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Design of “Zn2+ salt-bondings” crosslinked carboxylated styrene butadiene rubber with reprocessing and recycling ability via rearrangements of ionic crosslinkings Chuanhui Xu, Xunhui Huang, Conghui Li, Yukun Chen, Baofeng Lin, and Xingquan Liang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01897 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Sustainable Chemistry & Engineering

Design of “Zn2+ salt-bondings” crosslinked carboxylated styrene butadiene rubber with reprocessing and recycling ability via rearrangements of ionic crosslinkings Chuanhui Xu1,*, Xunhui Huang2, Conghui Li1, Yukun Chen2, Baofeng Lin1, Xingquan Liang1 1

School of Chemistry and Chemical Engineering, Guangxi University, No. 100, Daxuedong Road,

Xixiangtang District, Nanning, 530004, China 2

The Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (South

China University of Technology), Wushan RD.,Tianhe District, Guangzhou, 510640, China

Corresponding Author: Chuanhui Xu

[email protected]

ABSTRACT: Constructing a reversible supramolecular network crosslinked by non-covalent bonds is an effective approach to realize self-healing as well as reprocessing and recycling for rubbers. Unfortunately, in most case the resultant non-covalent crosslinked rubbers cannot hold enough forces to meet the routine applications. In this paper, our strategy was based on a simple reaction between carboxy groups in carboxylated styrene butadiene rubber (XSBR) and zinc oxide (ZnO), where the formed Zn2+ salt bondings connect separate XSBR molecules. The further self-aggregation of ion pairs of Zn2+ salts resulted in an ionic crosslinked network, whose rearrangements brought XSBR excellent reprocessing/recycling ability. Additionally, the reclaimed XSBR exhibited valuable mechanical properties due to the compensation of additional formed new Zn2+ salt bondings during recycling. The fresh XSBR with 5wt% zinc oxide showed a tensile strength of 6.7MPa, and it was further increased to 10.3MPa after 3 times of recycling, which was far higher than most reported non-covalent supramolecular rubbers. This study thus opens up an avenue to further extending the recyclable crosslinked XSBR with considerable mechanical properties to various engineering applications.

KEYWORDS: recycling; ionic crosslinked network; carboxylated styrene butadiene rubber; zinc oxide; tensile behavior

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INTRODUCTION Developing eco-friendly recyclable rubber products is always a crucial and hot issue in sustainable development of rubber industry1. To meet the requirements of high mechanical performance in various practical applications, rubbers must be chemically crosslinked to form a 3D covalent network2. Afterwards however, the formation of chemical crosslinked network makes rubbers to be thermoset materials, which prevents their reshaping and recycling3. Tremendous efforts have been made to solve this irreversible problem of the chemical network4. One widely employed method in rubber industry is the devulcanization of sulfur-crosslinked network under a thermal and mechanical treatment5,6. Unfortunately, the chain scission of rubber molecules is inevitable in this process6, which is to the detriment of the mechanical properties of recycled rubbers. A more desirable way of recycling rubbers is to make them to be thermoplastic elastomers (TPE), e.g. thermoplastic vulcanizates (TPV)7,8,9. This type of materials, usually consisting a plastic continuous phase and a dispersed rubber phase, behaves as crosslinked rubbers at room temperature but owns the processability and recyclability at high temperatures. However, this technical solution somewhat deviates from the topic of chemical network in rubbers. Another popular approach to achieve recyclability is to construct reversible networks which can be ruptured and reconstructed under an external stimulus such as heat or force10-17. So far, the commonly applied available functional units for reversible networks include dynamic reversible covalent bonds and non-covalent bonds18. The former involves Diels–Alder (DA) reactions19,20, disulde-thiol exchange reactions21,22, disulfide rearrangements23, bond exchange reactions (BER)24,25 and other reversible chemical linkages26. The rearrangements of network at high temperatures are the essential to recycle rubbers. Although the covalent bonds bring high mechanical strengths, the molecule chains of rubbers have a strict requirement for functional modification before building such a reversible network27. Comparatively, the non-covalent bonds are more feasible and convenient owing to their easy rupture and rebuilding nature28-32. For instances, Maes et al.33 compounded fatty diacids and triacids to form a hydrogen bonding crosslinked network which brought self-healing and potential reshaping performance to rubber. Wang et al.34 reported the preparation of reversible supramolecular network constructed by ionic hydrogen bondings which has higher stability than common hydrogen bondings. Xu et al.35 generated an ionic crosslinked natural rubber via the polymerization of zinc dimethacrylate, where the reversibility of ionic associations made it to be self-healable. However, from the viewpoint of rubber industrial applications, in most case rubbers with reversible non-covalent crosslinks cannot hold higher stress to meet the routine applications36,37. Addition of reinforcers absolutely increases the mechanical properties of rubbers, but, usually accompanying by a disadvantage that the nano-fillers tend to destroy the non-covalent crosslinkings, retarding the reconstruction of networks. The salt-bondings, another kind of non-covalent bond which should belong to ionic bondings38,39, may bring potential self-healing and recycling functions to rubbers, as well as the considerable mechanical properties. It is reported that massive salt-bondings in polymerized Zn2+ or Mg2+ based unsaturated carboxylic acid salt molecules had strong electrostatic interaction, which could restrict the mobility of circumambient rubber chains40. Simultaneously, the spontaneously aggregated ion clusters had high moduli which functioned as effective reinforcing agent40. This enlightens us to coordinate carboxylated rubbers with metal oxides41-43, where the 2


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reactions of carboxylic groups and metal oxides generate massive salt-bondings to realize recycling ability for rubbers. Based on idea above, in this paper, we selected carboxylated styrene butadiene rubber (XSBR) to coordinate with zinc oxide (ZnO) to form an ionic crosslinked network. XSBR, generally synthesized with styrene, butadiene and acrylic acid via emulsion polymerization, is promising for preparing recyclable rubbers because it has higher glass transition temperature and good mechanical performance even without reinforcement. Unfortunately, less attention has been paid to explore the potential recyclable ability of XSBR. In this work, XSBR was successfully crosslinked by Zn2+ salt-bondings, which exhibited excellent reprocessing and recycling ability. Additionally, the resultant XSBR showed good mechanical properties, which has potential applications in diversified realms. EXPERIMENTAL SECTION Raw Materials. Zinc oxide (analytical pure), with a purity of 99.5%, density of 5.6g/cm3, was purchased from XiLong Chemical Company (Guangzhou, China). Carboxylate butadiene-styrene (XSBR) latex with 48% solids (DL659) was obtained from Quanzhou Deli Chemical Co., Ltd. (Fujian, China). It is a random copolymer consisting of 33 wt% butadiene, 64 wt% styrene and 3 wt% of carboxylate functional monomer. The chemical structure of XSBR is shown as below. Other additives were of industrial grade and used as-received.

Preparation of recyclable XSBR. The XSBR was firstly coagulated from its latex, and then was washed with de-ionized water until neutral. Before coordinating with zinc oxide, the solid XSBR was completely dried under reduced pressure at 60 °C. The compounding was carried out in a two-roll mill at room temperature. Then the masticated compounds were dried under reduced pressure again to remove the possible water formed during compounding. Next, the dried compounds were masticated in the two-roll mill once gain to make sure the sufficient contact of carboxylic groups and zinc oxide. The above process was repeated for three times. At last, the XSBR compounds were hot compression molded to be 2mm-sheets according to the requirements of further experimental design. Characterization. The crosslinking of XSBR at various temperatures was studied by a UR-2010SD Rotorless Rheometer (U-CAN Dynatex Inc.), the elastic torque curves were recorded under an amplitude of 0.5° and a frequency of 1.67 Hz. The FT-IR spectrums were recorded by a Bruker Tensor 27 Spectrometer (Germany) in an attenuated total reflectance (ATR) model, the resolution was 4 cm-1 and the accumulation was 32 scans. Dynamic mechanical behaviors were performed on a Netzsch DMA 242C from -40 to 80°C in a tensile mode, the heating rate was 3°C/min and the oscillation frequency was 1 Hz. The TEM images were obtained by a JEM-100CX II transmission electron microscope (JEOL, Japan) with an accelerating voltage of 100 kV. The TEM samples were cryomicrotomed into about 100 nm thin sections by a Leica EMUC6 in liquid nitrogen atmosphere. The SEM observation was conducted on a Nova NanoSEM 430 (FEI Company, USA). XRD profiles of zinc oxide and XSBR composites were obtained using a D/MAX 2500V X-ray diffractometer, the operation conditions: Cu-Ka radiation λ= 0.1542 nm, 2θ ranged from 10 to 60°, scan rate of 5°/min. Tensile behaviors, containing consecutive loading-unloading cycles, were performed on a UT-2080 tensile machine (U-CAN 3



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Dynatex Inc.), the tensile speed was 50 mm/min. The relative weight loss was evaluated through a dissolution/swell experiment in toluene, which was used to discuss the crosslinking degree of XSBR/ZnO composites. The rubber samples were carefully weighted (M1) and then immersed in toluene at ambient temperature for 72h. Then the swollen samples were dried at 60°C until a constant weight (M2). The relative weight loss was calculated according to equation 144, which was used to discuss the crosslinking degree of XSBR composites. Relative weight loss= [(M1-M2)/M1]×100% (1) RESULTS AND DISCUSSION

Figure 1. (a) Schematic for a simple reaction of carboxy groups and zinc oxide to form an ideal network crosslinked by Zn2+ salt-bondings in XSBR; (b) schematic for ion multiplet, ion cluster and final ionic crosslinked network The core concept for preparing recyclable crosslinked XSBR in this paper is to form a potential ionic network which is evolved from salt-bondings. Our strategy is based on the simple reaction between carboxy groups in XSBR chains and zinc oxide, where the formed Zn2+ salts connect separate XSBR molecules to form a salt-bonding crosslinked network. The schematics of their reaction and the ideal network crosslinked by Zn2+ salt-bondings are illustrated in Figure 1a. In fact, this ideal Zn2+ salt-bondings crosslinked network is difficult to form. According to the Eisenberg–Hird Moore (EHM) model32,45, once the Zn2+ salts are formed, ion pairs tend to self-aggregate into ionic multiplets due to their powerful electrostatic interactions. Followed, clusters will be organized by several ionic multiplets, forcing a strong restriction on the mobility of adjacent polymer chains45. Those ion-rich domains have high moduli and serve as ionic crosslinking points to form a final network. At the same time, they function as effective reinforcers to enhance the mechanical properties of XSBR40. The schematic illustrations of the potential ion multiplet, ion cluster and final ionic crosslinked network in XSBR composites are shown in Figure 1b. The slipping and rebuilding nature of ionic crosslinkings allows the rubber segment rearrangements under external force and heat35, which turns XSBR into potential recyclable rubber materials.

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Figure 2. (a) Elastic torque of XSBR with various concentration of zinc oxide at 100°C; (b) schematic for effective contact between zinc oxide and separate carboxy groups; (c) schematic for shielding effect of excess zinc oxide; (d) elastic torque curves of XSBR with 5wt% zinc oxide cured at various temperatures; (e) relative weight loss of XSBR with 5wt% zinc oxide cured at various temperatures; (f) photographs of the XSBR with 5wt% zinc oxide cured at 100°C in the dissolution/swell experiment Crosslinking of XSBR is confirmed by the elastic torque curves of rheometer. Figure 2a shows the evolution of elastic torque of XSBR with various concentration of zinc oxide at 100°C. For comparison, neat XSBR underwent the same process of XSBR composites. It is well accepted that the increase in torque value represents the formation of crosslinkings in rubbers46. No apparent increase was observed in the torque of neat XSBR, suggesting that there were no effective crosslinkings occurred during curing. As expected, incorporation of zinc oxide brought a continuous increase in the elastic torque of XSBR, which strongly confirmed the occurrence of crosslinkings in rubber matrix. The addition of ZnO increased the viscosity of rubber and affected the minimal value of torque, however it was interesting to find that the highest torque value was observed for the XSBR with 5wt% zinc oxide (~4.3dNm at 120min), which was higher than that with 7 (~3.6 dNm) and 10wt% zinc oxide (3.7 dNm). This clearly suggested that the XSBR with 5wt% zinc oxide generated more crosslinkings. The ideal effective crosslinking in the XSBR composites was believed to be the formation of Zn2+ salts: one ZnO reacted with two -COOH groups which belonged to separate XSBR molecules to create one Zn2+ salt-bonding42,43. However, one important precondition for the formation of Zn2+ salt-bondings was the effective contact between ZnO and separate –COOH groups, as illustrated in Figure 2b. More zinc oxide absolutely had higher chance to contact –COOH, however, the ideal effective contact might be reduced due to the shielding effect of mass zinc oxide, as shown in Figure 2c. The XSBR with 10wt% zinc oxide (Figure S1) had a more white appearance than the one with 5wt% zinc oxide (Figure S2), revealing that addition of excess zinc oxide left lots of residual zinc oxide in XSBR matrix. Of course, the possible agglomeration of zinc oxide at a large content also contributed to the shielding effect. As a result, the shielding effect of excess zinc oxide on carboxylic groups hindered the 5



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formation of ideal Zn2+ salt-bondings. We also considered the effect of curing temperatures on the crosslinking of XSBR. Figure 2d shows the torque curves of XSBR with 5wt% zinc oxide cured at various temperatures. Attention should be first taken for the torque values obtained at 80 and 100°C. High temperature usually lowered the modulus of rubbers, however, the 100°C-torque was higher than 80°C-torque, which revealed that denser crosslinkings were achieved at 100°C. We believed that higher temperatures would facilitate the reaction between carboxylic groups and zinc oxide, hence generating more Zn2+ salt-bondings, consequently stronger ionic crosslinkings. This assumption was supported by a dissolution/swell experiment in toluene, as shown in Figure 2e. What must be pointed out was that the polarity of XSBR made it hardly dissolved in toluene at room temperature44. Therefore, we tried to maintain the integrity of swollen samples without any stirring, and calculated their relative weight loss carefully to discuss the sol fraction which was used to represent the crosslinking degree in this particular XSBR/ZnO system. As expected, the relative weight loss was significantly reduced when the temperature exceed 80°C, which indicated that an increased crosslink density was obtained at elevated temperatures. For example, the neat XSBR showed a relative weight loss as high as nearly 14% due to no crosslinkings in it, while incorporation of 5wt% zinc oxide at room temperature made the relative weight loss of XSBR/ZnO composite sharply decreased to 8.9%. With increasing the curing temperature, the relative weight loss was further decreased to 2.8% for 80°C, 0.4 % for 100°C, 1.1% for 130°C and 1.5% for 150°C. However, the ionic crosslinkings were quite sensitive to temperatures since a higher temperature would weaken the ionic associations47. This natural resulted in a significant reduced torque value when the temperatures exceed 100°C, which was a typical behavior of ionic crosslinked rubbers at high temperatures35, as shown in Figure 2d. Although the ionic crosslinkings could be reconstructed at relative low temperatures, the rupture at high temperatures slightly lowered the final crosslink density (see the above data of relative weight loss). We also provide the photographs of the 100°C-cured XSBR with 5wt% zinc oxide during dissolution/swell experiment in Figure 2f. It is clearly seen that the sample was only swollen and the toluene remained its limpidity, strongly suggesting a developed crosslinked network in the swollen sample. Nevertheless, both the torque curve and the dissolution/swell experiment results agreed that 100°C was the best choice to achieve a relative developed ionic crosslinked network in this XSBR/ZnO system. In the whole curing observation window, Zn2+ salt-bondings were successively formed, and followed with the progressive construction of ionic crosslinkings. This was a dynamic ionic association process which turned out a fluctuated curve shape in Figure 2d. However, the increasing curve progression in the whole curing observation window suggested that crosslinking was not completed even after 160min of curing treatment at 100 °C. Luckily, the residual zinc oxide had a positive contribution on the mechanical properties of the recycled XSBR which will be discussed later. The promoted reaction degree of zinc oxide at higher temperatures was also confirmed by the XRD results, as shown in Figure S3. The decreased intensity of characteristic diffraction peak of zinc oxide suggested that more of zinc oxide were consumed by carboxylic groups at higher temperatures, hence forming more Zn2+ salt-bondings.

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Figure 3. (a) FT-IR spectrum of XSBR composite: a-neat XSBR, b-XSBR/ZnO compounded at room temperature, c-XSBR/ZnO cured at 100°C and d-XSBR/ZnO cured at 150°C; (b) and (c) TEM images of XSBR with 5wt% zinc oxide cured at 100°C; (d) DMA results: storage modulus and loss tangent; (e) stress-strain curves of the XSBR with various concentration of zinc oxide (cured at 100°C for 120min) The reaction between carboxylic groups in XSBR and zinc oxide was explicitly confirmed by FT-IR measurement. As shown in Figure 3a, the characteristic absorption peak at approximately 1694 cm−1 in the spectrum of neat XSBR correspond to the stretching vibration of C=O in carboxyl group44,48. After compounding zinc oxide into XSBR, the absorption at 1694 cm−1 showed a change at high curing temperatures. At room temperature, the spectrum of XSBR composite was quite similar to that of the neat XSBR, revealing that the reaction of carboxylic groups and zinc oxide only occurred in a small scale. For the samples cured at 100 and 150°C, the absorption at 1694 cm−1 was disappeared, accompanied by the emergence of a new peak at 1551 cm−1. According to the report of Yin et al.49, the new absorption at 1551 cm−1 was due to the asymmetrical stretching vibration mode in coupled C=O and C-O bonds which attached to the same carbon (see the structure of Zn2+ salt bonding in Figure 1a). These changes in FT-IR spectrum confirmed the formation of Zn2+ salt bondings. To give an intuitive observation of the inside structure of recyclable XSBR, TEM images of the XSBR with 5wt% zinc oxide cured at 100°C are shown in Figure 3b and c. Although the obtained resolution and contrast were not high, it did not hamper the qualitative investigation. Dark nanoparticles with size of 40~50nm were observed, which were originated from the self-aggregated ion-rich domains, according to EHM model. Similar structures in TEM observation can be found in Nie40 and Lu’s50 reports, in which those ion-rich nano-domains served as effective reinforcing agent for rubbers. Therefore, in this XSBR/ZnO system, the observed similar nano-structures further confirmed the formation of ionic crosslinkings. 7



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Figure 3d show the temperature dependence of the storage modulus (E’) and loss tangent (tan δ) for neat XSBR and XSBR with 5wt% zinc oxide. Like most reinforced rubbers, E’ of XSBR was improved after incorporating 5wt% zinc oxide at ambient temperature due to the reinforcing effect of filler. The slight increase in E’ was due to that 5wt% was a quite low concentration for fillers in rubber industry. However, a further improved E’ was observed after its cure at 100°C. This finding can be interpreted by the fact that more Zn2+ salt bondings were generated at an elevated temperature, thus leading to the increase in E’. All of the samples show a Tan δ peak at around 20 °C, which was related to the glass transition temperature (Tg) of XSBR. Generally, incorporation of fillers would hinder the segmental motion of polymers, hence shifting the glass transition temperature to higher temperatures. However, incorporation of 5wt% zinc oxide at ambient temperature turned out an adverse fact that the Tan δ peak shifted from 17.8°C for neat XSBR to 15.2°C for XSBR/ZnO composite. Since zinc oxide is a Lewis acid with the vacant P orbitals which can ease the delocalization of electrons through the involved bonds51, it has a strong tendency to form hydrogen bondings with the carboxylic groups in XSBR, resulting in good filler-rubber interactions. This might function as an internal plasticization that the zinc oxide ruptured the virgin hydrogen bondings between XSBR molecules44,52, showing a reduced glass transition temperature. After curing at 100°C, the XSBR/ZnO composite showed an elevated Tan δ peak temperature at 18.1°C due to that the segmental relaxation of XSBR was restricted by the formation of more ionic crosslinkings. Figure 3e shows the stress-strain behaviors of the XSBR with various concentration of zinc oxide. Unlike the typical “S” shape for most soft rubbers, the stress-strain curves of recyclable XSBR show a linear shape that the increased stress was proportional to the strain. As seen, incorporation of zinc oxide brought a significant enhancement on the mechanical property of XSBR. With 5wt% zinc oxide, the tensile strength was increased from 4.1Mpa to 6.7MPa, which was far higher than most reported non-covalent supramolecular rubbers37. Another important finding was that the tensile behaviors of the recyclable XSBR were consistent with the results of elastic torque in Figure 2a. Obviously, the XSBR with 5wt% zinc oxide showed the best tensile property. It should be emphasized that the Zn2+ salt bondings contained ionic pairs tended to self-aggregate into ionic domains, which had high moduli to behave like reinforcing fillers40,50. This in-situ reinforcement combined a relative developed ionic crosslinked network should be responded to the best tensile property of XSBR with 5wt% zinc oxide.

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Figure 4. Illustration of typical reprocessing and recycling for the XSBR with 5wt% zinc oxide: (a) 1st recycle; (b) 2cn recycle and (c) 3rd recycle. The reprocessing and recycling capacity of the XSBR composites was evaluated according to the follow recycling experiments. The samples were cut into small pieces and then masticated in the two-roll mill for 5 minutes at room temperature. After that, the masticated rubbers were hot pressed at 100 °C for another 5 minutes. After the tensile measurement, the reprocessed samples were recycled again to the next generation. Three recycles of the XSBR with 5wt% zinc oxide are demonstrated in Figure 4. With the increased recycle times, the appearance of the sample became smooth, accompanying with a darker color change. The slipping and rupture of ionic crosslinkings during shearing facilitated the rearrangement of new ionic crosslinkings, which realized the reprocessing of the XSBR composites without the need of depolymerization for conventional sulfur-crosslinked rubbers. Additionally, the appearance changes were related to the further reactions of the residual zinc oxide and carboxylic groups, which could be deduced from the followed analysis on the tensile behaviors.

Figure 5. Typical stress-strain curves of the recycled XSBR with various concentration of zinc 9



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oxide: (a) 1wt%; (b) 3wt%; (c) 5wt%; (d) 7wt% and (e) 10wt% Figures 5a-e show the typical stress-strain behaviors of the recycled XSBR in comparison with their virgin samples. Although the reclaimed samples showed a fluctuated ultimate stress and break elongation at various recycles, the apparent enhanced stress-strain curves strongly confirmed the excellent tensile performance after 3 times of recycling. In general, the original inner structures of materials are inevitably destroyed under repeated shearing and heating, this always lead to a more or less reduced mechanical property for reclaimed materials even if the non-covalent associations can be reconstructed. However, the mechanical property of XSBR samples was improved after the first recycling. Note that the reprocessing process provided more effective contact for the residual zinc oxide and carboxylic groups, therefore the followed heating process generated additional new Zn2+ salt bondings in the reclaimed materials. This would compensate the irreversible ruptures of the virgin crosslinking structures in the rearrangement, which made the reconstructed networks stronger. In addition, the formation of new ion-rich domains which have been observed in Figures 3b and c of course contributed to the enhanced mechanical performance. For example, the fresh XSBR with 5wt% zinc oxide showed a virgin tensile strength of about 6.7MPa and elongation at break of 150%. After the 3rd recycling, its tensile strength was increased up to about 10.3MPa, with a considerable elongation at break of ~160%.

Figure 6. SEM images of the cryogenically fractured surface of the samples: neat XSBR (a); XSBR with 5wt% zinc oxide: (b) fresh sample; (c) 1st recycled sample and (d) 3rd recycled sample To further identify the additional reaction of residual zinc oxide and carboxylic groups during recycling, SEM observation for the cryogenically fractured surface of the recycled XSBR was conducted, using the XSBR with 5wt% zinc oxide as the test object. Compared with the neat XSBR in Figure 6a, the additional white points appear in Figure 6b represent the nanoparticles and small aggregates involved with zinc oxide, which are uniformly dispersed in the XSBR matrix. Notably, as shown in Figure 6c and d, the number and the size of white points decreased significantly with the increased recycling times, suggesting that more residual zinc oxide had been converted to Zn2+ salt. As a result, the new formed Zn2+ salt bondings involved in the reconstruction and consequent rearrangement of ionic croslinkings during recycling. This compensation effect suggested that an appropriate amount of excessive zinc oxide helped to 10


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improve the mechanical property of the reclaimed XSBR. What should be emphasized is that the heating process is critical to the mechanical property of the reclaimed XSBR. We provide the stress-strain curves of the XSBR with 7wt% zinc oxide in Figure S4 to illustrate the importance of heating process. Compared with Figure 5d, the recycled samples compression molded at room temperature showed no improvement on their tensile behaviors. The reclaimed sample even showed a slightly reduced tensile property after the 3rd recycling. This indicated that, without heating, the new formed Zn2+ salt bondings were limited, which were unable to compensate the irreversible rupture of the virgin crosslinking structures. Of course, the recycling and reprocessing could still be realized due to that the rearrangement of the ionic crosslinkings could also be occurred at room temperature.

Figure 7. (a) Illustration for welding experiment of isolated pieces of recyclable XSBR; (b) stress-strain curves of the fresh and welded samples; (c) photographs of the stretching course of welded sample In engineering applications, an accidental serious mechanical damage usually brings unwilling isolated interfaces to the serving products. In most case, the recycling and remolding of the products is not practical because of the unwilling changes of the serving dimensions and structures during remolding. In view of these facts, we further extended the reprocessing ability of recyclable XSBR to weld isolated break interfaces53. The repairing process of isolated break interfaces is illustrated in Figure 7a. A strip-shape sample was first cut into two pieces, and the two pieces were replaced together to ensure the cut faces contact again. Then, the crumbs of fresh recyclable XSBR were placed on the reconnected section. After a hot compression molding at 100°C for 10min, the cut pieces were successfully welded together. The result of tensile test for the welded sample is shown in Figure 7b. It is clearly seen that the stress-strain curve of the welded sample shows a perfect coincidence with the virgin curve of the fresh sample. It is also worthwhile to mention that the failure in the tested sample is not taken placed on the welded interface as shown in Figure 7c, which verified that the repaired sample had regained its integrity completely after welding. This derived particular healing application further indicated the excellent reprocessing and recycling ability of the Zn2+ salt-bondings crosslinked XSBR.

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Figure 8. (a) Loading-unloading curves of the XSBR with various concentration of zinc oxide at sequential increasing strains of 50%, 80% and 100%; (b) Loading-unloading curves of the XSBR with various concentration of zinc oxide at continuous three cycles of 100% strain; (c) the calculated dissipated energies according to loading-unloading curves The rearrangement of the ionic crosslinkings was discussed through the cyclic tensile experiments. Figure 8a shows the loading-unloading curves of the XSBR with various concentration of zinc oxide at sequential increasing strains of 50%, 80% and 100%. The hysteresis loops during loading-unloading cycles were increased with the increasing concentration of zinc oxide, suggesting the effective dissipation of energy by the rearrangement of ionic crosslinkings during stretching. In every cycle, the slipping, disassociation and reforming of ionic crosslinkings dissipated energy, which was responsible for the remarkable Mullin’s effect54. The rearrangement of ionic crosslinkings restricted the instantaneous network, which retarded the recovery of deformed XSBR chains and then resulted in a large permanent residual strain (εr)35. As discussed previous, the SXBR with 5wt% zinc oxide generate more ionic crosslinkings, which resulted in a large permanent sets in every cycles. For example, the εr of SXBR with 5wt% zinc oxide at the third cycle (εmax=100%) was as high as about 49.7%, while the SXBR with 10wt% of zinc oxide showed a reduced εr of about 31.2%. This experimental result is also consistent with the former discussed torque curves (Figure 2a). The shielding effect of mass zinc oxide reduced the formation of effective Zn2+ salt-bondings, and subsequently the ionic crosslinkings. As a result, the reduced rearrangement of the ionic crosslinkings turned out a reduced εr for the SXBR with 10wt% zinc oxide. Despite this, the formed ionic crosslinkings in the SXBR with 10wt% zinc oxide still brought a εr of about 31.2% at the third cycle, which was higher than that of the neat XSBR, 29.9%. The similar findings can be found in Figure 8b, the loading-unloading curves of the XSBR 12


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with 0, 5 and 10wt% zinc oxide at continuous three cycles of 100% strain. This further revealed that the ionic crosslinkings were destroyed and reformed during deformation, and subsequent rearrangement of ionic crosslinkings limited the recovery of the network, exhibiting a large hysteresis. Correspondingly, the dissipated energies were shown in Figure 8c, which were calculated by areas from the loading-unloading curves18. Obviously, the largest dissipated energy of 182.4 MJ/m3 was found for the SXBR with 5wt% zinc oxide in the first cycle due to the highest ionic crosslink density. The SXBR with 10wt% of zinc oxide showed a dissipated energy of 85.6 MJ/m3 which was less than half of the SXBR with 5wt% zinc oxide. This energy dissipation process further revealed that the rearrangement of a developed ionic crosslinked network in the SXBR with 5wt% zinc oxide was responsible for the remarkable reprocessing and recycling ability. CONCLUSIONS This work reported a recyclable and reprocessable Zn2+ salt-bondings crosslinked XSBR which was prepared based on the simple reaction of carboxy groups and zinc oxide. The formed Zn2+ salts tended to self-aggregate into ionic multiplets or clusters. Those ion-rich domains had high moduli and strong restriction on adjacent polymer chains, serving as ionic crosslinking points to form a final network, as well as effective reinforcers to enhance the mechanical properties of XSBR. The slipping and rebuilding nature of ionic crosslinkings allowed the rearrangements under external force and heat, which realized the remarkable reprocessing and recycling ability of XSBR. Although a higher temperature promoted the conversion of Zn2+ salt-bondings, it weakened the ionic crosslinkings, which a limitation that the rubber developed in this work could not work at elevated temperature. In addition, mass excess zinc oxide had a shielding effect on carboxylic groups which hindered the formation of ideal Zn2+ salt-bondings. However, an appropriate amount of excessive zinc oxide helped to improve the mechanical property of reclaimed XSBR. It was found that the SXBR with 5wt% zinc oxide cured at 100 °C obtained more ionic crosslinkings and the best mechanical properties. Its tensile strength was increased from 6.7 of the fresh sample to 10.3MPa of the 3rd recycled sample, which was far higher than most reported non-covalent supramolecular rubbers. In addition, the excellent reprocessing and recycling ability of the XSBR composites could be further extended to weld isolated break interfaces. The prepared recyclable and reprocessable XSBR with valuable mechanical properties thus offer the potential versatility to engineering applications, and contribute to the sustainable development in chemical industry.

Supporting Information. Figure S1. Photographs of XSBR with 10wt% zinc oxide. Figure S2. Photographs of XSBR with 5wt% zinc oxide Figure S3. XRD patterns of the XSBR with 5wt% zinc oxide cured at various temperatures. Figure S4. Typical stress-strain curves of the recycled XSBR with 7wt% zinc oxide without heating process.

ACKNOWLEDGMENT 13



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This work was financially supported by the Natural Science Foundation of Guangxi Province (2016GXNSFAA380145).

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For Table of Contents Use Only Design of “Zn2+ salt-bondings” crosslinked carboxylated styrene butadiene rubber with reprocessing and recycling ability via rearrangements of ionic crosslinkings Chuanhui Xu1,*, Xunhui Huang2, Conghui Li1, Yukun Chen2, Baofeng Lin1, Xingquan Liang1 1

School of Chemistry and Chemical Engineering, Guangxi University, No. 100, Daxuedong Road,

Xixiangtang District, Nanning, 530004, China 2

The Key Laboratory of Polymer Processing Engineering, Ministry of Education, China (South

China University of Technology), Wushan RD.,Tianhe District, Guangzhou, 510640, China

Corresponding Author: Chuanhui Xu [email protected]

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Synopsis. “Zn2+ salt-bondings” crosslinked carboxylated styrene butadiene rubber with reprocessing and recycling ability contributes to sustainable development in material industry.

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Cross-Linked Carboxylated Styrene Butadiene ... - ACS Publications

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