ZnO Interfaces by

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Nanoimaging and Spectroscopic Analysis of Rubber/ZnO Interfaces by Energy-Filtering Transmission Electron Microscopy Shin Horiuchi* Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan

Hidehiko Dohi SRI Research & DeVelopments Ltd., 2-1-1, Tsutsui-cho, Chuo-ku, Kobe 651-0071, Japan ReceiVed August 24, 2005. In Final Form: February 19, 2006

Energy-filtering transmission electron microscopy (EFTEM) was employed for investigating interactions between rubber and ZnO particles in the accelerated vulcanization process. Combining elemental mapping and electron energy loss spectroscopy (EELS) by EFTEM enabled the characterization of the interfaces with spatial resolutions of less than 10 nm and with high elemental detection sensitivity. We found that a sulfur- and zinc-rich compound was generated around ZnO particles, and that product was then revealed to be ZnS-generated as a byproduct in the accelerated vulcanization process. Through this study, it is indicated that the accelerated vulcanization with ZnO does not occur uniformly in the rubber matrix; it occurs locally around ZnO particles at a higher reaction rate, implying that the rubber network structure is not uniform on the nanoscale.

Introduction Studies on interfaces of polymer/polymer and polymer/ inorganic substance are of great importance to polymer science and industry. Interfaces play major roles in determining numerous material properties such as mechanical, adhesion, transportation, and optoelectrical. Compared to surface analysis, there have been limited techniques that can be applied to interfacial characterization. To employ conventional techniques for interface characterization such as neutron reflectometry, ellipsometry, and secondary ion mass spectrometry, samples must be constructed as bilayers with pure materials. Deuterated or labeled samples are in some cases required for measurement. Therefore, those conventional techniques cannot be directly and practically applied to materials used in industry, and thus a technique that enables the characterization of interfaces without special sample setups is desired. We have been employing EFTEM for the characterization of nanostructures in polymers and composites.1-5 EFTEM is a powerful tool for investigating material nanostructures.6-8 It allows us to create elemental distribution images by electron spectroscopic imaging (ESI) and to perform a quantitative chemical analysis by electron energy loss spectroscopy (EELS).9 * Corresponding author. E-mail: [email protected]. Tel: +81-29861-6281. Fax: +81-29-861-4773. (1) Horiuchi, S.; Hanada, T.; Yase, K.; Ougizawa, T. Macromolecules 1999, 32, 1312. (2) Horiuchi, S.; Ishii, Y. Polym. J. 2000, 32, 339. (3) Horiuchi, S.; Fujita, T.; Hayakawa, T.; Nakao, Y. Langmuir 2003, 19, 2963. (4) Horiuchi, S.; Hamanaka, T.; Aoki, T.; Miyakawa, T.; Narita, R.; Wakabayashi, H. J. Electron Microsc. 2003, 52, 255. (5) Horiuchi, S.; Yin, D.; Ougizawa, T. Macromol. Chem. Phys. 2005, 206, 725. (6) Wang, Z. L. In Characterization of Nanophase Materials; Wang, Z. L., Ed.; Wiley-VHC: Weinheim, Germany, 2000; p 37. (7) Costa, C. A. R.; Leite, C. A. P.; de Souza, E. F.; Galembeck, F. Langmuir 2001, 17, 189. (8) Rom, I.; Hofer, F.; Bucher, E.; Sitte, W.; Gatterer, K.; Fritzer, H. P.; Popitsch, A. Chem. Mater. 2002, 14, 135. (9) Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer: Berlin, 1995.

For materials with high resistivity against electron beams such as metals or semiconductors, elemental analysis with a spatial resolution of less than 1 nm has been achieved.10,11 EFTEM has also been employed for the characterization of polymer/polymer interfaces.1,5,12,13 However, the instability of polymer specimens against electron beams limits high-resolution nanoanalysis by EFTEM. Polymer specimens have been known to suffer from the cleavage of chemical bonds, loss of mass, deformation, and drift of specimens by the irradiation of electron beam. These limit high-resolution and high-precision quantitative analysis.14 Although it is not possible to avoid such specimen damage completely, we have achieved a spatial resolution of less than 10 nm for the analysis of polymer/polymer interfaces in bilayer samples by minimizing specimen damage.5 Next, we are working on the investigation of “buried” interfaces in polymer matrixes by EFTEM, which appear in polymer composites and blends. It has been difficult to approach such interfaces by conventional characterization techniques because the interfaces are randomly located within the polymer matrix. In this article, we report, as our first attempt, an analysis of the interfaces between rubber and ZnO particles in vulcanized rubber mixtures by EFTEM. ZnO has recently attracted considerable attention with respect to applications to photonic and electronic devices. However, it has also been used as a functional filler in polymer composites. In the early part of the 20th century, it was found that vulcanization is accelerated by certain organic compounds, which are called accelerators, and that ZnO combined with stearic acid (StAc), which is called an activator, was found to reduce the vulcanization time and improve the properties of (10) Hofer, F.; Warbichler, P. In Transmission Electron Energy Loss Spectroscopy in Material Science and the EELS ATLAS, 2nd ed.; Ahn, C., Ed.; Wiley-VCH: Weinheim, Germany, 2004; p 159. (11) Wang, Z. L. AdV. Mater. 2003, 15, 1497. (12) Tremblay, A.; Tremblay, S.; Favis, B. D.; Selmani, A.; L’Espe´rance, G. Macromolecules 1995, 28, 4771. (13) Siangchaew, K.; Libera, M. Macromolecules 1999, 32, 3051. (14) Reimer, L. Transmission Electron Microscopy, 4th ed.; Springer: Berlin, 1997; p 463.

10.1021/la052308f CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006

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Table 1. Rubber Mixtures Investigated in This Study sample no.

SBR (g)

ZnO (g)

S8 (g)

acceleratora (g)

StAc (g)

1 2 3 4 5

100 100 100 100 100

3 3 3 3 3

1.5 1.5 0 1.5 1.5

1.5 1.5 1.5 0 0

2.0 2.0 2.0 2.0 0

a

vulcanization 170 °C for 10 min 170 °C for 10 min 170 °C for 10 min 170 °C for 10 min

N-tert-Butyl-2-bensothiazolyl-sulfinamide (TBBS).

rubber.15 Rubbers used in industry are composite materials involving complicated network structures with various kinds of organic and inorganic substances. Therefore, various kinds of interfaces are involved in the rubber materials and play important roles in determining their properties. Although vulcanization is a well-established chemical process, a clear understanding of the complex reaction schemes and the resulting network structures continues to be a challenge at this fundamental level. The crosslinked structures are formed in a complicated sequence of reactions, and it is even difficult to find persuasive evidence for the kind of mechanisms involved. Throughout this study, we also evaluated the spatial resolution, elemental detection sensitivity, and quantitative accuracy of the elemental mapping along with the accompanying EELS analysis for polymer/filler interfaces. Experimental Section All materials used in this study were commercial products and were used as received. An emulsion-type styrene-butadiene rubber (SBR) was used as a rubber material, which was supplied by Zeon Corp. (Japan) as a grade name of SBR1502. Sulfur (S8) was a 200mesh powder form supplied by Tsurumi Chem. Co., Ltd. (Japan). ZnO was standard grade with an average particle size of 270 nm and was supplied by Mitsui Mining & Smelting Co., Ltd. (Japan). StAc and N-tert-butyl-2-benzothiazolyl-sulfenamide (TBBS) used as an accelerator were supplied by NOF Corp. (Japan) and by Ouchi Shinko Chem. Ind. Co. (Japan), respectively. The rubber mixtures were compounded in an 8 in. open roll mixer according to the formulation shown in Table 1. The open roll was cooled by circulating water with a temperature of 50 °C. Mixing was continued for 5-10 min until no aggregates larger than about 0.5 mm in diameter could be seen in the stretched films of the mixtures. The uniformity of the mixtures thus prepared was also confirmed by the TEM observation, which revealed that micrometer-scale aggregates of sulfur and/or the accelerator did not exist in the rubber. Then, the compounded mixtures were thermally treated at 170 °C for 10 min in a hot press machine for vulcanization. Industrial mixtures, used in sulfur vulcanization, are rather complicated because they contain more than 10 different components as fillers or additives. In this study, we investigated rubber/ZnO interactions during the vulcanization process with the simple rubber mixtures containing ZnO, sulfur, an accelerator, and an activator. To investigate the changes in the interface by vulcanization, we studied the mixtures before and after vulcanization that are labeled in Table 1 as samples 1 and 2. To study the effects of a specific component on vulcanization, a set of mixtures (samples 3-5) were prepared with intentional absence of certain compounds in each case as shown in the Table. Thin sections of the samples, about 100 nm thick, were prepared by cryoultramicrotomy at -60 °C and collected on a 600-mesh copper grid for EFTEM analysis. An LEO922 in-column-type energy-filtering transmission electron microscope with a LaB6 cathode and equipped with an Omega-type energy filter was used at an accelerating voltage of 200 keV. The detailed instrumental setup was described in our previous papers.4,5 All observations were carried out cryogenically at 120 K using a (15) Krejsa, M. R.; Koenig, J. L. In Rubber Technology Handbook; Cheremisnoff, N. P., Ed.; CRC Press: Boca Raton, FL, 1993; Chapter 11, p 476.

Gatan 613-DH single-tilt liquid-nitrogen-cooled holder to minimize radiation damage to the specimens. To obtain high-resolution elemental maps and to perform quantitative chemical analysis, we employed an Image-EELS technique. Figure 1 shows the scheme of the process employed in this study. First, a set of energy-filtered images were recorded sequentially across a wide range of energy loss to construct a 3-D data set containing spatial information, I(x, y), and spectral information, I(E). The range of energy loss, the energy width for each image, and the energy increment between the neighboring images depended on the element of interest. For sulfur and oxygen, the energy width and the energy increment were set at 5 and 3 eV, respectively. For zinc, the energy width was set at 10 eV, and the energy increment was set at 5 eV. Image-EELS enables us to acquire EELS spectra from areas of interest in any shape in an image.4,5,10,16-20 EELS spectra from the regions of interest in an image can be synthesized by calculating the average gray values of the same pixels in each energy-filtered image over the whole range of acquired images. The image analysis system extracts intensities at the same pixel in each image across the series and reconstructs an EELS spectrum by plotting the intensities against the corresponding energy-loss values. The drift of the specimen was corrected by shifting the individual images pixelwise over the entire images acquired. Elemental mapping is based on the fact that each core-loss edge of an EELS spectrum occurs at the energy that is characteristic of a specific element. A core-loss edge is superimposed on a background (BG) due to other energy losses, and extracting elemental information for mapping necessitates their separation from the BG. The BG curve is estimated using two energy windows (E1 and E2) by assuming a power law or exponential law dependence as follows, where factors A and r are calculated pixel by pixel from signals I(E1) and I(E2). power law exponential law

I(E) ) AE-r

(1)

I(E) ) A exp(-rE)

(2)

To perform quantitative elemental analysis, we must accurately subtract the BG contribution from the spectrum. Also, to obtain a high-resolution elemental map, it is necessary to improve the signalto-noise (S/N) ratio of the image. The proper energy position and the energy width of a core-loss image give an elemental map with a high S/N ratio on the edge shapes of an element of interest.21 Thus, we optimized the quality of elemental maps by the following scheme: Referring to the EELS spectra calculated by Image-EELS, two images to be used for the BG fitting were appropriately selected from the stack of images, and then one of the BG fitting equations as shown above was chosen to give the better BG fitting result. Then, the core-loss image was selected to give an elemental map with a high S/N ratio. In this scheme, the energy width of each image for elemental mapping was adjusted by combining successive images into one image.

Results and Discussion ZnO/Rubber Interface before Vulcanization. First, the sample before vulcanization (sample 1) was analyzed as shown in Figure 2. Figure 2a is a zero-loss image of a ZnO particle distributed in the SBR rubber matrix, which is formed by the unscattered and elastically scattered electrons with an energy range of 0 ( 10 eV. Image-EELS was performed in the two energy ranges that included the sulfur and the zinc L2,3-ionization edges at around 160 and 1020 eV, respectively. Figure 2b and c shows the sulfur and zinc distribution images created by the three-window method, where the three appropriate energy-loss positions for the mapping were selected by referring to the Image(16) Ko¨rtije, K. H. J. Microsc. 1994, 74, 149. (17) Ko¨rtije, K. H. Scanning Microsc. 1994, Suppl. 8, 277. (18) Abolhassani-Dadras, S.; Vazquez-Nin, G. H.; Echeverria, O. M.; Fakan, S. J. Microsc. 1996, 183, 215. (19) Thomas, P. J.; Midgley, P. A. Ultramicroscopy 2001, 88, 179. (20) Thomas, P. J.; Midgley, P. A. Ultramicroscopy 2001, 88, 187. (21) Kothleitner, G.; Hofer, F. Micron 1998, 29, 349.

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Figure 1. Illustration representing the scheme of EFTEM analysis for high-resolution elemental mapping and EELS employed in this study.

Figure 2. Elemental maps and Image-EELS spectra obtained from the sample before vulcanization (sample 1). (a) Zero-loss filtered image and (b) sulfur and (c) zinc distribution images of a ZnO particles dispersed in the rubber matrix. (d) Energy-filtered image at 250 ( 10 eV, on which the regions subjected to Image-EELS analysis are presented. (e and g) As-obtained Image-EELS spectra containing sulfur and zinc L2,3-ionization edges acquired from the regions shown in (d). (f and h) BG-subtracted spectra of corresponding spectra shown in e and g. The dotted lines are BG fitting curves.

EELS spectra shown in Figure 2e-h. For sulfur mapping, two energy windows at 125 and 150 eV were used for the BG fitting, and the energy window at 210 eV was used for the core-loss image. For zinc mapping, the energy windows at 950 and 1000 eV were used for the BG fitting, and the energy window at 1150 eV was used for the core-loss image. Also, the BG fitting equation was selected to give a better fitting result for the spectra synthesized by Image-EELS. In this case, the exponential law was found to give a better fit for the sulfur ionization edges, and the power law gave a better fit for the zinc ionization edges. The regions on the ZnO particle and in the rubber matrix as defined in Figure 2d gave the spectra in the two energy ranges as presented in Figure 2e and g, where the BG fitting curves are shown as

dotted lines. Figure 2f and h shows the spectra after the subtraction of the BG components from the corresponding “as-obtained” spectra. The sulfur L2,3-ionization edges are especially hard to identify in the as-obtained spectra because the energy-loss region is close to the plasmon-loss region yielding high, steep BG curves. After the subtraction of the BG components, the ionization edges could be clearly seen as shown in Figure 2f, and the trend in their peak intensities could be correlated to the pixel intensities in the sulfur distribution image shown in Figure 2b. The order of the sulfur content among the evaluated regions displayed in Figure 2d is known to be 1 > 3 > 4 > 2. That is, sulfur element tended to be enriched on the ZnO particle’s surface rather than in the rubber matrix. In this case, a part of the ZnO surface that was assigned to region 1 obviously exhibits the highest sulfur content among the evaluated regions, whereas region 2 was almost sulfurfree. The rubber matrix, however, was sulfur-poor, but it contained sulfur at a certain low level. The localization of elemental sulfur could apparently be distinguished on the EELS spectra with high spatial resolution. The analysis of the sample before vulcanization indicates that solely dispersed aggregates of S8 and/or the accelerator did not exist but were adsorbed on the ZnO particles after sufficient mixing, whereas a small number of them were dissolved in the rubber. Unfortunately, nitrogen could not be detected with sufficient signal intensity because the nitrogen K-ionization edge appeared at the energy loss (395 eV) just beyond the large carbon-K ionization edge (285 eV), which obstructed the detection of the weak signals originating from the ionization of nitrogen atoms. Therefore, it was difficult to distinguish the location of S8 from that of the accelerator here. As expected, the zinc distribution image (Figure 2c) does correspond to the ZnO particle, and the Image-EELS analysis (Figure 2g and h) shows that elemental zinc was not detected from the region in the rubber matrix. ZnO/Rubber Interface after Vulcanization. Figure 3 shows typical ZnO particles observed in the vulcanized sample (sample 2), where the upper row shows the zero-loss images and the bottom row shows the corresponding energy-filtered images at 250 ( 10 eV, which clearly shows the existence of products produced around the ZnO particles as a result of vulcanization. We selected particles that were smaller than average in the following analysis because both the particle and the interfacial region are included in a picture with appropriate magnification. We have confirmed that the product was generated around the

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Figure 3. Typical ZnO particles distributed in the rubber matrix after vulcanization (sample 2). The upper row shows the zero-loss filtering images, and the bottom row shows the corresponding energyfiltered images at 250 ( 10 eV.

ZnO particles regardless of the particle size. Those products were hardly observed in the zero-loss images but could be seen in the inelastically scattered images. This effect of ESI is called “contrast tuning”.22 Tuning of the energy-loss value can produce an image at optimum conditions in which all components in a specimen are presented with comparable contrast and intensity within the available gray levels. In a specimen where carbon is the majority element, optimum contrast is usually achieved at around 250 eV, which is an energy-loss level below the carbon ionization K-edge at 285 eV. To identify the chemical structure of the product surrounding the ZnO particles, Image-EELS and elemental mapping were carried out. Figure 4a-d shows an energy-filtered image of a ZnO particle at 250 ( 10 eV and the corresponding distribution images of sulfur, zinc, and oxygen, respectively. The corresponding Image-EELS spectra acquired from the regions indicated in Figure 4a are shown in the bottom row. The elemental maps indicate that the product of interest was rich in both sulfur and zinc. The corresponding Image-EELS spectra allowed us to perform quantitative analysis in terms of the elemental distributions in the maps. The sulfur L2,3-ionization peaks, thus obtained, indicated that sulfur was not only located in the products that were generated around the ZnO particles (region 2 and 3) but also distributed in the rubber matrix (region 4). As compared to the sample before vulcanization (Figure 2), the sulfur content in the rubber matrix increased after vulcanization. This may indicate that the thermal treatment at 170 °C for 10 min promoted the dissolution of sulfur into the rubber to cross link the rubber molecules. This implies that the vulcanization reaction is initiated on the ZnO particle’s surface with the adsorbed sulfur compounds. The zinc distribution image and the corresponding Image-EELS spectra showed that elemental zinc is localized in the limited area around the ZnO particle and is not detected in the rubber matrix. Oxygen, however, seems to be equally distributed in the rubber matrix and is not localized in the zinc- and sulfur-rich areas around the ZnO particles. Thus, we may reasonably speculate that the elemental oxygen distributed in the rubber matrix came mainly from StAc and not from ZnO. This speculation is also supported by the differences in the shapes of the oxygen ionization edges between the spectra from the ZnO particle (region 1) and from the zinc-rich regions around the particle (regions 2 and 3). For a comparison of the shapes of (22) Reimer, L. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer: Berlin, 1995; p 377.

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spectra, the spectra obtained from regions 2 and 3 are magnified 3× and displayed as dotted lines. It is known that the ionization edge structure on an EELS spectrum provides information on the local environment of an excited atom associated with the valence, oxidation state, chemical bonding, and electronic properties.23 The obtained spectra were synthesized from the data with the energy interval equivalent to the energy increment in the recording of the energy-filtered images by Image-EELS. Therefore, the spectra provided by Image-EELS do not possess such high-energy resolution as those obtained from parallel EELS recording.24 However, the spectra displayed in Figure 4d exhibit a shift in the top of the peaks to a higher energy-loss position than the spectra from the ZnO particle. Considering these results, we assume that the sulfur- and zincrich compound found around the ZnO particles was produced as a result of the reaction among S8, TBBS added as an accelerator, and ZnO that took place on the ZnO surface. To identify this compound and to investigate the role of the respective components in the accelerated vulcanization process, we analyze the interfaces between the ZnO particles and the rubber in the samples in which some of the components necessary for accelerated vulcanization were absent. Accelerator and Activator Effects on ZnO/Rubber Interfacial Structure. Figure 5a and b shows sulfur distribution images on a ZnO particle in sample 3, which was compounded without S8. Thus, no cross linking of the rubber took place because of the absence of S8. The area indicated by the white square in Figure 5a is shown as a magnified image in Figure 5b. Figure 5c and d shows sulfur and zinc L2,3-ionization edges created by Image-EELS in the regions defined in the images. The sulfur distribution images show that the accelerator was localized on the ZnO surface, but the Image-EELS spectra indicate that the accelerator was also dissolved in the rubber matrix at a certain lower level. Obviously, the product produced around the ZnO particles by the accelerated vulcanization in sample 2 (Figure 3) was not found in this sample. The spectra in Figure 5d show that the zinc L2,3-ionization edge can be detected clearly from the rubber matrix by magnifying 100 times as displayed by the dotted lines. This indicates that elemental zinc was distributed uniformly in the rubber matrix at a much lower level than in the bulk ZnO particles. From the results shown in Figure 5, we can evaluate the spatial resolution of the elemental mapping and the elemental detection sensitivity in this study. Figure 5d indicates that the zinc L2,3ionization edge with an intensity approximately 100 times weaker than that of bulk ZnO can be detected. If the region on the ZnO particle in the image includes elemental zinc at 50 atom %, then elemental zinc contained in the rubber matrix would be roughly less than 0.5 atom %. The results shown in Figure 5b and c indicate that the two neighboring small areas with a width of less than 10 nm on the ZnO surface can be distinguishable in terms of the sulfur content. That is, the one region (region 1) is sulfurrich, and the other (region 2) is nearly sulfur-free as shown in the BG-subtracted EELS spectra in Figure 5c. Therefore, we can confirm that the spatial resolution of the elemental maps obtained in this study was less than 10 nm. The sulfur maps shown in Figures 2 and 5 suggest that S8 and/or the accelerator are not uniformly distributed on the ZnO surface. This implies that the ZnO surface properties are not (23) Brydson, R.; Sauer, H.; Engel, W. In Transmission Electron Energy Loss Spectroscopy in Material Science and the EELS ATLAS, 2nd ed.; Ahn, C., Ed.; Wiley-VCH: Weinheim, Germany, 2004; p 223. (24) Brydson, R. In Electron Energy Loss Spectroscopy; BIOS: Oxford, U.K., 2001; p 46.

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Figure 4. Elemental maps and Image-EELS spectra obtained from the vulcanized sample (sample 2). (a) Energy-filtered image at 250 ( 10 eV. The regions indicated in the image are subjected to Image-EELS analysis. Elemental distribution images of (b) sulfur, (c) zinc, and (d) oxygen. The corresponding element-specific ionization edges are shown in the bottom row. The spectra shown in the bottom row are BG-subtracted Image-EELS spectra acquired from the regions indicated in part a. The oxygen K-ionization spectra taken from regions 2 and 3 are shown by 3× magnification (dotted lines in part d).

Figure 5. (a) Sulfur distribution image on a ZnO particle in vulcanized rubber without S8 (sample 3). (b) Enlarged view of the area indicated in part a as a white square. (c and d) BG-subtracted Image-EELS spectra showing sulfur and zinc L2,3-ionization edges, respectively, acquired from the regions indicated in parts a and b.

uniform, which causes the differences in the interactions of ZnO with S8 and with the accelerator. Sample 4, which was compounded without the presence of the accelerator, also shows different behaviors from the other vulcanized samples in terms of the sulfur and the zinc distributions as shown in Figure 6. Parts a and b of Figure 6 are a zero-loss image and an energy-filtered image at 250 ( 10 eV, respectively. One can see a product that is spreading from the ZnO particle. However, the size of this phase is significantly larger than the compounds produced in sample 2 (Figure 3). The sulfur, zinc, and oxygen distribution images shown in parts c-e of Figures 6, respectively, reveal that the zinc distribution in the rubber

matrix (Figure 6d) matches the phase observed in the energyfiltered image shown in Figure 6b. We confirmed by ImageEELS that the zinc L2,3-ionization edge was not detected in the region outside of this phase in the rubber matrix. However, sulfur and oxygen seemed to be distributed over the entire area of the rubber matrix. The oxygen seemed to be distributed evenly in the rubber (Figure 6e), whereas sulfur was distributed in the zinc-rich area at a slightly higher concentration (Figure 6c). We also confirmed that sample 5, in which both the accelerator and StAc were not compounded, did not exhibit the widely diffusing zinc-rich phase as could be seen in sample 3. The role of StAc in the vulcanization has been known to promote the production of the zinc ion (Zn2+) through the formation of zinc stearate.25 Therefore, it is speculated that the zinc-rich phase shown in Figure 6 was produced from the diffusion of the zinc salt and was not the same product as found in sample 2. Also, we can understand that S8 is necessary for the formation of the zinc-rich phase found in the sample 4 (Figure 6) because this kind of product is not found in the sample in the absence of S8 (Figure 5). Mechanism of the Accelerated Vulcanization. Many mechanisms concerning the role of ZnO as an activator in sulfur vulcanization have been proposed over the years.26,27 Most of this work was carried out using low-molecular-weight compounds as representative models.28 It has been confirmed that ZnO with a fatty acid has a pronounced effect on the enhancement of the reaction rate of the cross linking of rubber. It is a curious fact, however, that ZnO, which is incorporated as solid particles and is highly incompatible with organic rubbers, plays such an important role in accelerated vulcanization. This is the first time that we have learned of the interaction of ZnO particles with rubber compounds. The interaction of sulfur (S8) with ZnO, even before vulcanization, is quite apparent as shown in Figure 2. Here, we explore the mechanism of the accelerated vulcanization by (25) Hummel, K.; Santos Rodoriguez, F. J. Kautsch. Gummi Kunstst. 2001, 54, 122. (26) Heideman, G.; Datta, R. N.; Noordermeer, J. W. M. Rubber Chem. Technol. 2004, 77, 512. (27) Coran, A. Y. J. Appl. Polym. Sci. 2003, 87, 24. (28) Wolfe, J. R.; Pugh, T. L.; Killian, A. S. Rubber Chem. Technol. 1968, 41, 1329.

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Figure 6. (a and b) Zero-loss and energy-filtered images of a ZnO particle distributed in rubber vulcanized without the accelerator (sample 4), respectively. Elemental maps of (c) sulfur, (d) zinc, and (e) oxygen.

considering the results obtained in this study and the proposed reaction schemes in previous work. The proposed reaction scheme of accelerated vulcanization in the presence of ZnO using TBBS as an accelerator is briefly shown as follows:

TBBS is thermally decomposed into 2-mercaptobenzothiazole (MBT) and then quickly converted into 2,2′-dithiobenzothiazole

(MBTS) (eq 3a). Sulfur atoms are inserted into MBTS to form polysulfidic MBTS, (eq 3b), where the interaction of Zn2+ with the accelerator results in the formation of the complex and sulfur insertion occurs rapidly. As shown in the reaction scheme (eq 3c), the cross-link precursor, which consists of an acceleratorterminated polysulfidic group attached to a rubber chain, is formed as a product of the reaction between the polysulfidic MBTS and the rubber chain, and then MBT and ZnS are formed as shown in eq 3c. In the presence of Zn2+, the zinc chelation as shown in the scheme (eq 3d) influences the position of the S-S bond that is most likely to break as a result of the stabilization of the other sulfur bonds by resonance. The sulfur-sulfur bond in the cross-link precursor breaks, and then the rubber is cross linked with polysulfide and another new cross-link precursor is produced as shown in the scheme (eq 3e). In this scheme, we can understand that Zn2+ plays important roles in many steps in the accelerated vulcanization and also how ZnS is produced as a byproduct. Indeed, the evidence of the generation of ZnS during vulcanization has been reported in

Nanoimaging of Rubber/ZnO Interfaces

the study of mold fouling of rubber compounds.29 Therefore, the product that we found around the ZnO particles in the vulcanized sample is ZnS. ZnS can be detected only in the localized area around the ZnO particles. This implies that the accelerated vulcanization catalyzed with Zn2+ does not take place uniformly over the entire area of the rubber matrix; it occurs locally around the ZnO particles at a higher reaction rate. It is assumed that vulcanization in the absence of Zn2+ also takes place in the area far from ZnO particles. In the accelerated vulcanization scheme in the absence of ZnO, it was proposed that the reaction between the accelerator and S8 proceeds via a radical pathway to form polysufidic MBTS.26 Therefore, it is suggested that the vulcanization process involves both ionic and radical reaction pathways; therefore, the network structure of the accelerated vulcanized rubber is not uniform on the nanoscale. We also showed the effect of StAc on the promotion of Zn2+ production as shown in Figure 6, which suggests that Zn2+ is widely diffused into the rubber matrix when the accelerator is absent. The formation of ionic intermediates in rubbers is also supported by current measurements during vulcanization.25 This phenomenon indicates that the Zn2+ ions in the absence of the accelerator are not consumed because the intermediates with the accelerator are not formed and thus the dissociation of Zn2+ ions into the rubber is promoted. However, the concentration of Zn2+ ions dissociated in the rubber was revealed to be significantly (29) Bukhina, M. F.; Morozov, Y. L.; van de Ven, P. M.; Noordermeer, J. W. M. Kautsch. Gummi Kunstst. 2003, 56, 172.

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low in the sample compounded without S8 (sample 3), as shown in Figure 5. The formation of a zinc-accelerator complex presumably hinders the dissociation of the ions.26

Conclusions The interactions between the rubber matrix and ZnO filler particles added as an activator in the accelerated vulcanization process are visualized by EFTEM. The results shown in this article indicate that the spatial resolution of the elemental maps is less than 10 nm and that the detection sensitivity for elements is about 0.5 atom %. Elemental mapping and Image-EELS revealed that a sulfur- and zinc-rich product is produced in the localized area around the ZnO particles. The proposed reaction schemes in previous work indicate that the product we found around the ZnO particles was ZnS-generated as a byproduct of accelerated vulcanization. This suggests that the vulcanization activated by ZnO occurs mainly in the local area around the ZnO particles and does not occur uniformly in the rubber matrix. This evidence implies that vulcanization does not take place in a single process in rubber mixtures and that the rubber network structure is not uniform. Also, we showed the adsorption of sulfur on the ZnO surface before vulcanization and the function of StAc in vulcanization to promote the production of Zn2+ ions. Therefore, this modern analytical tool is expected to provide new valuable insights into the complex mechanism of sulfur vulcanization and the nanostructures of rubbers. LA052308F