*2.1. Materials*

Solution styrene butadiene rubber (SSBR) BUNA 2525-2 VSL HM containing 25% vinyl content and 25% styrene content was supplied by Lanxess, Cologne, Germany. Tetrahydrofuran (THF), N-butyl amine, sulfur, zinc oxide, and stearic acid were acquired from Acros Organics, Germany. The vulcanizing accelerators *<sup>N</sup>*-cyclohexyl 2-benzothioazolesulfonamide (CBS-Vulkacit CZ) and diphenyl guanidine (DPG) were delivered by Lanxess, Germany. The precursor for the in-situ silica generation tetraethoxyorthosilicate (TEOS) was obtained from Sigma Aldrich (Munich, Germany) with a purity of 90%. The precipitated silica (Ultrasil VN-3) exhibiting a BET surface of 175 m<sup>2</sup>/g and a

purity of 99% and the silane coupling agen<sup>t</sup> bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPT) were kindly provided by Evonik Industries, Essen, Germany.

### *2.2. Preparation of Silica Rubber Composites*

The preparation of the in-situ SSBR silica composites was performed in two separate steps. In the first step, in-situ silica-rubber masterbatches were produced and, in the second one, this polymer-silica masterbatch was mixed with the other rubber chemicals. For preparing masterbatches, 30 g of SSBR was dissolved in 300 mL of THF and, then, 0.1 mole of TEOS and 0.2 moles of water were added into a round flask. After forming a standard mixture, 0.025 moles of n-butylamine catalyst were added and the homogeneous solution was stirred at 60 ◦C for 4 h. The viscous liquid obtained was treated in an ultrasonic bath for 10 min to prevent any pre-agglomeration of silica particles inside the rubber. This solution was then carefully given into 900 mL of ethanol. This process immediately solidifies and precipitates the rubber mass. The precipitate was filtered and dried for two days at 40 ◦C in a circulating air oven. Using di fferent amounts of TEOS and water (always at the molar ratio 1:2) leads to a controlled variability of volume fractions of silica in the rubber. Scheme 1 shows the chemical reaction for the formation of the silica particles and the particle growth [20].

**Scheme 1.** Silica particle formation through sol–gel reaction reprinted from Reference [20] with permission from the Royal Society of Chemistry.

The second stage of preparation of in-situ silica composites consists of two sub-steps. In the first sub-step, in an internal mixer (HAAKE Rheomix 600P internal mixer, Karlsruhe, Germany), few grams of raw SSBR are added to the dried in-situ silica SSBR masterbatch. This is done to fine-tune the silica concentration in rubber and also to achieve the desired fill factor of 0.7 (70% filling). After adequate thorough blending, other rubber components as zinc oxide (2 phr), stearic acid (3 phr), and the TESPT coupling agen<sup>t</sup> were added sequentially. This compounding process was carried out at 110 ◦C and at 80 rpm for 6 min. The mixture was dumped at a temperature of ~140 ◦C. In the second sub-step, the vulcanizing chemicals CBS (1.4 phr), DPG (1.7 phr), and sulfur (1.4 phr) were mixed into the compound in a two roll mill (Polymix-110L, Servitec, Wustermark, Germany) at 50 ◦C for 10 min with a constant friction of a 1:1.2 ratio.

Likewise, commercial precipitated silica composites were prepared by the incremental addition of zinc oxide, stearic acid, silica (Ultrasil-VN3), and silane TESPT into the SSBR rubber in the internal mixer. Mixing was carried out at 110 ◦C and 80 rpm for 10 min. For the addition of vulcanization chemicals in the two-roll mill, a process similar to that used for in-situ silica was applied. The finished compounds of in-situ and precipitated silica were then allowed to age for 24 h at room temperature. The stored rubber compounds were rheometrically analyzed in a rubber process analyzer (Scarabaeus

SIS-V50, Scarabaeus GmbH, Wetzlar, Germany) to determine their optimal vulcanization time TC90. Vulcanization of these rubber samples was conducted at 160 ◦C into sheets of 2-mm thickness in a heated compression molding press (Fontijne TP1000, Delft, The Netherlands) with a force of 150 kN for optimum vulcanization time TC90 plus 1 min per mm thickness of the sample. The prepared silica-rubber samples are abbreviated as i-30, i-30s, x-30, x-30s. 'i' and 'x' represents in-situ and commercial precipitated silica filled composites, respectively. The number refers to the amount of silica in rubber (mentioned in 'phr'—parts per hundred rubber) and the suffix 's' after the number denotes rubber composites containing a TESPT silane coupling agent.
