Article pubs.acs.org/Macromolecules
Carbon Nanodots as High-Functionality Cross-Linkers for Bioinspired Engineering of Multiple Sacrificial Units toward Strong yet Tough Elastomers Siwu Wu, Min Qiu, Zhenghai Tang, Jie Liu, and Baochun Guo* Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *
ABSTRACT: It is still a huge challenge to implement multiple energy dissipation mechanisms into polymers toward strong yet tough elastomers. Here, we describe a biomimetic design for diene-rubber by incorporating carbon nanodots (CDs) into a chemically cross-linked network. The high-functionality CDs serve as both physical and chemical cross-linkers, which give rise to a covalent network that interlinks multiple chains with nonuniform lengths, and interfacial hydrogen bonds. Upon stretching, the hydrogen bonds preferentially detach, leading to the orientation of short covalent bridging, which contributes the forward onset of strain-induced crystallization. The subsequent rupture of short covalent bridging, together with the successive detachment of hydrogen bonds result in further orientation of hidden length, which enhances the crystallinity. Consequently, the samples exhibit an integrated improvement of strength and toughness, and intact stretchability. We envisage that this strategy may provide a new avenue to implement biomimetic design for high-performance elastomers through multiple energy dissipation mechanisms.
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INTRODUCTION Biological materials in plant and animal tissues, such as bone, muscle, silk, and byssus, are usually super strong and tough in order to withstand huge and dynamic loads from the surrounding environment.1 It has been revealed that the hierarchical structuring involves sacrificial bonds allows the adaptation and optimization of the materials upon deformation through molecular-scale energy dissipation mechanism to yield extraordinary performance.2,3 Inspired by naturally formed structures, several successful strategies including double network, hybrid-cross-linked structure, and high-functionality cross-linkers (HFC) have been implemented to strengthen and toughen polymer materials by virtue of the concept of energy dissipation mechanism.4−7 Among these, the strategy of incorporating HFC into polymer network has been extensively developed to achieve hydrogels with high strength and extensibility.7−9 In these systems, multiple chains with nonuniform lengths are attached to HFC via covalent bonds/ physical interactions (physical absorption, hydrogen bonds, or electrostatic interaction). As the system is subjected to deformation, the short chain bridges between adjacent crosslinkers served as sacrificial units can be preferentially ruptured/ detached to enable energy dissipation while the long chains can still maintain the structural integrity.10 However, most reported materials with HFC have been limited to hydrogel systems and usually dissipated energy through one single mechanism which cannot be well-rounded in every aspect. For example, the hydrogels physically cross-linked by HFC are usually not tough enough because of the poor interfacial interaction,11,12 while poor recovery performance is observed in the systems that are © 2017 American Chemical Society
chemically cross-linked by HFC due to the irreversible rupture of covalent linkages.13,14 It is believed that the integration of multiple energy dissipation mechanisms into polymer matrix may turn into a promising and executable strategy toward highperformance polymers. Rubbers are indispensable and fascinating materials due to their applications in a variety of industries such as tire, electronics, aerospace, and military. From a practical point of view, the applications of rubbers are severely limited by the poor mechanical property of raw gum. Incorporation of various nanofillers is a common practice to enhance the mechanical performance of rubber matrix.15,16 However, nanofilling has been limited by several crucial problems including high filler loading, processing difficulty, and elaborate regulation of dispersion/interface. To address this problem, we implemented sacrificial bonds strategy to achieve high-performance dienerubbers by incorporating metal−ligand or hydrogen bond motifs into a chemically cross-linked network lately.17−21 This bioinspired design not only simultaneously strengthens and toughens the soft matrix but also endows the resultant materials with adaptive recovery and flex-cracking resistance. Still, only one energy mechanism was involved in our previous studies. Therefore, it is believed that the implementation of multiple energy dissipation mechanisms into rubber may be a viable strategy toward robust elastomer with integrated mechanical performance. Received: March 5, 2017 Revised: April 7, 2017 Published: April 13, 2017 3244
DOI: 10.1021/acs.macromol.7b00483 Macromolecules 2017, 50, 3244−3253
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Preparation of Rubber Composites. A certain amount of CDs was admixed with g-IR at 50 rpm at 80 °C for 30 min in an internal mixer. After cooling to room temperature, the compound was incorporated with curing ingredients in an open two-roll mill, followed by hot-pressing at 143 °C for the optimum vulcanizing time which was determined by a U-CAN UR-2030 vulcameter. The specific formulation is listed as g-IR 100 g, zinc oxide 5 g, stearic acid 2 g, N-cyclohexyl-2-benzothiazole sulfonamide (CZ) 1.5 g, CDs, and sulfur variable. The samples are referred to as g-IR-x, where x denotes x phr (parts per hundreds of gum) of CDs and contains a fixed sulfur content of 1.5 phr. Meanwhile, g-IR-3-y refers to as the samples contain a fixed CDs content of 3 phr and a variable sulfur content of y phr. For comparison, IR formulations incorporating with 40 phr N330 carbon black or VN3 silica were prepared. In silica-filled IR formulation, additional accelerator diphenylguanidine (1 phr) and TESPT (3 phr) were added. Other curing ingredients are the same as those of g-IR-x samples. Characterization. Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Vertex 70 FTIR spectrometer equipped with a temperature accessory. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250 Mutiltechnique Surface Analysis equipped with an Al Kα radiation source. Swelling behaviors in toluene or tetrahydrofuran/ methanol were characterized through the observation on morphological changes of a disc sample along increasing immersing time at room temperature. The samples were filtered with 400 mesh screen to remove the dissolved portion; then fresh solvent was added to ensure the sufficient swelling. These procedures were repeated every 24 h. After equilibrium swelling (72 h), all samples were dried thoroughly at 80 °C and then weighted again to determine the gel fraction. The cross-linking density of the samples was determined by the equilibrium swelling method and was calculated using the Flory−Rehner equation.24 All samples were immersed in the tetrahydrofuran/ methanol solution until equilibrium swelling (72 h). The tensile stress−strain test was measured at 25 °C by using a Gotech AI-7000 S servo control system universal testing machine following ISO standard 37-2005. Fracture toughness (MJ/m3) was calculated from the area below the tensile stress−strain curve until fracture. For the cyclic loading−unloading test, all measurements were carried out at 100 mm/min at 25 °C, and the hysteresis was calculated from the area between the loading and unloading curves. Stressrelaxation experiments were performed on a TA Q800 dynamic mechanical analyzer in strain rate mode to monitor the decay of stress. All specimens were extended to a predetermined strain at a strain rate of 500%/min and held at different temperatures. The viscoelasticity of the present system was evaluated by a TA Q800 dynamic mechanical analyzer in tensile mode with the preset frequency and strain of 1 Hz and 0.5%, respectively. The testing temperature ranged from −80 to 80 °C with a heating rate of 3 °C/ min. Rheological tests were performed on an Anton Paar MCR302 rheometer at 25 °C using a 20 mm flat parallel plate. For frequency sweep test, a dynamic range from 0.01 to 628 rad/s was conducted at a small strain of 0.5%. Strain sweep in the range of 0.001−100% was measured at an angular frequency ω = 5 rad/s. Soak time for each test was 600 s. In situ synchrotron wide-angle X-ray diffraction (WAXD) measurements were performed on the platform of BL14B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), using X-ray with a wavelength of 1.2398 Å. The two-dimensional WAXD patterns were recorded by a Mar225 CCD X-ray detector with a flat screen and a pixel size of 0.073 mm. The sample-to-detector distance was maintained at 482 mm. All tests were conducted during synchronously stretching at room temperature (approximately 20 °C). A custommade tabletop stretching device was mounted on the testing platform, allowing the symmetric deformation of the rectangular specimens and to illuminate the same sample position during stretching. The initial distance between the clamps was 10 mm, and the deformation rate was 10 mm/min. The nominal strain α, in terms of the deformation ratio,
In this work, we describe a biomimetic design of strong yet tough diene-rubber by incorporating amine-passivated carbon nanodots (CDs) as HFC to engineer multiple sacrificial units into a chemically cross-linked network. For this purpose, functionalized CDs were synthesized by using organic amine as surface passivation agents, on which numerous primary amino and amide moieties were immobilized on the surface of CDs.22 This amine-passivated CDs can be classified as an active hydrogen compound which exhibits unparalleled potential to serve as HFC in polymer. Furthermore, anhydride moieties were introduced to a commercially available cis-1,4-polyisoprene, which enabled us to construct both covalent bridge and transiently hydrogen-bonding anchoring between the rubber skeleton and CDs. The reversible hydrogen-bonding association endued the system with thermal-induced recovery capacity. Meanwhile, these linkages served as multiple sacrificial units to dissipate energy upon multiple mechanisms, leading to an integrated improvement of strength, modulus, and toughness of the system, together with intact stretchability. In addition, the incorporation of CDs as HFC led to forward onset strain of crystallization and promoted crystallinity simultaneously, which was distinct from the strain-induced crystallization (SIC) behavior of the systems with conventional fillers.
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EXPERIMENTAL SECTION
Materials. Polyisoprene rubber (IR, Trademark Nipol IR2200) with 98 wt % cis-1,4-polyisoprene (Mn of 5 × 105 g/mol) was manufactured by Zeon Co., Ltd., Tokyo, Japan. Maleic anhydride (MAH), critic acid (CA), and 1,2-ethylenediamine (EDA) were purchased from Beijing InnoChem Science & Technology Co., Ltd., Beijing, China. Carbon black (N330, Trademark VULCAN 3) was fabricated by Shanghai Cabot Chemical Co., Ltd., Shanghai, China. Silica (VN3, Trademark ULTRASIL VN 3) and silane bis(triethoxysilylpropyl)tetrasulfide (TESPT, Trademark Si 69) were supplied by Evonik Specialty Chemicals (Shanghai) Co., Ltd., Shanghai, China. All rubber additives were industrial grade and used as received. Synthesis of Amine-Passivated Carbon Nanodots. CDs were synthesized using a commercial household microwave according to previously published methods.22,23 Specifically, 1.0 g of CA was dissolved in 10 mL of phosphate solution, followed by dropwise adding a certain amount of EDA under mildly stirring (−COOH/− NH2 molar ratio for CDs and CDs-1 were 3:2 and 3:1, respectively). The mixture was then placed into a household microwave oven and heated for 3 min at 700 W, and a red-brown solid was obtained. After cooling to room temperature, the solid was diluted with distilled water and washed with excess anhydrous ethanol repeatedly for three times to remove byproducts and unreacted chemicals. The purified CDs were dried in vacuo at 50 °C for further characterization and application. Preparation of Maleic Anhydride Grafting Polyisoprene Rubber. In situ grafting MAH onto IR was carried out by meltblending according to previously published methods, with some modifications.18 Specifically, 30.0 g of IR (0.44 mol, based on isoprene unit), 4.32 g of MAH (10.0 mol % of IR), 0.30 g of antioxidant Nphenyl-N′-1,3-dimethylbutyl-p-phenylenediamine (6PPD, 1.0 wt % of IR), 3.00 g of aromatic oil (10.0 wt % of IR), and 4.68 g of xylenes (10.0 mol % of IR) were added successively and mixed at 50 rpm at 80 °C for 30 min in a laboratory batch mixer. The resultant mixture was further admixed at 50 rpm at 180 °C for 60 min to obtain maleic anhydride grafting polyisoprene rubber (g-IR). After cooling to room temperature, the mixture was dissolved in excess toluene and then precipitated in acetonitrile. The resulting precipitate was washed thoroughly with excess acetonitrile and dried in vacuo at 50 °C overnight. The molar ratio of grafted maleic anhydride moiety was determined as 3.95 mol % through the titration method. 3245
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Figure 1. (a) FTIR spectra of the uncured g-IR and g-IR-3 before and after hot-pressing (HP) at 143 °C for 30 min. (b) Photographs of the swelling behaviors of the uncured g-IR-3 (upper: before hot-pressing; bottom: after hot-pressing at 143 °C for 30 min) in toluene at various times. (c) FTIR spectra of the g-IR-3 in the amide I (left) and II (right) regions recorded as a function of decreasing temperature.
groups centered at 1710, 1645, and 1560 cm−1 come into being. The conversion is thoroughly promoted after further hotpressing the compound, indicating the covalent grafting of polymer chains on the surface of CDs. The swelling behaviors in toluene were investigated to further verify the existence of covalent network cross-linked with CDs in g-IR/CDs compounds. As shown in Figure S4, the disc of blank g-IR is almost dissolved into toluene under mildly stirring within 2 h. In comparison, the g-IR-3 without hot-pressing can only swell in toluene (Figure 1c). After further hot-pressing, the shape of the g-IR-3 disc can be effectively maintained, and the degree of swelling is substantially decreased even after 72 h stirring (the corresponding gel fractions of g-IR samples are tabulated in Table S1). The high gel fraction of g-IR-3 also implies the formation of covalently cross-linked network through the additive CDs served as chemical HFC. In addition, to verify the formation of hydrogen bonding anchoring between g-IR and CDs, temperature-dependent FTIR spectra for g-IR-3 were recorded as shown in Figure 1b. With increasing temperature, the amide I band (CO stretching vibration) appears to shift to higher frequency (1654 to 1660 cm−1 from 30 to 150 °C), accompanied by decrease in intensity. Meanwhile, the amide II band (N−H deformation vibration) is observed to shift to lower frequency and decrease in intensity as the temperature is increased. The shift and reduction in intensity of the amide band with temperature should result from the dissociation of hydrogen bonds which leads to the reduction in the average strength of hydrogen bonds.36,37 Therefore, the CDs can be regarded as both physical and chemical high-functionality crosslinkers to enable the introduction of multiple sacrificial units into the g-IR skeleton. Dynamic Features of Sacrificial Hydrogen-Bonding Anchoring in g-IR/CDs Composites. The introduction of transiently hydrogen-bonding anchoring can endow the samples with reversibility, which was demonstrated by cyclic tensile tests. As g-IR-3 sample is stretched to a predefined strain of 150%, prominent hysteresis is observed in comparison with the sulfur-cured g-IR sample (Figure S5), indicating large amounts of energy dissipation by unzipping hydrogen-bonding anchoring.2 However, successive loading/unloading cycles would lead to a significant decline of hysteresis and stress. Even after resting at room temperature for 30 min, the spontaneous recovery is still negligible. The phenomenon is consistent with other reported systems combining hydrogenbonding network with covalently cross-linked network, which should be ascribed to the reattachment of hydrogen bonds
was determined by α = (l − l0) × 100%/l0, where l0 is the original clamp−clamp distance and l is the real-time clamp−clamp distance during stretching. Each specimen was continuously stretched to the predetermined maximum strain of 550% in case of rupture. All measured patterns were corrected for air background and further analyzed by Fit2D software based on the previously published methods.25−27
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RESULTS AND DISCUSSION Characterization of the Synthesized Carbon Nanodots. The morphology of CDs is assessed by HRTEM. As shown in Figure S1, the synthesized CDs exhibit a quasispherical geometry with an average diameter of 1−3 nm and successive lattice fringes with a spacing distance of 0.21 nm, which is in accordance with the (100) facet of graphite.28−30 FTIR and high-resolution XPS spectra were performed to gain further insight into the organic structure and bonding manner of the elements of the synthesized CDs. Compared to the source citric acid, the typical sharp absorptions centered at 1657 and 1561 cm−1 in the spectrum of CDs (Figure S2) are assigned to the CO stretching and N−H blending vibration of amide group. In addition, the broad band in the 3160−3360 cm−1 region is ascribed to the N−H stretching vibration of the secondary amine group, respectively.31 In accord with the FTIR result, three types of N-related bonding can be identified in the deconvoluted high-resolution N 1s spectrum of the synthesized CDs (Figure S3). The peaks centered at 399.6, 400.3, and 401.5 eV are assigned to the amine groups, amide−carbonyl groups, and graphitic N, respectively.32−34 These above investigations have verified the presence of primary amine and amide functional groups on the surface of the synthesized CDs. Construction of Multiple Sacrificial Units in g-IR/CDs Compounds. Previously, we have demonstrated the introduction of a weak or transient network could be an efficient strategy to endow chemically cross-linked elastomers with supramechanical performance and unexpected functionality.17,18 The weak or transient junctions can sustain and sacrifice an initial load and then preferentially rupture before the failure of the covalent network of elastomers.4,35 In the present study, the CDs were employed as an active hydrogen compound to construct both covalent bridge and transiently hydrogen-bonding anchoring between the rubber skeleton and CDs. First, the formation of covalent linkage between the amino groups on CDs and anhydride moieties on g-IR was substantiated by FTIR (Figure 1a). With the addition of CDs to g-IR, the absorption peaks of anhydride moieties at 1865 and 1785 cm−1 decline mildly while the peaks of carboxyl and amide 3246
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Figure 2. (a) Recovery of samples stored at 80 °C for different durations in cyclic loading−unloading tests. (b) The work of the second loading− unloading cycle, W2nd, normalized by that of the first loading−unloading cycle, W1st, measured for samples stored for different durations at different temperatures. (c) Stress-relaxation profiles of g-IR samples with various CDs content at 25 °C. The samples for the test were extended to a strain of 100% and held for 15 min. Frequency (d) and strain (e) dependence of the storage modulus G′ and loss modulus G′′ for g-IR samples.
Scheme 1. (a) In g-IR-X Samples, CDs Serve as HFC for Incorporating Reversibly Interfacial Hydrogen Bonds and Covalently Bridging Chains between Adjacent CDs into a Chemically Sulfur-Cross-Linked Network; (b) Hydrogen Bonds Can Reversibly Detach and Re-Form during Loading/Unloading Test; after the Detachment of Hydrogen-Bonding Anchoring, Large Amounts of Energy Are Dissipated via Reversible Cross-Linking Mechanism; (c) after Further Stretching, the Relatively Short Chains That Covalently Bridged between Adjacent CDs Suffer from Rupture To Dissipate Energy through Chain-Fracture Mechanism, Accompanied by the Orientation of Long Chains
retards sufficient recovery by weakening the elastic resilience of the polymer chains.4,38 Therefore, storing the specimen at elevated temperature can induce the dissociation of hydrogen bonds, which can enable a faster spontaneous recovery. As shown in Figure 2a, the hysteresis gradually recovers to the first loading cycle with incremental storage time at 80 °C. Meanwhile, the recovery process also exhibits a temperature dependence that better recovery can be achieved at an elevated temperature with the same storage time. After storing at 80 °C for 20 min, 90% of the original hysteresis for cycle 1 is recovered (Figure 2b). The stress relaxation experiment was performed to further illustrate the energy dissipative capability of sacrificial hydro-
gen-bonding anchoring (Figure 2c). With the incorporation of CDs, the g-IR-x samples exhibit much faster stress relaxation rate in comparison with the blank g-IR. The relaxation rate is gradually improved with increasing content of CDs, which is proportional to the increment of hysteresis area in cyclic loading−unloading experiments (Figure S6). The superior ability of the g-IR-x system to dissipate energy also provided convincing evidence for the dissociation of transient hydrogen bonds upon deformation. Rheological measurements at room temperature were conducted to analyze the storage (G′) and loss modulus (G″) at various strains and frequencies in order to get insight into the network structure of the present systems. Frequency 3247
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Figure 3. Representative stress−strain curves of (a) g-IR-x and (b) g-IR-3-y samples. (c) Fracture toughness of g-IR-x and g-IR-3-y samples. (d) Comparison of g-IR-3 with literature data for filled composites based on IR or natural rubber, in terms of tensile modulus (stress at 300% strain), filler content, and variation of EAB (elongation at break).
sweep measurements (Figure 2d) reveal that the G′ for blank gIR exhibits a clear frequency-independent feature and is distinctly higher than G″ over the frequency range, which are typical characteristic of permanently cross-linked polymer.39 However, the incorporation of CDs leads to a comparable increase of G′, presumably due to the contribution of multiple networks to the higher cross-linking density. More importantly, the sacrificial hydrogen bonds have more pronounced effect on the frequency dependence of G″. As the frequency increases, an obvious increase in the G″ of g-IR-3 is observed due to the dissociation of transient hydrogen bonds under shear stress.4 In addition, the samples were also subjected to strain sweep test as shown in Figure 2e. For blank g-IR sample, both G′ and G″ slightly decline at higher strains, which is resulted from the softening and disentanglement of polymer chains upon shearing deformation. Noteworthy is that the G′ of g-IR-3 decreases more rapidly, leading to a crossover of G′ and G″ at the strain of 2.5%. In some reported systems with noncovalently cross-linked network, the crossover has been considered to be an indication of the collapse of the overall network structure to the liquid-like state. Beyond this critical stain, the systems become more viscous and fluid-like at higher strains, which proves the dissociation of the transient bonds.40−42 In the present system, the chemically cross-linked network structure in g-IR-3 should remain stable during the strain sweep test. However, as the strain amplitude increases, the rupture of transient hydrogen bonds will release the coiled chain lengths hidden from the applied load and facilitate the motility of the chain segments around the anchoring. Hence, the crossover of G′ and G″ may infer the local softening of the network due to the rupture of the sacrificial hydrogen bonds. A
similar phenomenon has been observed in some previous systems composed of both permanent and transient crosslinks.17,43−45 Mechanical Properties and Strain-Induced Crystallization. As for elastomeric materials, the foremost performance toward engineering applications is to possess robust mechanical properties. Though incorporating high-functionality CDs as both chemical and physical cross-linkers into g-IR, transiently hydrogen-bonding anchoring and covalent chain bridge are implanted between the rubber skeleton and CDs (Scheme 1a). These interlinkages can both serve as sacrificial units, which preferentially detach/rupture from CDs to substantially dissipate energy with different mechanisms. Upon deformation, the weak hydrogen bonds are first disabled due to the relatively low dissociation energy,35,46 which relaxes stretched chains and therefore dissipates mechanical energy relied on the reversible cross-linking mechanism (Scheme 1b). With larger deformation, the relatively short linkages that covalently bridge between adjacent CDs are highly stretched which end up with rupture to dissipate the stored energy relied on the chain-fracture mechanism (Scheme 1c). The rupture of these covalently bridges upon deformation is demonstrated by comparing the overall cross-linking density and stress relaxation behavior of the sample before and after prestretching (Figure S7). Both of the above energy dissipation mechanisms have been well studied and individually implemented to toughen hydrogels.5−7,10 It should be pointed out that due to the detachment/rupture of above sacrificial units, the coiled chains hidden from applied load will be released, and sustain larger deformation during the whole process, which may enable larger energy dissipation. Noteworthy is that the sulfur-cross-linked 3248
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Macromolecules network can still maintain the high elasticity and structural integrity of the sample even under large deformation. With the architecture design of multiple sacrificial units into a sulfur-cross-linked network, the obtained g-IR-x composites exhibited a dramatic increase in tensile modulus and tensile strength after the incorporation of CDs (Figure 3a and the corresponding mechanical data are tabulated in Table S2). For instance, g-IR-3 achieved approximately 2-fold increases in tensile modulus, tensile strength, and toughness, while the stretchability has a comparable improvement in the meantime. It is quite challengeable to integrate moduli, ultimate strength, and stretchability of elastomeric materials simultaneously. In general, the incorporation of nanoparticles can lead to a certain reinforcement on the mechanical properties of elastomeric materials but usually impair the extensibility because the strong particle−polymer chain interaction inevitably constrains the mobility of polymer network and then rigidifies the flexible matrix.47−49 The present strategy could be a practicable countermove to circumvent the problem. Compared with other IR composites filled with conventional fillers (40 phr carbon black, silica or silane-modified silica, respectively, as shown in Figure S8) and the reported composites based on IR or natural rubber (Figure 3d, detailed date are listed in Table S3), the present strategy provides a unique avenue to integrate strength, toughness, and intact stretchability in rubbers with a relatively lower filler content. Furthermore, to achieve effective reinforcement of the soft elastomer matrix, large numbers of conventional fillers have to be added, which usually tends to impair the viscoelasticity of the matrix. As shown in Figure S9, the storage modulus and loss tangent of the present system in the rubbery plateau region are almost unchanged in comparison with the blank sample. This indicates the addition of carbon nanodots has little effect on the viscoelasticity of elastomer matrix, which is in contrast to the composite filled with conventional carbon black. In addition, by regulating the structure parameters of chemically cross-linked network (sulfur content) and multiple sacrificial units (CDs content), mechanical performance for gIR/CDs composites can be facilely tuned. As the sulfur content decreases, the extensibility and toughness of g-IR-3-y successively increase while the modulus shows in a crosscurrent (Figure 3b,c). This should be attributed to the fact that the reduction of chemically cross-linking density lengthens the chain segments between two adjacent junctions, so that the loosely network can extensively be extended after the rupture of sacrificial units. In light of the above observations, the incorporation of CDs into g-IR has been demonstrated to be a facile yet efficient strategy to implement multiple energy dissipation mechanisms toward robust elastomers integrating toughness, modulus, and strength properties. As the parental material for g-IR, synthetic IR exhibits unique strain-induced crystallization (SIC) behavior which is account for its outstanding mechanical performance and has been extensively studied.50,51 The SIC capability is governed by multiple factors such as cross-linking network, fillers, strain rate, and temperature, etc.52,53 However, the effect of multiple sacrificial units on SIC has rarely been studied, and the corresponding mechanism is still far from being clearly understood. Hence, in situ synchrotron WAXD was performed to reveal the influence of multiple sacrificial units on the SIC behavior of g-IR-x. In this case, the diffraction intensity near the meridian was normalized and azimuthally integrated in a cake from 85° to 100° (inset in Figure 4a). As an example, the cake
Figure 4. (a) Meridional cake integrated intensity as a function of the 2θ taken from the WAXD pattern of the stretched vulcanized g-IR at α = 550%. Right inset shows integration limits from 85° to 100°. (b) Crystallinity index Xc of g-IR samples at room temperature as a function of strain and corresponding WAXD patterns of g-IR samples collected upon stretching.
integrated intensity as a function of 2θ for g-IR at α = 550% is shown in Figure 4a. The resultant profiles were deconvoluted involving the diffraction peaks of the 200 and 120 planes and the amorphous halo, respectively. The mass fraction crystallinity index Xc was evaluated by the equation Ac Xc = × 100% Ac + A a where Ac is the total area under crystalline diffraction peaks and Aa is the area under the amorphous halo. Figure 4b shows the typical WAXD patterns and calculated Xc of g-IR-x samples as a function of strain. As the sample is stretched, detectable crystalline diffraction of g-IR starts to appear at α = 400%, and the crystallinity index increases gradually to reach a maximum value of 16.4% at α = 550%. As expected, the onset strain of crystallization (αc) of the samples is relatively forward due to the introduction of multiple sacrificial units. The αc of g-IR-3 and g-IR-5 shift to 350% and 250%, respectively, which are comparable to those for the IR composites with conventional fillers.50,54 This is reasonable because the incorporated CDs serve as physical cross-linkers, giving rise to a covalent network that interlinks multiple chains with nonuniform lengths between each other. The relatively short linkages that covalently bridges between adjacent CDs are easier to be highly stretched and thus much prefer to orient to initiate the crystallization. The influence of the introduced interparticle 3249
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Figure 5. (a) FTIR spectra of the uncured g-IR-3 and g-IR-3′ after hot-pressing at 143 °C for 30 min. (b) Photographs of the swelling behaviors of the uncured g-IR-3 (upper) and g-IR-3′ (bottom) after hot-pressing at 143 °C for 30 min in tetrahydrofuran/methanol solution at various times. (c) Tensile loading−unloading curves of g-IR-3 and g-IR-3′. (d) The work of the second loading−unloading cycle, W2nd, normalized by that of the first loading−unloading cycle, W1st, measured for samples stored for different durations at 80 °C. (e) Stress−strain curves of g-IR-3 and g-IR-3′. Insets correspond to WAXD patterns of g-IR-3 (left) and g-IR-3′ (right) collected upon stretching.
reasonable to conclude that the introduction of multiple sacrificial units by using CDs as HFC is an alternative avenue to improve SIC, which is coincident with our previous study.19 Pivotal Effect of Covalent Linkages between Two Adjacent CDs on Overall Performance. We have already demonstrated herein before that multiple polymer chains covalently interlink two adjacent CDs by the addition reaction of primary amine groups and anhydride moieties. To specify the effect of these covalent linkages on the overall performance of g-IR-x samples, another kind of CDs (CDs-1) without primary amine groups on the surface was synthesized and then incorporated with g-IR (referred to as g-IR-3′) to suppress the above addition reaction. In the FTIR spectrum of CDs-1, only carboxyl and amide groups are observed, as indicated by the presence of typical absorptions around 1710 and 1565 cm−1, respectively (Figure S2). This is further confirmed by the result of the corresponding XPS spectra (Figure S3). As shown in Figure 5a, the absorption bands assigned to the grafted anhydride moieties at 1865 and 1785 cm−1 remain almost unchanged even after g-IR-3′ compound is subjected to hotpressing. These spectral results clearly demonstrate that the secondary amine groups of CDs-1 are unable to react with anhydride moieties on g-IR chains to generate covalent interlinkages. However, the abundant amide groups of CDs-1 were still capable to forming hydrogen-bonding blocks with the anhydride moieties on g-IR chains, which can be proved through the influence of solvent on the morphology of the compound. Figure S10 shows that the disc of g-IR-3′ compound with hot-pressing forms swollen gels in toluene due to the blocking of strong hydrogen bonds limits the extension of polymer chains in this aprotic toluene. However, in a tetrahydrofuran/methanol solution, the sample is gradually disassembled and finally dissolves (Figure 5b). This is because the strength of hydrogen bonds is drastically weakened,
covalent bridges on SIC is testified and discussed below. Meanwhile, the addition of fillers will generally increase the heterogeneity within the matrix which gives rise to a amplification effect on the local strain around filler particle and facilitates the nucleation of crystallite.50,53 However, it is widely accepted that the above heterogeneity also promotes the cavitation to suppress the crystallization through decreasing the effective volume for crystal growth, leading to a decline of the overall crystallinity.55,56 This seems contradictory to the results as shown in Figure 4b, which exhibits monotonous increase of crystallinity index along increasing CDs content. As mentioned above, the incorporation of CDs particles is bound to generate heterogeneous distribution of stress within the matrix, leading to an inevitable enlargement of local strain around the particles during stretching. Subsequently, the highly oriented and relatively short chains start to reorganize, and then the ordering of these chains certainly decreases the configurational entropy of the system and thus decreases the entropy of fusion. Thus, the melting temperature becomes higher than room temperature if the heat of fusion is independent of the deformation, which is the so-called “supercooling effect”.26,57 This phenomenon tends to induce a local volume decrease and then intensify the heterogeneity of stress distribution.56 Alternatively, the multiple sacrificial units in g-IR-x samples are certainly subjected to detachment or even rupture under such local deformation, which may sufficiently dissipate energy and serve to smooth out the stress distribution.58−60 Hence, the unzipping of sacrificial units can reduce the stress concentration which is favorable to cavitation and then eventually contribute to the crystal growth through energy dissipation mechanism. In addition, the release and orientation of coiled chain lengths hidden from the applied load by the sacrificial units are likely to reduce the amorphous chain portion in the random-coil-like state and increase the overall crystallinity. Therefore, it is 3250
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Macromolecules
stretching. The unzipping of multiple sacrificial units could smoot out the stress concentration and dissipate energy upon deformation, leading to a superior and integrated improvement of strength, modulus, and toughness together with intact stretchability. Meanwhile, the mechanical performance of g-IR could be conveniently tuned by adjusting sulfur and CDs contents. In addition, the SIC behavior of the present system was distinct from the polyisoprene composites filled with conventional fillers, in which forward onset strain of crystallization and promoted crystallinity were simultaneously accomplished. Because the relatively short chains covalently bridged between the adjacent CDs preferred to orientate and then initiated the crystallization upon smaller deformation. While at larger deformation, the detachment/rupture of multiple sacrificial units gave rise to substantial energy dissipation and the release and extension of coiled chain length hidden from the applied load. Accordingly, the overall crystallinity was also promoted, which also accounted for the outstanding mechanical performance. The present elastomer can be defined as hybrid-cross-linked system which the chemical network ensured the elasticity and the multiple sacrificial units enabled the energy dissipation with different mechanisms. We envisaged that this strategy might provide a new avenue to design robust elastomer by integrating multiple energy dissipation mechanisms.
resulting in the complete disintegration of the bulk. In contrast, g-IR-3 compound with hot-pressing only exhibits a distinct swelling, regardless of the solvent used, further confirming the high-functionality CDs can act as chemical cross-linkers that gives rise to multiple covalent linkages between each other. Therefore, as the samples are deformed, relatively short chains may be ruptured from the CDs surface, serving as another kind of sacrificial units to enhance the overall energy dissipation through chain-fracture mechanism. As expected, g-IR-3 achieves a definite improvement in hysteresis compared to g-IR-3′ in cyclic loading−unloading tests (Figure 5c). The time dependence of hysteresis ratio at 80 °C indicates that the introduction of such covalent linkages remarkably improves the recovery process (Figure 5d). This is probably related to competition between the elasticity of the covalent network and the strength of the re-formed hydrogen bonds in the sample. The incorporation of CDs allows for additional covalent crosslinking and thus enhances the elastic contraction of the chain skeleton, leading to quick recovery.35 In addition, to further uncover the contribution of different sacrificial units to the mechanical properties, the SIC behaviors of vulcanized g-IR with different CDs were investigated. Figure 5e shows the typical stress−strain curves of g-IR with different CDs and selected WAXD patterns at various strains. Obviously, g-IR-3 exhibits a relative and integrated increases in strength, modulus, and toughness compared to those of g-IR-3′ (Table S2). It is worth noting that the discernible crystalline diffraction maxima cannot be detected until α = 375% for g-IR-3′. This provides convincing evident for the above conclusion that the relatively short chains covalently bridged adjacent CDs in g-IR-3 prefer to orientate at relatively smaller strain. Meanwhile, the subsequent increase of crystallinity index of g-IR-3′ is relatively lower than that of g-IR-3 (Figure S11). At larger strain, the multiple covalent linkages with nonuniform lengths can serve as another kind of sacrificial unit that may rupture to reduce stress concentration and thus promote the crystallinity. In conclusion, the SIC behavior of present system is closely related to the existence of multiple sacrificial units. During stretching, the multiple sacrificial units could preferentially detach/rupture from carbon nanodots to dissipate energy, leading to an initial reinforcement of the soft matrix (as indicated by the increase of tensile modulus) at small strain. Subsequently, the orientation of short covalent linkages and coiled chain-length hidden from the applied load give rise to distinctive SIC behavior (forward onset strain of crystallization together with promoted overall crystallinity) of the system, which is believed to contribute to the prominent enhancement of mechanical performance at larger deformation.25,50,54
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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.7b00483. HRTEM image, FTIR and XPS spectra of CDs, recovery of the g-IR-3 specimen, tensile loading−unloading curves of g-IR samples, comparison of the stress−strain curves for the sample with conventional fillers, cross-linking density, stress relaxation and dynamic mechanical profiles of g-IR samples, swelling behaviors of the uncured g-IR3, crystallinity index Xc of g-IR samples with different CDs, calculated gel fractions and detailed mechanical properties of g-IR samples (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel +86 20 87113374; Fax +86 20 22236688 (B.G.).
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ORCID
Baochun Guo: 0000-0002-4734-1895
CONCLUSIONS In summary, we have described a unique strategy to achieve robust elastomer with integrated mechanical properties by implementing multiple energy dissipation mechanisms into a chemically cross-linked architecture network. The incorporated high-functionality CDs served as both physical and chemical cross-linkers, which gave rise to interfacial hydrogen bonds and a covalent network that interlinked multiple chains with nonuniform lengths. Both of the interlinkages could serve as sacrificial units which preferentially detached/ruptured from CDs to dissipate substantial energy via different mechanisms. The dynamic nature of the hydrogen-bonding anchoring allowed them to rupture and re-form which endowed the gIR sample with significant reversible recovery during cyclic
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (2015CB654703), National Natural Science Foundation of China (51673065, 51333003, U1462116, and 51473050), and Natural Science Foundation of Guangdong Province (2014A030310435 and 2014A030311051). The authors thank beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time and the help from Dr. Yi Liu (Shanghai Inst. Appl. Phys., CAS) during experiments. 3251
DOI: 10.1021/acs.macromol.7b00483 Macromolecules 2017, 50, 3244−3253
Article
Macromolecules
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