The effect of POSS nanoparticles on crosslinking of styrene-butadiene rubber nanocomposites

The effect of octaisobutyl-polyhedral oligomeric silsesquioxane (OIB-POSS) as a nanosized reinforcement on the cure kinetics, crosslinking density, and mechanical properties of styrene-butadiene rubber (SBR) nanocomposites was examined in this study. For this purpose, SBR compounds with various OIB-POSS nanoparticle loadings at 1, 3, and 5 phr were prepared and their results were compared with a reference compound without OIB-POSS. When 1 phr of OIB-POSS was added to the rubber matrix, the elongation at break values and tensile strength of the corresponding nanocomposite increased by 24.1% and 29.2% compared to the reference sample, respectively. The presence of OIB-POSS nanoparticles and their random distribution in the SBR matrix was confirmed by transmission electron microscopy. The crosslinking density of nanocomposites was calculated by the Flory-Rehner method and a decrease was observed with the addition of OIB-POSS nanoparticles. In addition, thermal aging process as 70 °C for 70 h was applied to vulcanized samples. It was noted that the mechanical properties of SBR/OIB-POSS nanocomposites remarkably improved, whereas their crosslinking densities gradually decreased after thermal aging.

In this study, it was aimed to use OIB-POSS as a potential reinforcing agent for SBR, unlike our previous study, in which POSS nanoparticles containing reactive groups were chemically bonded to the SBR matrix and involved in the vulcanization mechanism. The effect of OIB-POSS nanoparticles added to the SBR matrix as nonreactive reinforcement nanofillers at 1, 3, and 5 phr on the rubber compounds was investigated through crosslinking, mechanical, physical, and rheological properties. The change in the properties of SBR nanocomposites after the thermal aging process was also examined.

Preparation of SBR/OIB-POSS nanocomposites
A laboratory type banbury was used for the preparation of SBR/OIB-POSS nanocomposites according to the procedure of our previous study [5]. The details of formulation are listed in Table 1. First, SBR was masticated in banbury for 2 min to prepare rubber nanocomposites. Subsequently, OIB-POSS nanofiller selected to be 1, 3, or 5 phr was added to the masticated SBR and compounded in the same mixer for 1 min. This compound was mixed with ZnO as activator, and SA as lubricant for 0.5 min. Then, mixing of stabilizers (TMQ, IPPD, and ozone wax) was carried out for another 0.5 min. Finally, the remaining components (CBS, S, and TMTD) were included into the compound and mixed for another 1 min. Overall, all SBR/OIB-POSS compounds were obtained after 5 min of total mixing process at about 80 °C.
The SBR/OIB-POSS compounds were successively vulcanized using a hydraulic hot press at 160 °C. During the vulcanization, their optimum cure times were determined by a moving die rheometer (MDR). Test specimens were obtained by cutting in accordance with the required dimensions and standards of tests to be applied to the compounds. Rubber nanocomposites were kept in an air-circulating oven for 70 h at 70 °C to investigate the effect of thermal aging.

Characterization
The rheometer parameters of the SBR/OIB-POSS nanocomposites and the reference compound were determined by a moving die rheometer (MDR, Alpha Technologies) in accordance with ASTM D5289 (2019). The chemical structures of  SBR/OIB-POSS nanocomposites were characterized by ATR technique using Spectrum Two, Perkin-Elmer (USA) Fourier-Transform Infrared Spectrum (FT-IR) equipment at room temperature. Mechanical properties of reference sample and rubber nanocomposites were determined with an Instron Universal Tester (Model 3345) in accordance with ASTM D412 (2019). The speed of the crosshead was applied to be 500 mm/min. Zwick Shore A type durometer in accordance with ASTM D2240 (2015) was used to test the hardness of vulcanized nanocomposites. Also, compressions set tests of samples were performed properly with ASTM D395 (2018). To examine the dispersion of OIB-POSS nanoparticles in the rubber matrix and the morphology of SBR/OIB-POSS nanocomposites, transmission electron microscopy (TEM) analysis was performed with the instrument Joel JEM-2100 (UHR) Gatan, (USA) at 300 kV. Crosslinking densities of cured and aged SBR/OIB-POSS nanocomposites were determined by equilibrium solventswelling ratios in toluene, densities of the rubber matrix and toluene [18], volume fractions of the polymer and solvent [19], and polymer-solvent interaction parameter [19][20][21] according to Flory-Rehner equation.

Results and discussion
In this study, influence of nonfunctionalized OIB-POSS nanoparticles on the rheological, physical, mechanical, morphological, and crosslinking properties SBR nanocomposites were investigated. For this purpose, the OIB-POSS nanofiller at 1, 3, and 5 phr was intensively mixed with SBR and then were vulcanized at 160 °C. The effect of thermal aging (70 hours at 70 °C) on SBR/OIB-POSS nanocomposites was also investigated.

Rheological characteristics
Rheometer data with important parameters of the reference compound and rubber nanocomposites were determined at 160 °C. The average results of rheometer tests repeated three times are presented in Table 2. The minimum torque (ML) values were proportional to the composition viscosity.
The ML values of the SBR nanocomposites and the reference sample were quite similar; however, the maximum torque (MH) values were remarkably decreased by increasing OIB-POSS loading. In addition, the cure extent (CE) value, which is related to the crosslinking degree of rubber and calculated from the difference between MH and ML values in the literature, decreased as the amount of OIB-POSS increased [22]. This may be because bulky POSS cages both increased the steric barrier between double bonds and sulfur atoms and decreased the crosslinking density in the rubber nanocomposites. Moreover, the optimum curing time (t 90 ) values of SBR/OIB-POSS nanocomposites did not change significantly with the increase of OIB-POSS amount compared to the reference compound. The absence of a significant change in optimum curing time in the presence of OIB-POSS could be attributed to the fact that sulfur was not consumed in the early stage of vulcanization and there was no dominant reaction between OIB-POSS and sulfur.
One of the important parameters characterizing the vulcanization of rubber compounds is the cure rate index (CRI) and it was calculated with the given equation. (equation 1) According to the rheometer test performed at 160 °C, the CRI values of SBR/OIB-POSS nanocomposites were significantly lower than those of the reference sample. Therefore, it could be concluded that the presence of OIB-POSS in the SBR matrix reduced the curing rate. The decrease in CRI values during the vulcanization reaction indicated to reduce the crosslink density in the rubber nanocomposites [8]. the increase in hydroxyl and carbonyl bands of these products was the proof of the successful hydroxyl and carbonyl bonds. In our case, the FT-IR bands assigned the hydroxyl and carbonyl groups were clearly visible in the spectrum of the aged (SBR/OIB-POSS-1 (A)) sample.

Mechanical properties
The mechanical properties of SBR/OIB-POSS nanocomposites, in comparison with the conventional SBR reference compound, are shown in Figures 4 and 5 and summarized in Table 3. The average results of mechanical tests repeated five times are given with standard deviations. Among the prepared samples, the tensile strength of SBR/OIB-POSS-1 nanocomposite increased from 1.41 MPa to 1.75 MPa, with an improvement of 24.1% compared to the reference specimen. The dispersion or agglomeration of nanoparticles, interaction between SBR matrix and nanofillers, and particle distribution in cross-linked three-dimensional networks are of great importance in terms of mechanical strength of rubber nanocomposites [5]. Also, the tensile strength of the compounds is highly dependent on the amount and type of filler [4]. So, it was concluded that the good interaction between the nano-sized OIB-POSS reinforcement and the SBR matrix led to high mechanical strength. Compared to the reference sample, the elongation at break values increased by 29.2% when 1 phr OIB-POSS was added to the rubber matrix. The elongation at break and tensile stress values of the obtained nanocomposites gradually decreased due to possible agglomerations and lower degree of crosslinking with the addition of more OIB-POSS, which could be supported by

FT-IR characterization
The insertion of OIB-POSS into the SBR matrix was studied by FT-IR spectroscopy equipped with a diamond attenuated total reflection (ATR) instrument at room temperature. The FT-IR spectra of the reference and SBR/OIB-POSS-1 nanocomposite samples in nonvulcanized, vulcanized (V), and aged (A) steps are given in Figures 2 and 3 [7,20,23]. The complete disappearance of peak at 1638 cm -1 assigned to double bond of SBR matrix was the proof of the successful curing process (REF V). Moreover, the increase in peak intensity of bonds of hydroxyl and carbonyl groups in the aged reference compound spectrum (REF A) verified oxidative aging ( Figure 2). Additionally, the stretching vibrations of aliphatic C-H peaks at 2917 cm -1 and 2847 cm -1 were evidently seen for SBR matrix. The bands at 1638 cm -1 and 968 cm -1 were attributed to the cis-CH= and trans-CH=CH-peaks of SBR, respectively. The aromatic substitution, another characteristic peak of SBR, was detected at 698 cm -1 [7,[24][25][26]. The presence of OIB-POSS in the SBR nanocomposites was confirmed by detecting the Si-O stretching band at 1129 cm -1 for the POSS groups in the FT-IR spectrum of the SBR/OIB-POSS-1 nanocomposite [27][28][29]. The successful vulcanization process was also verified by the absence of the peak at 1638 cm -1 in the FT-IR spectrum of vulcanized (SBR/OIB-POSS-1 (V)). In the literature, thermal oxidative aging of SBR results in the formation of oxygenated chemicals such as anhydrides, peresters, carboxylic acids, ethers, and alcohols [30,31]. Therefore,   previous rheological data (Table 2). However, even in the SBR/OIB-POSS-5 nanocomposite, which was expected to exhibit the most possible agglomeration, both elongation at break and tensile stress values were greater than those of the reference sample. Tensile stress and elongation at break values of SBR/OIB-POSS nanocomposites decreased after thermal aging. It was also noted that the mechanical properties of aged SBR/OIB-POSS nanocomposite samples were significantly higher than the reference sample. On the other hand, the 50% and 100% tensile modulus values of the SBR/ OIB-POSS nanocomposites were lower than the reference compound ( Figure 4) due to the decrease in crosslinking density by addition of OIB-POSS into the rubber compounds. After thermal aging process, both 50% and 100% tensile modulus of aged SBR/OIB-POSS nanocomposites were greater than those of the vulcanized samples [30,[32][33][34]. The 50% and 100% tensile modulus values of the SBR/OIB-POSS nanocomposites were lower than the reference compound ( Figure 5). The addition of OIB-POSS to the rubber compounds gave rise to a decrease in crosslinking density and modulus values. In addition, 50% and 100% modulus values of rubber nanocomposites increased due to increased stiffness after thermal aging [30,[32][33][34].
Shore A hardness and compression tests were performed according to ASTM D2240 (2015) and D395 method B (2018) to determine the effect of OIB-POSS nanoparticle loading on the resistance to indentation, permanent deformation of samples and to observe how the elastic properties change after prolonged compression at room temperature for 22 h and after thermal aging process at 70 °C for 70 h [35]. It was noted that the addition of OIB-POSS nanoparticles into the rubber compounds gave rise to a decrease in hardness values ( Figure 6). Additionally, thermal aging process increased stiffness of the nanocomposite samples yielding the higher Shore A hardness values. It was also revealed that the permanent deformation value decreased when 1 phr of OIB-POSS was added to the reference sample but increased gradually with the addition of more OIB-POSS nanoparticles, which could be due to the change in crosslink density (Table 3) [36].

Crosslinking density
The crosslinking densities (ν) of both vulcanized and aged SBR/OIB-POSS nanocomposites were determined by swelling measurements in toluene solvent using Flory-Rehner equation. The average results of the crosslink densities calculated from the three swelling tests are given in Figure 7 with standard deviations. It was determined that the crosslink densities of the nanocomposites decreased with increasing OIB-POSS concentration up to 3 phr. This could be attributed to possible agglomerations and inhomogeneous crosslinking points, which was also consistent with the rheological and mechanical tests data. During the thermal aging process of SBR, several complicated interactive reactions can take place, which causes the chain scission of SBR compound to soften and lose its elastic properties, while reducing the crosslink density. Therefore, the observed decrease in crosslink density of the samples after thermal aging of rubber nanocomposites could be attributed to potential chain breaking due to heat during the thermo-oxidative aging process [37,38].   The reinforcement mechanisms in rubber nanocomposites are complex due to the multicomponent heterogeneous system and cross-linked structure. In addition to the interactions between filler-filler and filler-rubber, the particle size, specific area and dispersion level also affects the reinforcement of composites. Therefore, the decrease in crosslinking density cannot normally be explained by a single simple theory. Currently, the reinforcing mechanisms of the rubber nanocomposites have drawn significant interest with an emergence of novel fillers and various matrix/filler combinations [39].

Morphological properties
The morphological properties of the SBR/OIB-POSS nanocomposites were evaluated by TEM analysis from the rupture surface of the corresponding tensile sample. TEM images of the SBR/OIB-POSS-5 nanocomposite demonstrated fairly continuous and homogeneous distribution with some very rare OIB-POSS aggregates visible as black clusters between 50-200 nm (Figure 8-a). These dark clusters were intersections of the silicate layer bundles dispersed in styrene-butadiene rubber. Furthermore, at a higher magnification (Figure 8b), cubic OIB-POSS nanoparticles with diameters ranging from 2 to 5 nm, close in size to a single POSS molecules, were randomly distributed. Moreover, some aggregated OIB-POSS nanoparticles were still present in the corresponding rubber nanocomposite. In conclusion, the existence of OIB-POSS nanoparticles and their random distribution in the SBR matrix were confirmed by TEM analysis.

Conclusion
In this study, a series SBR/OIB-POSS nanocomposites with 1, 3, and 5 phr on the rubber compounds were prepared using a laboratory type banbury mixer, and their rheological, physical, mechanical, and crosslinking properties were investigated along with the reference SBR compound without nanofillers. The presence of OIB-POSS in the SBR nanocomposites was confirmed by detecting the Si-O stretching band at 1129 cm -1 for the POSS groups in the FT-IR spectrum of the SBR/OIB-POSS-1 nanocomposite. The successful vulcanization process was also verified by the absence of the peak at 1638 cm -1 in the FT-IR spectrum of vulcanized nanocomposite. As a result of the rheological test, it was determined that the addition of OIB-POSS to the rubber recipe reduced the CE value. The mechanical strength increased with the addition of 1 phr OIB-POSS to the SBR matrix, but further increase in the amount of POSS nanoparticle led to a negative effect on the mechanical properties due to possible agglomerations. The tensile strength of SBR/OIB-POSS-1 nanocomposite increased from 1.41 MPa to 1.75 MPa, with an improvement of 24.1% and the elongation of break values of corresponding nanocomposite increased by 29.2% compared to the reference sample, respectively. This increase in mechanical properties implied that the reinforcement effect was more dominant than the negative crosslinking effect. Accordingly, the crosslink densities of nanocomposites calculated by the Flory-Rehner method decreased with the addition of OIB-POSS, as predicted. TEM micrographs verified the presence and random dispersion of OIB-POSS in the SBR matrix. In addition, after the thermal aging process, the rigidity of the SBR/OIB-POSS nanocomposites increased while the mechanical strength decreased.

Acknowledgment
This work was supported by the Scientific Research Fund of Yalova University (Project No: 2018/DR/0007) for the financial support.