Locating a Silane Coupling Agent in Silica-Filled Rubber Composites

Oct 17, 2007 - We could then appropriately determine the energy windows for pre-edge and post-edge images for the elemental mapping of the two element...
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Langmuir 2007, 23, 12344-12349

Locating a Silane Coupling Agent in Silica-Filled Rubber Composites by EFTEM Hidehiko Dohi† and Shin Horiuchi*,‡ SRI Research & DeVelopment Ltd., 2-1-1, Tsutsui-cho, Chuo-ku, Kobe 651-0071, Japan, and Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed May 25, 2007. In Final Form: August 8, 2007 A silane coupling agent (SA) was added to silica/rubber composites at different mixing temperatures and the formation of a coupling layer at the silica/rubber interface was investigated by energy-filtering transmission electron microscopy. Bis(triethoxysilypropyl)tetrasulfane (TESPT), which was used as the SA, reacted with the silanol groups on the silica surface and with styrene-butadiene rubber to form an interfacial coupling layer. The silicon and sulfur elemental distributions were analyzed by electron energy loss spectroscopy (EELS) and elemental mapping. The amount of TESPT trapped in the rubber matrix could be qualitatively estimated by EELS, and the in situ formed coupling layer could be characterized by elemental mapping. The result indicated that the formation of the coupling layer was affected by the mixing temperature. The technique described here will contribute to the study of interfaceproperty relationships and the evaluation of the role of SAs in polymeric composites.

1. Introduction Variety types of fillers have been intensively added to polymeric materials to improve the static and dynamic mechanical properties of compounds. For rubber materials, carbon black and silica are the most widely used filler in the rubber industry to achieve sufficient stiffness, modulus, tear strength, abrasion, and fatigue resistance.1,2 Recently, silica has been recognized as an important filler for rubber reinforcement and is used as a partial or even complete replacement for carbon black fillers. One reason is that a silica-filled tire shows low hysteresis when compared with a carbon-filled tire.3,4 However, silica has numerous hydrophilic silanol groups on its surface, resulting in a strong filler-filler interaction and a poor filler-polymer interaction. Therefore, silica prefers to form an agglomerate as a secondary structure by hydrogen bonds between silanol groups on the silica surface. To produce silica-filled rubber compounds with high performances, the compatibility between silica and rubber has to be enhanced. To effectively transfer a load from the matrix to the reinforcing filler, the interface between the two materials in the composites should be highly adhesive. For this purpose, a silane coupling agent (SA) is commonly used to chemically modify silica surfaces and promote interactions between hydrophilic silica surfaces and the hydrophobic rubber phase.5-7 For silica-filled rubber, bis(triethoxysilypropyl)tetrasulfane (TESPT) has conventionally been used to strengthen the silica/ * To whom correspondence should be addressed: Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan. Tel.: 8129-861-6281. Fax: 81-29-861-4773. E-mail: [email protected]. † SRI Research & Development Ltd. ‡ National Institute of Advanced Industrial Science and Technology. (1) Mark, J. E.; Erman, B.; Eirich, F. R. The Science and Technology of Rubber, 3rd ed.; Elesevier Academic Press: London, 2005; p 367. (2) Wolff, S. Rubber Chem. Technol. 1996, 69, 325-346. (3) Waddell, W. H.; O’Haver, J. H.; Evans, L. R.; Harwell, J. H. J. Appl. Polym. Sci. 1995, 55, 1627. (4) Luginsland, H.-D.; Frohlich, J.; Wehmeier, A. Rubber Chem. Technol. 2002, 75, 563-579. (5) Reuvekamp, L. A. E. M.; Ten Brinke, J. W.; Van Swaaij, P. J.; Noordermeer, J. W. M. Rubber Chem. Technol. 2002, 75, 187-198. (6) Alex, R.; Mathew, N. M.; De, P. P.; De, S. K. Kautsch. Gummi Kunstst. 1989, 42, 674. (7) Dannenberg, E. M. Rubber Chem. Technol. 1975, 48, 410-444.

Figure 1. Schematic illustration of the coupling reaction of TESPT in a silica/rubber composite and of the expected formation of the coupling layer at the interface between the silica and the rubber.

rubber interface.8,9 It is expected that the ethoxy groups of TESPT condense with the silanol groups present on the silica surface and the tetrasulfane group of TESPT covalently bonds with the rubber. As shown in Figure 1, it is believed that in situ reactions among the three components form the TESPT monolayer at the interface between silica and rubber, which couples the silica with the rubber matrix via covalent bonds; thus, the degree of adhesion at the silica/rubber interface can be enhanced. However, the reactions and interactions that occur during the mixing process are more complicated than expected: Polycondensation possibly occurs within the coupling agent, resulting in the formation of multilayers at the interface or in the suppression of the reaction between the SA and the silanol groups on silica.10 Moreover, the SA might be trapped in the rubber matrix through the dissolution and chemical reaction between the SA and the rubber. To obtain (8) Goerl, U.; Muenzenberg, J. Presented at Rubber Division Meeting, American Chemical Society, Anaheim, CA, May 6-9, 1997. (9) Byers, J. T. Rubber Chem. Technol. 2002, 75, 527-547. (10) Benkoski, J. J.; Kramer, E. J.; Yim, H.; Kent, M. S.; Hall, J. Langmuir 2004, 20, 3246-3258.

10.1021/la701537k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007

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and no S. Therefore, the investigation of the elemental distributions of these two elements in silica-filled SBR composites enables us to locate TESPT used as a SA, leading to the evaluation of the formation of the coupling layer and the reactivity of TESPT. 2. Experimental Section Figure 2. Zero-loss images of the binary silica/SBR composite (a) and the composite with the SA mixed at 110 (b) and 150 °C (c). Table 1. Bound Rubber Content, Sulfur Content, and Tan δ of Silica-Rubber Composites

sample no.

composition SBR/silica/ TESPT (g)

mixing temperature (°C)

bound rubber content (%)

1 2 3

100/20/0 100/20/2 100/20/2

110 110 150

7.7 9.8 18.3

sulfur content in bound rubber (%)

tan δ at 60 °C

0 0.7 1.1

0.146 0.139 0.135

the preferable situation in which the added SA is appropriately located at the interface and effectively works to strengthen the interface (as depicted in Figure 1), the composition of the rubber mixtures and the compounding condition have to be optimized.11 The chemical and physical interactions of SAs in silica-rubber composites have been studied using 29Si-CP/MAS solid-state NMR,12 bound rubber,13,14 X-ray photoelectron spectroscopy,15 gas chromatography,16 and high-performance liquid chromatography.8 Those analytical techniques, however, cannot offer the possibility to perform nanoscale analysis of filler/matrix interfaces in composites. The effect of the SA on the structure and properties of silica-filled composites is one of the important subjects in the field of material science and technology. In this study, we investigate the effect of the mixing conditions on the coupling reactions of TESPT between silica and styrenebutadiene rubber (SBR) by energy-filtering transmission electron microscopy (EFTEM). EFTEM allows us to characterize the local chemical structures of materials by employing elemental mapping and electron energy loss spectroscopy (EELS), which provides us with valuable information on polymer nanostructures in terms of phase characteristics17-19 and interfacial structures.20-23 With regard to rubber structures, we have reported EFTEM analysis of ZnO-rubber interfaces for the investigation of the accelerated vulcanization mechanism24 and the heterogeneity of vulcanized rubber structures.25 As shown in Figure 1, TESPT contains both silicon (Si) and sulfur (S), while silica contains Si (11) Eisenbach, C. D.; Ribbe, A. Kautsch. Gummi Kunstst. 1994, 47, 477480. (12) Hunsche, A.; Go¨rl, U.; Muller, A.; Knaak, M.; Go¨bel, T. Kautsch. Gummi Kunstst. 1997, 50, 881-889. (13) Dannenberg, E. M. Rubber Chem. Technol. 1986, 59, 512-524. (14) Rajeev, R. S.; De, S. K. Rubber Chem. Technol. 2002, 75, 475-509. (15) Salvi, A. M.; Pucciariello, R.; Guascito, M. R.; Villani, V.; Intermite, L. Surf. Interface Anal. 2002, 33, 850-861. (16) Castellano, M.; Conzatti, L.; Turturro, A.; Costa, G.; Busca, G. J. Phys. Chem. B 2007, 111, 4495-4502. (17) Horiuchi, S.; Ishii, Y. Polym. J. 2000, 32, 339-347. (18) Horiuchi, S.; Fujita, T.; Hayakawa, T.; Nakao, Y. Langmuir 2003, 19, 2963-2973. (19) Liao, Y.; Horiuchi, S.; Nunoshige, J.; Akahoshi, H.; Ueda, M. Polymer 2007, 48, 3749-3758. (20) Horiuchi, S.; Hanada, T.; Yase, K.; Ougizawa, T. Macromolecules 1999, 32, 1312-1314. (21) Horiuchi, S.; Hamanaka, T.; Aoki, T.; Miyakawa, T.; Narita, R.; Wakabayashi, H. J. Electron Microsc. 2003, 52, 255-266. (22) Horiuchi, S.; Yin, D.; Ougizawa, T. Macromol. Chem. Phys. 2005, 206, 725-731. (23) Horiuchi, S.; Liao, Y.; Yin, D.; Ougizawa, T. Macromol. Rapid Commun. 2007, 28, 915-921. (24) Horiuchi, S.; Dohi, H. Langmuir 2006, 22, 4607-4613. (25) Dohi, H.; Horiuchi, S. Polymer 2007, 48, 2526-2530.

All materials used in this study were commercial products and were used as received. Emulsion-type SBR with the grade name SBR1502 was supplied by Zeon Corp. (Japan). Precipitated silica (Ultrasil VN3) and TESPT (Si69) were supplied by Degussa GmbH (Germany). An amount of 100 g of SBR, 20 g of silica, and 2 g of TESPT was compounded in a Banbury mixer at the two mixing temperatures of 110 and 150 °C for 5 min. The mixing temperature was controlled by the rotation speed of the mixer. A simple binary mixture of SBR and silica was also prepared by the same procedure at 110 °C. The bound rubber, representing the portion of rubber adsorbed onto the silica, was prepared by extracting the samples in toluene for 48 h at room temperature in a Soxhelt-like system. The samples were then filtered by a 100 mesh filter, and the filtrate was dried in a vacuum oven. The bound rubber contents were calculated by measuring the change in weight before and after the extraction. The portion remaining after the extraction contained all the added silica with the rubber attached on the silica surface and occluded into the silica aggregations. The contents of TESPT in the bound rubbers were estimated by measuring the sulfur contents. The rubber and sulfur contents in the extracted unbounded portions were estimated by a thermogravimetric analyzer (TGA-50, Shimadzu Corp., Japan) and a sulfur analyzer (EMIA-822, Horiba Ltd., Japan), respectively. Then the sulfur contents in the bound rubbers were calculated. Thin sections of the samples (approximately 100 nm thick) were prepared by cryo-ultramicrotomy at -60 °C and collected on a 600 mesh copper grid for the EFTEM analysis. Subsequently, gold nanoparticles with a diameter of 10 nm were dispersed on the specimens by dropping aqueous Au colloid (Polyscience Inc., U.S.A.) on the specimens; these nanoparticles were used as markers for focus adjustment and drift correction during observations. An LEO922 (Carl Ziess SMT, Germany) in-column-type energyfiltering transmission electron microscope with a LaB6 cathode and an omega-type energy filter was used at an accelerating voltage of 200 keV. Image recording and processing was performed using a 2K × 2K slow scan CCD camera, Proscan HSC2 (Proscan Co. Ltd.), and an image processing system, analySIS (Soft Imaging System, GmbH, Germany), on a PC connected to the microscope. Energyfiltered images were recorded in the binning mode,in which 4 × 4 pixels were summed up into one effective pixel giving 512 × 512 pixel images. All observations were cryogenically carried out at 120 K to minimize radiation damage to the specimens. For obtaining high-resolution elemental maps and for performing quantitative EELS analysis, we employed Image-EELS technique. The details and the advantages of this technique have been described in our previous works.19,21-25 First, a set of energy-filtered images was recorded sequentially across a wide range of energy loss to construct a 3-D dataset containing spatial information, I(x,y), and spectral information, I(E). Image-EELS enables us to acquire EELS spectra from areas of interest in any shapes in an image. 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 the 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. The elements of interest in this study were Si and S, which exhibit L2,3-edges at around 100 and 160 eV, respectively. Thus, energyfiltered images with an energy window of 5 eV were sequentially recorded across the energy range from 80 to 200 eV with an energy increment of 3 eV, which gave a stack of 40 energy-filtered images at the same position with different energy loss levels. We could then

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Figure 3. Energy-filtered images at 200 ( 2.5 eV and EELS spectra including the Si L2,3-edges acquired from the regions indicated in the corresponding images of the binary silica/SBR composite (a) and the composite with the SA mixed at 110 (b) and 150 °C (c). As-obtained EELS spectra are shown in the middle column together with the background-fitting curves and the background-subtracted spectra are shown in the right column. appropriately determine the energy windows for pre-edge and postedge images for the elemental mapping of the two elements using the EELS spectra and select the corresponding images from the stack of images. For the measurements of dynamic mechanical properties, sulfur and an accelerator (N-tert-2-benzothiazolyl-sulfenamide, TBBS) were added to the uncured mixture in an open roll and were vulcanized at 170 °C for 10 min in a hot press machine. Tan δ at 60 °C was measured by a viscoelastic spectrometer (Ueshima Seisakusyo, VR7110) at a frequency of 10 Hz.

3. Results and Discussion Figures 2a-2c show the zero-loss images of the simple SBR/ silica (100/20) binary composite and SBR/silica/TESPT (100/ 20/2) composites compounded at 110 and 150 °C, respectively. The three images show no significant differences in terms of the distribution of silica. However, the bound rubber content, sulfur content, and tan δ indicate significant differences among the three samples as shown in Table 1. The composite mixed at the higher temperature (sample 3) shows the highest rubber bound content and sulfur content, suggesting that TESPT facilitates strong interactions between rubber and silica. The sulfur contents are directly related to the TESPT contents in the bound rubber, indicating that the mixing at the higher temperature facilitates the coupling reaction. TESPT is thus assumed to effectively covalently bond both the silica and the rubber phase. On the other hand, the bound rubber content of the binary compound (sample 1) is mainly attributed to the occluded rubber portion

within the silica agglomerate. Tan δ represents one of the dynamic mechanical properties of the composites. Low values of tan δ can be attributed to low Payne effect, which can be achieved by the hindrance of the silica-silica network formation by the silane coupling layer on the silica surface.4-7 Tan δ values measured at 60 °C indicate that the composite mixed with TESPT at the higher temperature (sample 3) yields the lowest tan δ; this suggests that the interface between the silica and the rubber can be strengthened and thus the external stress can be effectively transferred from the matrix to the silica. These experimental results strongly suggest that the addition of TESPT can effectively improve the compatibility of silica with rubber and that mixing at a high temperature can facilitate the reaction of TESPT at the silica/rubber interface. However, the differences in the distribution of TESPT between the composites mixed at different temperatures cannot be identified by conventional transmission electron microscopy (TEM). Therefore, we attempt to locate the SA in the composites by elemental mapping and EELS. Figure 3 shows the EELS spectra acquired from regions across the silica/rubber interfacial zones in the three samples by means of “Image EELS”; the energy-filtered images at 200 ( 2.5 eV are shown in the left column, as-obtained spectra acquired from the regions indicated in the corresponding images are shown in the middle column, and background-subtracted spectra, presenting the Si L2,3-edges, are shown in the right column. The fitting curves for the background subtraction, which are indicated as the dotted curves in the middle column, were calculated in

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Figure 6. Schematic illustration representing the path of the electron beam through the silica particle covered with the SA layer and the projected images of Map A and Map B representing the Si distribution and Si + S distribution, respectively. The division of Map B by Map A can emphasize the S-rich layer at the edge of the silica particle.

Figure 4. Background-subtracted EELS spectra showing the Si L2,3-edges of the composites with and without the SA (a). Elemental maps of the composite with the SA mixed at 110 °C created using the energy-filtered images at 135 ( 10 (b) and 175 ( 10 eV (c) as post-edge images with the same pre-edge image at 85 ( 10 eV energy loss.

Figure 5. Pixel intensity profiles measured along the lines drawn in Figures 4b and 4c.

accordance with the power law function, which is expressed as I ) A‚E-r where I is the intensity, E is the energy loss, and A and r are two fitting parameters.26 The EELS spectra were obtained from regions with 10 nm diameters with sufficient intensities of Si L2,3-edges (L-shell excitation for the ionization of 2p electrons) that appear at 100 eV energy loss. In the binary silica/SBR composite (Figure 3a), the spectra obtained from the rubber phase show no ionization edges, which confirms the fact that the background is accurately fitted. On the other hand, the samples (26) Hofer, F.; Warbichler, P. In Transmission Electron Energy Loss Spectrometry in Material Science and the EELS ATLA; Ahn, C. C., Ed.; WileyVCH: Weinheim, 2004; Chapter 6.

with SAs (Figure 3b and 3c) yield Si L2,3-edges from the rubber phases, indicating that the locations of the SAs in the composites are not limited to the silica-rubber interfaces and certain amounts of the SAs are included in the rubber matrices. The intensities of the ionization edges obtained for the rubber phase mixed at 110 °C (Figure 3b) seem to be slightly stronger than those obtained from the sample mixed at 150 °C. Therefore, the EELS spectra qualitatively indicate that the amount of dissolved SA in the rubber matrix is higher in the composites mixed at the lower temperature (Figure 3b) as compared to that in the composites mixed at the higher temperature (Figure 3c). This tendency agrees with that of the bound rubber contents shown in Table 1. The spectra thus obtained by “Image EELS” seem to be noisy because those were constructed only with 40 data extracted from the stack of the images. However, we could obtain the clear Si L2,3-ionization edges from the narrow regions with 10 nm diameter and also could detect qualitative differences in the amounts of the dissolved SA within the rubber phase. We can thus gain information by this technique in terms of the location of SA, which cannot be revealed by conventional TEM. The “Parallel EELS”, which is an alternative technique for recording an EELS spectrum, could provide higher energy-resolved spectra.27 In parallel EELS, a spectrum is directly captured by the slow-scan CCD camera, and then the image analysis system measures the intensity and converts it into a spectrum. However, in this technique, regions to be analyzed have to be carefully determined by inserting an aperture and the drift of a specimen during the acquisition of a spectrum would cause the poor site selectivity. On the other hand, “Image EELS” enabled us to obtain spectra in arbitrary regions with high spatial resolution less than 10 nm as shown in our previous works.22,24 The S L2,3-edge that should appear at around 160 eV energy loss cannot be observed due to the strong Si L2,3-ionization, which occurs in the energy loss region below the S L2,3-edge. In EELS, the shapes of inner-shell ionization are generally complicated forms with a sharp rise at the edge followed by a long decay. The S L2,3-edge is superimposed on the long decay of Si L2,3-ionization owing to the vicinity of the Si L2,3-edge. This situation makes it difficult to subtract the background (27) Egerton, R. F.; Leapman, R. D. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995; p 271.

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Figure 7. Energy-filtered images at 200 ( 2.5 eV of the binary silica/SBR composite (a), the composite with the SA mixed at 110 (b) and 150 °C (c), and the corresponding calculated “Map B/Map A” images showing the formation of the coupling layer at the interface ((d), (e), and (f)).

intensity for the S L2,3-ionization and to separate these two ionizations. However, the comparison between the spectra obtained from the composite without SA (sample 1) and that obtained from the composite with SA (sample 3) clearly shows the difference in the signal intensities in the energy loss region in which the S L2,3-ionization event occurs (160-200 eV), as shown in Figure 4a. These two spectra are obtained for the regions containing the silica fillers and are normalized at their highest intensities at 130 eV. The shapes of the spectra are similar in the energy loss region close to the edge (100-140 eV), while the signal intensities of the composite with the SA in the higher energy loss region beyond 150 eV are greater than those of the composite without an SA. The silica/SBR binary composite contains no S, and hence the EELS spectrum contains no signals generated by S L2,3-ionization. Therefore, it is reasonable to interpret the high signal intensities in the high energy loss region (140-250 eV) in the composite with the SA as being caused by the S L2,3-ionization superimposed on the Si L2,3-ionization. To view the two-dimensional distributions of Si and S, two elemental maps were created using the same pre-edge image at 85 ( 10 eV with two post-edge images at 135 ( 10 and 175 ( 10 eV as shown in Figures 4b and 4c, respectively. Those two elemental maps were created by a “two-window jump ratio” method, where a post-edge imaged is divided by a pre-edge image.26 The elemental map shown in Figure 4b corresponds to a Si distribution image (Si map) and that shown in Figure 4c corresponds to the distribution of both Si and S (Si + S map). These two elemental maps are almost similar, but the edges of the silica fillers presented in the Si + S map (Figure 4c) seem to be sharper than those in the Si map. This is observed in a more quantitative way in the changes in the pixel intensities across the silica fillers as presented in Figure 5, where the profiles were measured on the two maps along the lines shown in Figures 4b and 4c. The sharper ups and downs can be observed in the profile obtained from the “Map B” (red line) than that from the “Map A” (green line), while the “Map A” gives higher intensities in the inner parts of the fillers. Although these differences are small, it is suggested that sulfurcontaining SA is located at the interface at relatively high

concentrations. A similar situation has been reported in the coreshell structures of monodispersed spherical silica particles investigated by EFTEM, where the silanol groups on the surface were characterized by Si and O elemental maps.28 Figure 6 illustrates how the two elemental maps represent the structure of the silica fillers covered with the TESPT layer; the cross section of a silica particle covered with TESPT and the path of the electron beam within the particle are depicted. When the silica particle is completely covered with the TESPT layer and is entirely embedded in the specimen, the incident electrons travel through both the silica particle and the TESPT layer, as shown therein. Considering this geometry, it can be recognized that it is not possible to separate the S-rich TESPT map from the Si-rich silica map because the created images are the projections of particles with the electrons passing through both the parts. The volume fraction of the TESPT layer is larger at the edge of the particle as compared to that at the center of the particle. Thus, the concentration ratio S/Si is larger at the edge of the particle as compared to that inside the particle. Therefore, the pixel intensities in the silica/SBR boundary region in the Si map (Map A) are lower than those inside the particle, while the reverse holds true in the Si + S map (Map B). This tendency can be identified in the elemental maps shown in Figures 4b and 4c, but the discrepancy between the two maps is not sufficiently large. However, the division of Map B by Map A is expected to emphasize the S-rich TESPT layer, as illustrated in Figure 6. Figures 7a, 7b, and 7c show the energy-filtered images at 200 ( 2.5 eV for samples 1, 2, and 3, respectively, and the corresponding images created in accordance with the procedure described in Figure 6 are presented in Figures 7d-7f. To avoid the overflow of the resulting images by the interimage calculation (Map B/Map A), Map B is multiplied by 256 and then divided by Map A. If the gray value of Map A is 0, the corresponding pixels in Map B are divided by 1. The resulting image is then displayed in the range from 0 to 255, i.e., as an 8-bit gray-value (28) Costa, C. A. R.; Leite, C. A. P.; Souza, E. F.; Galembeck, F. Langmuir 2001, 17, 189-194.

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respectively. The spectra were then created from the representative regions as depicted in the images. The inner part (shown as the red circle) and the edge of the filler (shown as yellow) were inspected by “Image EELS”. One of the attractive features of “Image EELS” technique is the possibility of the creation of spectra from regions with such irregular shapes. All the spectra are normalized at 130 eV. One can see the differences in the intensities in the energy loss region higher than 160 eV (as indicated by an arrow), which is attributed to the contribution of the S ionization, in the sample mixed at the lower temperature. The bright regions at the edge of the filler give higher intensities in the high energy loss region than the inner region in the sample mixed at the lower temperature, while the sample mixed at the higher temperature shows no significant differences in the shapes of the spectra between the two regions. Therefore, it can be mentioned that the image contrast obtained by the interimage calculation proposed in this study represents the difference in the concentration ratios of Si and S, and also represents the interfacial structures coupled with SA. “Image EELS” technique allows us to perform both elemental mapping and EELS using the same image data set, and moreover, we can evaluate the validity of the elemental maps by EELS. Figure 8. Calculated “Map B/Map A” images of the composites with SA mixed at 110 (a) and 150 °C (b) and EELS spectra acquired from the regions indicated in the corresponding images by the “Image EELS” technique.

image. As shown in Figure 7d, the number of bright pixels in the binary silica/SBR composite is quite small. For the binary composite (sample 1), Map A and Map B should represent the same Si distribution, which would confirm that the interimage calculation accurately represents the elemental distribution, as expected. The composite with the SA mixed at the lower temperature (sample 2) shows bright pixels in the calculated image, as shown in Figure 7e, where the pixels appearing at the edge of the silica particles are almost bright. On the other hand, in the composite with the SA mixed at the higher temperature (sample 3), bright pixels are distributed on the entire surface of the silica filler, as shown in Figure 7f. These results indicate that the TESPT layer is formed more densely on the silica surface by mixing at the higher temperature or that a thick TESPT layer is formed by polycondensation between the ethoxy groups of TESPT or by the intercalation of TESPT into the silica. To confirm that the calculated images shown in Figures 7d7f represent the true structures and include no artifacts, we again evaluate the typical regions that appear in the calculated images by EELS spectra. Figures 8a and 8b are the images calculated by “Map B/Map A” of the different regions of the samples mixed at the lower and higher temperatures, respectively. The images present the same features as shown in Figures 7e and 7f,

4. Conclusion EELS and elemental mapping by EFTEM enabled us to locate TESPT, which was added as an SA to the silica/rubber composite. Conventional TEM could not show any differences in terms of the dispersions of the silica in the rubber among the samples evaluated in this study, while the EELS analysis could detect the SA dissolved in the rubber matrix and the elemental mapping of Si and S could provide information on the formation of the coupling layer. Although the S L2,3-edge at 160 eV could not be identified due to it being concealed by the large Si L2,3-edge at 100 eV, the combination of the two elemental maps, where one corresponds to the Si distribution and the other corresponds to the distribution of both Si and S, qualitatively revealed the effect of the mixing temperature on the formation of the coupling layer. In the composite mixed at the higher temperature, it is assumed that a larger number of silanol groups on the silica surface reacted with TESPT, resulting in a dense or thick coupling layer at the interface. This speculation is supported by the bound rubber contents and tan δ measurements. The technique described here will contribute to the study of interface-property relationships and the evaluation of the role of SAs in polymeric composites. Acknowledgment. One of the authors (H.D.) gratefully thanks Prof. Tetsuo Asakura, Department of Biotechnology, Tokyo University of Agriculture and Technology, for useful discussions. The authors thank M. Sakai, N. Tsukamori, H. Nakamae, H. Kimura, and M. Kotani, SRI R&D Ltd., for their kind cooperation. LA701537K