applications. Moreover, the composite hydrogel shows reasonably high swelling ratio, implying the promising applications especially in biomedical field.

ACKNOWLEDGMENTS The authors acknowledge the financial supports from the Natural Science Foundation of China (51273166) and the Fundamental Research Funds for Xiamen University (20720172007).

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For Table of Contents Use Only 12

60

3

U ( kJ/m )

50

σb (kPa)

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

Page 26 of 36

8 40

30

σb

U

4

20 0

2

4

6

8

10

A M -M C C c o n te n t (% )

Promising reinforcement for hydrogel of the surfaced modified microcrystalline cellulose particle, an inexpensive and sustainable resource material.


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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

Transmittance (%)


MCC

897

1639

AM-MCC

1161

3349

897

1677 1619 1161

3349

PAM

3422

1648 1617

PDMAA-G0

3445

1630 14971356

PDMAA-G1.5 3466

4000

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3500

3000

2500

1357 1642 1495 2000 1500 1000 -1

Wavenumber (cm )


500

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Figure 3 SEM images of the hydrogels: PDMAA-G0 (a), PDMAA-G0.5 (b), PDMAA-G1.0 (c), PDMAA-G1.5 (d), PDMAA-G2.0 (e) and PDMAA-G2.5 (f). The arrows indicate the AM-MCC particles. 47x55mm (600 x 600 DPI)




Page 30 of 36

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50 40

 (kPa)

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


(a)

30 20 10 0 0

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 (%)


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2000


60

2000

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b

30

U

b

12 10 1500

b (%)

U (kJ/m3)

(b)

b or E (kPa)

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

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8

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b (%)

1500

b (kPa)

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


70

1400 60 1300

50 50

100

150

Strain rate (mm/min)


1200 200


40

initial rest 5 min rest 15 min rest 30 min rest 45 min

30

20

20

Toughness ( kJ/m3)

σ (kPa)

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

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PDMAA-G0 PDMAA-G0.5 PDMAA-G1.0 PDMAA-G1.5 PDMAA-G2.0 PDMAA-G2.5

4000

3000

4000

2000

3000 2000

1000 1000

Equilibrium SR (%)

SR (%)

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0

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0

6

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Time (h)


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9


600 500 400

 (kPa)

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

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AM-MCC content (%)


8

Microcrystalline Cellulose Surface-Modified with ... - ACS Publications

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