*3.1. Morphology*

Figure 3a displays the x-ray diffraction analysis (XRD) of in-situ and precipitated silica powder samples. The broad XRD band is located in the 2θ region of 15◦ to 35◦. This confirms that the physical state of both silica types is entirely amorphous and there is no significant difference in the crystalline structure.

A dynamic light scattering measurement (DLS) was performed to characterize the two silica types in particle size (hydrodynamic diameter) and morphology. The precipitated silica powder particles (Ultrasil VN3) in Figure 3a display a bimodal distribution with a wide spectrum of different particle sizes. These silica particles are aggregated and agglomerated. The aggregate and agglomerate diameter sizes range from 150 to 1500 nm. The DLS results for the in-situ derived silica particles (see Figure 3b) give an average diameter of around 200 to 500 nm. The in-situ silica particles exhibit a mainly monodisperse or unimodal narrow particle distribution.

**Figure 3.** (**a**) X-ray diffraction patterns of in-situ silica and precipitated silica powders. Particle size histograms of (**b**) precipitated silica (Ultrasil-VN3) and (**c**) in-situ silica, from dynamic light scattering measurements.

SEM images of two silica powders and their respective SSBR rubber composites are given in Figure 4. Figure 4a shows how the precipitated silica particles look i.e., a network or chain-like structures with highly aggregated morphology. In Figure 4c, the in-situ silica particles display spherical aggregates with particle sizes from 200 to 400 nm. Nevertheless, the average primary particle sizes of both silica types appear to be in the order of ~10 to 15 nm. Figure 4b,d show the SEM pictograms of brittle fracture surfaces of 30 phr filled commercial precipitated silica and in-situ silica filled SSBR composites, respectively. The fractured images highlight the state of silica dispersion, which appears

to be significantly different for either of the silica. An improved dispersion is found in the in-situ silica system.

**Figure 4.** SEM images of (**a**) pristine commercial precipitated silica powder, and (**b**) its respective SSBR composite. (**c**) In-situ silica powder (extracted from un-crosslinked rubber) and (**d**) its respective SSBR composite.

### *3.2. Friction Coe*ffi*cients* μ *and Friction Curves*

Figure 5 shows the experimental friction results measured at different sliding velocities with a normal load of 2 bar. The experimental results show that the unfilled elastomer exhibits a higher friction coefficient. An incorporation of silica in SSBR reduces the friction coefficient slightly. Precipitated silica without any silane exhibits a lower coefficient of friction in comparison to their respective silane-modified systems. The application of silane coupling agents to the system filled with precipitated silica has significantly improved the friction behavior in the high velocity range. The in-situ silica-filled systems exhibited lower friction coefficient values when compared to the precipitated silica system. It should be noted that incorporation of the silane coupling agen<sup>t</sup> into the in-situ silica system did not affect the friction behavior. This result supports our earlier findings that in-situ silica has the capability to reinforce the rubber matrix without the need of any silane [20,21,31]. For good ABS (anti-lock braking or anti-skid braking) wet braking, the friction coefficient of the rubber system must be higher (or nearly equal to 1). Therefore, from the analysis, samples are rated as per their wet traction performance: Gum > x-30s > x-30 > i-30s > i-30.

On the other hand, rubber composites must display a lower friction coefficient to achieve lower rolling resistance properties. Lower rolling resistance contributes to better fuel efficiency of the vehicle. Lower rolling resistance can be observed when the rubber compounds display lower hysteresis and smaller tan δ in the temperature range around ca 60 ◦C as well as with a reduced rolling friction. From this perspective, the in-situ silica system will offer better low rolling resistance [21]. The samples rated as per their low rolling resistance (larger the better): Gum < x-30s < x-30 < i-30s < i-30.

**Figure 5.** Friction behavior of unfilled rubber, in-situ silica, and precipitated silica with and without addition of a silane coupling agent. The dotted line corresponds to ABS wet braking conditions at 40 ◦C and a slipping velocity of 1 m/s.
