Structure and Mechanical Properties of Polybutadiene Thin Films

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Structure and Mechanical Properties of Polybutadiene Thin Films Bound to Surface-Modified Carbon Interface Koichiro Hori, Norifumi L Yamada, Yoshihisa Fujii, Tomomi Masui, Hiroyuki Kishimoto, and Hideki Seto Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01457 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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Structure and Mechanical Properties of Polybutadiene Thin Films Bound to SurfaceModified Carbon Interface Koichiro Hori1,*, Norifumi L. Yamada1, Yoshihisa Fujii2, Tomomi Masui3, Hiroyuki Kishimoto3, Hideki Seto1 1

Institute of Materials Structure Science, High Energy Accelerator Research Organization, 203-

1 Shirakata, Tokai, Ibaraki 319-1106, Japan 2

Department of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577

Kurimamachiya, Tsu, Mie 514-8507, Japan 3

Sumitomo Rubber Industries Ltd., 1-1, 2-chome, Tsutsui-cho, Chuo-ku, Kobe 651-0071, Japan

Keywords: bound rubber, interface, neutron reflectivity, X-ray reflectivity, wear test

The structure and mechanical properties of polybutadiene (PB) film on bare and surfacemodified carbon films were examined. There was an interfacial layer of PB near the carbon layer whose density was higher (lower) than that of the bulk material on the hydrophobic (hydrophilic) carbon surface. To glean information about the structure and mechanical properties of PB at the carbon interface, a residual layer (RL) adhering on the carbon surface, which was considered to

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be a model of "bound rubber layer," was obtained by rinsing the PB film with toluene. The density and thickness of the RLs were identical to those of the interfacial layer of the PB film. In accordance with the change in the density, normal stress of the RLs evaluated by atomic force microscopy was also dependent on the surface free energy: the RLs on the hydrophobic carbon were hard like glass, whereas those on the hydrophilic carbon were soft like rubber. Similarly, wear test revealed that the RL on the hydrophilic carbon could be peeled off by scratching under a certain stress, while the RLs on the hydrophobic carbons were resistant to scratching.

Introduction The interface between polymers and inorganic materials is of pivotal academic and industrial interest. An example is the interphase in nanocomposites, a class of materials composed of polymers (matrix) and inorganic nanopowders (fillers).[1-3] In general, the performance of a nanocomposite strongly depends on not only the physical properties of the matrix but also the interaction between the matrix and the filler material. It is noteworthy that the physical response of a composite can be modulated via the interactions resulting from the attachment/detachment of polymers on fillers.[1,4,5] This means that an understanding of the interfaces between the polymer matrix and inorganic fillers is essential to construct highly functionalized nanocomposites. A specific example are tire materials; because the rubber for tires consists mainly of a polymer elastomer matrix and carbon black fillers, an understanding of the interface between the polymer and carbon is important for improving quality.[6-8] It is well known that a nanometer-thick polymer layer, the so-called bound rubber layer (BRL), is formed on carbon fillers, which is


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resistant to dissolution even in a good solvent for the polymer.[9] As in the case of other nanocomposites, it is empirically known that the BRL plays an important role in improving the tensile strength, tear strength, fatigue resistance, and wear resistance of tires[10]. There have been significant efforts in the industry to manufacture tires having a good performance by making a surface-modified filler.[11,12] As the structure and/or thermal molecular motion of the polymer at the filler/polymer interface are affected by the interactions with the filler, it is believed that surface modification changes the property of the BRL, resulting in an improvement of the performance of the tires.[11,13,14] However, this is still a hypothesis because of insufficient evidence and the accumulation of empirical data, e.g., on the influence of surface modification on the BRL, and the relationship between the BRL and the tire performance need to be evaluated to develop a guiding principle for improving tire performance. The structure and dynamics of BRLs have recently attracted attention, especially in the fields of polymer science and interfacial science. Low field 1H nuclear magnetic resonance (NMR) spectroscopy revealed that segmental motion of the elastomer molecules was suppressed in the vicinity of the carbon filler.[15-19] Atomic force microscopy (AFM) and electron microscopy images have indicated that the thickness of the BRL is 5–10 nm and is dependent on the chemical species of the filler.[20-22] Although the number of reports on the general BRL framework is increasing, it is still unclear how the surface modification of fillers affects the structural and mechanical properties of BRLs. To answer this fundamental but industrially crucial question, the hydrophilization treatment of carbon is performed to change the surface free energy, and the influence on the structure and mechanical properties of an elastomer bound to the surface, model BRL, is investigated by X-ray and neutron reflectometry, normal stress measurement, and wear test.


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Experimental 1. Materials and sample preparation 1,4-cis polybutadiene (PB, purchased from Ube Industries, Ltd.) with a number-average molecular weight (Mn) of 190 kg·mol−1 and molecular weight distribution (Mw/Mn) of 2.3, and deuterated 1,4-cis PB (d-PB, purchased from Polymer Source, Inc.) with a Mn of 90 k and Mw/Mn of 1.05 were used. The glass transition temperature (Tg) of PB and d-PB evaluated by differential scanning calorimetry (DSC) were 170 K and 173 K, respectively. Si wafers with 2-inch diameters were used as substrates, and model carbon films were prepared on the Si wafers by plasma-enhanced chemical vapor deposition using a home-made instrument. The deposition was performed using acetylene gas under a pressure of 10 Pa and a radio frequency power of 20 W for 80 s. The thickness of the carbon layers evaluated by X-ray reflectivity (XR) measurement (see next subsection for details) were approximately 55 nm (Figure S1). Then, the carbon films were irradiated with 6 W low pressure mercury lamp with a wavelength of 185 nm and 254 nm in steam atmosphere at room temperature to change the surface energy. In the UV irradiation, samples were placed under the lamp at the distance of approximately 10 mm. To investigate the effect of the surface modification, three kinds of carbon layers were prepared by changing the irradiation time from 0 to 1 h, and surface chemical composition of the carbon films was investigated by X-ray photoelectron spectroscopy (XPS, BL6N1 of Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan), with photon energy of 1.75 to 6 keV. In the XPS C1s core level spectra, peaks corresponding to neutral, hydroxyl and carboxyl carbon were observed, and the second and the last peak intensities were increased after the UV


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irradiation (Figure S2). To evaluate the surface free energies of the carbon films, contact angles of water and glycerol droplets were measured on the carbon films as shown in Figure S3. The contact angles taken at more than five different locations were listed in Table S1, and the surface free energies of the carbon films evaluated with the values using the Fowkes equation were approximately 30.1 ± 0.1, 45.2 ± 0.1, and 57.9 ± 0.3 mJ·m−2, respectively. Here, we call these carbon layers carbon-30, carbon-45, and carbon-58, respectively. PB films with a thickness of approximately 55 nm were spin-coated on the carbon layers from a toluene solution and dried under vacuum for more than 12 h at room temperature. After the evaluation by XR, the PB films were rinsed with a large amount of toluene 5 times to obtain residual layers (RLs) adhering on a carbon surface, which are model BRLs.[23] Then, the specimens were dried under vacuum for more than 8 h at room temperature and evaluated by XR again. The flatness and uniformity of the samples were confirmed by AFM (Nanonavi IIs, Hitachi High-Technologies Inc.) observations: the root-mean-square roughness of all the carbon layers, PB films, and RLs surfaces were less than 0.5 nm in the topographic images and no phase angle contrast is seen at the surfaces in the phase images with a scanning area of 10 × 10 µm2. 2. XR measurements An electron density profile of the films along the direction normal to the surface was examined by XR measurements using a Rigaku Smartlab diffractometer (Rigaku Co., Ltd.) with the incident angle θ varied from 0.30° to 3.0° in steps of 0.01°. The incident X-ray with a wavelength (λ) of 0.154 nm was guided into the specimen with a footprint of 20 × 20 mm2. The reflectivity profiles depending on the absolute value of the scattering vector, q = (4π/λ)·sinθ, were analyzed by fitting using the least-squares method on the basis of the electron density (ρe)


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profile along the depth direction using the Parratt32 software, which is a freeware program from the Hahn-Meitner Institut (HMI). For the fitting, we assumed stacking of layers having different

ρe and thickness (h) with a Gaussian roughness (σ) at each interface to represent the PB films on the carbon layer. A q resolution of 1% and statistic error of 0.4% were taken into account in the fitting, in which the statistic error was evaluated from standard deviation of the data at the total reflection region. 3. Neutron reflectivity (NR) measurements Interfacial diffusion of polymeric chains between the RLs and d-PB films on the RLs was examined by NR measurements with the SOFIA reflectometer installed at the Materials and Life Science Experimental Faculty, Japan Proton Accelerator Research Complex (MLF, J-PARC). For the measurements, the d-PB films with a thickness of approximately 100 nm were spincoated on the RLs from the toluene solution and dried under vacuum for more than 12 h at room temperature. After the measurements for the dried samples were completed, the specimens were annealed under vacuum for more than 8 h at 453 K, and NR measurements were performed after annealing. Incident neutrons with a λ ranging from 0.25 to 0.88 nm were guided into the specimen with a θ of 0.3, 0.7, and 1.6° and a footprint of 40 × 30 mm2. The obtained profiles were analyzed in a manner similar to XR analysis, and the scattering length density (b/V, b is the scattering length of the nucleus and V is the unit volume) of the neutrons was used to calculate the reflectivity. A q resolution of 3% was used in the fitting. 4. Mechanical property measurements So far, many attempts have been made to examine the mechanical properties of polymer thin films. For example, an indentation (nano-indentation) after pressing the surface of a specimen


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was used to evaluate the elastic modulus,[24] and lateral force microscopy (LFM) was used for surface Tg evaluation.[25] However, the RLs in this work are so thin that their mechanical properties cannot be effectively analyzed using these methods: the properties of the bottom layer greatly affect the deformation of the thin RL upon nano-indentation and the force response of the thin RL is too weak to be detected by LFM. Therefore, the normal force of the RLs was simply measured on the basis of force-distance curves derived from AFM observations at room temperature, as proposed by Aimé et al.[26] A cantilever made of Si with a spring constant and Young’s modulus of 1.6 N·m−1 and 169 GPa, respectively, was used to evaluate the normal force (the spring constant in the specification was used in the evaluation), and an approach-retract scan was performed at a speed of 20 nm·s−1. To avoid contamination of the cantilever by the PB films, each measurement was performed using a fresh cantilever. In addition, a wear test was performed to evaluate the wear resistance of RLs by AFM at a constant force. A cantilever made of Si with a spring constant and Young’s modulus of 42 N·m−1 and 169 GPa was used. Scratching to a length of 5 µm was performed with a tip scan speed of 2.0 µm·s−1 at a normal load of 10 µN for 10 cycles. Then, a 2 × 2 µm surface was observed for imaging the wear mark by using the same cantilever tip, and the force-distance curves at the wear mark were measured using the same cantilever and procedure as those for the normal force measurement mentioned before. For comparison, a carbon film surface was also scratched and observed by the same procedure.

Results and discussion


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Figure 1(a) shows the XR profiles for the PB films on the carbon layers. Open symbols denote experimental data and solid curves show the calculated reflectivity based on the model electron density (ρe) profiles shown in Figure 1(b). For clarity, each data set of the PB films is offset by two decades. In the fitting, thickness (h), ρe, and the interfacial roughness of SiOx (ρe = 7.84×102 nm−3) and the carbon layer (ρe = 5.32×102 nm−3) were fixed to be those obtained by the XR result without PB. Here, we confirmed that ρe of the carbon layer does not change through the UV irradiation beforehand. Note here that the XR profiles cannot be reproduced well, especially in the cases of carbon-30 and carbon-58, under the assumption that one PB layer having a uniform density is on the carbon layer. Therefore, we introduced an interfacial layer PB1 between the bulk layer PB2 and the carbon layer as shown in Figure 1(c); reflectivity profiles in good agreement with the experimental data were obtained under the constraint that ρe of the PB2 (ρePB2) layer was fixed at 3.04 × 102 nm−3 calculated from the density of the bulk PB (0.89 g·cm−3).[27] The XR result suggests that the interfacial layer of PB has a density different from that of the bulk material, which is consistent with the results of previous experimental studies.[28,29] Interestingly, the obtained fitting parameters summarized in Table 1 show that the ρe of the PB1 (ρePB1) layer changed systematically with the surface property whereas the h of the PB1 (hePB1) unchanged within the statistic error: ρe1 on carbon-58, i.e., the hydrophilic carbon, was the smallest, while that on carbon-45 was intermediate, and that on carbon-30, i.e., the hydrophobic carbon, was the largest. Indeed, several simulations and theoretical works predict that the interaction can change the density of a matrix in the vicinity of a filler surface: a decrease in density is predicted for non-adsorptive surfaces, while an increase in density is predicted for strongly adsorptive surfaces.[30-33] To investigate the effect of the surface energy


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further, we washed the PB films with toluene and evaluated the structures of the RLs on the carbon films.

(a) 1

100

1. carbon-58 2. carbon-45 3. carbon-30

(b) 3

10−4

10−6

10−8

10−10

0

0.5

1.0

1.5

2.0

q / nm−1

Electron density･10−2 / nm−3

2

10−2

Reflectivity

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(c)

3

σPB2

3 1

2

σPB1

2

air PB2 (ρePB2)

hPB2

PB1 (ρePB1)

hPB1

carbon film 1. 2. 3.

1

0

0

carbon-58 carbon-45 carbon-30 10

20

SiOx Si 30

40

50

60

Distance from carbon film / nm

Figure 1 (a) X-ray reflectivity of PB films on a carbon film. Open symbols denote experimental data. Solid curves are calculated reflectivities on the basis of model electron density profiles shown in panel (b). (c) Schematic illustration of the model used for the film.

Table 1 Fitting parameters used in the XR measurement of PB films on carbon layer. hPB1 / nm ρePB1×10−2 / nm−3 σPB1 / nm hPB2 / nm

σPB2 / nm

PB on carbon-58 6.9 ± 0.6

2.93 ± 0.10

3.3 ±0.2 45.0 ±1.6 0.47 ± 0.10


3.09 ± 0.08

2.8 ±0.1 47.3 ±1.9 0.40 ± 0.15


3.28 ±0.08

3.2 ±0.2 48.5 ±1.5 0.39 ± 0.14

Figure 2(a) shows the XR profiles of the PB films on the carbon layers after rinsing. As RLs were required to reproduce the profiles, we performed the fitting under the assumption that one PB layer was on the carbon film. Here, only h, ρe, and surface roughness of the RL were adjustable parameters in the fitting and the other parameters of the carbon films were held


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constant as before. The schematic fitting model is illustrated in Figure 2(c). As is seen in Figure 2(b), the ρe values of the RLs were also dependent on the surface free energy of the carbon layers. Remarkably, the values of ρe were almost the same as those for the corresponding PB1 layers on the same carbon films. This result supports the existence of interfacial PB layers observed before the rinsing the PB films, and suggest that they are identical to the RLs after rinsing.

(a) 1

100


(b) 3

10−4

10−6

10−8

10−10

0

0.5

1.0

1.5

2.0

q / nm−1

Electron density10−2 / nm−3

2

10−2

Reflectivity

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(c)

3

σRL 2

1 1. 2. 3.

1

0

0

3

2

air RL (ρeRL)

hRL

carbon film

carbon-58 carbon-45 carbon-30

SiOx Si 5

10

Distance from carbon film / nm

Figure 2 (a) X-ray reflectivity of RL on carbon film. Solid curves show calculated reflectivities on the basis of model electron density profiles shown in panel (b). (c) Schematic illustration of the model for the RL. Table 2 summarizes film used thickness and mass density (ρ) of PB1 and RL. ρ was calculated from

ρe by using eq.(1).

ρ = ρ e M ⋅ {N A ∑ (ni zi )}

−1

,

(1)

where NA and M are the Avogadro constant and molecular weight, respectively, and ni and zi are the number of atoms and the atomic number of element i, respectively. Whereas the ρ values of PB1 and RL on carbon-58 were smaller than that of the bulk, those of PB1 and RL on carbon-45


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and carbon-30 were larger. Notably, the ρ values of PB1 and RL on carbon-30 were 0.98 g·cm−3, approximately 10% larger than that of the bulk. Provided PB maintains the glassy state even at room temperature (297 K), the ρ can be estimated to be 0.98 g·cm−3 from the thermal expansion coefficient and the density in the glassy state.[34] The increase in the mass density of the RLs leads to a decrease in the free volume of the molecules, and a suppression of the thermal molecular motion. In other words, the suppression of the thermal motion because of the interaction with the carbon surface could change the density of the RLs. This result motivated us to examine the thermal molecular motion of the RLs.

Table 2 Film thickness and density of the PB1 layers and RLs. hPB1 / nm ρPB1 / g･cm−3

hRL / nm ρRL / g･cm−3

PB1(carbon-58) 6.9 ± 0.6

0.87±0.01

RL(carbon-58) 6.3±0.7

0.88±0.01

PB1(carbon-45) 7.3 ± 0.4

0.92±0.01

RL(carbon-45) 7.4±0.8

0.92±0.01

PB1(carbon-30) 6.8 ± 0.9

0.98±0.01

RL(carbon-30) 6.6±0.7

0.98±0.01

For exploring the chain mobility of PB molecules in the RLs, a d-PB layer was mounted on the RL and the interface was evaluated by NR measurements. Unlike X-rays, substitution of hydrogen by deuterium in organic materials allows us to change the neutron optical index of the material without changing its physical and chemical properties. NR is a powerful tool for evaluating the interfacial diffusion of d-PB and h-PB caused by annealing. Figure 3 shows the NR profiles of the RL/d-PB bilayers on the carbon films before and after annealing at 453 K for more than 8 h. The schematic fitting model is illustrated in Figure 3(c). Here, we focused on the scattering length density (b/V) of RL, (b/V)RL, and the interfacial roughness, σint. Table 3


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summarizes the (b/V)RL and σint values before and after annealing. For comparison, (b/V) of the RL before adding d-PB calculated using the density in Table 2 are also listed in Table 3. First, (b/V)RL of the RL on carbon-58 after the formation of a d-PB layer was larger than that of the RL without the d-PB layer, and it increased upon annealing. This indicates that a part of the d-PB chains has already penetrated into the RL during spin coating of the d-PB layer, and the intermixing of the molecules between the RL and d-PB layers are enhanced by the annealing. In contrast, no change in (b/V)RL was observed and only σint increased upon annealing in the case of the RL on carbon-45 and carbon-30 (the change in the NR profiles was mainly caused by the change in the roughness of the d-PB surface, σd-PB, for all the cases). Note here that σint of carbon-45 increased to a greater extent than that of carbon-30; that is, the PB molecules in the RL on the carbon-45 were slightly more mobile than those on carbon-30. These results suggest that the mobility of the PB molecules in the RLs is strongly suppressed depending on the surface energy of the carbon films.

(b)

(a) 1

100

101 1



10−1 2

2

10−2

Reflectivity

10−3

Reflectivity

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3

10−4

10−6

3

(c) σdPB

10−5

σint 10−7

air d-PB

hd-PB

RL (b/V)RL carbon film

10−8

10−9

10−10

10−11

SiOx Si

0

0.5

1.0

q / nm−1

1.5

2.0

0

0.5

1.0

1.5

2.0

q / nm−1

Figure 3 Neutron reflectivity curves of (RL/d-PB) bilayer on the carbon film (a) before and (b) after annealing. Solid curves show calculated reflectivities on the basis of model scattering length density profiles. (c) Schematic illustration of the model used for the bilayer. ACS Paragon Plus Environment

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Table 3 Scattering length density of RLs (b/V)RL and interfacial roughness of (RL/d-PB) interface σint before and after annealing. For comparison, (b/V)RL of the RL before coating of the d-PB layer estimated from the density in Table 2 is also listed. RL without d-PB layer (b/V)RL / nm−2

Before annealing (b/V)RL / nm−2

(b/V) RL / nm−2

(0.43 ±

RL(carbon-45)

(0.47 ± 0.05)×10−4

(0.47 ± 0.05)×10−4 1.4 ± 0.2 (0.48 ± 0.05)×10−4

2.4 ± 0.4

RL(carbon-30)

(0.50 ± 0.06)×10−4

(0.50 ± 0.05)×10−4 0.9 ± 0.3 (0.50 ± 0.05)×10−4

1.2 ± 0.4

0.05)×10−4

1.3 ± 0.3 (2.28 ±

σint / nm

RL(carbon-58)

0.08)×10−4

(1.02 ±

σint / nm

After annealing

0.06)×10−4

2.6 ± 0.5

To further examine the influence of surface modification on the mechanical properties, we employed AFM for investigating the Young’s modulus of the RLs. When a cantilever tip approaches a specimen, the normal force drops upon contact because of the van der Waals force between them. Upon closer contact of the cantilever, the difference in the Young’s modulus can be detected as an increase in the normal force because the slope of the force-distance curve monotonically increases with the Young’s modulus. For example, the normal force immediately increased after contact with the carbon surface, whereas it increased very slowly in the case of the bulk rubbery PB surface (Figure S4). This indicates that the Young’s modulus of the carbon film is much higher than that of PB. For comparison, the force-distance curve of bulk polystyrene (PS, Tg is approximately 370 K), which is glassy at room temperature, was also measured, and it was confirmed that the spring constant of the cantilever was suitable for distinguishing the Young’s moduli of the rubbery and glassy polymers on the basis of a clear change in the slope. In addition, a large hysteresis was observed on the rubbery surface because the polymer adheres to the cantilever upon retraction. Figure 4 shows the force-distance curves of the RLs. In the case of the RL on carbon-58, the normal force slightly increased immediately after contact with the RL surface on approach, the slope becomes gradually steeper with closer contact, and the force curve presents a large


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hysteresis upon retraction. This suggests that the cantilever first detected the normal force of the RL, which has a thickness of 6.3 nm in the rubbery state; it then detected that of the carbon under the RL upon further contact and finally detected that modulated by the rubbery RL adhesion upon retraction. In contrast, the slope of the force-distance curves immediately increased for the RLs on carbon-45 and carbon-30. Table 4 summarizes the slope of the normal force against the Z-sample position on contact. The slopes of carbon-45 and carbon-30 were less than that of the carbon film but obviously more than that of the rubbery PB, and it was comparable to that of the glassy PS film. This increment of the slope, in other words, the enhancement of the stiffness of the RL on the hydrophobic carbons, implies the elevation of Tg far above the bulk one.[35, 36] Also, the Young's modulus (E) of the RLs were roughly estimated by Johnson–Kendall–Roberts two-point method at Z-sample position is −5 nm, namely, within a range of elastic deformation.[37, 38] As a result, E of RL on carbon-58 was a few hundreds MPa, which is smaller than those of RL on carbon-45 and carbon-30 having the E of a few GPa. This result also supports that the RLs on carbon-45 and carbon-30 seem not to be the rubbery state. So far, the change in the polymer dynamics at various interfaces has been intensively studied and many experimental works have reported the increase in Tg and the suppression of the dynamics above the bulk Tg [31,39-46]. To the best of our knowledge, the molecular orientation of the phenyl rings of a PS film at an interface with a sapphire substrate did not change at 100 K above the bulk Tg [31], but our result suggests a greater elevation of the Tg. As a complete study of the elevation of the Tg at the interface lies outside the scope of this work, we leave these details for future study, and we will instead focus on the mechanical properties of the RLs here.


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10

Normal force / nN

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5 0

1

−5

2

−10

1. 2. 3.

−15

carbon-58 carbon-45 carbon-30

−20 −50

3

0

50

Z-sample position / nm

Figure 4 Force-distance curve of RL on carbon films. Downwards arrows denote positions where slope of normal force against Z-sample position were obtained.

Table 4 Slope of normal force against Z-sample position for the samples. Slope / N･m−1 RL on carbon-58

0.02±0.01

RL on carbon-45

0.31±0.02

RL on carbon-30

0.31±0.02

Bulk carbon-58

0.38±0.02

Bulk carbon-30

0.38±0.02

Bulk PB

0.03±0.01

Bulk PS

0.32±0.02

Figure 5 summarizes the result of the wear test. As shown in Figure 5(a–c), the RL surfaces on carbon-30, carbon-45, and carbon-58 before scratching were flat with a root-mean-square


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roughness (RMS) of 0.3, 0.3, and 0.4 nm, respectively. These RMS values were almost identical to that obtained for the carbon film, 0.2 nm, shown in Figure 5(d). After the wear test, a scratch track was observed at the surface of the RL on carbon-58 (Figure 5(g)), although the carbon film and the RLs on the carbon-30 and carbon-45 remained smooth (Figure 5(e, f, and h)). The depth of the scratch was approximately 5 nm as shown in Figure 5(k), which is comparable to the thickness of the RL, 6.3 nm (see Table 2).This indicates that almost all of the RL was peeled off by scratching. In fact, the slope of the force-distance curves measured on the scratch was consistent with that of the carbon film (Figure S5). Thus, we concluded that the RLs on the hydrophobic carbons that have a high density and are not in a rubbery state showed good wear resistance, whereas the wear resistance of those on the hydrophilic carbon with a low density and in a rubbery state was poor.

µm

0

2

0

2

(f)

µm

(e)

0.0

0

(i)

10

Before After

0

−10

0

Height / nm

Height / nm

10

0

1 Distance / µm

2

µm

0

1 Distance / µm

2

µm

2

(h)

0 10

0

1 Distance / µm

2

2

µm

(l)

Before After

0

−10

0

2

µm

(k)

Before After

0

−10

0 10

(j)

2

0

2

µm

µm

2

0

2

µm

0

2

(g)

Height / nm

0

0

0

2

µm

2

(d)

µm

20

0

2

carbon film

2

µm

0

(c)

Height / nm

nm

0

0.0

RL on carbon-58

2

µm

(b)

µm

RL on carbon-45

2

µm

(a)

µm

RL on carbon-30 3.0

nm

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Before After

0

−10

0

1 Distance / µm

2

Figure 5 Topographical AFM images of the RLs and carbon film (a–d) before and (e– h) after scratching. (i–l) Sectional view along the lines in the AFM images.


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Conclusion In this work, interfacial layers of PB films on bare and surface-modified carbon films and RLs on the carbon films were examined. Figure 6 summarizes a schematic representation of the experimental results. An interfacial layer of PB was present near the carbon layer and the density and thickness were identical to those of the RL after rinsing, in which case, a denser layer was formed on the carbon having a more hydrophobic surface. The thermal diffusion between the RL and d-PB top layer, investigated by NR, indicated that the denser layer did not intermix significantly with the top layer. A consistent result was obtained for normal stress measurements based on AFM: the RLs on hydrophobic carbons were not in a rubbery state but were presumably in a glassy state, whereas those on hydrophilic carbon were in a rubbery state. If the RLs on the hydrophobic carbons were glassy at room temperature, the transition temperature increased by more than 120 K. Similarly, the wear test revealed that the RL on the hydrophilic carbon could be peeled off by scratching, whereas the RLs on the hydrophobic carbons were resistant to scratching.


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(a) Deuterated PB

not intermixing

PB Interfacial layer Hydrophobic carbon film

rinse

not rubbery

RL

not peeled RL Carbon film

(b) Deuterated PB

intermixing

PB Interfacial layer Hydrophilic carbon film

rinse

rubbery

RL

peeled RL Carbon film

Figure 6 Schematic representation of a possible structure and mechanical properties of RLs on (a) hydrophobic and (b) hydrophilic carbon films.

From these results, we conclude that the surface modification of the carbon affects the interaction between the PB molecules and the carbon surface, and the density and mobility of the PB molecules bound to the surface are altered depending on the strength of the interaction. In addition, a drastic modification of the mechanical properties of the RL on the hydrophobic carbon is confirmed by the normal stress measurement and wear test. This result seems


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consistent with an empirically known fact that the hydrophilization treatment makes the reinforcement of PB/carbon nanocomposites worse.[47,48] We believe that the method we proposed here is a new tool for the investigation of BRLs because it can be applied for evaluation of BRLs at various interfaces, and a deeper understanding of BRLs will play an important role in the material design of new-generation tires.

AUTHOR INFORMATION

Corresponding Author [email protected]

Author Contributions KH designed the study, analyzed data, and wrote the initial draft of the manuscript. NLY, YF, TM, HK and HS contributed to interpretation of data, and assisted in the preparation of the manuscript. All other authors have contributed to data collection and interpretation, and critically reviewed the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. NR measurements were performed on BL-16 at the Materials and Life Science Facility, JPARC, Japan, under program no. 2014S08 and no. 2016A0072.


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High density bound rubber on hydrophobic carbon

Low density bound rubber on hydrophilic carbon


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Figure 1 (a) X-ray reflectivity of PB films on a carbon film. Open symbols denote experimental data. Solid curves are calculated reflectivities on the basis of model electron density profiles shown in panel (b). (c) Schematic illustration of the model used for the film. 70x29mm (300 x 300 DPI)


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Figure 2 (a) X-ray reflectivity of RL on carbon film. Solid curves show calculated reflectivities on the basis of model electron density profiles shown in panel (b). (c) Schematic illustration of the model used for the RL. 71x29mm (300 x 300 DPI)


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Figure 3 Neutron reflectivity curves of (RL/d-PB) bilayer on the carbon film (a) before and (b) after annealing. Solid curves show calculated reflectivities on the basis of model scattering length density profiles. (c) Schematic illustration of the model used for the bilayer. 67x26mm (300 x 300 DPI)


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Figure 4 Force-distance curve of RL on carbon films. 67x54mm (300 x 300 DPI)


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Figure 5 Topographical AFM images of the RLs and carbon film (a–d) before and (e–h) after scratching. (i–l) Sectional view along the lines in the AFM images. 112x74mm (300 x 300 DPI)


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Figure 6 Schematic representation of a possible structure and mechanical properties of RLs on (a) hydrophobic and (b) hydrophilic carbon films. 189x210mm (300 x 300 DPI)


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Figure S1 X-ray reflectivity curve of carbon film on Si wafer. Solid curves are calculated reflectivity on the basis of model electron density profiles shown in the panel (b). 73x31mm (300 x 300 DPI)


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Figure S3 Photographs showing water (a-c) and glycerol (d-f) droplet on the surface of carbon-30 (a, d), 45 (b, e), and 58 (c, f) films. 29x10mm (300 x 300 DPI)


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Figure S4 Force-distance curve of carbon film, bulk PB and polystyrene film. 67x54mm (300 x 300 DPI)


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Figure S5 (a) Topographical images of RL on carbon-58. (b) Force-distance curve of the RL. The measurement points are depicted in panel (a). For comparison, the force-distance curve of carbon film is also shown in panel (b). 73x32mm (300 x 300 DPI)


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TOC 32x12mm (300 x 300 DPI)


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Figure S2 X-ray photo electron spectroscopy C1s core level spectra of (a) carbon-30 and (b) carbon-58 at emission angle of 45°. Both neutral C1s peaks were assigned a binding energy of 284.7 eV to correct for the charging energy shift. Red line denote an experimental data. Black dotted lines represent decomposed neutral, hydroxyl and carbonyl carbon peaks, and a black solid line corresponds to sum of them. 26x11mm (300 x 300 DPI)


Structure and Mechanical Properties of Polybutadiene Thin Films

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