Direct observation of dynamic interaction between a functional group in a single SBR chain and an inorganic matter surface

As a composite of hybrid organic-inorganic materials, blending hydrophilic silica microparticles with oil-extended rubber can improve vehicle tire performance but the nanometer scale effects of microparticle inclusion have not been thoroughly studied. Here, we used atomic force microscopy (AFM) video imaging to closely investigate the behavior of functionalized and unmodified styrene-butadiene rubber (SBR), as models for tire rubber, on mica surfaces. The hydrophilic silica microparticle surface could be simulated by a mica substrate because both have silanol groups on their surface. Using AFM video imaging, we tracked the behavior of individual SBR polymer chains on mica surfaces to reveal how polymer modification affects the interaction of SBR with mica surfaces. We measured the diffusion coefficients and spring constants of single SBR polymer chains for the first time, demonstrating that it is possible to parameterize the relationship between the molecular dynamic structure of a polymer and rubber properties of the vulcanized compound.

. Diffusion coefficients of a single SBR chain

Synthesis of unmodified SBR and carboxyl--functionalized SBR
Chart S1 According to a previously reported method, 1,2 unmodified styrene--butadiene rubber (SBR) (Chart S1) was synthesized by a living anionic polymerization reaction of styrene and 1,3--butadiene using n--butyllithium as the initiator. After completion of the copolymerization reaction, in order to synthesize carboxyl--functionalized SBR (Chart S1), 3--mercaptopropionic acid (the thiol, 2) and lauroyl peroxide (radical initiator, 3) were added under stirring. 3 The product was dried under reduced pressure. Four equivalents of 3--mercaptopropionic acid (2) to one equivalent of the living anionic polymerization initiator were added to the reaction system.
The content of the styrene unit in the polymer was determined from the refractive index using the Abbe refractometer (NAR--3T, ATAGO, Tokyo, Japan) according to international standard ISO 2453:1991. The amount of vinyl unit in the polymer was determined from the infrared absorption intensity. The absorption peak at around 911 cm --1 is that of the 1,2--vinyl unit, the peak at around 735 cm --1 is that of the 1,4--cis unit, and the peak at around 967 cm --1 is that of the 1,4--trans unit. Fourier transform infrared spectrometer (FT/IR--470 plus, JASCO, Tokyo, Japan) was used. Regarding the reaction mechanism (Scheme S1), after hydrogen abstraction from thiol 2 by radical initiator 3, the resulting thiyl radical 4 was added to vinyl group 1 in the SBR chain. The generated radical 5 abstracted a hydrogen atom from another thiol to form a stable structure (6). In each reaction cycle, a thiyl radical (4) was generated anew, which attacked another vinyl group, inducing a chain reaction. Because 3--mercaptopropionic acid was used as the thiol (2) in this reaction, carboxyl groups were introduced into the polymer chain.

AFM video imaging and the analytical method
According to the previously reported method, 1,2 an unmodified SBR and a carboxyl-functionalized SBR (Chart S1 and Scheme S1) were synthesized. A dilute tetrahydrofuran (THF) solution (< 1 × 10 −6 w/v) of unmodified SBR (M w : 1.54 × 10 5 , M w /M n : 1.02) and carboxyl--functionalized SBR (SBR--(COOH)--: M w : 2.14 × 10 5 , M w /M n : 1.17) was prepared, respectively. Dehydrated THF (Kanto Chemical, Tokyo, Japan) was used to prevent aggregation of the SBR polymer chain. A freshly cleaved mica surface of the muscovite substrate (Nilaco, Tokyo, Japan) was obtained using adhesive tape, and any adsorbed water on the mica surface was removed by rinsing with dehydrated THF in a dry air atmosphere (RH < 25%). The samples were prepared by casting the dilute THF solution of the polymer (1 μL) onto a mica substrate. The substrate was rinsed with THF (1.0 mL) after standing for ca. 20 s to remove excess polymer chains and leave isolated chains on the substrate. As another method, a sample was prepared by spin--casting (1,500 rpm) the dilute polymer solution (1 μL) onto a mica substrate in a dry air atmosphere.
Through the bonding/interaction of the exposed hydroxyl groups on the mica surface with the functional groups in the SBR polymer chains, each single chain of the functionalized SBR polymer was dispersed as shown in Fig. 1B and adsorbed onto the mica surface in a suitably stretched state. If the dilute solution of the polymer is cast/dried on the substrate, the polymer chains aggregate and form globules easily, so the above technique is necessary.

Chart S2.
We modified the specifications of a fast--scanning atomic force microscope (NVB500, Olympus, Tokyo, Japan) in dynamic (tapping) mode to observe isolated polymer chains. 4

Spring constant of a single polymer chain
The relationship between the energy stored by a spring (E) and thermal energy (k B T) can be described as follows: .
Here, k chain is the spring constant, and x is the displacement of the spring. The In terms of molecular stiffness, this value is close to the soft stiffness of a biomacromolecular chain of the myosin subfragment 2 of single myosin molecules in myofilaments. 8 In addition, the force at x = 17.2 nm is, [pN]. In addition, the force at x = 2.84 nm is, [pN].
Furthermore, the spring constant and the force of each segment between the four anchor points (a, b, c, and d, see Fig. 4A) were also calculated.
The spring constant k ab , k bc , and k cd were 6.55, 11.0, and 8.30 [pN/nm], respectively. For example, spring constant between anchor points a and b was indicated as k ab .
Here, the displacement of the spring x ab , x bc , and x cd were 1.12, 0.866, and 0.995 [nm].
Chart S3 shows the classical spring model.

Relationships between tire performance and single polymer chains
The macromolecular movement of the functionalized SBR in an organic medium on mica is considered to resemble that of an actual functionalized SBR with silica in a tire.
Therefore, observing and quantifying such behavior should reveal the factors affecting tire performance, such as energy loss and wet grip, at a molecular level. The wet--grip performance (tan δ at 0 °C) of a vulcanized silica--blended functionalized SBR was 1.4 times higher than that of a silica--blended unmodified SBR. 2 This result strongly correlates with D of a single polymer chain (Table S1). The D values of a single chain of carboxyl--functionalized SBR ranged from 0.03 to 5.97 nm 2 /s (Fig. S6), while those of an unmodified SBR were from 0.09 to 33.2 nm 2 /s (Fig. 2D). Because the D values of carboxyl--functionalized SBR were low and within a narrow range, its glass transition temperature (T g ) was higher than that of the unmodified SBR. This result indicates that the macromolecular motion of carboxyl--functionalized SBR was strongly controlled by the interaction between the functional groups and mica (or silica) in an organic medium at 25 °C. In addition, the controlled polymer chain motion of the functionalized SBR was measured as an increase in spring constant (k; N/m). This is the origin of the high tan δ (maximum) of silica--blended functionalized SBR. These measured values for single polymer chains on mica in an organic medium correlate with T g values (as the temperature at maximum tan δ) of SBRs in silica--blended compounds. Actually, T g of a silica--blended oil--extended rubber containing carboxyl--functionalized SBR was -14 °C, and T g of that with unmodified SBR was -20 °C. 2 The rolling resistance of tire performance (tan δ at 60 °C) of a vulcanized compound of a silica--blended rubber containing carboxyl--functionalized SBR was smaller than that of silica--blended rubber of unmodified SBR. 2 This result also strongly correlates with D of a single polymer chain (Table S1). The D values (D 2 --D 18 ) of carboxyl--functionalized SBR were lower than those of unmodified SBR; that is, by introducing the functional groups into the SBR polymer chain, excessive movement in the polymer chain containing the chain end was moderately suppressed.