Double-Network Formation and Mechanical Enhancement of

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Double-Network Formation and Mechanical Enhancement of Reducing End-Modified Cellulose Nanocrystals to the Thermoplastic Elastomer Based on Click Reaction and Bulk Cross-Linking Han Tao,† Alain Dufresne,§ and Ning Lin*,†,‡ †

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, P. R. China Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230601, P. R. China § Univ. Grenoble Alpes, CNRS, Grenoble INP* (* Institute of Engineering Univ. Grenoble Alpes), LGP2, F-38000 Grenoble, France Downloaded via BUFFALO STATE on July 31, 2019 at 00:46:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: In addition to being a renewable nanomaterial, cellulose nanocrystals (CNCs) exhibit a high specific modulus and are widely used as a reinforcing phase (filler) to improve the mechanical performance of polymeric materials. In these composite systems, the filler−matrix, filler−filler, and matrix− matrix interactions are critical factors that govern the mechanical properties of the composites. Inspired by the idea of combining these three interactions, we design a novel composite system of reducing an end-modified CNCenhanced thermoplastic elastomer [styrene−butadiene−styrene copolymer (SBS)] with click reaction and bulk crosslinking. The strong linkage between the nanocrystals and SBS (filler−matrix) is first achieved by the thiol−ene click reaction induced by UV irradiation in the liquid compounding process, accompanied by the preservation of surface hydroxyl groups on nanocrystals and therefore the formation of a stable percolation network (filler−filler). The matrix−matrix network is further constructed in the composite by chemical self-cross-linking of bulk SBS with a post-irradiation treatment during molding process. Benefiting from these three strong interactions, a remarkable improvement in mechanical performance is accomplished for the fabricated composite, exhibiting simultaneous increases in strength (239%), modulus (411%), work of fracture (330%), and elongation at break (7%) in comparison with those for the pure SBS material. Finally, the percolation, Halpin−Kardos, and double-network models with three interactions are applied to compare the theoretical and experimental data for mechanical properties and further discuss the enhancing mechanism for the composites.

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INTRODUCTION Supported by the development of nanoscience, the composite science attracts much interest for material researchers particularly with the presence of diverse nanoparticles serving as nanoreinforcing fillers.1−3 The mechanical properties of composites can be improved several times compared to the neat matrix even at low loading levels, typically exhibiting a remarkable increase in tensile modulus and storage modulus. Cellulose nanocrystal (CNC) is an emerging nanomaterial possessing numerous advantages, for instance, the reported renewability, biodegradability, biocompatibility, nontoxicity, high rigidity and crystallinity, high modulus, and low density.4,5 The ultrahigh specific modulus of CNC (about 85 GPa·cm3/ g)5,6 endows it as a promising candidate as “green” filler for mechanical improvement of composites. In fact, during the last 20 years, CNC has been widely used to enhance various matrices involving natural and synthesized polymers7−9 since the first report for poly(styrene-co-butyl acrylate)/CNC composite.10 To completely play the nanoenhancing effect of rigid CNC in composites, the matching matrix is an important © XXXX American Chemical Society

consideration, such as rubber elastomer, which is a sort of high-ductility but low-modulus material particularly demanding the enhancement of rigid nanoparticles. The conventional strategies for constructing rubber/CNC composites are concerned with the direct aqueous compounding between pristine CNC and rubber latex,11−16 or regulating the surface property of hydrophilic CNC with modification first and then physical compounding with rubber in organic solvents.17,18 Both approaches lack strong interactions between the filler and the matrix and therefore result in the increase of only the strength and modulus but at the expense of ductility for the obtained composites. A similar trade-off between stiffness and extensibility is also observed for strengthening elastomers by cross-linking and leads to a typical embrittlement and decreased ductility for the resultant materials.19 With the formation of strong interfacial interaction between CNC Received: June 13, 2019 Revised: July 14, 2019

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DOI: 10.1021/acs.macromol.9b01213 Macromolecules XXXX, XXX, XXX−XXX

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and rubber, the ideal composite system can provide simultaneous enhancement in strength and modulus together with the preservation of high ductility. Another strategy for the fabrication of rubber/CNC composites consists in promoting physical adsorption or chemical linkage between hydrophilic CNC and hydrophobic rubber. The hydrogen-bonding interaction was reported to be achieved based on the surface −OH groups of CNC and introduced −OH groups in natural rubber (NR) by its chemical modification.20−24 The tensile strength and modulus of composites were reported to be markedly increased in comparison with pure NR material. Alternatively, covalent linkage can provide strong interaction between CNC and rubber, by the thiol−ene click reaction between alkenemodified CNC (mCNC) and NR or poly(butadiene), and therefore exhibit increase of tensile strength, strain-to-failure, and work of fracture for the fabricated composites.25−27 However, the constructed interactions between CNC and rubber are realized commonly through −OH consumption or shielding at the surface of CNC, which is the driving force for the formation of a stable CNC percolating network. The nanoenhancement of CNC in rubber-based composites demands an architecture of both strong linkages between CNC and rubber matrix, together with sufficient interaction among CNCs. Recently, we proposed the reducing end modification on CNC by aldimine condensation and further covalent linking onto NR based on the thiol−ene click reaction, which contributed to the simultaneous enhancement of strength and ductility for the obtained composite.28 The reducing end modification provided active −SH groups on the reducing end of CNC and chemically linked to NR matrix induced by UV irradiation, which preserved the surface −OH groups of CNC and therefore the formation of a stable percolating network in the composites. On the basis of this design, both percolating network from filler (reducing end-mCNC) and strong interfacial adhesion between filler and matrix (NR) can be achieved, with the enhancement of strength and modulus while retaining high ductility of the rubber material. It is worth noting that most of the current studies on the topic of rubber/CNC composites miss the bulk cross-linking of unsaturated bonds (CC) for the rubber matrix, which can result in possible improvement of the mechanical properties of the resultant composites. In this work, we propose the combination of covalent linkage from reducing end mCNC and styrene−butadiene−styrene copolymer (SBS) and meanwhile realizing the self-cross-linking from bulk SBS for mechanical enhancement of the obtained composites. SBS is a typical thermoplastic elastomer taking the features of both high deformation of rubbers and processability and recyclability of thermoplastics. It is widely used as a polymeric matrix for the development of composites particularly without vulcanization.29 In this study, the UV irradiation initiated the click reaction between mCNC and SBS during liquid compounding and then further induced the bulk cross-linking of SBS in casting evaporation. The critical parameter of UVirradiated duration was discussed to achieve the gradual reaction between the filler and the matrix and then the matrix itself. In comparison with pure SBS material, the fabricated SBS/mCNC composite exhibited a remarkable improvement in tensile strength, Young’s modulus, work of fracture, and storage modulus, while preserving high ductility.

Article

EXPERIMENTAL SECTION

Materials. SBS was purchased from SINOPEC Cor. (Hunan, China) as a rubber matrix (SBS1201) with 30.1 wt % styrene content. Cysteamine hydrochloride (CH, NH2(CH2)2SH·HCl, 98%), sodium triacetoxyborohydride (Ac, 90%), potassium chloride (KCl), and sodium carbonate (anhydrous Na2CO3, ≥99%) were supplied by Aladdin Cor. (Shanghai, China) for reducing end modification of CNC. Other chemicals, for example, benzoin dimethyl ether (DMPA, 99%), tetrahydrofuran (THF, 99%), sulfuric acid (H2SO4, 98%), and hydrochloric acid (HCl, 37%) were also purchased from Aladdin Cor. and used without purification. Preparation of CNC and Its Reducing End Modification (mCNC). CNCs were extracted from cotton linter by the typical H2SO4 hydrolysis as reported in our previous study.30 Briefly, the purified cotton fibers (treated with diluted alkaline solution) were hydrolyzed by 65 wt % H2SO4 solution at 45 °C for 1 h, followed by successive centrifugation and washing steps against distilled water to remove free acid. The homogeneous suspension containing the individual CNC was obtained after further purification by dialysis in distilled water for 5 days. With regard to the reducing end modification, the aldimine condensation was performed to introduce the additional thiol groups (−SH) on the reducing end of CNC based on the covalent reaction between active aldehyde groups (CNC) and amino groups (CH).28 The Na2CO3 buffer solution was added into the CNC aqueous suspension (10 mg/mL) to regulate the pH at 9.2. The CH (100 μmol) and reducing agent Ac (45 mmol) were dissolved in distilled water and carefully transferred into the CNC suspension for reducing end modification at 70 °C under magnetic stirring for 24 h.31 The other two parts of reagents with the same content of CH and Ac were separately added again into the CNC suspension for complete reaction during the following 48 h. All the reactions were carried out under the protection of nitrogen in order to prevent any destruction of active thiol groups. The reaction was terminated by the addition of 3 M HCl solution to neutralize the remaining Ac in the suspension and dialyzed against distilled water for 3 days. A desired amount of KCl solution was introduced into the suspension and kept under continuous magnetic stirring overnight to remove the physisorpted molecules on CNC.32 Finally, the reducing end mCNC possessing active thiol groups in aqueous suspension was obtained after further dialysis against distilled water for 1 day and stored in 4 °C. UV Irradiation (X min) for Click Reaction between mCNC and SBS during Liquid Compounding. The loading level of mCNC or CNC filler in SBS-based composites was controlled as 10 wt %, which matched the construction of rigid percolating network according to the calculation of percolation model (7.5 wt % critical percolation threshold33 as discussed in the next section). The mCNC aqueous suspension (10 mg/mL, 20 mL) was solvent-exchanged with acetone into 30 mL of THF and then 1.8 g of SBS powder was introduced as rubber matrix and 0.036 g of DMPA as photoinitiator. The mixture was protected from light by covering with aluminum paper and magnetically stirred for 4 h at room temperature to obtain a homogeneously dispersed suspension. Then, after the removal of aluminum paper, the suspension was exposed to high-intensity ultraviolet lamp (365 nm, SB-100PC/FC, Spectroline, USA) for 8 min to induce the click reaction between mCNC and SBS components. The nanocomposites (S/mCNC) were obtained after casting in molds and solvent evaporation at 40 °C for 12 h. As a contrast, the nanocomposites (S/CNC) were also prepared by liquid compounding between SBS and pristine CNC (10 wt %) with the same treatments and 8 min UV irradiation. The pure SBS materials were also prepared by dissolving 2 g of SBS powder in 30 mL of THF with the addition of 0.036 g DMPA under varied durations of UV irradiation. Post-UV Irradiation (Y min) for Self-Cross-Linking of SBS in Molds and Composite Preparation. Considering the effect of bulk cross-linking (SBS) to the mechanical enhancement of composites, the mixtures of SBS and mCNC or CNC after liquid compounding were further treated by post-UV irradiation during the curing process, with various UV irradiation durations of Y min (4, 12, and 22 min). B

DOI: 10.1021/acs.macromol.9b01213 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Reducing End Modification of CNC and Preparation Route for S/mCNC Composites in This Study

After self-cross-linking in molds, the composites were obtained by solvent evaporation at 40 °C for 12 h. Taken the similar treatments, the pure SBS materials suffering the post-cross-linking were also prepared for comparison. Characterization. Reducing End Modification and Morphological Observation. The end modification of CNC was proved by Xray photoelectron spectroscopy (XPS) based on the trace of sulfur element derived from the addition of thiol groups, which was recorded at 100 W power with a 20 eV pass energy on a ESCALAB 250Xi equipment (Thermo Fisher Scientific, USA). The CNC suspension was negatively stained with 2 wt % uranyl acetate solution and observed by transmission electron microscopy (TEM) on a Tecnai G2 F30 instrument (FEI, USA) at 300 kV. The end mCNC suspension (mCNC, 2 mL, 1 mg/mL) was treated by silver nitrate solution (200 μL, 10 mM) under the reduction of sodium borohydride (2 mL, 5 mM) to observe the Ag nanoparticles tagged at the end of mCNC by TEM, attributed to the affinity of end thiol groups to the metal particles. Click Reaction between mCNC and SBS during Liquid Compounding. The possible covalent bonding between mCNC and SBS induced by UV irradiation was proved by Fourier transform infrared spectroscopy (FTIR) and 13C solid-state nuclear magnetic resonance. The collected product from the reaction between mCNC and SBS was completely purified by successive washing against THF at least three times to remove any unreacted reagent or physisorbed rubber. The freeze-dried product was characterized by an FTIR iS5 spectrometer (Nicolet, Madison, USA) with anhydrous KBr in the range of 4000−400 cm−1 and a Bruker AVANCE 400 NMR spectrometer with a 4 mm CP-MAS probe at 12 000 Hz spinning speed and 6 s relaxation delays. Gel Fraction and Self-Cross-Linking Extent of SBS under Post-UV Irradiation. In order to investigate the possible self-cross-linking of SBS induced by UV irradiation, the apparent viscosity of pure SBS solutions (6.67 mg/mL in THF) was recorded on a MCR 102 rheometer (MCR102, Anton Paar, Austria) with a 0.3 mm gap parallel plate sensor under a shear frequency of 0.1−100 rad/s. The self-crosslinking of SBS in the obtained films was further analyzed by the measurement of the gel fraction (treated in toluene) and characterization by attenuated total reflectance (ATR)−FTIR technique for the obtained materials. Briefly, strip-shaped film specimens were weighed as the initial weight (M0) and soaked in toluene for varied durations at room temperature. The insoluble polymer was collected by filtration and dried at room temperature until reaching a constant weight (Mt).

Therefore, the gel fraction (G%) of irradiated SBS materials can be calculated by the following equation G(%) =

Mt × 100% M0

(1)

The extent of self-cross-linking for SBS component induced by post-UV irradiation was further qualitatively determined by ATR− FTIR. All the prepared materials were treated under vacuum-drying at room temperature overnight before performing the ATR−FTIR analysis (Nicolet, Madison, USA) in the range of 4000−650 cm−1. The characteristic peak located at 910 cm−1 corresponds to the pendant vinyl groups of SBS for self-cross-linking and therefore chosen as the point of calculation for the different samples.34 The extent of self-cross-linking for SBS can be calculated by the following equation X (%) =

A u − Ac × 100% Au

(2)

where X (%) is the self-cross-linking extent and Au and Ac represent the area of the peaks (900−925 cm−1) for the uncross-linked material (SBS-1) and post-irradiated materials, respectively. Mechanical Performance of Fabricated Materials. The mechanical properties for pure SBS materials, S/CNC and S/mCNC composites, were accessed by tensile tests with a load cell of 5000 N (WDW-5D, Jinan Chuan Bai Instrument Cor., China). All the films were carefully cut as specimens 40 mm long and 4 mm wide and stored in a desiccator for 1 week before measurements. The uniaxial tensile tests were performed at 20 °C and 50% relative humidity using a cross head speed of 200 mm/min to investigate the tensile strength, Young’s modulus, elongation at break, and fracture work. Cyclic tensile tests were performed, in which the specimen was submitted to repeated stretching/recovery to 100, 300, and 500% elongation with a controlled cross head speed of 50 mm/min, and the stress−strain behavior was recorded for one to five cycles. The recovery ratio (R %) was defined as the recovering percentage of the material after stretching to 500% elongation, which can be calculated according to the following equation

ij l yz R(%) = jjj1 − s zzz × 100% j 5l0 z{ k

C

(3) DOI: 10.1021/acs.macromol.9b01213 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. XPS spectra for CNC and mCNC (A) and expanded view for S 2p signal in the range from 174 to 160 eV (B) and TEM images for CNC (C) and mCNC with end-capped Ag nanoparticles (D). where ls represents the length of the specimen after stretching and recovery and l0 is the initial length of the specimen. All measurements were carried out at least five times for each sample. Storage Modulus, Enhancing Mechanism, and Models Discussion. The storage modulus of fabricated materials was measured by dynamic mechanical analysis (DMA, Q800, USA) under the temperature range from −120 to 120 °C at a heating rate of 5 °C/min and 1 Hz frequency. To investigate the enhancing mechanism of the composite systems, the classic percolation model and Halpin−Kardos model were first applied to analyze the storage modulus and discuss the nanoenhancing effect of the filler. Furthermore, the stress−strain curves of the prepared composites were fitted with the doublenetwork model to discuss the possible formation and effect of filler− filler and matrix−matrix networks.

composites (with SBS and CNC components), and S/mCNC series composites (with SBS and reducing end-modified mCNC components), following the nomenclature reported in Table S1 according to the different UV irradiation treatments and durations. The reducing end modification of CNC was proved by XPS analysis and TEM observation tracing the presence of end additional −SH groups. As shown in Figure 1A,B, typical characteristic signals for O 1s, C 1s, and S 2p located at 531, 287, and 168 eV, respectively, were observed on the general spectra for nanocrystals before and after modification. From the expanded view of spectra ranging from 174 to 160 eV for the S 2p characteristic signal (Figure 1B), two separated peaks at 168 and 163 eV can be observed on the spectrum for mCNC, ascribed to the sulfurs in the original −OSO3H groups from H2SO4 hydrolysis and in additional −SH groups from reducing end modification.32 On the basis of the adsorption to metal nanoparticles via −SH groups at the reducing end of mCNC, the reducing end modification of nanocrystals can be further proved by direct morphological observation on TEM images.31 As shown in Figure 1C, the pristine CNC derived from native cotton exhibits a typical rodlike morphology, with a length of 100−300 nm, diameter of 10−20 nm, and measured aspect ratio (L/d) of 11.5 according to the Nanomeasurer software. Rather, numerous spherical nanoparticles (sub-5 nm) attributed to reduced Ag nanoparticles are observed at the end of rodlike nanoparticles (Figure 1D), indicating the adsorption from successful reducing end modification. Is the 8 min UV irradiation time used in the study sufficient to achieve the click reaction between mCNC and SBS during liquid compounding? An experiment was designed to investigate this issue. After UV irradiation for 8 min under continuous stirring, the mixture of SBS and mCNC was collected by centrifugation and successive washing against THF to completely remove any free SBS component. As shown in Figure 2A, the purified product of SBS and mCNC

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RESULTS AND DISCUSSION Reducing End Modification of CNC and Click Reaction between mCNC and SBS. The preparation route for the composites investigated in this work includes three steps, namely reducing end modification of CNC, reactive compounding between reducing end mCNC and SBS, and chemical self-cross-linking post-treatment for bulk SBS matrix. As shown in Scheme 1, the purpose of reducing end modification consists in introducing active thiol groups (−SH) at the reducing end of CNC with the preservation of surface hydroxyl groups (−OH) on nanocrystals. The additional −SH groups are further serving as reactive species for covalent linking between the mCNC filler and SBS matrix and is induced by the UV irradiation for their click reaction. The duration of UV irradiation (X min) is a critical point needed to be carefully investigated in this part, which must ensure the complete reaction between mCNC and SBS but prevent the self-cross-linking of bulk SBS during liquid compounding. Regarding the molding process, the duration of UV irradiation (Y min) should be discussed for the self-cross-linking of SBS matrix based on the chemical reaction among its unsaturated vinyl bonds. Three series of materials were prepared as SBSseries materials (with pure SBS component), S/CNC series D

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Figure 2. (A) FTIR spectra and (B) 13C solid NMR spectra for pristine CNC (a), purified mixture of SBS and CNC (b), and purified mixture of SBS and mCNC (c) after 8 min UV irradiation.

mixture treated under 8 min UV irradiation exhibits the typical features of benzene stretching vibration with two additional peaks located at 1600 and 1554 cm−1 on its FTIR spectrum.35 As a contrast, the product of SBS and CNC mixture with the same treatment exhibits features quite similar to pristine CNC on its FTIR spectra, demonstrating the weak interaction between unmodified CNC and SBS matrix and therefore easy removal of free SBS component during purification. Furthermore, the characteristic signals of polybutadiene located at 130 and 33 ppm attributed to C2′, C3′ and C1′, C4′ (vinyl backbone)36 can also be observed on the 13C solid NMR spectrum for the purified mixture of S/mCNC-8min (Figure 2B), indicating the success of covalent reaction between both components induced by the 8 min UV irradiation. The peaks for the purified S/mCNC-8min mixture located at 40−37 ppm are ascribed to the carbon signals of unreacted pendant vinyl (C5′, C6′) in the SBS copolymer.37 However, without the presence of SBS characteristic, only the features of cellulose (ascribed to glucose of CNC C1, 2, 3, 4, 5, and 6) can be observed on the spectrum for the S/CNC-8min product after purification. In fact, the purified S/CNC-8min and S/mCNC-8min products also exhibit distinct dispersion states in THF suspensions. As shown in Figure S1, the purified S/CNC8min product apparently settles after centrifugation at 3000 rpm for 5 min because of the immiscibility between pristine CNC and THF. However, attributed to the dissolubility of end-grafted SBS segment, the purified S/mCNC-8min product can homogeneously dispersed in THF even after the centrifugal treatment. The apparent difference between both products in suspension further proves the chemical linkage between mCNC and SBS during liquid compounding induced by UV irradiation for 8 min. Mechanical Performance and Comparison of SBS-, S/ CNC-, and S/mCNC-Series Composites. The obtained materials exhibit different appearances concerning the shrinkage observation when comparing SBS materials and S/ CNC or S/mCNC composites, as shown in Figure 3. It was reported that the presence of rigid filler (CNC) can improve the dimension stability and therefore reduce the shrinkage of composites38 resulting from the obstruction effect to molecular chain mobility of the polymeric matrix.39 In the present study, the pure SBS materials exhibited an apparent shrinkage particularly suffering high-intensity UV irradiation during the molding process (SBS-c1, c2, c3), whereas the composites containing CNC or mCNC display smooth surfaces and

Figure 3. Digital photographs showing the appearance of obtained materials.

uniform appearance attributed to dimension stability during the self-cross-linking process. The typical stress−strain curves for fabricated materials are shown in Figure 4 to make the comparison of different mechanical performances. The effect of UV irradiation imposed to the substance during liquid compounding process or molding process to the mechanical properties of materials can be observed in the case of S/mCNC-8min composites (Figure 4A). It is apparent that the composite prepared by the UV processing during liquid compounding (suspension) exhibits higher strength and ductility than those of the composite prepared by the UV processing during molding (Table S2), which indicates a closer contact and reaction between mCNC and SBS in liquid compounding process than that in molding process induced by UV irradiation. Figure 4B summarizes the effect of different UV irradiation durations on the mechanical properties for pure SBS materials. The 8 min UV treatment results in weak influence on the bulk SBS structure during liquid irradiation, displaying similar mechanical performance for SBS-1 and SBS-2 materials. The further UV treatment molding process can induce possible chemical self-cross-linking of SBS chains and therefore improve the mechanical properties of the materials (SBS-c1, c2, and c3). On the basis of the dominant nanoreinforcing effect from rigid CNC, a typical stress increase and strain reduction is obtained for all S/CNC-series composites (Figure 4C), which is the general result for traditional composite systems obtained from the physical compounding between rigid filler and polymeric matrix. The optimal mechanical performance is observed for the S/mCNC-series composites with a remarkable enhancement in strength and ductility in comparison with the pure SBS material (Figure 4D), which can be attributed to the strong E

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Figure 4. Stress−strain curves for S/mCNC-8min composites with UV irradiation during liquid compounding or molding process (A), pure SBSseries materials (B), S/CNC-series composites (C), and S/mCNC-series composites (D).

Figure 5. Mechanical properties for prepared materials: tensile strength (A), Young’s modulus (B), elongation at break (C), and work of fracture (D).

exhibiting 239, 411, 7, and 330% increase in σb, E, εb, and Wf values in comparison with those for pure SBS material. In view of the high work of fracture for the composites, cyclic tensile tests were performed for the fabricated materials to investigate their mechanical recovery at strain levels of 100, 300, and 500%. As shown in Figure 6, all the materials present the typical Mullins effect on the cyclic stress−strain curves, as the classic mechanical response for rubber-based materials. In comparison with other materials, the composite S/mCNC-c2 exhibits the maximum stress and largest hysteresis loops for the three strain levels, demonstrating its strong energy-absorbing capacity. Ascribed to the stress-transfer effect from rigid nanoparticles (mCNC), stable microstructures are constructed in composites S/mCNC-2 and S/mCNC-c2, which therefore maintain high stress levels even for the five-cycle test. The

linkages between the filler (mCNC) and the matrix (SBS) together with the self-cross-linking among the bulk matrix. The specific enhancing mechanisms for S/mCNC-series composites will be discussed in the next section. The typical mechanical parameters for the obtained materials, viz., tensile strength σb, Young’s modulus E, elongation at break εb, and work of fracture Wf, are shown in Figure 5. Evidently, the S/mCNC-series composites with the covalent linkage between mCNC and SBS (8 min UV irradiation) exhibit improved mechanical properties with simultaneous increase of the four parameters in comparison with the pure SBS material. This mechanical performance is further strengthened by the SBS self-cross-linking induced by post-UV irradiation to achieve extraordinary mechanical performance for S/mCNC-c2 (8 + 12 min) composite, F

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Figure 6. Stress−strain curves during cyclic tensile tests (one, three, and five cycles) for SBS-2 (A), SBS-c2 (B), S/CNC-2 (C), S/CNC-c2 (D), S/ mCNC-2 (E), and S/mCNC-c2 (F).

Figure 7. Chemical cross-linking of pendant vinyl groups in SBS induced by UV irradiation and DMPA catalysis.

interactions for both nanocrystals in the matrix, which will be further discussed in the next section. Self-Cross-Linking (Matrix−Matrix) of Bulk SBS in Composites. The free-radical mechanism was reported for the chemical cross-linking of SBS induced by UV irradiation and DMPA photoinitiation.40 As shown in Figure 7, both intramolecular and intermolecular cross-linkings are expected to take place among pendant vinyl groups on polybutadiene segments of SBS, leading to possible cyclization and macromolecular linkage.34 In previous reports, the duration of UV irradiation to induce bulk cross-linking of SBS ranged from several seconds to hours, resulting in varied extents of self-

structural stability can be further analyzed by the recovery ratio of materials, representing the strain lag (indicated in the images) under large deformation (500%). Obviously, the materials SBS-2 and SBS-c2 together with the composites S/ mCNC-2 and S/mCNC-c2 preserve the superior structural recovery with high recovery ratios >90%, whereas the composites S/CNC-2 and S/CNC-c2 display apparent recovery lag with decreased recovery ratios to 71 and 83%, respectively (Figure S2). These different behaviors regarding the mechanical recovery for S/mCNC and S/CNC composites indicate the distinct dispersion states and interfacial G

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Figure 8. Viscosity for pure SBS dissolved in THF solution (6.67 wt %) (A) and gel fraction of fabricated SBS-series materials (B) under various UV irradiation durations.

Figure 9. FTIR spectra for SBS-series materials (A), S/CNC-series composites (B), and S/mCNC-series composites (C) treated by different UV irradiation durations and calculated extent of bulk SBS cross-linking (D).

for SBS-c3 material with more than 80% gel fraction. The different gel fractions of fabricated SBS-based materials prove the possible regulation of bulk SBS cross-linking through the control of UV irradiation during molding process, which is another enhancing factor for the mechanical improvement of composites as mentioned before. The photoinitiated self-cross-linking of SBS was reported to mainly depend on the chemical reaction of pendant vinyl groups under UV irradiation, while leaving the unreacted backbone double bonds.40 As indicated by their FTIR spectra (Figure 9A), the intensity of the characteristic peak located at 910 cm−1 for pure SBS-series materials gradually decreases with the increase of UV-irradiated duration, which demonstrates the consumption of pendant vinyl groups of SBS in these materials. A similar result is observed on the infrared spectra for S/mCNC-series composites (Figure 9C), indicating the possible control of self-cross-linking for SBS components in these composites by regulating the UV irradiation. However, the reaction among pendant vinyl groups is restricted for S/ CNC-series composites, which exhibited only weak changes in peak intensity (910 cm−1) on their infrared spectra (Figure 9B). This result may be explained by the seriously spatial hindrance caused by self-aggregation of pristine CNC in these composites, reflected by the presence of characteristic bands

cross-linking for the increase of molecular weight and insoluble composition in solvent. The self-cross-linking of SBS component is characterized by viscosity change during UV irradiation in liquid (THF) and gel fraction of materials during UV irradiation in mold, as shown in Figure 8. An apparent increase of viscosity is observed for the SBS solution treated by 10 min UV irradiation, indicating a critical intensity of UV photoinitiation for the SBS self-crosslinking during liquid irradiation. Combined with the previous results, the duration of UV irradiation during liquid compounding was therefore optimized as 8 min to ensure the sufficient click reaction between mCNC and SBS components and avoid any insoluble composition caused by the self-cross-linking of bulk SBS in this process. The selfcross-linking of SBS is expected to be achieved by UV irradiation during the molding process, which can be reflected by the insolubility of the material when immersed in toluene (SBS-c1, c2, and c3). The calculated gel fraction for SBS-c1 material gradually decreases from 32% (at 2 h initial dissolution) to 0% (>10 h dissolution), indicating the low self-cross-linking extent or even only intra-cross-linking for this preparation. The moderate extent of self-cross-linking with the equilibrium gel fraction of about 32% (at 48 h dissolution) is reserved for SBS-c2 material, in contrast to over-cross-linking H

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Figure 10. Evolution of the storage modulus (log E′) vs temperature for S/CNC-series composites (A) and S/mCNC-series composites (C); loss factor (tan δ) vs temperature for S/CNC-series composites (B) and S/mCNC-series composites (D).

for hydroxyl groups (3338 cm−1) on their infrared spectra. It is worth noting that any oxidation of pendant vinyl groups is unexpected and indeed the side reaction is avoided as shown by the absence of a peak at 1694 cm−1 for carbonyl bands on the infrared spectra for all obtained materials.34 The calculated extent of self-cross-linking for SBS component in the pure SBSseries materials and S/mCNC-series composites (Figure 9D) are consistent with the previous conclusion as low, moderate, and over SBS self-cross-linking induced by (8 + 4), (8 + 12), and (8 + 22) min UV irradiation. Percolation Mechanism (Filler−Filler) and DoubleNetwork Mechanism for Mechanical Enhancement. The dispersion state of CNC in composites is an important factor for the construction of a percolating network, based on the driving force of hydrogen-bonding interaction among surface hydroxyl groups.8 The aggregation of hydrophilic nanocrystals (pristine CNC) in hydrophobic matrix (SBS) may result in the shielding of surface hydroxyl groups and therefore weaken the formation of percolating network. As a contrast, the dispersion and compatibility of mCNC in SBS are efficiently improved by the reducing end modification and reactive compounding strategy, which has been proved in our previous study28 and also observed in this case (Figure S3). The storage modulus (E′) of fabricated materials can be obtained from DMA, a classic characterization to reveal the molecular motion and microstructure of polymeric composites. As shown in Figure 10, the storage modulus value for all

composites increases with the addition of 10 wt % CNC or mCNC in both the glassy and rubbery regions, resulting from the restriction effect to free motion of polymeric chains by rigid nanoparticles. The increase in storage modulus for S/ mCNC-series composites is more notable than for S/CNCseries composites because of the improved compatibility between mCNC and SBS by covalent linkage. Typically, the storage modulus at 25 °C for the S/mCNC-c2 composite increased to 34.3 MPa, which is 600 and 40% higher in comparison with the value for the pure SBS-1 material and the S/CNC-c2 composite, respectively. This apparent enhancement in the storage modulus for S/mCNC-c2 composite can be mainly attributed to the possible formation of a rigid network (percolating structure) from mCNCs, which will be specifically discussed in the next section. It is worth noting that the treatment of SBS self-cross-linking seems to have a weak effect on the storage modulus in the case of S/mCNC-series composites because of the good comparability and rigid network that play a dominant role achieved under the 8 min UV pre-irradiation during liquid compounding. Increased storage modulus based on the self-cross-linking of bulk SBS can be observed from the DMA results for pure SBS-series materials (Figure S4), which revealed E′ increase for SBS-c1, c2, and c3 materials in comparison with the untreated material SBS-1. Regarding the change in loss factor (tan δ), the typical thermodynamic behavior of polymeric composites filled with nanoparticles is observed, with peak magnitude of the I

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storage modulus for S/CNC-series composites ranges between the theoretical modulus obtained from the percolation model and Halpin−Kardos model (7.99 MPa, black line), as a result of high-modulus filler but low incompatibility for the composites. Focusing on the mechanical performance of S/ mCNC-series composites, the proposed strategy of covalent compounding with reducing end-modified nanocrystals (mCNC) and SBS matrix provides a strong filler−matrix interaction and meanwhile preserves the surface hydroxyl groups of the nanocrystals as the driving force to construct the percolating network. The present approach appears promising to solve the contradiction between interfacial adhesion and network architecture of the composites in this topic. On the basis of the previous discussion, both strong matrix− matrix and filler−filler interactions are achieved for the S/ mCNC-c2 composite, which endows it superior mechanical performance with simultaneous enhancement of strength, modulus, ductility, and work of fracture. If both interactions in the S/mCNC-c2 composite are regarded as two “supernetworks”, the classic double-network mechanism can be applied to explain its mechanical properties. The doublenetwork mechanism was proposed by Duschl et al. in 1993 to predict the stress (σ)−strain (λ) behavior of elastomer-based composites,41 according to the following equations

relaxation process associated with the glass transition of the matrix decreasing by about 30% for all composites in comparison with that of the pure material SBS-1. In the glassy transition region, the energy loss for the composites is mainly ascribed to the bulk matrix (rubber) with the molecular motion in the state of highly viscoelastic retardation. The addition of rigid fillers (CNC or mCNC) restricts the free motion of surrounded rubber chains and reduces the effective volume of rubbery matrix in the composite, which therefore results in an intensity decrease of loss factor for the prepared composites. The possible formation of a rigid network among mCNCs in the composites needs to be further analyzed by percolation and mean-field mechanisms with the comparison between experimental and predicted data from different models. The theoretical calculation of the storage modulus (EC′ ) for composites is based on percolation model and Halpin−Kardos model according to the universal equations as follows. EC′ (percolation model) =

(1 − 2ψ + ψX r)Es′Er′ + (1 − X r)ψEr′2 (1 − X r)Er′ + (X r − ψ )Es′

EC′ (Halpin − Kardos model) =

4U5(U1 − U5) U1

(4)

σtotal = σ1 × (1 − Φ) + σ2 × Φ

(5)

Information and detailed calculation of varied parameters can be obtained in the Supporting Information. It should be emphasized that (i) the percolation model predicts the storage modulus of composites involving filler−filler interaction while the Halpin−Kardos model excludes this interaction; (ii) in this study, we controlled the loading level to 10 wt % (6.4 vol %) in composites to meet exactly the critical percolation threshold condition (calculated as 6.0% volume fraction) for percolation model; and (iii) the matrix−matrix interaction (SBS component) in composites will be synergistically discussed in the double-network mechanism and therefore not considered in this part. Figure 11 shows the results for the storage modulus for S/ mCNC-series and S/CNC-series composites comparing the

λ=

(6)

2b12 × b2 2

(1 − Φ)b2 2 + Φb12 ÅÄÅ ÑÉÑ1/2 Å 1 1 2 2Ñ ÑÑ × ÅÅÅÅ(1 − Φ) × × + Φ × × λ λ Ñ 1 2 Ñ ÅÅÇ ÑÑÖ 2b12 2b2 2 (7)

The specific information on the parameters and values in both equations are summarized in the Supporting Information (Table S3). The double-network model consists of two independent networks in the composites, involving the SBS network formed by self-cross-linking and the mCNC network established by hydrogen bonds. As shown in Figure 12, the

Figure 12. Stress−strain curves for S/mCNC-c2 and S/mCNC-2 composites and SBS-c2 material: experimental results (full curves) and theoretical calculations (dots) by double-network model.

Figure 11. Comparison between experimental storage modulus (E′) at 25 °C for S/mCNC-series and S/CNC-series composites and theoretical modulus calculated according to the percolation model and Halpin−Kardos model.

experimental stress−strain curves (full lines) for the S/mCNCc2 and SBS-c2 materials are towel-predicted by the computed results (dots), indicating a good applicability of the doublenetwork model to this study. Considering the previous discussion on the matrix−matrix and filler−filler networks, the double-network structure is expected to be formed in the S/mCNC-c2 composite and therefore achieves the compre-

experimental data (DMA) and theoretical calculations. It is clearly evidenced that a stable percolating network forms for the S/mCNC-series composites, exhibiting the highest experimental values for the storage modulus above the predicted modulus (27.88 MPa, blue line) from the percolation model. However, the experimental values for the J

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CONCLUSIONS In this study, the elastomeric (SBS) composites reinforced with the rigid nanoparticle (mCNC) were prepared through double-network formation based on click reaction and bulk cross-linking. The main challenge of this work was to separately conduct the two steps of SBS/mCNC click reaction and subsequent bulk SBS self-cross-linking during liquid compounding and molding processes, respectively, which can therefore ensure the construction of strong filler−matrix, filler−filler, and matrix−matrix interactions in the system. The interfacial adhesion between mCNC and SBS was achieved by the reactive compounding based on the covalent linkage between SBS and reducing end of mCNC. This click reaction was recommended to be arranged during the liquid compounding process induced by UV irradiation (8 min) but not during the molding process in order to obtain a sufficient contact and reaction efficiency. The further UV postirradiation (12 min) was performed during the molding process to complete the self-cross-linking of bulk SBS in composites. Together with the formation of the rigid filler− filler network (percolation) at a 10 wt % loading level of mCNC, the fabricated S/mCNC-c2 composite exhibited the superior improvement in mechanical properties, with simultaneous increase in tensile strength, Young’s modulus, work of fracture, storage modulus, and elongation at break.

hensive improvement of the mechanical performance. For the S/mCNC-2 composite and the SBS-c2 material, only an individual network is constructed (a filler−filler network for S/ mCNC-2 and a matrix−matrix network for SBS-c2), resulting in moderate improvement of the mechanical performance. It should be pointed out that when making the theoretical calculation for the unfilled material SBS-c2, the parameters related to filler’s network is determined as zero, so indeed only matrix−matrix network is involved for this material.

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DISCUSSION The performance of a material is the external expression of its structure, and the structure of a material is the internal condition for its performance. For polymeric composites, three types of interactions originating from the dispersed phase (filler) and continuous phase (matrix) are the critical factors to determine their mechanical performances, including filler− matrix, filler−filler, and matrix−matrix interactions. In the case of CNC-reinforced composites, many studies are devoted to improving the interfacial adhesion between the filler and the matrix because of the intrinsic incompatibility of the hydrophilic filler (pristine CNC) and hydrophobic matrix (majority of synthesized polymers). The traditional approach consists in performing surface modification of CNC, which however represents consumption or shielding costs for the surface hydroxyl groups and therefore results in a weakness of filler−filler interaction for the composites. Another shortage is the missing involvement of matrix−matrix interaction concerning the physical entanglement or chemical self-crosslinking to the mechanical performance of the composites. In this study, strong adhesion between the filler and the matrix was achieved by covalent linkage from thiol−ene click reaction for reducing end-modified nanocrystals (mCNC) and SBS. The reducing end modification of CNC preserved their surface hydroxyl groups as the potential driving force for the construction of a stable percolation network (filler−filler interaction) in the composite. The matrix−matrix interaction was achieved by moderate self-cross-linking of bulk SBS, and it further enhanced the mechanical properties of the composites. As shown in Figure 13, this structural design can involve all components and remarkably improve the mechanical performance of the obtained composite (S/mCNC-c2) with a simultaneous enhancement in strength, modulus, and fracture work while retaining the high ductility of the elastomer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01213. Equations and calculations for the mechanical properties of prepared materials based on the percolation model, Halpin−Kardos model, and double-network model; nomenclature and different UV energies (J) for the prepared materials; mechanical properties of S/mCNC composites prepared by UV irradiation during liquid compounding or molding process; parameters, symbols, and implications in the double-network model; digital photos for purified S/CNC-8min and S/mCNC-8min mixtures after centrifugation; recovery ratio for prepared materials; appearance of prepared composites; and DMA results for SBS-series materials (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +86-27-87152611. Fax: +86-27-87152611. ORCID

Alain Dufresne: 0000-0001-8181-1849 Ning Lin: 0000-0002-7367-8037 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51603159). The authors also wish to acknowledge the financial support of Anhui Province Key Laboratory of Environment-friendly Polymer Materials (KF2019004). LGP2 is part of LabEx Tec 21 (Investissements d’Avenirgrant agreement no. ANR-11-LABX-0030) and of

Figure 13. Schematic double-network structure formed in the S/ mCNC-c2 composite. K

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