Article pubs.acs.org/JPCC
Tip-Enhanced Raman Spectroscopy Study of Local Interactions at the Interface of Styrene−Butadiene Rubber/Multiwalled Carbon Nanotube Nanocomposites Toshiaki Suzuki,† Xinlei Yan,† Yasutaka Kitahama,† Harumi Sato,† Tamitake Itoh,‡ Takeshi Miura,§ and Yukihiro Ozaki*,† †
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan ‡ Nano-Bioanalysis Group, Health Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Takamatsu, Kagawa 761-0395, Japan § UNISOKU Co. Ltd., 2-4-3, Kasugano, Hirakata, Osaka 573-0131, Japan ABSTRACT: Tip-enhanced Raman scattering (TERS) spectral measurements of nanocomposite styrene−butadiene rubber (SBR) and multiwalled carbon nanotubes (MWCNTs) films were used to explore the local molecular interaction between nanocomposites. TERS spectra from geographically separated points were attributable to SBR or MWCNTs, showing great potential for investigating local film inhomogeneity within several tens of nanometers. Such inhomogeneity has never been observed by confocal Raman measurements. TERS bands due to SBR phenyl groups were strong when MWCNT bands were strong, whereas vinyl-group TERS bands were strong when the MWCNT bands were weak. Analysis of the findings suggests that the local distribution of polymer chains is modified with changes in the orientation of the phenyl rings by π−π interactions between the polymer chains and the MWCNTs.
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INTRODUCTION Recently, various kinds of nanomaterials, such as titania, layered silicate, clay fiber, carbon blacks, and carbon nanotubes, have been used as fillers to reinforce the physical properties of polymers; these mixtures are called polymer nanocomposites.1−7 Among polymer nanocomposites, polymer/carbon nanotube (CNT) materials have attracted great interest because they can significantly improve the mechanical, physical, and electrical properties of polymers.8,9 For example, nanocomposites of styrene−butadiene rubber (SBR) with multiwalled carbon nanotubes (MWCNTs) show improved mechanical, electrical, and thermal properties as compared to those of pure SBR.10 The physical and chemical properties of polymer nanocomposites are greatly affected by the distribution and decentralization of the CNTs. These effects can be induced by the interaction between a polymer and a nanofiller and/or a change in the local structure of the polymer.10,11 However, the mechanism by which the properties are improved has not been fully elucidated. Spectroscopic techniques are usually powerful tools for directly exploring molecular interactions and structures,12 but the interactions in nanocomposites occur at the interface between a polymer and a nanofiller. Thus, high spatial resolution, on the order of less than several tens of nanometers, is required to investigate the interactions, but normal spectroscopic techniques are insufficient for the task. For example, normal Raman experiments can determine only average information from nanocomposites and can detect © 2012 American Chemical Society
differences in a region several tens of nanometers in scale only with great difficulty. Tip-enhanced Raman scattering (TERS) is a method that combines scanning probe microscopy and surface-enhanced Raman scattering (SERS).13−15 TERS spectroscopy has a high spatial resolution, beyond the diffraction limit of light, because its spatial resolution is determined by the tip radius.16,17 Therefore, TERS is a powerful tool for investigating very small portions of complicated materials such as graphene,18,19 carbon nanotubes,20−22 DNA,23,24 proteins,15,25 and polymer blend films.26,27 The present study aimed to explore the interactions and local structure changes at an interface of SBR/MWCNT nanocomposites using TERS. We observed position-dependent TERS spectra variations for the nanocomposites. Raman spectroscopy has already been used to elucidate the structure and physical properties of SBR/MWCNTs. For example, Bokobza et al. studied SBR polymer nanocomposites containing MWCNTs and carbon black using Raman spectroscopy and discussed the dispersion of the fillers.28 Yan et al. reported the self-rearranging behavior of MWCNTs upon laser heating in SBR/MWCNT nanocomposites.29 These studies were very important, but they could not provide new insight Received: September 17, 2012 Revised: December 24, 2012 Published: December 31, 2012 1436
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into the nature of the intermolecular interactions and interface structure of SBR/MWCNT nanocomposites. TERS has considerable promise in exploring these properties by measuring position-dependent spectra of SBR/MWCNTs. The present study might open a new avenue for investigating polymer nanomaterial interfaces.
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EXPERIMENTAL PROCEDURE SBR/MWCNT nanocomposites containing 1 phr MWCNTs were generous gifts from Prof L. Bokobza (ESPCI ParisTech, Paris, France). The unit “phr” stands for parts per hundred parts of rubber by weight. The method for preparing the nanocomposites was reported elsewhere.29 The SBR employed in this study contained 25 wt % styrene units. The butadiene phase contained cis (10%), trans (17%), and vinyl (73%) configurations. The MWCNTs were obtained from Arkéma and used without further purification. An experimental configuration consisting of a reflectionmode Raman microscope and an atomic force microscopy (AFM) instrument (Photon design, Nanostar NFRSM800) was used. The system was arranged to introduce excitation light from the top of the sample and to collect backscattered signals using an objective lens. The top illumination and detection system allowed for the observation of opaque samples as well as transparent samples. The tip radius of the probe was about 100 nm, and therefore, the spatial resolution of the present experiments was estimated to be about 100 nm. The 514.5 nm line from an argon ion laser (Spectra Physics, Stabilite 2017-06S) was used as the excitation light. The excitation laser beam was focused through an objective lens with a long working distance [×90, numerical aperture (NA) = 0.71] onto a sample. The TERS probes were purchased from UNISOKU Co. Ltd. (Hirakata, Osaka, Japan). A tungsten needle coated with silver was used as the TERS tip needle and was attached to a quartz tuning fork of a shear-force-based AFM instrument at an angle of 45°. The TERS tip approached the sample, and an AFM image was first collected by tip scanning using noncontact mode. Then, the tip was parked at a desired spot, and a 100 μW laser beam was focused onto the sample. The backscattered light was collected by the objective lens and introduced into the spectrometer. A Raman signal was first collected under tip-retracted conditions and then under tip-approaching conditions at the same point. A TERS spectrum was calculated by subtracting the spectrum observed under the tip-retracted conditions from that collected under tip-approaching conditions.
Figure 1. Raman spectra of (a) pure SBR, (b) pure MWCNTs, and (c) 1 phr SBR/MWCNT nanocomposites.
assignment of the Raman bands of SBR/MWCNT nanocomposites is summarized in Table 1. Compared with the pureMWCNT spectrum, the G band in the SBR/MWCNT spectrum shifted to a higher wavenumber. According to previous studies, a shift in the G band of a Raman spectrum of CNTs was explained by the shrinkage of the CNTs induced by the pressure from the matrix of the polymer.29,32 The Raman signals assigned to the polymer were not shifted. Panels a and b of Figure 2 show Raman spectra of SBR/ MWCNT nanocomposites observed under tip-approaching and Table 1. Assignment of Raman Bands of 1 phr SBR/ MWCNT Nanocomposites wavenumber (cm−1) 1004 1032 1354 1604 1641 1668 2698 2847 2906 2990 3064
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RESULTS AND DISCUSSION Raman spectra of pure SBR, pure MWCNTs, and 1 phr SBR/ MWCNT nanocomposites are shown in panels a−c, respectively, of Figure 1. From a comparison of the three spectra in Figure 1, Raman bands at 2698, 1604, and 1354 cm−1 in the Raman spectrum of SBR/MWCNTs were assigned to the G′, G, and D bands, respectively, of the MWCNTs.30 Raman bands at 2990, 1668, and 1641 cm−1 in the same spectrum were due to the vinyl C−H stretching and trans and cis CC stretching modes of SBR, respectively.31 These signals originated from the carbon−carbon double bonds of the SBR polymer chain. The Raman peak at 3064 cm−1 and those at 1032 and 1004 cm−1 arose from the aromatic C−H stretching and the phenyl ring modes of SBR, respectively, and that at 2906 cm−1 was ascribed to aliphatic C−H groups. The
assignment phenyl ring breathing phenyl ring stretching D band G band cis CC stretching trans CC stretching G′-band aliphatic CH symmetric stretching aliphatic CH asymmetric stretching vinyl C−H stretching aromatic C−H stretching
SBR (phenyl) SBR (phenyl) MWCNTs MWCNTs SBR (CC) SBR (CC) MWCNTs SBR (main chain) SBR (main chain) SBR (CC) SBR (phenyl)
tip-retracted conditions, respectively. The Raman signal was enhanced when the TERS tip approached. A TERS spectrum was calculated by subtracting the spectrum observed under tipretracted conditions from that collected under tip-approaching conditions as shown in Figure 2c. Figure 3 presents AFM images of the surface of 1 phr SBR/ MWCNT nanocomposites measured using the TERS probe. From these images, one can see some structures on the surface of the polymer nanocomposites. The sizes of the structures are about tens or hundreds of nanometers, and thus, they are too large to be assigned to MWCNTs. Moreover, similar structures that originate from the roughness on the surface were also observed in an AFM image of pure SBR. Thus, we cannot determine the dispersion of the fillers in the SBR/MWCNT 1437
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Figure 2. Raman spectra of SBR/MWCNT nanocomposites measured under (a) tip-approaching and (b) tip-retracted conditions. (c) Difference spectrum calculated by subtracting spectrum b from spectrum a (TERS spectrum).
Figure 4. (a) Raman and (b) TERS spectra of 1 phr SBR/MWCNT nanocomposites measured at the eight points shown in Figure 3a. Figure 3. AFM images of 1 phr SBR/MWCNT nanocomposites: (a) topography image, (b) frequency-shift image. The eight points where Raman and TERS spectra were measured are shown in panel a.
cm−1. The signals at 3064, 2990, 2904, and 2847 were assigned to SBR, whereas those at 2698, 1604, and 1354 cm−1 were due to MWCNTs. These results demonstrate the potential of TERS spectroscopy in exploring position-dependent structural variations. These differences can be explained by the distribution of fillers on the scale of several tens of nanometers in the polymer nanocomposites. The MWCNT content, at 1 phr, was considerably lower than the polymer content. Therefore, it was expected that there would be areas containing only SBR as well as areas that contained both SBR and MWCNTs. A TEM image of similar nanocomposites that contained 4 phr CNTs was reported. According to the image, the area where CNTs were not included was on the order of 1 μm.11 Thus, it is very likely that the TERS measurements detected the distribution clearly. Panels a and b of Figure 5 show the 1800−800 and 3200− 2500 cm−1 regions of TERS spectra obtained at positions 7 and 3, respectively, in Figure 4b. In the corresponding Raman spectra for positions 7 and 3 in Figure 4a, the signal intensity at 1004 cm−1 was stronger than those at 1668 and 1641 cm−1. In contrast, in the TERS spectra, the signal intensities at 1668 and 1641 cm−1 were stronger than that at 1004 cm−1 (Figures 5a). In addition, in the higher-wavenumber region, four Raman peaks at 3064, 2990, 2906, and 2847 cm−1 due to SBR were observed in the normal Raman spectrum (Figure 4a). However, in the TERS spectrum in Figure 5a, the signal at 3064 cm−1 was nearly missing, and the relative intensity of the signal at 2990 cm−1 became stronger than that of the corresponding signal in the Raman spectrum. The signals at 3064 and 1004 cm−1 that
nanocomposites by using the topography and frequency-shift images. Figure 4a displays normal Raman spectra of 1 phr SBR/ MWCNT nanocomposites collected at points 1−8 shown in Figure 3a. The Raman spectra in Figure 4a changed depending on their point of origin. For example, at position 7, the Raman signals assigned to SBR were relatively strong. On the other hand, the spectrum collected at position 3 showed relatively strong MWCNT signals. However, both CNT and SBR signals were always observed at each point, and the point-dependent spectral differences were rather small. From the Raman spectra in Figure 4a, it was difficult to extract structural information or information about the interaction at the interface between the MWCNTs and the SBR. Figure 4b shows TERS spectra collected at the same points at which the Raman spectra in Figure 4a were measured. The TERS spectra measured at the eight points were markedly different from each other. For example, in the spectrum at position 7, TERS signals at 2990, 2904, 2847, 1668, and 1641 cm−1 assigned to SBR were observed; however, a TERS signal at 1354 cm−1 due to MWCNTs (D band) was considerably weaker than the corresponding peak in the Raman spectrum. TERS signals ascribed to the G band (1604 cm−1) and G′ band (2698 cm−1) were not observed in the spectrum collected at position 7. On the other hand, at position 3, TERS signals were observed at 3064, 2990, 2904, 2847, 2698, 1604, and 1354 1438
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both signals at 2990 (vinyl) and 3064 (phenyl) cm−1 increased with an increase in the intensity of the 1604 cm−1 signal (Figure 6c,d). These results suggest that the local distribution of polymer nanocomposites at the interface between the polymer and the filler was different from that of polymer areas without CNTs. The spatial resolution of TERS measurements is determined by the tip radius. The tip radius used was about 100 nm. It was reported that the intensity of a TERS signal decays exponentially from a surface with a decay length of 2.8 nm.33 The area of enhanced field can be estimated by using the tip radius and the enhanced field width. Under theses conditions, the spatial resolution of these experiments was estimated to be about 100 nm. The depth resolution of TERS is much smaller than the spatial resolution (∼ 100 nm). It was reported that the intensity of the TERS signal decays exponentially from a surface with a decay length of 2.8 nm.33 Thus, it is very likely that the TERS experiments measured a region several nanometers in depth from the surface of the nanocomposites. The SBR sample contained 25 wt % styrene units. In some TERS spectra where SBR signals were mainly observed, signals from the phenyl rings were not observed. The sizes of the styrene and butadiene units were far smaller (less than 1 nm) than the spatial resolution of our TERS experiments. Therefore, it is difficult to explain the dramatic spectral changes only in terms of the difference in the distribution of styrene units. The TERS spectral differences generated by SBR between the spectra in Figure 5a,b suggest that the local orientation of the polymer nanocomposites at the interface between the polymer and the filler was different from that of the polymer areas without MWCNTs in the surface region. The observed results regarding the spectral variations induced by SBR led us to consider the following possibility: In general, a Raman signal is strongly enhanced if its polarizability component is perpendicular to a metal surface. An enhanced electric field decays exponentially from a tip surface. Therefore, the TERS experiment measures only the surface region of the polymer nanocomposites. Figure 7
Figure 5. Enlarged TERS spectra of 1 phr SBR/MWCNTs from Figure 4: (a) spectrum 7, (b) spectrum 3.
were observed to be relatively weak in the TERS spectrum arose from the aromatic C−H groups and the phenyl rings of the polymer side chains, respectively. In contrast to Figure 5a, the TERS signals in Figure 5b due to MWCNTs at 2698, 1604, and 1354 cm−1 appeared strongly, and a peak at 3064 cm−1 due to aromatic C−H was clearly observed. The intensities of these bands are plotted in Figure 6. In the TERS spectra, the intensity of the 2990 cm−1 signal (vinyl) decreased as the intensity of the 1604 cm−1 signal (G band) increased (Figure 6a). On the other hand, the signal intensity at 3064 cm−1 (phenyl) increased as the intensity of the G-band signal increased (Figure 6b). In the Raman spectra,
Figure 7. Schematic diagram of the surface structure of SBR/ MWCNT nanocomposites: (a) far from and (b) near the CNT area. At the interface between the polymer and the filler, the phenyl ring should be oriented perpendicular to the surface through a π−π interaction between the CNT and the phenyl ring.
illustrates the expected structure of the surface of the SBR/ MWCNT nanocomposites. In areas of polymer nanocomposites without MWCNTs, it was expected that the CC groups and the phenyl rings would be parallel to the polymer surface. In this orientation, the CC signal would be much more enhanced than that of the phenyl ring. However, at the interface of the polymer and the filler (Figure 7b), it is likely
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Figure 6. Peak intensity plots of (a) the vinyl band (2990 cm ) versus the G band (1604 cm−1) in the TERS spectrum, (b) the phenyl band (3064 cm−1) versus the G band (1604 cm−1) in the TERS spectrum, (c) the vinyl band (2990 cm−1) versus the G band (1604 cm−1) in the normal Raman spectrum, and (d) the phenyl band (3064 cm−1) versus the G band (1604 cm−1) in the normal Raman spectrum. 1439
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that MWCNTs had a significant interaction with the phenyl rings of the polymer through π−π interactions. These interactions induce differences in the orientations of the polymer chain and its side residue. It is very likely that the orientation of the phenyl rings at the interface is perpendicular to the surface because of the π−π interaction between the CNTs and the phenyl rings shown in Figure 7b. It seems that the TERS spectra detected these changes very clearly. To explore these results more thoroughly, additional experiments, such as imaging of SBR/MWCNT nanocomposites, are required.
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CONCLUSIONS The present study demonstrated that TERS spectroscopy has great potential in exploring interactions at interfaces in polymer nanocomposites. We measured TERS spectra of SBR/ MWCNT nanocomposites at different positions and observed marked position-dependent spectral variations. In the TERS spectra, the relative intensities of the MWCNT and SBR bands changed from place to place, and the relative intensities of the vinyl and phenyl bands also changed by location. The former finding reflects the dispersion of MWCNTs in the polymer nanocomposites, and the latter finding suggests that the π−π interaction between the phenyl rings and MWCNTs affects the orientation of the phenyl rings, leading to a modification of the local orientation of the polymer chains.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81-795-65-8349. Fax: +81-795-65-9077. E-mail: ozaki@ kwansei.ac.jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009−2013. We thank Prof. Liliane Bokobza, ESPCI ParisTech (École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris), for kindly providing the samples.
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REFERENCES
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