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Elastomer Reinforced with Regenerated Chitin from Alkaline/Urea Aqueous System Peng Yu, Hui He, Yuanfang Luo, Demin Jia, and Alain Dufresne ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08294 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Applied Materials & Interfaces

Elastomer Reinforced with Regenerated Chitin from Alkaline/Urea Aqueous System Peng Yu, †,‡ Hui He, †* Yuanfang Luo, † Demin Jia, † Alain Dufresne ‡* †

Department of Polymer Materials and Engineering, South China University of

Technology, Guangzhou, 510640, People's Republic of China ‡

Univ. Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France

KEYWORDS. Regenerated chitin; elastomer; coagulation; interpenetrating polymer networks; reinforcement ABSTRACT Novel hybrid elastomer/regenerated chitin (R-chitin) composites were developed, for the first time, by introducing chitin solution (dissolved in alkaline/urea aqueous solution at low temperature) into rubber latex, and then co-coagulating using ethanol as the co-coagulant. During the rapid co-precipitation process, the chitin solution showed rapid coagulant-induced gelation and a porous chitin phase was generated, while the rubber latex particles were synchronously demulsificated to form the rubbery phase. The two phases interlaced and interpenetrated simultaneously to form an Interpenetrating Polymer Network (IPN) structure, which was evidenced by SEM observation. The ensuing compound was also characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and swelling experiments. The unique porous structure of R-chitin could result in strong physical 1

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entanglements and interlocks between filler and matrix, thus a highly efficient load transfer between the filler and the matrix was achieved. Accordingly, R-chitin endows the elastomer with a remarkable reinforcement. We envisage that this work may contribute new insights on novel design of chitin-based elastomer hybrids with IPN structure. INTRODUCTION Elastomers are acknowledged as strategically important materials because of their unique entropic elasticity and various irreplaceable applications. Therefore, the reinforcement of rubbers via incorporation of reinforcing fillers is indispensable because most neat rubbers suffer from poor mechanical strength.1 Among possible fillers, carbon black (CB) is the widely used reinforcing filler in the rubber industry for more than one century because of its excellent reinforcement and low cost. However, the source of CB is petroleum-based while the fossil resource is limited, and CB causes pollution and gives to the rubber a monotonous black color.2 Therefore, significant efforts have been devoted to seek for environmentally-friendly reinforcing fillers that are readily available, light-weight, inexpensive, and renewable. Chitin, a structural and natural polymer which can be found in the exoskeleton of crustacean, cuticle of insects, and cell wall of fungi, is, next to cellulose, the most bountiful biopolymer on Earth.3-4 Apart from its application in drug delivery,5 tissue engineering,6 environmental pollutants remediation7 and optoelectronics,8 chitin is also attracting a great deal of attention as a green reinforcing agent in both synthetic and natural polymeric matrices.9-10 However, compared to cellulose, the attempts to 2


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apply chitin in elastomer composites is relatively scarce. Most researchers focused on the use of chitin nanocrystals as a reinforcing phase in elastomer composites.11-17 In these previous reports, the chitin nanocrystals were firstly suspended in water, and the chitin suspension was mixed with polymer latex/solution to achieve a fine dispersion within the polymeric matrix. As expected, chitin nanocrystal shows a prominent reinforcing effect towards the elastomer due to its rigid attribute. Recently, the aqueous alkali/urea system was developed as a novel and green chemical medium to dissolve chitin by freezing and thawing procedure,18-21 on the basis that the NaOH hydrogen-bonded chitin complex can be surrounded by urea hydrates to form a water-soluble sheath-like structure adopting an extended chain, which leads to chitin dissolution,18,20 and the chitin could regenerate from this system by heating22 or adding coagulant.23 Various materials, such as graphene oxide,24 carbon nanotubes,25 silver molybdate,26 hydroxyapatite27 and poly(vinyl alcohol)28 were used to form hybrids with regenerated chitin, for improving the mechanical or functional performances of ensuing composites. However, studies devoted to the preparation of elastomer/R-chitin are scarce. It is well-known that most rubbers have a latex form. Herein, we propose a simple method to prepare rubber/chitin hybrids by co-precipitating the mixture of the chitin alkali/urea aqueous solution and rubber latex. According to the authors' knowledge, such kind of chitin/rubber composite has rarely been exploited. The present work intends to investigate the reinforcing capability of regenerated chitin (R-chitin) towards the elastomer, and the natural rubber (NR) was chosen as the polymer matrix because (i) it is the most important industrial raw 3



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material with great economic importance, (ii) NR is a perfect matrix to be used as a model system to study the effect of filler reinforcement, owing to its high flexibility and low stiffness.29-30 The results revealed that the porous R-chitin phase interlaced and interpenetrated with the NR phase, and a unique R-chitin/NR IPN structure was in-situ formed in the hybrid. Although the poor compatibility of NR and R-chitin (NR is a hydrophobic polymer while chitin is a hydrophilic polymer), no surfactant addition or any chemical modification were introduced in the present system. Moreover, the R-chitin phase is in a microscale instead of nanoscale, but the R-chitin was still found to have a conspicuous reinforcing effect towards NR. This was ascribed to the R-chitin's unique network-like porous structure which could induce a physical entanglement and interpenetration between filler and matrix. It is believed that R-chitin could bear much external force when an exerted stress is applied to the material. Thus, the reinforcement will not be so much dependent on the interfacial properties, polarity discrepancy and nanoscale dispersion, which were traditionally considered as crucial factors for the reinforcement of elastomers.31 This simple reinforcing strategy, irrespective of the great polarity discrepancy between non-polar NR and polar R-chitin, is highly desirable. The mechanical performance, crystallinity and swelling behavior of NR/R-chitin hybrids were investigated as well as their thermal stability. MATERIALS AND METHODS Materials. Commercial chitin (practical grade, formula: (C8H13NO5)n, color: off white to light yellow to light beige, form: flakes, source: shrimp shells, % acetylated ≥ 4


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95 %) was purchased from Sigma-Aldrich (USA). Natural rubber (NR) was kindly supplied as NR latex (type: CENTEX FA, Full Ammonia, pH 10~11, dry rubber content 59~61%) by Centrotrade Deutschland GmbH (Eschborn, Germany). NaOH and urea (purity≥99.5%, BioScience-Grade) were bought from Carl Roth (Karlsruhe, Germany). Ethanol and other agents were purchased from local sources. Purification of chitin. Commercial chitin was purifed to remove most of the proteins and impurities according to the related references.10-11 Briefly, the samples were first boiled and stirred in a 5% KOH solution for 6 h to remove most of the proteins. This suspension was subsequently kept at room temperature overnight under stirring, filtered, and washed several times with distilled water. Chitin samples were then bleached with 17 g of NaClO2 in 1 L of water containing 0.3 M sodium acetate buffer (the pH of the bleaching solution was adjusted to 4 by adding CH3COOH) for 6 h at 80°C. The bleaching solution was changed every 2 h followed by abundant rinsing of the sample with distilled water. After the samples were bleached, the suspension was kept in a 5% KOH solution for 48 h to further remove residual proteins. The resulting purified chitin suspension was washed with distilled water, and then dried at 50°C until a constant weight. Processing of composites. A schematic of the preparation route for NR/chitin hybrids is illustrated in Figure 1.

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Figure 1. Schematic diagram of preparation of NR/R-chitin hybrids. Firstly, the purifed chitin was dissolved in the NaOH/urea aqueous system by freezing and thawing method32 to form a 4 wt% transparent solution. Then, the chitin solution was centrifuged at 8,000 rpm for 10 min at 5°C to remove the slightly undissolved fraction and bubbles. A desired amount of chitin solution was immediately added into the diluted NR latex (the solid content was 30%) under stirring to obtain a uniform chitin/NR mixture at room temperature. Subsequently, the well-mixed mixture was co-precipitated by adding ethanol. The sediment was filtered first and then immersed in distilled water for one week with changing water to remove the residual urea and NaOH. After that, the sediment was dried in a oven at 50°C until a constant weight. The NR/R-chitin sediments were firstly pre-heated for 3 min in a hydraulic press (Mécanique Outillage, Model Saint-Eloi, France), and subsequently hot-pressed under 42 bars at 145°C for another 3 min to obtain films. After that, the films were 6


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immediately cold-pressed at room temperature for 7 min to avoid shrinkage. The as-prepared films were conditioned at 23.5°C and 50% relative humidity (RH) for tensile tests. Composite samples were denoted NR/xR-Chitin, where x represents the chitin:NR ratio (in %). Characterization. X-ray diffraction (XRD) analysis was performed to investigate the crystallinity of chitin and NR/R-chitin films. The XRD experiments were conducted with a diffractometer (X'Pert PROMPD, PAN analytical, Netherlands) at a step interval of 0.02° with Cu-Kα radiation (40 kV, 40 mA). The crystallinity index (CrI) was determined by the following empirical equation: 33

CrI =

୍భభబ ି୍౗ౣ

(1)

× 100

୍భభబ

where I110 is the max intensity of the peak at 2θ = 19.3° and Iam is the intensity of the amorphous diffraction around 2θ = 16°. Scanning electron micrographs (SEM) of the composites were conducted on a Quanta 250 FEI microscope instrument. The NR and NR/chitin sediments were cut with a razor blade to examine their cross-section. After that, the surface was sputter coated with gold/palladium before examining under SEM. The swelling study was carried out as follows: 0.5 g neat NR, 0.55 g NR/10R-chitin, 0.65 g NR/30R-chitin, 0.75 g NR/50R-chitin, 0.85 g NR/70 R-chitin and 1 g NR/100 R-chitin sediments (the rubber content in all samples was 0.5 g) were divided into scraps and dipped in toluene for one week at room temperature. The photos of the samples during swelling at various times were recorded with a camera.

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Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TGA-6 equipment at a heating rate of 10°C.min-1 under N2 atmosphere in the temperature range 30-700°C. Tensile tests were carried out with a RSA3 (TA Instruments, USA) equipment with a 100 N load cell. The samples were prepared by cutting strips from the films, 30 mm long and the distance between the jaws was 10 mm. The width and the thickness of the samples were measured before each measurement. All experiments were performed at room temperature with a cross-head speed of 1 mm.s-1. The results were averaged on 5 measurements. Optical microscopy was carried out to observe the morphology of the tensile fracture surfaces for the NR and NR/chitin films with a ZEISS (Carl Ziess AG, Jena, Germany) AX10 optical microscope. RESULTS AND DISCUSSION The X-ray diffraction patterns for raw chitin, purified chitin and R-chitin are shown in Figure 2a. All samples show the typical characteristic peaks of α-chitin, which agrees well with the results of previous works.22-23 For raw chitin and purified chitin, there are six diffraction peaks at 9.3°, 12.8°, 19.3°, 20.7°, 23.4° and 26.4° assigned to (020), (021), (110), (120), (130) and (013) planes, respectively. However, for regenerated chitin, the diffraction peaks around 20.7° and 23.4° are hardly visible. In addition, the diffraction peaks for R-chitin are much weaker and broader than those for purified chitin or raw chitin, indicating a decrease in the crystallinity and crystallite size.24, 32 This result could also be evidenced through the crystallinity index (CrI), which was 8


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determined according to Eq. (1). The CrI value for raw chitin, purified chitin and R-chitin is 89.2, 92.3, and 76.3, respectively. The CrI of purified chitin is higher than that of raw chitin because of the removal of part of amorphous chitin domains, proteins and other impurities during the purification process. The CrI of R-chitin is much lower than that of raw chitin or purified chitin, as expected. This might be due to that the rapid self-association and realignment of chitin molecules during the fast precipitation process, which could lead to an inadequate development of the crystallization of chitin chains.

Figure 2. XRD patterns for (a) chitin and (b) rubber compounds: raw chitin (□ □), purified chitin (○ ○), regenerated chitin (△ △), NR (■), NR/10R-chitin (●), NR/30R-chitin (▲), NR/50R-chitin (▼), NR/70R-chitin (◆), NR/100R-chitin (★). Figure 2b shows the XRD patterns for rubber composites. NR does not exhibit any diffraction peak and displays typical behavior of fully amorphous polymer. It is characterized by a broad hump located around 18~19°. The characteristic peaks for α-chitin, at 9.3° (020), 12.8° (021), 19.3° (110) and 26.4° (013) can be observed for the hybrids when adding R-chitin. As expected, these peaks become more intense with 9



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the increase of the R-chitin content in the sample. The XRD results demonstrated that R-chitin was successfully incorporated into the NR matrix and its α-chitin crystalline structure was retained, which is expected to reinforce the NR material with the chitin rigid attribute. SEM observation was performed to investigate the morphology and structure of the different samples. As displayed in Figure 3, at low filler concentrations (NR/10R-chitin), a typical sea-island morphology and quasi-interpenetrating polymer network was identified. The individual units existing as dispersed phase consist of R-chitin, which exhibited a microporous structure at the microscale instead of nanoscale. NR forms the continuous phase. At higher filler contents (NR/70R-chitin and NR/100R-chitin), both the rubbery phase and filler phase establish a continuous phase, and it is clearly observed that the rubber phase interprenetrated the pores of the R-chitin phase. The two phases interlaced and interlocked but were not covalently bonded to each other to form a fully-IPN structure. It should be noted that the drying method could affect the pore size of the R-chitin. Direct drying in air caused significant shrinkage for neat R-chitin, resulting in an essentially porosity-free material.23 However, the porous structure of R-chitin in the NR/R-chitin blend (direct drying) can be clearly observed. It indicates that hybridization of NR and R-chitin could significantly increase its pore size. Similar results were reported when introducing graphene oxide (GO) into R-chitin.24

10


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Figure 3. SEM images of the cross-section for NR/xR-chitin sediments (before hot-pressing): (a) NR, (b) NR/10R-chitin, (c) NR/30R-chitin, and (d) NR/50R-chitin. The yellow arrows indicate the NR phase that interpenetrates the R-chitin phase, the green arrows indicate the R-chitin pores, and the blue circle indicates the interlock/interlacement between NR and R-chitin.

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Figure 3(continued). SEM images of the cross-section for NR/xR-chitin sediments (before hot-pressing): (d) NR/50R-chitin, (e) NR/70R-chitin, and (f) NR/100R-chitin. The yellow arrows indicate the NR phase that interpenetrate s the R-chitin phase, the green arrows indicate the R-chitin pores, and the blue circles indicate the interlock/interlacement between NR and R-chitin. Since chitin is a hydrophilic polymer containing large amounts of −OH groups while NR is a diene-based elastomer with hydrophobic nature, it is not surprising that a gap between the R-chitin and NR domains was observed (Fig. 3b), indicating the poor interfacial bonding between the filler and the matrix. However, the porous R-chitin 12


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could strongly influence the volume fraction of the constrained region of rubber due to the high physical interlacement, entanglement and interlock between R-chitin and NR, resulting in strong interaction between the filler and the matrix, which could be beneficial for the reinforcement of the elastomer composite. Also, one can imagine that the hot-pressing process could remove or minimized the gap between R-chitin and NR domains. Since NR could flow under heat and pressure during the hot-pressing process, it is reasonably convincing that the NR molecular chains can fill the R-chitin pores and the gaps between two adjacent particles, which would theoretically promote further interlacements/entanglements between NR and R-chitin. We can therefore infer that it would be a tight structure after hot-pressing so that the IPN structure cannot be identified by SEM anymore although it still exists. Figure 4 shows the photographs of NR and NR/xR-chitin compounds when immersed in toluene. It is well-known that NR can be easily swollen and dissolved in toluene while R-chitin cannot be swollen in this medium. As shown in Figure 4, it is observed that neat NR is almost dissolved in toluene within 3 hours. The NR/10R-chitin sample can be fully swollen in the vial, while the ensuing solution is not transparent compared to neat NR. When the R-chitin content is higher than or equal to 30 phr, the NR/R-chitin compound can even not be fully swollen in tolune. It is clearly observed that the upper phase of the vial is the transparent toluene solution while the lower phase of the vial is the NR/R-chitin compound (NR/30,50,70,100R-chitin). It is obvious that NR/R-chitin composites have a better resistance to swelling compared to NR, and this resistance was enhanced when increasing the R-chitin content, as 13



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expected. Apart the non-swelling capability of toluene towards chitin, one can imagine that the porous R-chitin structure could effectively constrain/immobilize the rubbery phase. Although the interfacial bonding between chitin and NR is poor (as discussed in the SEM analysis section), there will be more rubber domains trapped or caged in the rigid 3D R-chitin network with the increase of R-chitin content. R-chitin could impose a strong restriction to the mobility of the constrained rubber molecules, which could be sufficiently strong to restrict the swelling of trapped rubber. Moreover, the R-chitin network could lead to increased tortuosity of the diffusion pathway for toluene molecules. The sample with the stronger well-dispersed R-chitin network has the more compact complex structure, while the penetrating power of the solvent remains constant. The swelling study clearly demonstrates that there is an intimate interaction between porous R-chitin and rubber.

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Figure 4. Photographs of NR/xR-chitin composites swelling in toluene for various times. The TGA and DTG curves for the various samples are presented in Figure 5. Table 1 shows the thermal degradation characteristics of the samples. For R-chitin, the slight weight loss below 100℃ was ascribed to the release of moisture associated to the hydrophilic character of chitin,27, 34 and its weight decreased sharply from 250 to 400°C due to thermal degradation. For neat NR, the thermal decomposition occurred mainly in the range 300 to 500°C. The thermal degradation behavior for all NR/R-chitin composites with a single main mass loss step is similar, and no obvious separate degradation stage can be found in any DTG curve. A continuous shift of the degradation temperature to lower values can be observed for NR/R-chitin composites 15



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when increasing the R-chitin content. For instance, the T5%, T10%, T50% and Tmax values for NR/100R-chitin are 294.9, 323.4, 379.9 and 366.2°C, respectively, which are about 53.7, 35.8, 9 and 11°C lower than for neat NR. The results show that the thermal stability of NR/R-chitin composites gradually deteriorates with the loading of R-chitin, which was obviously ascribed to the lower thermal stability of the R-chitin component compared to NR. Upon adding R-chitin to the composite, an increase of the char content is also observed because of the higher char residue for neat R-chitin compared to NR.

Figure 5. (a) TGA and (b) DTG curves for NR/R-chitin composites. Table 1. Thermal degradation characteristics for the NR/R-chitin composites.* Samples

T5% (°C) T10% (°C) T50% (°C)

Tmax (°C)

Char Content (at 700°C)

R-chitin

176.7

302.3

364.9

347.2

31.8

NR

348.6

359.2

388.9

377.2

0

NR/10R-chitin

338.1

353.7

387.8

376.8

1.7

NR/30R-chitin

323.5

343.7

386.1

373.5

4.3

NR/50R-chitin

318.7

341.2

385.4

372.4

6.1

NR/70R-chitin

297.1

327.1

381.9

367.6

9.1

16


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NR/100R-chitin

294.9

323.4

379.9

366.2

11.7

*T5% - 5% weight loss temperature, T10% - 10% weight loss temperature, T50% - 50% weight loss temperature, Tmax - DTG peak degradation temperature.

The representative stress-strain curves for the composites are plotted in Figure 6, and the mechanical properties extracted from these curves are summarized in Table 2. With the incorporation of R-chitin, the stiffness of the composites tended to be improved. For instance, the Young's modulus, stress at 100 % strain and tensile strength are increased from 0.64, 0.52 and 0.89 for neat NR to 21.7, 4.94 and 5.85 MPa, respectively, when 30 phr R-chitin was added. This considerable improvement in the stiffness of the material can be possibly attributed to: (i) the inclusion of rigid chitin into the soft NR matrix that will definitely increase the global modulus and strength of the composite, (ii) the increasing amount of rubber domains trapped or caged in the chitin network when increasing the R-chitin content, and the trapped rubber would be at least partially ‘dead’, behaving like a filler, (iii) the unique porous structure of R-chitin that could induce strong physical entanglements between both components, with the consequence of a more effective stress transfer from NR to the R-chitin phase before catastrophic breakage occurs. However, the elongation at break for NR/R-chitin composites progressively decreased with the increase in R-chitin content. The increase in strength and modulus usually compromises the stretchability of the material. During external loading, the porous R-chitin could suppress the flexibility, mobility and slippage of the NR macromolecular chains. Accordingly, rubber extensibility was hindered. Also, a progressive increase in the toughness of the material was observed upon R-chitin addition up to 20 phr due to the dramatic 17



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increase of the sample' modulus. Above 20 phr R-chitin, the toughness started to decrease because of the serious drop of the materials' strain at break. However, it is worth noting that the toughness value for all NR/R-chitin composites is much higher than for neat NR, indicating that more energy is needed to break the NR/R-chitin hybrids compared to neat NR.

Figure 6. Typical tensile stress-strain curves for NR and NR/R-chitin composites: NR (■), NR/5R-chitin (●), NR/10R-chitin (▲), NR/15R-chitin (▼),NR/20R-chitin (◆), NR/25R-chitin (□ □) , NR/30R-chitin (○ ○). (The insert photo shows the stress-whitening scenario for NR/R-chitin). Table 2. Mechanical properties for NR/R-chitin composites (the standard error is presented in brackets). Young's modulus (MPa)

Stress at 100 % strain (MPa)

Stress at 300 % strain (MPa)

Tensile strength (MPa)

NR

0.64 (0.14)

0.52 (0.11)

0.63 (0.10)

0.89 (0.12)

625 (41)

4.28

NR/5R-chitin

1.72 (0.30)

0.72 (0.10)

1.25 (0.17)

1.88 (0.16)

568 (28)

7.08

NR/10R-chitin

2.90 (0.25)

1.28 (0.12)

2.21 (0.11)

3.27 (0.13)

511 (36)

11.0

Samples

18


Elongation Toughness* at break (%) (MJ/m3)

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NR/15R-chitin

4.62 (0.40)

1.91 (0.10)

3.28 (0.14)

4.34 (0.17)

492 (34)

13.9

NR/20R-chitin

10.1 (0.91)

3.30 (0.14)

5.40 (0.18)

6.29 (0.30)

461(39)

20.1

NR/25R-chitin

14.5 (1.2)

3.85 (0.18)

6.43 (0.22)

6.81 (0.26)

351 (30)

16.1

NR/30R-chitin

21.7 (2.4)

4.94 (0.29)

/

5.85 (0.31)

199 (25)

9.84

* Toughness is calculated from the area under the stress-strain curve before rupture.

Noticeably, the NR/R-chitin samples were transparent (see typical photographs for NR/25R-chitin before and after stretching which are inserted in Fig. 6), indicating the fine dispersion of R-chitin in the NR matrix. Also, a stress-whitening scenario was observed for the NR/R-chitin composites, indicating that there is an intense rubber molecular chain sliding and reconformation during the tensile testing. Figure 7 shows the tensile fracture surface of NR/R-chitin composites. The NR/R-chitin composites exhibit a much more rugged fracture surface compared to neat NR. Again, this difference could be ascribed to the interlacement and entanglement of porous R-chitin and NR chains, the R-chitin phase tending to confine the rubbery component. The material could therefore bear a higher external force upon load application, which would induce an obvious stress whitening and plastic deformation. A schematic representation of the hypothetical failure development for the composite is illustrated in Figure 8.

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Figure 7. Optical microscopy images of the tensile fracture surface for NR/R-chitin composites: (a) NR, (b) NR/10R-chitin, (c) NR/20R-chitin, and (d) NR/30R-chitin,.

Figure 8. Schematic representation of the hypothetical failure development for (a) NR and (b) NR/R-chitin composite during tensile testing.

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As shown in Figure 8, the rigid chitin network strongly influences the volume fraction of the constrained rubbery domains, which makes both components tightly interlocked and stress transfer more effective. It leads to a more tortuous pathway for the crack propagation and intense plastic deformation, which, in turn, results in a high energy dissipation and conspicuous reinforcement towards the elastomer. Tensile tests have clearly demonstrated that R-chitin could be novel and green reinforcing filler for elastomers. CONCLUSIONS We reported a simple and green approach, i.e. co-precipitation method, to fabricate rubber/R-chitin composites with a novel IPN structure. During the co-precipitation process, the chitin inclusion complex associated with NaOH, urea and water was broken by the coagulant, leading to the in-situ and bottom-up formation of a porous R-chitin phase, while the NR latex particles were synchronously demulsificated to form the polymeric matrix. XRD shows that the crystalline structure of chitin is not affected by the procedure. TGA shows that the thermal stability of the hybrids deteriorates when increasing the R-chitin content because of the lower thermal stability of R-chitin compared to NR. The incorporation of chitin endows the rubber with a more compact structure with limited solvent penetration property. The tensile tests clearly show that R-chitin has a striking reinforcement effect towards the elastomer. It is obvious that the fine dispersion and unique porous structure of R-chitin were responsible for the observed reinforcement and morphological characteristics of the blends. Further work will focus on the hybridization of R-chitin 21



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and other filler (such as graphene, carbon nanotube or carbon black, etc) on the structure and properties of the elastomer. Basically, this work offers a new general design strategy to use polysaccharides (such as regenerated cellulose, chitin, chitosan) from alkaline/urea aqueous system to reinforce elastomers through a green yet simple co-precipitation of polysaccharide alkaline-urea aqueous solution and rubber latex. AUTHOR INFORMATION Corresponding Authors * Alain Dufresne, Email: [email protected]. * Hui He, Email: [email protected]. Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †,‡ These authors contributed equally. ACKNOWLEDGMENTS The authors acknowledge financial support from China Scholarship Council (CSC no. 201606150006), Science and Technology Project of Guangdong Province (2015B010122002) and Science and Technology Project of Guangzhou (201508020090). LGP2 is part of the LabEx Tec 21 (Investissementsd’Avenir - grant agreement n°ANR-11-LABX-0030) and of the PolyNat Carnot Institut (Investissementsd’Avenir - grant agreement n°ANR-11-CARN-030-01). The authors 22


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are grateful to PhD Fatma Larbi for help on the purification of chitin. REFERENCES (1) Tang, Z.; Huang, J.; Wu, X.; Guo, B.; Zhang, L.; Liu, F. Interface Engineering toward Promoting Silanization by Ionic Liquid for High-Performance Rubber/Silica Composites. Ind. Eng. Chem. Res. 2015, 54, 10747-10756. (2) Angellier, H.; Molina-Boisseau, S.; Lebrun, L.; Dufresne, A. Processing and Structural Properties of Waxy Maize Starch Nanocrystals Reinforced Natural Rubber. Macromolecules 2005, 38, 3783-3792. (3) Muzzarelli, R. A. Chitin. Elsevier: 2013. (4) Morin, A.; Dufresne, A. Nanocomposites of Chitin Whiskers from Riftia Tubes and Poly(caprolactone). Macromolecules 2002, 35, 2190-2199. (5) Rejinold N, S.; Chennazhi, K. P.; Tamura, H.; Nair, S. V.; Rangasamy, J. Multifunctional Chitin Nanogels for Simultaneous Drug Delivery, Bioimaging, and Biosensing. ACS Appl. Mater. Interfaces 2011, 3, 3654-3665. (6) Arun Kumar, R.; Sivashanmugam, A.; Deepthi, S.; Iseki, S.; Chennazhi, K. P.; Nair, S. V.; Jayakumar, R. Injectable Chitin-Poly(ε-caprolactone)/ Nanohydroxyapatite Composite Microgels Prepared by Simple Regeneration Technique for Bone Tissue Engineering. ACS Appl. Mater. Interfaces 2015, 7, 9399-9409. (7) Duan, B.; Gao, H.; He, M.; Zhang, L. Hydrophobic Modification on Surface of Chitin Sponges for Highly Effective Separation of Oil. ACS Appl. Mater.

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(8) Lee, D.; Lim, Y.-W.; Im, H.-G.; Jeong, S.; Ji, S.; Kim, Y.; Choi, G.-M.; Park, J.-U.; Lee, J.-Y.; Jin, J. A Bio-inspired Transparent Laminated Composite Film for Flexible Green Optoelectronics. ACS Appl. Mater. Interfaces 2017, in press. (9) Paillet, M.; Dufresne, A. Chitin Whisker Reinforced Thermoplastic Nanocomposites. Macromolecules 2001, 34, 6527-6530. (10) Lu, Y.; Weng, L.; Zhang, L. Morphology and Properties of Soy Protein Isolate Thermoplastics Reinforced with Chitin Whiskers. Biomacromolecules 2004, 5, 1046-1051. (11) Gopalan Nair, K.; Dufresne, A. Crab Shell Chitin Whisker Reinforced Natural Rubber Nanocomposites. 1. Processing and swelling behavior. Biomacromolecules 2003, 4, 657-665. (12) Gopalan Nair, K.; Dufresne, A. Crab Shell Chitin Whisker Reinforced Natural Rubber Nanocomposites. 2. Mechanical Behavior. Biomacromolecules 2003, 4, 666-674. (13) Gopalan Nair, K.; Dufresne, A.; Gandini, A.; Belgacem, M.N. Crab Shell Chitin Whiskers Reinforced Natural Rubber Nanocomposites. 3. Effect of Chemical Modification of Chitin Whiskers. Biomacromolecules 2003, 4, 1835-1842. (14) Liu, M.; Peng, Q.; Luo, B.; Zhou, C. The Improvement of Mechanical Performance and Water-response of Carboxylated SBR by Chitin Nanocrystals.

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(22) Duan, B.; Chang, C.; Zhang, L. Structure and Properties of Films Fabricated from Chitin Solution by Coagulating with Heating. J. Appl. Polym. Sci. 2014, 131, 39538. (23) Ding, B.; Cai, J.; Huang, J.; Zhang, L.; Chen, Y.; Shi, X.; Du, Y.; Kuga, S. Facile Preparation of Robust and Biocompatible Chitin Aerogels. J. Mater. Chem. 2012,

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Graphical Abstract

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Urea

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