Design of self-healable supramolecular hybrid network based on carboxylated styrene butadiene rubber and nano-chitosan

Accepted Manuscript Title: Design of Self-Healable Supramolecular Hybrid Network Based on Carboxylated Styrene Butadiene Rubber and Nano-Chitosan Authors: Chuanhui Xu, Jiada Nie, Wenchao Wu, Lihua Fu, Baofeng Lin PII: DOI: Reference:

S0144-8617(18)31277-3 https://doi.org/10.1016/j.carbpol.2018.10.080 CARP 14212

To appear in: Received date: Revised date: Accepted date:

24-7-2018 25-9-2018 24-10-2018

Please cite this article as: Xu C, Nie J, Wu W, Fu L, Lin B, Design of Self-Healable Supramolecular Hybrid Network Based on Carboxylated Styrene Butadiene Rubber and Nano-Chitosan, Carbohydrate Polymers (2018), https://doi.org/10.1016/j.carbpol.2018.10.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Design of Self-Healable Supramolecular Hybrid Network Based on Carboxylated Styrene Butadiene Rubber and Nano-Chitosan Chuanhui Xu*, Jiada Nie, Wenchao Wu, Lihua Fu, Baofeng Lin Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

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Corresponding Author: Chuanhui Xu [email protected]

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Graphical Abstract

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Highlights

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We fabricated a salt-bond crosslinked supramolecular hybrid network Nano-chitosan served as crosslinking points in constructing the network Enhanced mechanical properties without sacrifice of self-healing property Tensile strength of composite is more than 2-time that of neat rubber matrix Healing efficiency achieved 92% after healing for 24h

ABSTRACT In this paper, carboxylated styrene butadiene rubber (XSBR)/nano-chitosan (NCS) composites were prepared via a salt-forming reaction, in which NCS participated in constructing a 1

supramolecular hybrid network. NCS served as multiple-functional cross-linker as well as reinforcer for XSBR. This turned out that the mechanical properties of XSBR could be improved by NCS, and simultaneously, the self-healing property was retained to utmost extent. With 20 wt% NCS, the tensile strength of NCS/XSBR composite was increased to 1.3 MPa, more than 2-time that of the neat XSBR, meanwhile, the healing efficiency was as high as to 92% after healing at ambient temperature for 24 h.

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KEYWORDS: ionic bonds; self-healing; supramolecular network; mechanical property; rubber; chitosan

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INTRODUCTION Self-healing is one of the most noteworthy bio-inspired concepts for synthetic materials (Potier, Guinault, Delalande, Sanchez, Ribot & Rozes, 2014). In recent years, a large quantity of self-healing materials has been raised (Xiang, Rong & Zhang, 2016; Van Tittelboom & De Belie, 2013). Once being damaged, these smart materials are able to repair themselves spontaneously or with the aid of a stimulus such as heat (Xu, Cui, Fu, & Lin, 2018; Du, Jin, Pan, Fan, Lai & Sun, 2018; Uzumcu, Guney & Okay, 2018). Conventional vulcanized rubbers do not possess self-healing behavior due to their nonreversible crosslinked networks (Hernandez, Mar Bernal, Grande, Zhong, van der Zwaag & Garcia, 2017). In order to acquire self-healing function, dynamic bonds are used to construct reversible network in rubbers (Xu, Cao, Lin, Liang & Chen, 2016). The reversible dynamic bonds include dynamic covalent bonds (Wang, Zhou, Wang, Xu, Tang & Yang, 2018; Frisch, Marschner, Goldmann & Barner-Kowollik, 2018; Yang et al., 2018; Chakma, Rodrigues Possarle, Digby, Zhang, Sparks & Konkolewicz, 2017; Santana, Grande, van der Zwaag & García, 2017; Lafont, van Zeijl & van der Zwaag, 2012; Wang, Deng, Zhou, Li & Chen, 2017), such as Diels-Alder bonds (Kuang, Liu, Dong, Liu, Xu & Wang, 2015; Moazzen, Zohuriaan-Mehr, Jahanmardi & Kabiri, 2018), Schiffbase bonds (Zhang, Rong, & Zhang, 2018; Liu et al., 2018), disulfide bonds (Jian, Hu, Zhou & Xiao, 2018; Xu & Chen, 2017), and noncovalent bonds, such as hydrogen bonding (Sordo, Mougnier, Loureiro, Tournilhac & Michaud, 2015; Wang et al., 2015; Cao, Zhang, Lu, Luo & Zhang, 2017; Bian, Wang & Yang, 2017), ionic bonding (Xu, Huang, Li, Chen, Lin & Liang, 2016; Das et al., 2015), molecular interdiffusion (Getachew, Kim & Kim, 2016; Billiet, Hillewaere, Teixeira & Du Prez, 2013), metal-ligand coordination (Weng, Thanneeru & He, 2018; Yu, Zhao, Zhou, Zhang & Zhao, 2017). Compared with reversible covalent bonds, self-healing approaches based on reversible noncovalent bonded supramolecular network are easier to be realized and have been reported in a large number of document (An, Lee, Yarin & Yoon, 2018). For example, Chen et al. (Chen, Kushner, Williams & Guan, 2012) described a diblock copolymers by compounding poly (n-butyl acrylate)b-polystyrene (PBA-b-PS) and 2-ureido-4-pyrimidinone groups (SB-UPy), in which the quadruple H-bonding established a reversible supramolecular network to realize the self-healing behavior. Chen et al. (Xu, Cao, Lin, Liang & Chen, 2016) transformed natural rubber to be self-healable material by controlling peroxide-induced vulcanization to create ionic crosslinked supramolecular network. The reversible ionic crosslinks induced the healing processes which enabled the cut sample to be fully recovered. Wang et al. (Wang et al., 2015) compounded polybutadiene (PB) bearing carboxylic acid and amine groups to obtain salt-hydrogen bonded supramolecular network which enabled materials to be healed. Unfortunately, noncovalent supramolecular rubber network usually 2

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suffers from kPa level of tensile strength, because the supramolecular network must keep sufficient flexibility to allow the chain diffusion during healing process (Burnworth et al., 2011; Herbst, Döhler, Michael & Binder, 2013). Design of a supramolecular hybrid network combining covalent and noncovalent crosslinks has been demonstrated to be a useful approach to overcome this problem (Wietor, Dimopoulos, Govaert, van Benthem, de With & Sijbesma, 2009). Tournilhac and Michaud (Sordo, Mougnier, Loureiro, Tournilhac & Michaud, 2015) track the mechanical strength and self-healing property via controlling the number of hydrogen-bonds and covalent crosslinks in supramolecular hybrid network. The system with 25% of tetrafunctional 4,4′-methylenebis(N,Ndiglycidylaniline) (TGMDA) in the monomer composition showed 11% residual deformation after 80000 s creep/recovery cycles under 2 kPa stress, and 100% healing efficiency after 24 h at room temperature. However, when the chemical crosslinks are increased, the efficiency of self-healing is decreasing, which indicates that the chemical crosslinks have still an inevitable negative effect on self-healing. Addition of nanoparticles is another effective and convenient method to improve the strength of supramolecular hybrid network. Unfortunately, in most case extrinsic nanoparticles do not belong to the reversible system, which usually hinder or block the healing of network (Wietor, Dimopoulos, Govaert, van Benthem, de With & Sijbesma, 2009; Xu, Cao, Huang, Chen, Lin & Fu, 2017). Recently, sustainable development has been the theme of today’s scientific world. Polymer composites based on renewable resources have become a hot research topic (Cao, Yuan, Fu, & Chen, 2018; Nie et al., 2018). Chitosan, one of the most famous natural renewable carbohydrate polymers, has received much attentions being applied in biomedical engineering, pharmaceutical, cosmetic products and water treatment (Zhang, Li, Shi, Chen & Fan, 2018). Incorporation of chitosan particles into rubbers could improve the mechanical properties of rubber composites and endow them with much more biodegradability characteristics. Ismail et al. (Ismail, Shaari, & Othman, 2011) added chitosan particles (average particle size of 90 mm) into three different types of rubber (natural rubber (NR), epoxidized natural rubber (ENR) and styrene-butadiene rubber (SBR)) and found that the maximum torque (MH) values and tensile modulus of rubber/chitosan composites were improved with the increase of chitosan content (0~40 phr). Saboktakin and Saboktakin (2016) improved the mechanical and biodegradation properties of polybutadiene rubber by adding chitosan and silica nanoparticles into the 1,4-cis-polybutadiene rubber. They found that sample’s tensile modulus was increased to the maximum of 450MPa at 2wt % of chitosan while the tensile strength achieved the highest value at 3 wt% of chitosan. Lv et al. (Lv, Wang, Fang, Li, & Li, 2017) prepared NR/chitosan microsphere blends and found that the tensile strength and Tear strength of blend were increased to 5.61 MPa and 12.28 kN.m-1, respectively, when the chitosan loading was 10 wt% (the size of chitosan particle ranged between 160 and 1000 nm in diameter). Unfortunately, the compatibility between chitosan and above reported nonpolar rubbers is not ideal because of the abundant polar groups of chitosan. Carboxylic styrene butadiene rubber (XSBR), a promising polar rubber containing carboxylic acid groups, is usually used as binder in paper coatings, carpet backing, paints and mechanical rubber goods. Considering that chitosan has typical polycationic character due to its amine side group, nano-chitosan (NCS) particles could serve as polymeric ionic templates for generating reversible ionic crosslinks in XSBR matrix, as well as an effective reinforcer. The salt-forming reaction is readily taken placed between XSBR and NCS. At this time, NCS can act as multiple3

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functional crosslinking points in constructing the supramolecular network. Based on the above, in this paper, we put forward a simple strategy to fabricate a supramolecular hybrid network by introducing NCS particles into XSBR. The reaction of XSBR and NCS generated massive hydrogen-bonds and ionic bonds to realize the self-healing behavior of XSBR/NCS composite. Furthermore, the XSBR/NCS composite could save its self-healing capacity due to that NCS participated in the formation of hybrid network. As expected, this supramolecular network was strengthened by the NCS, simultaneously, the self-healing property was able to be retained to utmost extent. To our knowledge, it is the first time to report the self-healable supramolecular hybrid network based on XSBR and NCS, which have potential applications in smart bio-actuators and so on.

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EXPERIMENTAL SECTION Materials. XSBR latex, whose solid content of latex is 50 wt%, was purchased from Shanghai Gaoqiao Petrochemical Co., Ltd (China). The XSBR is a random copolymer which consists of 62 wt% styrene, 35 wt% butadiene and 3 wt% carboxyl functional monomer. The schematic diagram of the chemical structure of the XSBR is shown in Fig. S1 (Supporting Information). NCS powder, with an average particle size about 200 nm, deacetylation rate of 96%, was purchased from Haili biological products Co. Ltd (Shandong, China), and used without further treatment. The other chemicals were of analytical grade.

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Preparation of XSBR/NCS composites. XSBR/NCS composites were prepared by mechanical stirring of XSBR latex and NCS powder at room temperature. Specific procedure is as follows: NCS was added into XSBR latex tardily under vigorous stirring (rotational speed of 2000 r/min) to achieve uniform dispersion. The stirring time was about 30 min and then the XSBR/NCS suspension was degassed in ultrasound for 60 min. After that, the suspension was cast onto a polyethylene (PE) mold and dried in a vacuum drying oven at 30 °C for 72 hours. The XSBR/NCS composite film was stored in drying bottle. The NCS contents in XSBR/NCS composites were 0, 5, 10, 20, 30 and 40 wt%.

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Characterizations. Light transmittances were determined in the wavelength range of 200 to 1000 nm using an UV1901 spectrometer (Shanghai Yuke instrument co., ltd., China). Samples were prepared as film with ~0.7 mm thickness. FTIR spectra was recorded on a Nexus-470 Spectrometer (Thermo Fisher Scientific, USA), scanning wavenumbers ranging from 750 to 4000 cm-1, attenuated total reflectance (ATR) mode, resolution 4 cm-1 and 64 scans for each measurement. All the test samples were completely dried to eliminate the interference of water. SEM images were obtained by XL30-ESEM (Netherlands). Samples were soaked in liquid nitrogen and then brittle fractured. The fractured surfaces were treated by spray-gold before the test. AFM observation was carried out using an ICON/INNOVA (Bruker, GER) under the contact mode. The measurements were performed in a scan rate of 0.5 HZ. Tensile tests. Tensile tests were carried out at room temperature from dumbbell-shaped sample of 20 mm width, 75 mm gauge length, and 2 mm thickness, on a UT-2080 tensile machine (U-CAN Dynatex Inc) at the speed of 500 mm/min. The load was recorded by a 1 KN load cell. The 4

specimens of each sample are measured at least three times. The strain of the cyclic tensile test was set to 50, 100, 150 and 200%.

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Self-healing test. The composites film (20 mm width, 75 mm gauge length, and 2 mm thickness) was cut into two separate parts using a clean razor blade, leaving two smooth cutting surfaces. Then the fresh surfaces were brought into contact together under appropriate press at once. The specimens were healing at ambient temperature for healing 24 h and subsequently subjected to stress-strain tests. Healing efficiency was determined by comparing the tensile strength ratio of the healed samples and the pristine samples (Yang & Urban, 2013).

water uptake ratios =

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Water uptake measurement. The specimens were immersed in deionized water at ambient temperature for 72 hours to achieve full water absorption. The weights of specimens pre- and postswelling were carefully measured on an analytical balance. The water uptake ratios were determined from the following equation: mass of wet sample-mass of dry sample mass of dry sample

×100%

(1)

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Equilibrium swelling experiment. The cross-link density, swelling ratio, relative weight loss of XSBR/NCS composites were determined in equilibrium swelling experiment. The specimens were sliced into small pieces and weighted (m0). Then, the specimens were immersed in toluene at 23 ℃ for 3 days. Subsequently, the swollen pieces were put on the filter paper for the purpose of removing the solution on the surface and immediately weighted (m1). Finally, the samples were dried in a vacuum drying oven at 40 °C for 72 hours and weighted again (m2). The cross-link density was calculated according to the Flory-Rehner equation (Wang, Zhou, Wang, Xu, Tang & Yang, 2018). -[ln(1-Φr ) +Φr +χΦr 2 ]= V0 n(Φr 1⁄3 - Φr ⁄2 ) (2) Where n represents the cross-link density; χ is the Flory-Huggins polymer-solvent interaction term (0.393 for toluene); V0 represents the molar volume of toluene (106.2 𝑐𝑚3), Φr is the volume fraction of rubber after swelling, which can be obtained from Bala et al. (Bala, Samantaray, Srivastava & Nando, 2004).

Φr =

m2 ⁄ρ2

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m2 ⁄ρ2 + (m2 -m1 )ρ1

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Where ρ1 and ρ2 are forthe densities of toluene (0.865 g/ ml ) and rubber (0.94 g/ ml ),

respectively. The relative weight loss was obtained according to the following formula. Relative weight loss =

m0 -m2 m0

×100%

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RESULTS AND DISCUSSION The key of our strategy for constructing a reversible supramolecular hybrid network is to generate ionic bonds in XSBR via adding NCS without any traditional vulcanizing agents. As shown in Fig. 1, NCS serve as multiple-functional crosslinking points in the supramolecular network, which makes NCS participated in the network construction. At the same time, the salt-forming reaction between XSBR and NCS generates massive ionic bonds in the supramolecular hybrid network. 5

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Fig. 1. Schematic illustration of salt-forming reaction between XSBR and NCS and evolution of supramolecular hybrid network in XSBR/NCS composites The crosslinking points in network depends on the amount of NCS. If NCS is distributed uniformly, a supramolecular hybrid network with uniformly distributed crosslinks will be constructed. Based on Eisenberg–Hird Moore (EHM) model (He, Mighri, Guiver & Kaliaguine, 2016), ionic bonds tend to be self-aggregated into ionic multiplets which forced restrictions on the mobility of adjacent rubber chains. This restriction effect functions as ionic bond crosslinking. Consequently, the nature of reestablishment of ionic bond crosslinking endows supramolecular hybrid network with self-healing capacity. It should be noted that an ideal dispersion of NCS will played another key role in strengthening the mechanical properties of XSBR, thus the NCS content would adjust the mechanical properties of XSBR/NCS composites.

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Fig. 2. (a-c) FTIR spectra of neat XSBR, NCS, XSBR/NCS composites; (d) Chemical reaction of XSBR and NCS. The formation of salt bonds between NCS and XSBR were confirmed by FT-IR (Fig. 2a-c) and their schematic illustration are shown in Fig. 2d. It is seen from Fig. 2a that the spectrums of XSBR/NCS composites are quite similar to that of the neat XSBR in the whole scanning wavenumbers range. However, the signs of the chemical reaction between amino group and carboxyl group can be found in an enlarged section of the spectrum in the range of 1800-900 cm-1 (Fig. 2b) and 3150-2850 cm-1 (Fig. 2c). As shown in Fig. 2b, the absorptions at 1650, 1600 and 1315 cm-1 in the FTIR spectrum of NCS correspond to the absorptions for amide I, bending vibration of N-H and amide III, respectively (Peniche, Argüelles-monal, Davidenko, Sastre, Gallardo & Roman, 1999). The absorptions at 1120 and 1031 cm-1, representing the skeletal vibrations involving the CO stretching, are the characteristic of saccharide structure of chitosan (Argüelles-monal & PenicheCovas, 1988). The band at 1705 cm-1 in neat XSBR represents the characteristic absorption peak of C=O of carboxyl group (Sordo, Mougnier, Loureiro, Tournilhac & Michaud, 2015). As for the XSBR/NCS composites, this characteristic absorption peak was shifted to 1702 cm-1, which suggested that the chemical environment of C=O in XSBR had been changed after incorporation of NCS (Fu, Wu, Xu, Cao, Wang & Guo, 2018). At the same time, the absorptions at 1590 (asymmetrical COO- stretching) and 1407 cm-1 (symmetrical COO- stretching) are attributed to the complexation between the amino groups of NCS and the carboxylic groups of XSBR. Furthermore, compared with neat XSBR and NCS, a new absorption peak appeared at 3105 cm-1 in composites is attributed to the [NH3]+ (Gao & Liu, 2018) (Fig. 2c), which confirms the formation of carboxylate ammonium salt. The above FTIR results are similar to the report of Peniche et al. (Peniche, Argüelles-monal, Davidenko, Sastre, Gallardo & Roman, 1999) in which the chitosan was mixed with polyacrylic acid at 37 °C. It has been reported that the dehydration of chitosan-acrylic acid 7

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salts could occur appreciably at 100 °C and the chitosan-acrylic acid ionic bonds were changed into amide bonds (Qu et al., 1997). However, in order to earn the reversible weak interactions from ionic bond crosslinking, we did not further convert ammonium carboxylate into amide linkage through high-temperature treatment. Considering that chitosan could bridge adjacent XSBR chains via [COO]-[NH3]+ and potential hydrogen bonds, the NCS particles or aggregates processed improved interactions with the XSBR matrix.

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Fig. 3. (a) Crosslink density and (b) relative weight loss of neat XSBR and XSBR/NCS composites; (c) photographs of samples before and after the swelling in toluene for 72 hours. (d) photographs of toluene-swollen gels and possible corresponding schematic diagram of network structure The formation of supramolecular network in XSBR/NCS composites was confirmed by equilibrium swelling experiments. The crosslink density, “n”, provides more scientific and intuitive data proof. As shown in Fig. 3a, n was 15.34 × 10-4 mol/cm-3 for the XSBR with 5 wt% NCS while it increases to 16.12 × 10-4 mol/cm-3 when NCS loading reached 40 wt%. This is a clear evidence that ionic bond crosslinks increase with increasing the NCS concentration. The relative weight loss of XSBR during swelling were also recorded which helps to understand the real structure in XSBR/NCS composites. As shown in Fig. 3b, the neat XSBR is dissolved in toluene and exhibits a weight loss of 100% due to no crosslinked network in it, while the relative weight loss of XSBR/NCS composites dropped sharply to 58.33% after incorporating 5 wt% NCS. With increasing the content of NCS, the relative weight loss of XSBR was constantly reduced to 52 % for 10 wt%, 50% for 20 wt%, 38% for 30 wt% and 36% for 40 wt% NCS. Note that the XSBR/NCS composite with 40 wt% NCS has still a large relative weight loss of 36%, this indicates that the crosslinking of XSBR molecules is maintained at a relative low degree. This is quite important for the selfhealing behavior that requires adequate chain movement to fulfill the molecule diffusion at fracture surfaces (Xu, Cao, Huang, Chen, Lin & Fu, 2017; Yang & Urban, 2013). The analysis of crosslink density and relative weight loss of XSBR suggested that a supramolecular network had been successfully constructed in XSBR/NCS composites. To give an intuitive understanding of the 8

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formation of supramolecular network, the photographs of equilibrium swelling experiment are provided in Fig. 3c. The neat XSBR was dissolved in toluene and resulted in a muddle solution while the composites with 10 and 40 wt% NCS were swollen in toluene, strongly suggesting that a crosslinked network was generated in the composites which against the dissolution of XSBR. The photographs of toluene-swollen gels and the corresponding schematic diagrams are shown in Fig. 3d. It is clearly seen that the strength of gel with 10 wt% NCS is quite poor. This is a fragile gel that it was split into several pieces when taking it out of the solution using a tweezer. As for the gel with 40 wt% NCS, a developed supramolecular hybrid network had been formed which endowed XSBR with improved mechanical property so that the gel did not break in the process of clamping.

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Fig. 4. SEM images of (a) neat XSBR and XSBR/NCS composites: (b and c) 20 wt% NCS; (d, e and f) 40 wt% NCS. The morphologies of fractured surface of XSBR/NCS composites were observed by SEM. In contrast to the smooth cross-section of neat XSBR in Fig. 4a, a lot of irregular white regions appear on the fractured surface of composite with 20 wt% NCS (Fig. 4b). As shown in the high magnification in Fig. 4c, those irregular white regions are composed by numerous white spots which are associated with NCS. The electrostatic interactions from ionic bonds on the surface of NCS might facilitate the enrichment of NCS during drying process of XSBR/NCS suspension. When the NCS concentration increased to 40 wt% (Fig. 4d, 4e and 4f), the irregular white regions became more obvious and the area had an apparent increase. As shown in the high magnification in Fig. 4d and 4f, the density of white spots increased significantly.

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Fig. 5. AFM 3D images of (a) neat XSBR, (b) and (c) XSBR with 20 wt% NCS; (d) height graph of XSBR with 20 wt% NCS and (e) height curve of selected NCS particle in (d). In order to further investigate the detail microstructure of composites, AFM observation was performed. By taking comparison between the 3D images of neat XSBR (Fig. 5a) and XSBR/NCS composite (Fig. 5b and 5c), it is evident that the observed particles are NCS. The observed size of single NCS particle in XSBR is about 200~400 nm and the estimated roughness parameter is 36.5 nm. We selected the biggest one in Fig. 5c to determine its height. As shown in Fig. 5d and 5e, the diameter is about 800 nm while the height of exposed NCS surface is not more than 50nm. This suggested that the majority of its body was embedded in XSBR matrix, showing a considerable compatibility between NCS and XSBR matrix. Considering that the average size of NCS is about 200 nm, this big one is composed of 2~3 singe NCS particles. Nevertheless, NCS did not substantially reunite in XSBR matrix, which provided good conditions for the salt-forming reaction. The AFM observation is in consistence with SEM analysis. The appearances of the XSBR/NCS composites are provided in Fig. S2a (Supporting Information). The color of composite becomes deeper with increasing the NCS content but is still translucent even with 40 wt% NCS. Fig. S2b (Supporting Information) shows the light transmittance of XSBR/NCS composites obtained through UV-vis spectrophotometer. With the increasing load of NCS, the transmittance of composites slowly decreases. In the visible wavelength range in 400-750 nm, the maximum light transmittance of neat XSBR is about 20 % difference from the lowest light transmittance of XSBR with 40 wt% NCS, which indicates that the composites still keep good light transmittance.

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Fig. 6. (a) Water uptake ratio of neat XSBR and XSBR/NCS composites; (b) photographs of samples before and after soaking in water for 72 hours; (c) schematic illustration of supramolecular network absorbing water Because of the hydrophilic groups on the surface of NCS, addition of NCS has a positive effect on the water absorbability of XSBR. As shown in Fig. 6a and 6b, neat XSBR has a water absorption rate of 19% due to its carboxyl groups. After absorbing water, all the samples became white and opaque. The water uptake rates of XSBR/NCS composites are much higher than that of neat XSBR. The maximum water uptake rate was as high as to 75% when the content of NCS reached 20 wt%. However, it was sharply reduced to 48% when the NCS load increased to 30 wt%. It is possible that a suitable crosslinking dense was able to catch more water under the assistance of hydrophilic NCS. High amount of NCS reduced the distance between crosslinking points. This would add difficulties in absorbing water in supramolecular network with higher crosslink density (Fig. 6c). Moreover, the filler-filler network had been formed when the NCS content reached 30 and 40 wt% (see the follow stress-strain analysis), which imposed strong restrictions on the XSBR molecular chains. This also helped to limit water uptake of XSBR/NCS composites.

Fig. 7. Typical stress-strain curves of XSBR/NCS composites. 11

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Typical stress-strain curves of XSBR/NCS composites are shown in Fig. 7 and the data of tensile strength, elongation at break and Young’ modulus is summarized in Table S1. The tensile strength of neat XSBR was only about 0.6 MPa. It was constantly increased to 1.3 MPa, more than 2-time that of the neat XSBR, when the NCS content was increased to 20 wt%. In light of the formation of supramolecular hybrid network in XSBR/NCS composites, the stress increased rapidly at low strain. In addition, the filler effect of dispersed NCS in XSBR also contributed to the enhanced tensile strength. What must be pointed out was that, although 1.3 MPa was a poor data for vulcanized rubbers, the system was not vulcanized by any conventional crosslinking agents. More importantly, incorporation of 20 wt% NCS not only improved the tensile behavior but also promoted the selfhealing of XSBR which will be discussed later. However, the tensile strength was reduced when the NCS load exceeded 20 wt%. For instance, the tensile strength of the one with 40 wt% of NCS was reduced directly to 0.5 MPa, which was lower than that of neat XSBR. This is due to the unfavorable aggregation of excess NCS. Note that its Young’ modulus was further increased to ~8 MPa, this suggested that a filler-filler network constructed by NCS had been formed (Liu, Cao, Yuan, & Chen, 2018). The formation of rigid filler-filler network inevitably reduced the water uptake capacity of supramolecular hybrid network (Fig. 6a). Nevertheless, the elongated at break was still maintained at above 600%. This implied that the XSBR/NCS composites have potential use in somewhere request high ductility and self-healing.

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Fig. 8. Cut samples after self-healing at 25 °C for 1 h: (a) neat XSBR under stretching; (b) XSBR/NCS composite (10 wt%) under stretching and twisting As well-known, reversible weak interaction is an effective method to realize self-healing behaviors for soft materials. The ionic bond interactions, with the assistance of hydrogen bonds, forced a strong restriction on the mobility of XSBR chains, which turned XSBR/NCS composites to be self-healing materials. The rectangular shape sample was cut into two separate parts and two cross sections were immediately contacted together under moderate pressure. Then, the sample was self-healing at 25 °C for 1 h. The healed sample was forcefully twisted and stretched as shown in Fig. 8. It can be seen that the composites had excellent self-healing property compared with the neat XSBR. In order to study the self-healing process intuitively, the interior (Fig. S3a, Supporting Information) and surface (Fig. S3b, Supporting Information) of the healed samples were observed through the Leika microscopy. Although the cutting line remained at the surface, it showed obvious 12

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shrinkage after an hour’s healing (Fig. S3b) compared with the fresh one. The interior of the cut position only had negligible traces after healing of 4 hours, showing excellent healing effect.

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Fig. 9. (a) Healing efficiency of XSBR and NCS/XSBR composites at ambient temperature for healing 24h; (b) Schematic of self-healing by reestablishment of ionic bond crosslinkings in supramolecular XSBR network. The tensile experiments were carried out on neat XSBR and NCS/XSBR composites after healing and the self-healing efficiency was determined by the comparison of stress before and after healing. As shown in Fig. 9a, the healing efficiency of composite increases continuously when the content of NCS is up to 20 wt%. Neat XSBR shows a 39 % healing efficiency due to its self-adhesion behavior from the polar carboxy groups while the XSBR with 20 wt% NCS exhibited an excellent healing efficiency of 92%. This was mainly attributed to the increased reversible ionic bond crosslinks in the supramolecular hybrid network. However, the formation of filler-filler network had a powerful hindrance on the self-healing behavior of rubbers. As seen, the self-healing efficiency for XSBR with 30 and 40 wt% NCS decreased significantly compared with the one with 20 wt% NCS. This is ascribed to the large amount of NCS limiting the mobility and the interfacial interpenetration of XSBR molecular chains, and consequent reconstitution of ionic bond crosslinking. Nevertheless, 20 wt% NCS seemed to be an optimal load for this XSBR system, which improved the mechanical property and self-healing of NCS/XSBR composite simultaneously. The schematic illustration of self-healing by reestablishment of ionic bond crosslinking in a supramolecular network is shown in Fig. 9b.

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Fig. 10 Cyclic tensile curve of the XSBR with different NCS contents at continuously increasing strain of 50%, 100%, 150% and 200%: the residual strain achieves the maximum 146% at 20 wt% NCS and then decreases at higher CNS loading The cyclic tensile curve is measured to further confirm the reconstitution of ionic bond crosslinkings networks. Fig. 10 shows the typical cyclic tensile curve of XSBR with different NCS contents at continuously increasing strain of 50%, 100%, 150% and 200%. The residual strains (εr) of XSBR/NCS composites are much higher than that of neat XSBR, which is attributed to disassociate and simultaneously reform of ionic bond crosslinkings when the composites were deformed. Reformed ionic bond crosslinkings had a strong restriction on the instantaneous network, which impeded the recovery of the XSBR network. It is clearly seen that the neat XSBR exhibits a recovery of strain, e.g., εr = 24 % for the fourth cycle (εmax = 200%). However, the residual strain of the XSBR with 20wt% NCS significantly increased up to 146%. This is attributed to that more ionic 14

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bond crosslinkings participated in the disassociation and simultaneous reformation of supramolecular hybrid network, which caused a greater degree of permanent set for every cyclic tensile (Xu, Cao, Huang, Chen, Lin & Fu, 2017). This network rearrangement based on ionic bond crosslinkings was beneficial to the self-healing behavior of XSBR/NCS composites. During healing process, the chain diffusion at the cut/heal position induced the reformation of ionic bond crosslinkings. More ionic bond crosslinkings participating in the network rearrangement turned out better healing efficiency. However, XSBR with 30 and 40 wt% of NCS showed a sharply reduced εr=49 % and 35%, respectively. The sharp decrease in residual strain suggested an improved elasticity of XSBR/NCS composites due to the strengthened supramolecular hybrid network by more NCS. This also strengthened the restriction of XSBR molecules which is unfavorable to the movement and the interfacial interpenetration of XSBR, consequently hindering the rearrangement of supramolecular hybrid network. Moreover, a developed filler-filler network had been formed in XBR with 30 and 40 wt% of NCS, which also caused a rapid decline in self-healing property. CONCLUSION By introducing NCS into XSBR matrix, we successfully prepared a supramolecular hybrid network in the NCS/XSBR composites. The carboxylate ammonium salts had been successfully formed from the chemical reaction between the amino groups of NCS and the carboxyl groups of XSBR. 20 wt% was a suitable load for this XSBR/NCS system. With this load, the tensile strength of NCS/XSBR composite was increased to 1.3 MPa, more than 2-time that of the neat XSBR, and, the water uptake achieved the maximum of 75%. Due to the existence of reversible ionic bond crosslinks in the supramolecular hybrid network, the XSBR/NCS composite exhibited significant self-healing behavior. Typically, the XSBR with 20 wt% NCS exhibited excellent self-healing behavior that its healing efficiency was as high as to 92% after healing at ambient temperature for 24 h. When the NCS content exceeded 20 wt%, the strengthened supramolecular hybrid network and formation of filler-filler network hindered the rearrangement of supramolecular hybrid network, which caused a rapid decline in self-healing properties.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21875047), Guangxi Natural Science Foundation (2016GXNSFAA380145) and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology Foundation (2017K002) and the Project Sponsored by the Scientific Research Foundation of Guangxi University (Grant No. XTZ140787). REFERENCES An, S., Lee, M. W., Yarin, A. L., & Yoon, S. S. (2018). A review on corrosion-protective extrinsic self-healing: Comparison of microcapsule-based systems and those based on core-shell vascular networks. Chemical Engineering Journal, 344, 206-220. Argüelles-monal,W., & Peniche-Covas, C. (1988). Study of interpolyelectrolyte reaction between chitosan and carboxymethyl cellulose, Macromolecular Rapid Communications 9, 693697. Bala, P., Samantaray, B. K., Srivastava, S. K., & Nando, G. B. (2004). Organomodified montmorillonite as filler in natural and synthetic rubber. Journal of Applied Polymer Sciene, 92(6), 3583-3592. 15

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