Unveiling Reinforcement and Toughening Mechanism of Filler

Article pubs.acs.org/Macromolecules

Unveiling Reinforcement and Toughening Mechanism of Filler Network in Natural Rubber with Synchrotron Radiation X‑ray NanoComputed Tomography Liang Chen, Weiming Zhou, Jie Lu, Jing Li, Wenhua Zhang, Ningdong Huang, Lihui Wu, and Liangbin Li* National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China S Supporting Information *

ABSTRACT: Double network structure constructed with filler network of carbon black and molecular network of natural rubber possesses excellent toughness and strength. However, due to lack of proper in situ imaging techniques to detect the structural evolutions under loading, the reinforcement mechanism of filler network is still under debate. Here in situ synchrotron radiation X-ray nano-computed tomography with high spatial resolution (100 nm) is employed to study structural evolution of carbon black in a large volume of natural rubber matrix. For the first time, strain-induced deformation, destruction, and reconstruction of filler network are directly observed under cyclic loading. Combining mechanical test, the reinforcing and toughening effect of filler network is quantitatively assigned to three mechanisms, namely elastic deformation, destruction, and friction of filler network. Elastic deformation mainly occurs at low strain for energy storage, while network destruction plays the dominant role at larger strain to dissipate strain energy. Additionally, friction is another energy dissipation mainly at low strain. between the aggregates and rubber matrix.17,18 Although the importance of CB network is well recognized in academy and industry, its reinforcing and toughening mechanism on rubber matrix is not fully understood. According to the empirical view, besides the strain-induced nanocavitation and crystallization in rubber under loading,19,20 the hard CB network may be similar to the weak network in double network hydrogels, which can also experience breakage and dissipate strain energy.21,22 To this end, finding the filler network in a rubber matrix, especially breakage and reaggregation of filler aggregates under loading, is the key to understanding the energy dissipation and reinforcement mechanism of filled rubber.23−26 Unfortunately, because of multiscale nature of filler structures (from several tens of nanometers up to several micrometers26) and the shortages of some techniques like transmission electron microscopy (TEM) with small imaging field (about 1 × 1 μm2)27−29 and visible light microscopy with poor spatial resolution, the attempts of directly viewing filler network in large volumes are still not successful, not to say the direct evidence of filler network destructing and reconstructing under loading. In this study, aiming to unveil the reinforcement and toughening mechanism of filler network, we employ the

1. INTRODUCTION To maximize the mechanical performance of polymeric materials, mixing different components together to create hierarchical structures is the most widely adapted approach in polymer industry as well as in nature.1−3 Double network structure constructed with either two polymeric chains or polymer and inorganic nanoparticles has been demonstrated to enhance toughness and strength of soft and brittle materials in orders of magnitude, which has been actively studied in the past 10 years. One example is hydrogels containing two polymeric networks with short and long mesh sizes4,5 or weak and strong bonds,6,7 respectively. When under stretch, the strongly bonded network keeps the overall shape of hydrogel intact, while the weakly bonded network with small mesh size or weak ionic bonds destructs and reconstructs dynamically to dissipate energy, enhancing the toughness and stretchability of polymer composites dramatically. Analogous to polymer−polymer double network, dispersing nanoparticles in polymer matrix can also create double network structures,8−10 which is a standard practice in rubber industry since its dawning era. An important example is natural rubber filled with carbon black (CB), which is irreplaceable in some key areas such as heavy track and airplane tires.11,12 Here the primary network is the cross-linking structure of rubber chains,13,14 while the other is filler network constructed with aggregates of CB particles2,15,16 as well as the interactions © XXXX American Chemical Society

Received: June 16, 2015 Revised: September 11, 2015

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the Beijing Synchrotron Radiation Facility (flux: 2 × 1012 Phs/s; view field: ∼60 × 60 μm2; spatial resolution: 100 nm) as described elsewhere.32 Briefly, the radiation from wiggler was first monochromated by a double-crystal monochromator. The selected quasimonochromatic X-rays were then focused by an elliptical capillary condenser. Coupled with a zone-plate optics, the whole system can perform absorption or phase contrast imagings with X-ray energies from 7 to 11 keV and photon fluxes about 1 × 1010 at 8 keV. In experiments, the rectangular-shaped rubber samples were first mounted between two clamps of a homemade miniature tensile tester which could draw thin films with different ratios. The samples were first imaged before stretch, then loaded to certain ratio with low stretching speed of 30 μm/s, kept still, and again imaged with the same method, and finally the samples come back to original under unloading and continued imaging. A series of 2D projections of samples were acquired at tilt angles ranging from −70° to +70° with an interval of 0.3° by the rotation of sample stage with X-ray energy of 8 keV. The exposure time of the 2D projection is 10 s every time. The imagings were then undergone subsequent processing for three-dimensional segmentation, reconstruction, and analysis as described in the Results and Discussion.

synchrotron radiation X-ray nano-computed tomography (Nano-CT) technique to study the destructing and reconstructing of CB network in natural rubber under cyclic loading. The high penetrating ability of X-rays coupled with mild sample imaging environment allows X-ray Nano-CT to in situ follow structural evolution in large volume with high spatial resolution.30,31 Based on the detailed structural information on CB network probed with X-ray nano-CT, the imposed tensile energy can be quantitatively attributed to elastic deformation and destruction of network and friction between filler network and rubber matrix. The experimental details on sample preparation and X-ray imaging measurement are presented in the Experimental Procedure section. According to structural information on filler network including elastic deformation, network destruction, and connectivity, we quantitatively evaluate the contributions of different toughening and reinforcing effects of CB on natural rubber.

2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. The natural rubber (NR) used in the study was ribbed smoked sheet (RSS) No. 1 from Indonesia, which is purchased from the Chinese Academy of Tropical Agricultural Science. The recipe and cure conditions for preparation of the rubber sample are shown in Table 1. The compounds were prepared using an internal

3. RESULTS AND DISCUSSION Because of the limitation of focal depth of X-ray imaging system, the samples were compressed to thin films with thickness of 50 μm. For X-ray imaging experiments, a series of 2-dimensional projections with tilt angles ranging from −70° to +70° and an interval of 0.3° were first constructed to three-dimensional imagings. Following the method of previous researcher,9 the three-dimensional images were segmented and reconstructed by Amira 5.3.0 by TGS Inc.,33 that different components in the rubber composites were given with different colors according to the contrast values, where red zones represent CB aggregates (with highest contrast values), blue zones represent natural rubber, and yellow zones represent pores in the rubber composites. During 3D reconstruction, the threshold chosen is very important to determine various components in the matrix. In order to find the proper threshold for reconstruction, first we have done several imaging experiments with known volume proportions of CB in a rubber matrix, and the volume values were compared with those calculated from the reconstruction results according to the threshold chosen. Then the threshold is modified and determined until the calculated values are very close to the actual values.34 Figures 1a,b show a representative imaging of CB dispersing, where the rubber matrix is made transparent. CB nanoparticles aggregating into aggregates with sizes from 100 to 1500 nm are observed in the rubber matrix. Moreover, the aggregates with rich branches on the surfaces present various shapes, such as

Table 1. Recipes and Cure Conditions of Vulcanized NR ingredients

loading level (phra)

natural rubber stearic acid ZnO accelerator TTb accelerator DTDMc accelerator DMd sulfur curing timee (min) CB

100 2 1 0.2 0.4 0.5 1 6 30

a

Parts by weight per hundred parts rubber (phr). bTetramethylthiuram disulfide. c4,4′-Dithiodimorpholine. dN-Cyclohexyl-2-benzothiazolesulfenamide. eCure temperature was 143 °C. mixer with an initial warming up during mastication and then were cured to a film with thickness of about 50 μm by a compression molding machine at 145 °C and 15 MPa for 15 min. The films were then cut into rectangular-shaped specimens with length and width of 15 and 3 mm, respectively, for uniaxial tensile testing and X-ray imaging. 2.2. X-ray 3-Dimensional Imaging. The synchrotron radiation X-ray imaging experiments were carried out on the 4W1A beamline at

Figure 1. (a) Highlighted dispersion of CB aggregates in the rubber matrix before stretching. Various structures with rich branches on the surface can be found. (b) Partial enlarged dispersion of CB aggregates in three-dimensional spaces. (c) Imagings of CB aggregates dispersing in rubber matrix at different strains and the corresponding engineering stress−strain curve. B

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Figure 2. (a) Size distributions of the aggregates before stretch. (b) Size distributions of CB aggregates counted from five slices. (c) Content of aggregates with sizes smaller than 400 nm (PS, ● and ○ for loading and unloading) and larger than 800 nm (PL, ■ and □ for loading and unloading) during cyclic loading. (d) Destructing (DR, ■) and reconstructing (CR, ●) ratios during loading and unloading, respectively.

Figure 3. Step of filler network construction. A small zone of filler network was constructed along the backbone of CB aggregates by ImageJ software and then dyed to red by Amira 5.3.0.

were obtained from statistical results rather than the direct changing observed the same spot during stretching. In order to eliminate the uncertainty during data statistics and reflect the changing trends of CB aggregates destruction and reformation under cyclic loading, every time five slices were selected randomly, and the average results were obtained (as shown in Figure 2b), where the error bars were recalculated to be about ±5% as indicated in Figures 2c and 2d. Figure 2c depicts PS and PL at different strains on loading. Upon loading, PS shows a monotonic increase. At strain of 0, PS is about 47.3%, which reaches to 68.2% at strain of 4. Meanwhile, PL decreases from 14.9% at 0 strain to 1.2% at strain of 4. The synchronous changing trends of PS and PL reveal that aggregates destructing occurs under loading, during which stress induces large aggregates splitting into small ones. After stretched to different strains, the samples are unloaded back to 0 strain and X-ray imagings are taken again. PS and PL from the unloaded samples are also plotted in Figure 2c, where the x-axis is the initial loading strains. After unloading, PS reduces back partially, which indicates the reaggregation of carbon black aggregates during unloading. For quantitative estimation on destructing and reconstructing of aggregates during cyclic loading, the relative variation of PS is selected as an indicator. The destructing ratio is defined as DR = (PSε − PS0)/PS0, where PSε and PS0 are the contents of aggregates with size smaller than 400 nm at strains of ε and 0, respectively. In analogue, the reconstructing ratio is written as CR = 1 − (PSε0 − PS0)/PS0, where PSε0 is the content of aggregate sizes smaller than 400 nm after loading to ε and unloading back to 0. As shown in Figure 2d, DR and CR increase and decrease with strain, respectively,

semilunar, cylindrical, elliptical, and spherical. Meanwhile, the imagings of CB aggregates dispersing in rubber matrix at different strains and the corresponding engineering stress− strain curve are listed in Figure 1c, which shows the increase of interaggregate distance as the strains increasing. In the following sections, strain-induced destructing and reconstructing at aggregates and filler network levels are analyzed, respectively. 3.1. Carbon Black Aggregates Destruction and Reconstruction under Cyclic Loading. In the counting of size distributions of CB aggregates in the rubber matrix, one slice of the reconstruction imaging was analyzed by ImageJ software35 with the assumption that aggregates are spherical, and then the diameter distributions of aggregates can be obtained. Figure 2a is the size distribution of CB aggregates before loading. The sizes of aggregates are mainly in the range of 100−400 nm and occupy about half of the total counted results, while large ones with sizes more than 800 nm take a minor fraction. Thus, the contents of aggregates with sizes smaller than 400 nm and larger than 800 nm (denoted as PS and PL) are selected to evaluate the destructing and constructing of aggregates during loading, respectively. The X-ray imaging is carried out on samples with cyclic strains of 0− 1−0, 0−2−0, 0−3−0, and 0−4−0. After being elongated to predetermined ratios, the samples were placed for relaxation for half an hour to decrease the relaxation causing structure changing after loading.36 Because of space restriction of the Xray imaging system, it is not possible for us to install a miniature tensile device capable to trace the same position for 3D imaging measurements during stretching; in the experiments, the data C

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Figure 4. Constructed filler networks during loading and unloading along the backbone of CB aggregates from one slice of reconstruction imaging: (a) is the filler network before stretch, and (b)−(e) are networks at strain of 1−4 while (b′)−(e′) are the networks of unloaded samples after stretched to respective strains. From the figures, we can see the filler networks breaking down on loading and partially recovering with stress relief.

at filler network level decreases with strain due to more severe destruction at larger strain (see Figure 4b′−e′). For quantitative evaluation on deformation, destruction, and reconstruction of network, the interdistance of the grids (mesh size) is analyzed at different strains under cyclic loading. The grid interdistance was obtained by ImageJ software by counting the grid diameters of the network. The average mesh sizes parallel and perpendicular to tensile direction are calculated (denoted as D∥ and D⊥), respectively, which are plotted vs strain as shown in Figure 5a. At 0 strain, D∥ and D⊥ are nearly

indicating that larger strains cause more destruction of CB aggregates. Moreover, the decrease of CR with strain suggests that more severe destruction occurring at large strains, which leads to lower recovering capability of CB aggregates during unloading. The above analysis demonstrates that CB aggregates can perform as dynamic bonds as that in double network hydrogels, which dissipate strain energy through destructing and reconstructing during loading and unloading. 3.2. Filler Network Structure Evolutions under Cyclic Loading. In large scale, CB aggregates form a threedimensional network, which undergoes deformation, destruction, and reconstruction during cyclic loading. Based on X-ray imaging during cyclic loading, the pseudo CB networks are constructed along the backbone of CB aggregates from reconstruction imaging. Briefly, a slice of the imaging was converted to binary imaging using ImageJ software; then the aggregates were skeletoned and dyed by Amira 5.3.0 software, and the filler network associating with the aggregate dispersing was obtained (as shown in Figure 3). Figure 4a shows CB network at strain of 0, and (b)−(e) are networks of samples at strains from 1 to 4 on loading while (b′)−(e′) are networks of samples unloaded back to 0 after stretched to respective strains. At 0 strain, X-ray imaging shows that nearly full-connected CB network forms in rubber matrix. Upon loading, filler network undergoes breakages and large strain causes more severe destruction (see Figure 4b−e). As shown in Figure 4e, the integrity of CB network is drastically reduced at strain of 4. After unloading back to 0 strain, filler network experiences partially recovering with stress removed. In line with the capability of aggregate reaggregation in unloading, the recovery

Figure 5. (a) Average mesh size of network parallel and perpendicular to tensile direction during cyclic loading. Loading (D∥, ■, and D⊥, ●) and unloading (D∥0, □, and D⊥0, ○). (b) Filler network connectivity during loading (◀) and unloading (▶).

the same with sizes of 966 and 977 nm, respectively. Upon loading, D∥ increases with strain monotonically up to 1831 nm at strain of 4, while D⊥ decreases to 830 nm at strain of 1 and then increases back at large strains and finally reaches 1430 nm at strain of 4. At low strain (below 1), the opposite evolution trends of D∥ and D⊥ indicate that CB network mainly suffers elastic deformation. Tensile force in drawing direction leads to D

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Macromolecules an increase of D∥, which is accompanied by a shrinkage of lateral dimension D⊥. At strain of 1, the drawing ratio λ∥ = 1 + (D∥ − D0)/D0 of CB network in tensile direction is 1.339, where D0 is the average mesh size at 0 strain. Taking the assumption of incompressibility,37 the shrinkage in lateral direction is λ⊥ = 1/√λ∥ = 0.864, which is unexpectedly in good agreement with the experimentally measured value (λ⊥ = 1 − 0.150 = 0.850). Nevertheless, CB network does not follow the macroscopic deformation of rubber composite or the rule of affine deformation.19,20,38,39 The macroscopic strain is 1, while CB network suffers a strain of 0.339, indicating that either sliding between rubber matrix and filler network or destructing of filler network does occur. Further increasing strains larger than 1, the destruction of CB network, instead of elastic deformation, plays a more dominant role, which leads to two or more grids combining together to form a new large grid and causes D∥ and D⊥ increasing simultaneously. After samples unloaded back to strain 0, the filler network mesh sizes parallel (D∥0) and perpendicular (D⊥0) to drawing direction are also plotted in Figure 5a. Here the x-axis is the strain for cyclic drawing while the data are collected at strain 0 after unloading. D∥0 is significantly lower than the corresponding D∥ obtained from samples under loading, indicating the occurrence of both reconstruction and elastic recovery of deformed and destructed network. The process of network destruction and reconstruction during cyclic loading is also evidenced by D⊥0 lower than D⊥ with large strains of 3 and 4, as pure elastic deformation would lead to opposite results. After unloading from large strains, lower D⊥0 than D⊥ is attributed to reconstruction of initially destructed grids, which leads to one large grid being divided into two or more grids and resulting into smaller average mesh size. While cyclic loading at low strains (below about 2) D⊥0 is larger than D⊥, demonstrating that partially elastic deformation indeed takes place during loading. Comparing mesh sizes under loading (D∥ and D⊥) and unloading (D∥0 and D⊥0) during cyclic tensile test at small and large strain regions, evidently both elastic deformation and destruction of filler network are induced by external loading, during which elastic deformation and destruction mainly takes place at small and large strain regions, respectively. This conclusion seems in line with our intuitive expectation on elastic filler network but not being directly observed before. 3.3. Filler Network Connectivity under Cyclic Loading. In addition to the average mesh size, network connectivity is also an important parameter to describe destructing and reconstructing of filler network during cyclic loading, which can be obtained with X-ray imaging analysis. With the aid of imaging analysis from X-ray nano-computed tomography, we obtain the size distributions ξi and dAve (the average diameter of CB aggregates). According to the face-centered arrangement of interparticles distance of dc ≈ (0.86ϕeff−1/3 − 1)dAve,40 we can consider that aggregates with sizes larger than dc will keep the network connected. The network connectivity at different strains can be calculated (provided in the Supporting Information) and plotted in Figure 5b. Before stretching, the filler network connectivity is about 95.2%, which decreases to about 80.6% at strain of 1 and finally about 39.1% at strain of 4. Unloading strain back to 0, the connectivity partially recovers to a higher value than that at the respective strains, which also reveals filler network destructing on loading and partially recovering with strain relief. 3.4. Energy Dissipation by Filler Network Evolution. With the above structural information on aggregate and

network levels during cyclic loading, we are able to analyze the reinforcing and toughening mechanism of CB filler on natural rubber quantitatively. The enhancement factor (RE) is defined as RE = (Wcom − Wrub)/Wrub, where Wcom and Wrub are the work done in tensile test on filled rubber composite and pure rubber, respectively (see Supporting Information, Figure 1). RE are about 10 with 30 phr N330 CB as plotted vs strain in Figure 6. We attribute RE to three different structure evolutions

Figure 6. Enhancement factor RE of CB filler network on rubber and the corresponding contributions of elastic deformation (Φnet−def), destruction (Φnet−des), and friction (Φnet−fri) at different strains.

of CB network, namely elastic deformation, destruction, and friction between network and rubber matrix, where elastic deformation stores energy while the latter two dissipate energy during tensile deformation. Combining structure information (including content of filler (ϕ), network connectivity (Pnet), average aggregate size (dave) and network mesh size (D∥ and D⊥)) and stress−strain information during cyclic loading, we calculate the contributions of elastic deformation (Φnet−def), destruction (Φnet−des), and friction (Φnet−fri) on RE according to equal-stress model,41 which are plotted in Figure 6 vs strain. The detailed calculation process is presented in the Supporting Information. Loading strain of 1, elastic deformation (Φnet−def), and friction (Φnet−fri) take the major share to dissipate strain energy, while filler network destruction (Φnet−des) plays a weak role in energy dissipation. Increasing strain, destruction (Φnet−des) increases almost linearly, which switches its role as the major contributor for toughening and reinforcement, while elastic deformation and friction contribute less energy dissipation. Nevertheless, though the contributions of three reinforcement and toughening mechanisms vary with strain, the synergistic effect of elastic deformation, destruction, and friction of CB network tune the enhancement factor RE varying in a narrow range from 10 to 17. Note natural rubber experiences stress-induced crystallization at large strains,42,43 which will shift crystallization critical ratios to smaller strains with the presence of filler network comparing to that in pure rubber.19 Thus, though we take the mechanical data from unfilled natural rubber as the reference, small deviation may still exist at larger strains. At strain lower than 2 before crystallization occurring, the attribution of energy is relative strict, while at large strain the complication of strain-induced crystallization is not taken into account yet, and the contribution of filler network may be slightly overestimated.

4. CONCLUSIONS Cyclic loading induced destructing and constructing of CB aggregates in natural rubber composite were studied by X-ray nano-computed tomography, and the corresponding pseudo filler network was constructed. On the basis of the CB network E

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structural information and mechanical properties, we attribute the reinforcement and toughening of CB network on rubber matrix to elastic deformation, destruction, and friction quantitatively. At low strain, elastic deformation and friction are the main contributors, while larger strain leads to more destruction of network, where energy dissipated by destruction plays the major role on toughening. Though the dramatic reinforcement of mechanical properties has been achieved in various double networks constructed with two polymers or polymer and inorganic fillers, the roles of different toughening and reinforcement mechanisms have not been analyzed quantitatively before due to lacking of microstructural information. With the aid of X-ray nano-computed tomography, current work provides a case study on reinforcement and toughening mechanism of nanofiller, which can be extended to other systems and help to explore the structure−property of composite.

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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.5b01301. Supplementary Figures 1−9 and Tables 1−4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (51325301, 51473151, 51227801, 51573177, 51120135002) and the Project 2013BB05 supported by NPL, CAEP. The synchrotron beamtime from National Synchrotron Radiation Lab (NSRL) and Beijing Synchrotron Radiation Facility (BSRF) is acknowledged.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b01301 Macromolecules XXXX, XXX, XXX−XXX