Unlocking the Potential Use of Reactive POSS as a Coagent for EPDM/PP-Based TPV

Thermoplastic vulcanizates (TPVs) are multifunctional materials consisting of two or more phases with solid elastomeric properties at room temperatures and fluid-like properties above their melting point. They are produced through a reactive blending process known as dynamic vulcanization. The most widely produced TPV type is ethylene propylene diene monomer/polypropylene (EPDM/PP), which is the focus of this study. The peroxides are mainly selected to be used in crosslinking of EPDM/PP-based TPV. However, they still have some disadvantages, such as the side reactions resulting in the beta chain scission of the PP phase and undesired disproportionation reactions. To eliminate these disadvantages, coagents are used. In this study, for the first time, the use of vinyl functionalized polyhedral oligomeric silsesquioxane (OV-POSS) nanoparticles was investigated as a potential coagent in EPDM/PP-based TPV production via peroxide-initiated dynamic vulcanization. The properties of the TPVs having POSS were compared with the conventional TPVs containing conventional coagents, such as triallyl cyanurate (TAC). POSS content and EPDM/PP ratio were investigated as the material parameters. Mechanical properties of EPDM/PP TPVs exhibited higher values in the presence of OV-POSS, which resulted from the active participation of OV-POSS into the three-dimensional network structure of EPDM/PP during dynamic vulcanization.

The most widely used crosslinking agents for EPDM/PP TPVs are peroxides and phenolic resins [19][20][21]. Dynamically vulcanization of the EPDM phase in the EPDM/PP Table 1. Materials used in the study.

Materials Commercial Name and Manufacturer Chemical Structure Physical Properties and Descriptions
Polypropylene (PP) Moplen HP456J, Lyondell Basel, Belgium acrylic functional groups were used as coagents to cross-link natural rubber (NR) wit sulfur [41,42]. The results showed that the crosslink density and crosslink reaction rat increase in the presence of POSS. In addition, the mechanical and physical properties ob tained in the compounds using POSSs are relatively developed. In another study, differen types of polyethylene (LDPE, HDPE, and LLDPE) were cross-linked by using POSS con taining -C = C-bonds in the presence of peroxide [51,52]. The studies show that POSSs ar more reactive and efficient than conventional coagents. The mechanical properties o crosslinked polyethylenes prepared using POSSs were better obtained than those pre pared using other coagents. POSSs containing -C=C-double bonds in their structure tha can participate in crosslinking have the potential to be used as a coagent in TPV produc tion.
In this study, an octavinyl functionalized POSS (OV-POSS) was utilized as a coagen for the first time in a TPV system. The vinyl groups on the nanocage of OV-POSS woul make it involved in the crosslinking reaction as a coagent and simultaneously improv the strength of the resulting TPV at a molecular level due to the molecular stiffness of th POSS cage. Dynamic vulcanization in the presence of peroxide was carried out in a labor atory micro-compounder (MC15 Xplore Instruments, Sittard, The Netherlands). The con tent of OV-POSS as a coagent, peroxide content, and EPDM/PP ratio was taken as th material parameters. The crosslinking density, compression set, tensile properties, mor phology, and rheology of the TPVs were investigated. The performance of nascent TPV was compared with that of conventional TPVs.

Materials
The materials used in the study are shown in Table 1. The literature indicates that P homopolymers with melt flow index (MFI) values between 1-12 are generally chosen fo EPDM/PP TPVs [1,18,53,54]. The critical point here is that the viscosity values of EPDM and PP must be close to each other at process conditions to ensure a good blending. Al hough the viscosity value of EPDM changes according to the oil content, its viscosity gen erally is higher than that of commodity thermoplastics. For this reason, it is more appro priate to use a PP having a relatively low MFI value, in other words, a high viscosity. Th EPDM used in the study has 50% paraffin oil content, a typical type of EPDM used indus trially in TPV production. OV-POSS and antioxidants (Irganos ® and Irgafos ® ) were pur chased from Hybrid Plastics and BASF, respectively. acrylic functional groups were used as coagents to cross-link natural rubber (NR) wi sulfur [41,42]. The results showed that the crosslink density and crosslink reaction ra increase in the presence of POSS. In addition, the mechanical and physical properties o tained in the compounds using POSSs are relatively developed. In another study, differe types of polyethylene (LDPE, HDPE, and LLDPE) were cross-linked by using POSS co taining -C = C-bonds in the presence of peroxide [51,52]. The studies show that POSSs a more reactive and efficient than conventional coagents. The mechanical properties crosslinked polyethylenes prepared using POSSs were better obtained than those pr pared using other coagents. POSSs containing -C=C-double bonds in their structure th can participate in crosslinking have the potential to be used as a coagent in TPV produ tion.
In this study, an octavinyl functionalized POSS (OV-POSS) was utilized as a coage for the first time in a TPV system. The vinyl groups on the nanocage of OV-POSS wou make it involved in the crosslinking reaction as a coagent and simultaneously improv the strength of the resulting TPV at a molecular level due to the molecular stiffness of th POSS cage. Dynamic vulcanization in the presence of peroxide was carried out in a labo atory micro-compounder (MC15 Xplore Instruments, Sittard, The Netherlands). The co tent of OV-POSS as a coagent, peroxide content, and EPDM/PP ratio was taken as th material parameters. The crosslinking density, compression set, tensile properties, mo phology, and rheology of the TPVs were investigated. The performance of nascent TP was compared with that of conventional TPVs.

Materials
The materials used in the study are shown in Table 1. The literature indicates that P homopolymers with melt flow index (MFI) values between 1-12 are generally chosen f EPDM/PP TPVs [1,18,53,54]. The critical point here is that the viscosity values of EPD and PP must be close to each other at process conditions to ensure a good blending. A hough the viscosity value of EPDM changes according to the oil content, its viscosity ge erally is higher than that of commodity thermoplastics. For this reason, it is more appr priate to use a PP having a relatively low MFI value, in other words, a high viscosity. Th EPDM used in the study has 50% paraffin oil content, a typical type of EPDM used indu trially in TPV production. OV-POSS and antioxidants (Irganos ® and Irgafos ® ) were pu chased from Hybrid Plastics and BASF, respectively. acrylic functional groups were used as coagents to cross-link natural rubber (NR) wit sulfur [41,42]. The results showed that the crosslink density and crosslink reaction rat increase in the presence of POSS. In addition, the mechanical and physical properties ob tained in the compounds using POSSs are relatively developed. In another study, differen types of polyethylene (LDPE, HDPE, and LLDPE) were cross-linked by using POSS con taining -C = C-bonds in the presence of peroxide [51,52]. The studies show that POSSs ar more reactive and efficient than conventional coagents. The mechanical properties o crosslinked polyethylenes prepared using POSSs were better obtained than those pre pared using other coagents. POSSs containing -C=C-double bonds in their structure tha can participate in crosslinking have the potential to be used as a coagent in TPV produc tion.
In this study, an octavinyl functionalized POSS (OV-POSS) was utilized as a coagen for the first time in a TPV system. The vinyl groups on the nanocage of OV-POSS woul make it involved in the crosslinking reaction as a coagent and simultaneously improv the strength of the resulting TPV at a molecular level due to the molecular stiffness of th POSS cage. Dynamic vulcanization in the presence of peroxide was carried out in a labo atory micro-compounder (MC15 Xplore Instruments, Sittard, The Netherlands). The con tent of OV-POSS as a coagent, peroxide content, and EPDM/PP ratio was taken as th material parameters. The crosslinking density, compression set, tensile properties, mo phology, and rheology of the TPVs were investigated. The performance of nascent TP was compared with that of conventional TPVs.

Materials
The materials used in the study are shown in Table 1. The literature indicates that P homopolymers with melt flow index (MFI) values between 1-12 are generally chosen fo EPDM/PP TPVs [1,18,53,54]. The critical point here is that the viscosity values of EPDM and PP must be close to each other at process conditions to ensure a good blending. Al hough the viscosity value of EPDM changes according to the oil content, its viscosity gen erally is higher than that of commodity thermoplastics. For this reason, it is more appro priate to use a PP having a relatively low MFI value, in other words, a high viscosity. Th EPDM used in the study has 50% paraffin oil content, a typical type of EPDM used indu trially in TPV production. OV-POSS and antioxidants (Irganos ® and Irgafos ® ) were pu chased from Hybrid Plastics and BASF, respectively.  The coagent/DCP ratio was taken as 1, 3, 5, and 7. The samples were abbreviated wit respect to the type and loading level of the coagent, such as 70/30 EPDM/PP/Coagent(X (Table 2). Here, X stands for the coagent/DCP ratio. For instance, when X is equal to coagent content then becomes 5 times higher than that of DCP. Moreover, DCP conten was kept constant at 1.75, 1.50, 1.25, 1.00, and 0.75 phr for a given EPDM/PP ratio, as see in Table 2

Processing
TPVs were prepared via melt blending in a vertically positioned twin-screw batch micro-compounder having a recirculation channel (15 mL micro-compounder-MC 15 HT, Xplore Instruments, Sittard, The Netherlands) ( Figure 1). The barrel temperature of the microcompounder was set to 175 • C. The compounding time was kept constant at 5 min. DCP was added to the system at the 3rd minute. At the end of the mixing period, the extrudate was transferred to the injection molding device (12 mL Xplore Injection molding machine). The molten compound was subsequently injection molded to obtain standard test samples. The mold and melt temperatures were 25 • C and 175 • C, respectively. The injection and holding pressures were set to 8 bars.

Processing
TPVs were prepared via melt blending in a vertically positioned twin-screw batch micro-compounder having a recirculation channel (15 mL micro-compounder-MC 15 HT, Xplore Instruments, Sittard, The Netherlands) ( Figure 1). The barrel temperature of the microcompounder was set to 175 °C. The compounding time was kept constant at 5 min. DCP was added to the system at the 3rd minute. At the end of the mixing period, the extrudate was transferred to the injection molding device (12 mL Xplore Injection molding machine). The molten compound was subsequently injection molded to obtain standard test samples. The mold and melt temperatures were 25 °C and 175 °C, respectively. The injection and holding pressures were set to 8 bars. The coagent/DCP ratio was taken as 1, 3, 5, and 7. The samples were abbreviated with respect to the type and loading level of the coagent, such as 70/30 EPDM/PP/Coagent(X) ( Table 2). Here, X stands for the coagent/DCP ratio. For instance, when X is equal to 5, coagent content then becomes 5 times higher than that of DCP. Moreover, DCP content was kept constant at 1.75, 1.50, 1.25, 1.00, and 0.75 phr for a given EPDM/PP ratio, as seen in Table 2. For instance, DCP content is 1.75 phr for 70/30 EPDM/PP sample, including 70 phr EPDM. The coagent/DCP ratio was taken as 1, 3, 5, and 7. The samples were abbreviated with respect to the type and loading level of the coagent, such as 70/30 EPDM/PP/Coagent(X) ( Table 2). Here, X stands for the coagent/DCP ratio. For instance, when X is equal to 5, coagent content then becomes 5 times higher than that of DCP. Moreover, DCP content was kept constant at 1.75, 1.50, 1.25, 1.00, and 0.75 phr for a given EPDM/PP ratio, as seen in Table 2. For instance, DCP content is 1.75 phr for 70/30 EPDM/PP sample, including 70 phr EPDM.

Characterization
An Anton Paar Modular Compact Rheometer, MCR 102, with plate-plate geometry of 25 mm diameter, was used to conduct the rheological tests. Frequency sweep measurements were carried out at 175 • C under a nitrogen atmosphere at a shear strain of 1% and an angular frequency range between 0.1 and 600 rad/s. The tensile properties were measured with an Instron Universal Testing Machine (Model 3345) according to ISO37 with type 2 sample geometry. The crosshead speed was 500 mm/min. Tensile strength, elongation at break, and 100% modulus values of TPVs were obtained from stress-strain graphs.
Compression set determination of the samples was conducted according to ASTM D395 with method B and type 2 geometry. The hardness of the samples was measured according to ASTM D2240 using a Zwick Shore-type durometer.
The overall crosslink density (CLD) of the EPDM phase in EPDM/PP TPVs was determined based on solvent-swelling measurements in cyclohexane at 25 • C according to the Flory-Rehner method (Equation (1)). Samples were submerged in acetone, then cyclohexane. After 24 h, the swollen sample was weighed, dried, and weighed again. From the degree of swelling, an overall CLD was calculated relative to the EPDM + PP phases as expressed by

Characterization
An Anton Paar Modular Compact Rheometer, MCR 102, with plate-plate geometry of 25 mm diameter, was used to conduct the rheological tests. Frequency sweep measurements were carried out at 175 °C under a nitrogen atmosphere at a shear strain of 1% and an angular frequency range between 0.1 and 600 rad/s. The tensile properties were measured with an Instron Universal Testing Machine (Model 3345) according to ISO37 with type 2 sample geometry. The crosshead speed was 500 mm/min. Tensile strength, elongation at break, and 100% modulus values of TPVs were obtained from stress-strain graphs.
Compression set determination of the samples was conducted according to ASTM D395 with method B and type 2 geometry. The hardness of the samples was measured according to ASTM D2240 using a Zwick Shore-type durometer.
The overall crosslink density (CLD) of the EPDM phase in EPDM/PP TPVs was determined based on solvent-swelling measurements in cyclohexane at 25 °C according to the Flory-Rehner method (Equation (1)). Samples were submerged in acetone, then cyclohexane. After 24 h, the swollen sample was weighed, dried, and weighed again. From the degree of swelling, an overall CLD was calculated relative to the EPDM + PP phases as expressed by Ʋe + PP. The latter was done to avoid the need to correct for a part of the PP being extracted as amorphous PP [55][56][57].
where "Mc" is the average molecular weight between the crosslinked points (g/mol), "ρ" is the density of samples (g/cm 3 ), "V0" is the molar volume of the solvent (for cyclohexane is 108.7 cm 3 /mol), "ꭕ" is polymer-swelling agent interaction parameter, or Flory-Huggins parameter, which in this case is 0.315 [28], and "φ" volume fraction of EPDM in the swollen network, which is expressed by Equation (2): where "W0" is the initial mass of the sample (g), "Ws" is the mass of the swollen sample (g), and "ρ 1 " and "ρ" are the density of the cyclohexane and EPDM (g/cm 3 ), respectively. Sample densities were measured with the Mettler Toledo density kit according to the Archimedes principle.
Swelling measurements were carried out to estimate the oil resistance according to ASTM D741. The mass change was measured after swelling in IRM903 standard oil for 70 h at 125 °C and dried for an hour at ambient temperature. The change in mass (Δm) can be calculated from Equation (3) using the final (mf) and initial (mi) weights of the specimen.
Δm % mf mi /mi 100 (3) e + PP. The latter was done to avoid the need to correct for a part of the PP being extracted as amorphous PP [55][56][57].

Characterization
An Anton Paar Modular Compact Rheometer, MCR 102, with plate-plate geome of 25 mm diameter, was used to conduct the rheological tests. Frequency sweep measu ments were carried out at 175 °C under a nitrogen atmosphere at a shear strain of 1% a an angular frequency range between 0.1 and 600 rad/s. The tensile properties were measured with an Instron Universal Testing Mach (Model 3345) according to ISO37 with type 2 sample geometry. The crosshead speed w 500 mm/min. Tensile strength, elongation at break, and 100% modulus values of TP were obtained from stress-strain graphs.
Compression set determination of the samples was conducted according to AST D395 with method B and type 2 geometry. The hardness of the samples was measu according to ASTM D2240 using a Zwick Shore-type durometer.
The overall crosslink density (CLD) of the EPDM phase in EPDM/PP TPVs was termined based on solvent-swelling measurements in cyclohexane at 25 °C according the Flory-Rehner method (Equation (1)). Samples were submerged in acetone, then cyc hexane. After 24 h, the swollen sample was weighed, dried, and weighed again. From degree of swelling, an overall CLD was calculated relative to the EPDM + PP phases expressed by Ʋe + PP. The latter was done to avoid the need to correct for a part of the being extracted as amorphous PP [55][56][57].
where "Mc" is the average molecular weight between the crosslinked points (g/mol), is the density of samples (g/cm 3 ), "V0" is the molar volume of the solvent (for cyclohexa is 108.7 cm 3 /mol), "ꭕ" is polymer-swelling agent interaction parameter, or Flory-Hugg parameter, which in this case is 0.315 [28], and "φ" volume fraction of EPDM in the sw len network, which is expressed by Equation (2): where "W0" is the initial mass of the sample (g), "Ws" is the mass of the swollen sam (g), and "ρ 1 " and "ρ" are the density of the cyclohexane and EPDM (g/cm 3 ), respective Sample densities were measured with the Mettler Toledo density kit according to the chimedes principle.
Swelling measurements were carried out to estimate the oil resistance according ASTM D741. The mass change was measured after swelling in IRM903 standard oil for h at 125 °C and dried for an hour at ambient temperature. The change in mass (Δm) be calculated from Equation (3) using the final (mf) and initial (mi) weights of the sp

Characterization
An Anton Paar Modular Compact Rheometer, MCR of 25 mm diameter, was used to conduct the rheological t ments were carried out at 175 °C under a nitrogen atmosp an angular frequency range between 0.1 and 600 rad/s. The tensile properties were measured with an Ins (Model 3345) according to ISO37 with type 2 sample geo 500 mm/min. Tensile strength, elongation at break, and were obtained from stress-strain graphs.
Compression set determination of the samples was D395 with method B and type 2 geometry. The hardnes according to ASTM D2240 using a Zwick Shore-type dur The overall crosslink density (CLD) of the EPDM p termined based on solvent-swelling measurements in cy the Flory-Rehner method (Equation (1)). Samples were su hexane. After 24 h, the swollen sample was weighed, drie degree of swelling, an overall CLD was calculated relati expressed by Ʋe + PP. The latter was done to avoid the ne being extracted as amorphous PP [55][56][57].
where "Mc" is the average molecular weight between the is the density of samples (g/cm 3 ), "V0" is the molar volum is 108.7 cm 3 /mol), "ꭕ" is polymer-swelling agent interacti parameter, which in this case is 0.315 [28], and "φ" volum len network, which is expressed by Equation (2): where "W0" is the initial mass of the sample (g), "Ws" is (g), and "ρ 1 " and "ρ" are the density of the cyclohexane a Sample densities were measured with the Mettler Toledo chimedes principle.
Swelling measurements were carried out to estimat ASTM D741. The mass change was measured after swelli h at 125 °C and dried for an hour at ambient temperatur be calculated from Equation (3) using the final (mf) and men.
Δm % mf mi /mi where "M c " is the average molecular weight between the crosslinked points (g/mol), "ρ" is the density of samples (g/cm 3 ), "V 0 " is the molar volume of the solvent (for cyclohexane is 108.7 cm 3 /mol), "

Characterization
An Anton Paar Modular Compact Rheometer, MCR 102, with plate-plate geometry of 25 mm diameter, was used to conduct the rheological tests. Frequency sweep measurements were carried out at 175 °C under a nitrogen atmosphere at a shear strain of 1% and an angular frequency range between 0.1 and 600 rad/s. The tensile properties were measured with an Instron Universal Testing Machine (Model 3345) according to ISO37 with type 2 sample geometry. The crosshead speed was 500 mm/min. Tensile strength, elongation at break, and 100% modulus values of TPVs were obtained from stress-strain graphs.
Compression set determination of the samples was conducted according to ASTM D395 with method B and type 2 geometry. The hardness of the samples was measured according to ASTM D2240 using a Zwick Shore-type durometer.
The overall crosslink density (CLD) of the EPDM phase in EPDM/PP TPVs was determined based on solvent-swelling measurements in cyclohexane at 25 °C according to the Flory-Rehner method (Equation (1)). Samples were submerged in acetone, then cyclohexane. After 24 h, the swollen sample was weighed, dried, and weighed again. From the degree of swelling, an overall CLD was calculated relative to the EPDM + PP phases as expressed by Ʋe + PP. The latter was done to avoid the need to correct for a part of the PP being extracted as amorphous PP [55][56][57].
where "Mc" is the average molecular weight between the crosslinked points (g/mol), "ρ" is the density of samples (g/cm 3 ), "V0" is the molar volume of the solvent (for cyclohexane is 108.7 cm 3 /mol), "ꭕ" is polymer-swelling agent interaction parameter, or Flory-Huggins parameter, which in this case is 0.315 [28], and "φ" volume fraction of EPDM in the swollen network, which is expressed by Equation (2): where "W0" is the initial mass of the sample (g), "Ws" is the mass of the swollen sample (g), and "ρ 1 " and "ρ" are the density of the cyclohexane and EPDM (g/cm 3 ), respectively. Sample densities were measured with the Mettler Toledo density kit according to the Archimedes principle.
Swelling measurements were carried out to estimate the oil resistance according to ASTM D741. The mass change was measured after swelling in IRM903 standard oil for 70 h at 125 °C and dried for an hour at ambient temperature. The change in mass (Δm) can be calculated from Equation (3) using the final (mf) and initial (mi) weights of the specimen.
Δm % mf mi /mi 100 " is polymer-swelling agent interaction parameter, or Flory-Huggins parameter, which in this case is 0.315 [28], and "ϕ" volume fraction of EPDM in the swollen network, which is expressed by Equation (2): where "W 0 " is the initial mass of the sample (g), "W s " is the mass of the swollen sample (g), and "ρ 1 " and "ρ" are the density of the cyclohexane and EPDM (g/cm 3  at 125 • C and dried for an hour at ambient temperature. The change in mass (∆m) can be calculated from Equation (3) using the final (mf) and initial (mi) weights of the specimen.
The surface morphologies of the TPVs were examined by the QUANTA 400F Field Emission model scanning electron microscope (SEM) to determine the phase sizes and interfacial interactions of the samples. The samples were cryogenically broken for SEM analysis, and the fracture surfaces were sputter-coated with gold. Moreover, the surface morphology of TPVs was also evaluated by atomic force microscopy (AFM, Nanosurf model) in dynamic force mode. A microtome was used to prepare ultrathin sections for AFM analyses.

Rheological Properties of the TPVs
The variation of complex viscosity (η*), storage (G ), and loss (G ) modulus concerning the EPDM/PP ratio and content of the TAC and OV-POSS as the coagent are shown in Figures 2-4 and S1-S3.
All the complex viscosity values (η*) decreased with increasing frequency (ω), indicating the shear-thinning behavior of the pseudoplastic liquid due to the deformation of the entanglement of the chains at high frequencies. The neat-PP exhibited a plateau region at low shear rates, where complex viscosity is slightly influenced by the increasing frequency, followed by a decreasing viscosity region at high shear rates due to the alignment of the molecules (Figure 2a). Similar behavior was also observed in 30/70 EPDM/PP blends independently from the content of OV-POSS. This can be attributed to the phase morphology of the TPV, where PP is the continuous phase and dominates the viscoelastic behavior. The apparent difference between coagent (TAC and OV-POSS) containing and non-containing TPVs is the linear dependency of the log(η*) to the log(ω) for 50/50 and 70/30 EPDM/PP TPV systems. This indicates that EPDM/PP melt-flow under controlled shear conditions obeys the power law above 1 rad/s. It can be seen that samples having high EPDM and coagent concentrations showed a pronounced viscosity up-turn at low shear rates (w < 1 rad/s). The rheological behavior of this dynamically vulcanized TPV is similar to those of block copolymers and highly particle-filled composites [58]. This can be attributed to the 3D network structures formed in the cured rubber particle and the hyperbranched PP matrix due to possible radical grafting reactions onto OV-POSS and rubber-PP grafting reactions. Increasing coagent content resulted in higher complex viscosities due to the enhanced crosslinking efficiency of the rubber phase, which enhances the elastic behavior of the resulting TPV. Compared to TPVs containing OV-POSS, TPVs containing TAC appears to have higher viscosities at low frequencies due to the higher crosslink density.

Rheological Properties of the TPVs
The variation of complex viscosity (η*), storage (G'), and loss (G") modulus concerning the EPDM/PP ratio and content of the TAC and OV-POSS as the coagent are shown in Figures 2-4 and S1-S3.    (d) (e)  All the complex viscosity values (η*) decreased with increasing frequency (ω), indicating the shear-thinning behavior of the pseudoplastic liquid due to the deformation of the entanglement of the chains at high frequencies. The neat-PP exhibited a plateau region at low shear rates, where complex viscosity is slightly influenced by the increasing frequency, followed by a decreasing viscosity region at high shear rates due to the alignment of the molecules (Figure 2a). Similar behavior was also observed in 30/70 EPDM/PP blends independently from the content of OV-POSS. This can be attributed to the phase morphology of the TPV, where PP is the continuous phase and dominates the viscoelastic behavior. The apparent difference between coagent (TAC and OV-POSS) containing and non-containing TPVs is the linear dependency of the log(η*) to the log(ω) for 50/50 and 70/30 It is known that the TPV's morphology and structure influence the rheological behavior. The viscoelastic properties of a binary polymer blend system depend on the dispersed phase's deformability and shape/size. Moreover, the deformability is also related to the size of the dispersed component. The elastic and viscous characteristics of the TPVs are well reflected by the change of G and G with angular frequency. Generally, at low frequencies, the G value provides information about the long-range relaxation (beyond entanglement distance). In contrast, the corresponding value at high frequency relies on short-range relaxation (motion with entanglement) [59]. Figures 3 and 4 show the variation of G and G values for selected compositions with frequency. It is seen that the values of G and G increase monotonously with increasing angular frequency for all blends. In addition, the increasing EPDM content resulted in higher G and G values in nearly the entire range of angular frequency values. However, depending on the EPDM/PP ratio and the coagent content, the rate of increase (the slope of the G (angular frequency)) changed. One should note that at low frequencies, the elasticity of the TPV was controlled by the rubber phase, but at higher frequencies, the elasticity was dominated by the PP phase [60,61]. Without a coagent, the slope of log(G ) vs. log(ω) was constant below 10 rad/s indicating a linear increase in G concerning ω; however, above 10 rad/s, the slope became smaller and approached zero. The G vs. angular frequency curves also approached zero. This shows that in the absence of a coagent, the network formed due to dynamic vulcanization can still undergo a relaxation at high frequencies. In addition, in the lack of a coagent, the G ' values were more petite or nearly the same as G values, pointing out that the TPVs had a viscosity. When only 1% OV-POSS was added to the TPV system, the character of the G vs. G curves changed due to the efficient dynamic crosslinking. In this case, especially for 50% and 70% EPDM-containing systems, the slope of the log(G ) vs. log(angular frequency) is minimal (even approaches zero) at lower frequencies (angular frequency < 1 rad/s). As the shear rate increased, the G values started to rise with a higher slope. The reason is that both the network formation in the rubber domains and the rubber-PP phase hinder the relaxation process in the presence of a coagent. This is more pronounced in the presence of 7% coagent, especially OV-POSS. In this case, due to the highly entangled network of rubber domains and rubber-PP in the presence of OV-POSS compared to TAC, the relaxation is suppressed, and the elasticity of the TPV becomes dominant.

Mechanical Properties and Crosslink Density of the TPVs
In a TPV system, the mechanical properties are associated with the rubber/plastic ratio, the crosslink density of the rubber phase, the size and distribution of the rubber phase, the thickness of the plastic ligaments, and the compatibility of the plastic and rubber phases [62][63][64][65][66]. When the studies in the literature were examined, it was observed that the tensile strength and hardness values increased with the increasing content in the thermoplastic phase ratio [62]. The crosslink density is one of the essential properties of elastomeric materials, and the mechanical properties of elastomers largely depend on the crosslink density of the elastomer phase [67,68]. The tensile strength and elongation at break values increase when crosslinking of EPDM reaches an optimum value, and the crosslinked rubber particles are homogeneously dispersed in the continuous phase in the EPDM/PP TPV systems. However, at very high crosslink densities, the mechanical properties are adversely affected due to the coarse distribution of the crosslinked rubber particles that could act as stress transfer in the matrix [69]. Therefore, the optimum crosslink density value should be sustained to obtain higher mechanical properties [62,70]. In addition, the particle size of the dispersed phase is important in this context [62,69].
The mechanical properties of nano-sized particle-reinforced polymer composites, such as POSS nanoparticle-containing polymer composites, depend highly on particle size, particle-matrix interfacial adhesion, and particle loading ratio [71]. Rigid inorganic particles often have higher stiffness than the polymer matrix. Therefore, the elastic modulus of the matrix can be improved with these nano or micro-sized particles [72][73][74][75][76]. However, the tensile strength of the material depends on the stress transfer that takes place between the particle and the matrix. In the case of well-bonded particles, the stress applied to the matrix is effectively transferred from the matrix to the particles [77]. This causes a noticeable increase in strength values [78][79][80][81][82]. However, micro or nanoparticles poorly bonded to the matrix cause a decrease in the strength of the material [83][84][85][86]. On the other hand, the most critical factor for the mechanical properties of TPVs is the homogeneous distribution of the crosslinked three-dimensional network.
The representative stress-strain curves of selected TPVs are given in Figure 5 and Figure S4. As can be seen, samples of EPDM/PP TPVs with high PP ratios exhibited higher mechanical properties, such as modulus and yield strength. The yield point and necking behavior were followed by a cold drawing before the fracturing in TPVs with high PP content. However, the neck formation was not observed with increasing EPDM, and the samples showed lower mechanical properties. A possible reason for reducing mechanical properties, such as elongation at break and 100% modulus, could be the chain scission reactions that would cause the molecular weight loss in PP with the addition of peroxide (EPDM/PP + DCP) independent of the mixing ratio to the EPDM/PP TPV. With the use of TAC and especially OV-POSS as a coagent, increases in the mechanical properties of EPDM/PP TPVs were observed with the rise in crosslink efficiency. EPDM/PP/OV-POSS TPVs exhibited relatively higher mechanical properties. This can be attributed to the formation of several crosslink bridges through the unsaturated double bonds of octavinyl groups on the highly stiff POSS cage that formed the links of a self-reinforced network structure. These findings are also consistent with the crosslink density test results given in Table 3 in the upcoming part. properties, such as elongation at break and 100% modulus, could be the chain scission reactions that would cause the molecular weight loss in PP with the addition of peroxide (EPDM/PP + DCP) independent of the mixing ratio to the EPDM/PP TPV. With the use of TAC and especially OV-POSS as a coagent, increases in the mechanical properties of EPDM/PP TPVs were observed with the rise in crosslink efficiency. EPDM/PP/OV-POSS TPVs exhibited relatively higher mechanical properties. This can be attributed to the formation of several crosslink bridges through the unsaturated double bonds of octavinyl groups on the highly stiff POSS cage that formed the links of a self-reinforced network structure. These findings are also consistent with the crosslink density test results given in Table 3 in the upcoming part.   The variation in the mechanical properties of the samples, such as elongation at break, 100% modulus, tensile strength, compression set, and hardness with respect to the composition, is given in Figures 6-10. It is seen that, especially when OV-POSS is used as a coagent, the tensile strength and the 100% modulus values are higher than that of the TPVs prepared with TAC (Figures 6 and 7). This can be attributed to the participation of OV-POSS in the crosslinking reactions, which resulted in improved mechanical properties. In addition, in the presence of OV-POSS having eight unsaturated vinyl groups per molecule, the chain scission reactions of PP were suppressed, resulting in higher mechanical properties. In addition, the possibly formed EPDM-OVPOSS-PP graft structures could act as a compatibilizer in EPDM/PP interphase and lead to phase compatibility. They could also bring some level of physical crosslinking in the structure of TPV. Therefore, the tensile strength and 100% modulus values increased. Similar findings were also reported in the literature, where a silane-based additive with radical vinyl groups was used [87]. With the addition of OV-POSS to the TPV system and the increase in the OV-POSS/DCP ratio, significant improvements in mechanical properties were obtained with the increase in crosslink density values. The improvement in crosslink density of EPDM/PP TPVs in the presence of OV-POSS was evaluated via the Flory-Rehner approach. As seen from Table 3, independent from EPDM/PP loading ratio, all samples exhibited higher crosslink density values with the addition of OV-POSS compared with EPDM/PP TPVs, including TAC as a coagent. This can be attributed to the forming of crosslink bridges between the constituents via vinyl groups of OV-POSS during the dynamic vulcanization process.  The variation in the mechanical properties of the samples, such as elongation at break, 100% modulus, tensile strength, compression set, and hardness with respect to the composition, is given in Figures 6-10. It is seen that, especially when OV-POSS is used as a coagent, the tensile strength and the 100% modulus values are higher than that of the TPVs prepared with TAC (Figures 6 and 7). This can be attributed to the participation of OV-POSS in the crosslinking reactions, which resulted in improved mechanical properties. In addition, in the presence of OV-POSS having eight unsaturated vinyl groups per molecule, the chain scission reactions of PP were suppressed, resulting in higher mechanical properties. In addition, the possibly formed EPDM-OVPOSS-PP graft structures could act as a compatibilizer in EPDM/PP interphase and lead to phase compatibility. They could also bring some level of physical crosslinking in the structure of TPV. Therefore, the tensile strength and 100% modulus values increased. Similar findings were also reported in the literature, where a silane-based additive with radical vinyl groups was used [87]. With the addition of OV-POSS to the TPV system and the increase in the OV-POSS/DCP ratio, significant improvements in mechanical properties were obtained with the increase in crosslink density values. The improvement in crosslink density of EPDM/PP TPVs in the presence of OV-POSS was evaluated via the Flory-Rehner approach. As seen from Table 3, independent from EPDM/PP loading ratio, all samples exhibited higher crosslink density values with the addition of OV-POSS compared with EPDM/PP TPVs, including TAC as a coagent. This can be attributed to the forming of crosslink bridges between the constituents via vinyl groups of OV-POSS during the dynamic vulcanization process.         Figure 8 shows the changes in elongation at break values EPDM/PP-based TPVs. It was observed that the elongation at break values increased in the presence of peroxide for all the EPDM/PP and EPDM/PP/Coagent(X) TPV systems compared to that of the EPDM/PP blends. This increase in the elongation at break values could be due to the finer dispersion of EPDM in the PP matrix due to the improved interfacial interaction between the components mentioned above. The relative decrements observed in elongation at break values with increased coagent/DCP ratio are due to increased crosslink density and partially larger particle sizes of the dispersed phase. Higher elongation at break values was observed in EPDM/PP TPV systems, including OV-POSS as a coagent, with high PP concentration (30/70 EPDM/PP). This can be attributed to the increase in the number of chain ends per unit volume due to the chain scission reactions occurring in the PP phase.
When the hardness values of EPDM/PP/Coagent(X) TPVs are examined, a similar trend was observed with the tensile strength and 100% modulus values, especially when OV-POSS was used as the coagent. This was due to the improvement in crosslink density of the EPDM/PP system in the presence of OV-POSS (Figure 9).    Figure 8 shows the changes in elongation at break values EPDM/PP-based TPVs. It was observed that the elongation at break values increased in the presence of peroxide for all the EPDM/PP and EPDM/PP/Coagent(X) TPV systems compared to that of the EPDM/PP blends. This increase in the elongation at break values could be due to the finer dispersion of EPDM in the PP matrix due to the improved interfacial interaction between the components mentioned above. The relative decrements observed in elongation at break values with increased coagent/DCP ratio are due to increased crosslink density and partially larger particle sizes of the dispersed phase. Higher elongation at break values was observed in EPDM/PP TPV systems, including OV-POSS as a coagent, with high PP concentration (30/70 EPDM/PP). This can be attributed to the increase in the number of chain ends per unit volume due to the chain scission reactions occurring in the PP phase.
When the hardness values of EPDM/PP/Coagent(X) TPVs are examined, a similar trend was observed with the tensile strength and 100% modulus values, especially when OV-POSS was used as the coagent. This was due to the improvement in crosslink density of the EPDM/PP system in the presence of OV-POSS (Figure 9).
The variation in the compression set values of TPVs with respect to coagent type and coagaent/DCP ratios are shown in Figure 10. The ability of a material to recover after being compressed and exhibit a lower compression set value is one of the fundamental features of a high-performance TPV. The improvement or deterioration of the compression set value is related to the crosslink density of the EPDM phase in the EPDM/PP system. In Figure 10. The variation in the compression set values of EPDM/PP TPVs with respect to EPDM/PP ratios, coagent types, and crosslinker system ratios.
The variation in the compression set values of TPVs with respect to coagent type and coagaent/DCP ratios are shown in Figure 10. The ability of a material to recover after being compressed and exhibit a lower compression set value is one of the fundamental features of a high-performance TPV. The improvement or deterioration of the compression set value is related to the crosslink density of the EPDM phase in the EPDM/PP system. In addition, the compression set value is significantly affected by the molecular structure of the continuous thermoplastic phase. In the current study, the compression set values deteriorated as the PP ratio increased in the TPVs. Improved compression set values were obtained for EPDM/PP/Coagent(X) TPV systems due to the enhanced crosslink efficiency. This is particularly pronounced in the presence of OV-POSS as a coagent. It was predicted that DCP molecules provide the formation of OV-POSS radicals involved in crosslinking. Therefore, chain scission reactions caused by DCP are suppressed. In addition, it was also suggested that some extent of the physical crosslinks could occur with the activation of OV-POSS. With the increasing OV-POSS concentration, the number of OV-POSS radicals increases; therefore, the amount of physical crosslinking also increases. This led to an improvement in compression set values. Moreover, the possible formation of an EPDM-OVPOSS-PP grafted structure on the EPDM and PP polymer chains leads to the formation of physical entanglements. Thus, it provides EPDM/PP system with lower compression set values.

Thermal Aging Resistance of the TPVs
In this section, the mechanical properties of the samples aged at 70 • C for 70 h were evaluated by compression tests. Table 4 shows the % changes in the compression set values of the blends and TPV systems with respect to coagent type and crosslinking system before and after aging. As seen from Table 4, the heat aging resistance of TPVs having OV-POSS as a coagent was higher than that of EPDM/PP blends both at 70 • C for 70 h and at room temperature for 22 h. The aging resistance of thermoplastic vulcanizates directly depends on the strong C-C bonds of TPVs. Moreover, with the increase in PP concentration in the structure of the EPDM/PP system, the aging resistance improves because the heat aging resistance of PP is higher than EPDM. On the other hand, with the increase in EPDM concentration, forming the network structure due to crosslinking with peroxide in TPVs also improves the heat aging resistance [62,88,89]. It can be postulated that the participation of OV-POSS in the network structure of EPDM/PP TPVs during dynamic vulcanization enhanced the thermal stability of EPDM/PP, which resulted in improved compression set values after aging.

Oil Resistance of the TPVs
The mass change of samples measured after the swelling test at 125 • C in IRM 903 standard oil for 70 h is given in Figure 11. Oil resistance in TPVs largely depends on the degree of the crosslinking of EPDM in EPDM/PP system and the formation of a threedimensional network structure that resists oil penetration into the matrix. In addition, the increase in the crystallinity of the PP phase is one of the factors that improve oil resistance [62,[90][91][92]. Polymers 2023, 15, x FOR PEER REVIEW 16 of 24 Figure 11. The variation in the oil resistance values of EPDM/PP TPVs with respect to EPDM/PP ratios, coagent types, and crosslinker system ratios. Figure 11 shows the variation of oil resistance values of the blends and TPV systems with respect to coagent types and crosslinking system ratios. As a comparison with EPDM/PP blends, the oil resistance of TPVs was improved regardless of the coagent type. Moreover, TPVs, including the highest content of PP, exhibited better oil resistance. This improvement was due to the increased crystallinity of PP in the presence of EPDM in the structure and the good oil resistance of PP. When the coagent types are compared, the oil resistance values of TPVs prepared with OV-POSS coagent are higher compared to TAC coagent. This was due to the increased crosslink density of TPVs and decreased particle sizes of the crosslinked dispersed phase of EPDM, which improved the interfacial interaction between the components in the presence of OV-POSS molecules. Figure 12 shows the SEM pictures obtained from cryogenically fractured surfaces of 70/30, 50/50, and 30/70 EPDM/PP REF blends and TPVs dynamically cured with DCP and containing a 0, 3, and 7 phr Coagent/DCP ratio. As shown in Figure 12, smooth surface morphology was observed regardless of the mixing ratio. For 70/30 EPDM/PP blend, spherical PP particles with an average particle size of 0.65 µm dispersed homogeneously in the matrix. However, when the EPDM concentration was increased to 50 phr, the average particle size of PP in both spherical and nodular structures increased. In 30/70 EPDM/PP blend, the average particle size of the dispersed phase of EPDM was found to be higher than that of 70/30 EPDM/PP system having a dispersed phase of PP. This was due to the higher viscosity of EPDM than that of PP. The presence of very large particles in the 30/70 EPDM/PP system after peroxide curing is remarkable. This is due to the fact that the addition of DCP to the 30/70 EPDM/PP blend resulted in chain scission of the PP phase; therefore, viscosity decreased and resulted in lower shear stress, which was necessary for the particle break-up process. Consequently, the coalescence of particles became more dominant than the break-up. Although similar findings were observed in vulcanizates with a high EPDM ratio, it was observed that the average particle size of the dispersed phase decreased. This can be attributed to the higher viscosity of EPDM and the consumption of peroxide for EPDM crosslinking rather than the chain scission of PP. Figure 11. The variation in the oil resistance values of EPDM/PP TPVs with respect to EPDM/PP ratios, coagent types, and crosslinker system ratios. Figure 11 shows the variation of oil resistance values of the blends and TPV systems with respect to coagent types and crosslinking system ratios. As a comparison with EPDM/PP blends, the oil resistance of TPVs was improved regardless of the coagent type. Moreover, TPVs, including the highest content of PP, exhibited better oil resistance. This improvement was due to the increased crystallinity of PP in the presence of EPDM in the structure and the good oil resistance of PP. When the coagent types are compared, the oil resistance values of TPVs prepared with OV-POSS coagent are higher compared to TAC coagent. This was due to the increased crosslink density of TPVs and decreased particle sizes of the crosslinked dispersed phase of EPDM, which improved the interfacial interaction between the components in the presence of OV-POSS molecules. Figure 12 shows the SEM pictures obtained from cryogenically fractured surfaces of 70/30, 50/50, and 30/70 EPDM/PP REF blends and TPVs dynamically cured with DCP and containing a 0, 3, and 7 phr Coagent/DCP ratio. As shown in Figure 12, smooth surface morphology was observed regardless of the mixing ratio. For 70/30 EPDM/PP blend, spherical PP particles with an average particle size of 0.65 µm dispersed homogeneously in the matrix. However, when the EPDM concentration was increased to 50 phr, the average particle size of PP in both spherical and nodular structures increased. In 30/70 EPDM/PP blend, the average particle size of the dispersed phase of EPDM was found to be higher than that of 70/30 EPDM/PP system having a dispersed phase of PP. This was due to the higher viscosity of EPDM than that of PP. The presence of very large particles in the 30/70 EPDM/PP system after peroxide curing is remarkable. This is due to the fact that the addition of DCP to the 30/70 EPDM/PP blend resulted in chain scission of the PP phase; therefore, viscosity decreased and resulted in lower shear stress, which was necessary for the particle break-up process. Consequently, the coalescence of particles became more dominant than the break-up. Although similar findings were observed in vulcanizates with a high EPDM ratio, it was observed that the average particle size of the dispersed phase decreased. This can be attributed to the higher viscosity of EPDM and the consumption of peroxide for EPDM crosslinking rather than the chain scission of PP. The effectiveness of the coagents used to increase the crosslinking efficiency can also be seen from the SEM pictures. As 7 phr TAC was added to the 70/30 EPDM/PP + DCP system, the average size of the dispersed EPDM particles decreased significantly and were homogeneously dispersed in the PP matrix. Additionally, the most notable result here is that the large spherical particles (circled in the SEM images) observed with the addition of DCP to the 30/70 EPDM/PP system are no longer visible. This indicates that the chain scission reactions occurring in the PP phase in the presence of DCP radicals are The effectiveness of the coagents used to increase the crosslinking efficiency can also be seen from the SEM pictures. As 7 phr TAC was added to the 70/30 EPDM/PP + DCP system, the average size of the dispersed EPDM particles decreased significantly and were homogeneously dispersed in the PP matrix. Additionally, the most notable result here is that the large spherical particles (circled in the SEM images) observed with the addition of DCP to the 30/70 EPDM/PP system are no longer visible. This indicates that the chain scission reactions occurring in the PP phase in the presence of DCP radicals are suppressed by the addition of TAC into the structure. For EPDM/PP TPVs having 7 phr OV-POSS coagent, almost a single-phase morphological structure was determined regardless of the mixing ratio. These findings show that the dispersed phase particle sizes decreased significantly depending on the composition of TPV in the presence of OV-POSS, indicating that OV-POSS is an effective coagent for PP/EPDM system.

Morphological Analysis by AFM
Two-and three-dimensional phase-topography AFM images of the samples are shown in Figures 13 and 14. For EPDM/PP blends with high EPDM content, PP (white) was dispersed as small particles in the EPDM (brown) main phase before dynamic curing. For 30/70 EPDM/PP blends, the dispersed particle size of EPDM was much larger This was due to the fact that the viscosity of EPDM is higher than that of PP, as mentioned before. suppressed by the addition of TAC into the structure. For EPDM/PP TPVs having 7 phr OV-POSS coagent, almost a single-phase morphological structure was determined regardless of the mixing ratio. These findings show that the dispersed phase particle sizes decreased significantly depending on the composition of TPV in the presence of OV-POSS, indicating that OV-POSS is an effective coagent for PP/EPDM system.

Morphological Analysis by AFM
Two-and three-dimensional phase-topography AFM images of the samples are shown in Figures 13 and 14. For EPDM/PP blends with high EPDM content, PP (white) was dispersed as small particles in the EPDM (brown) main phase before dynamic curing. For 30/70 EPDM/PP blends, the dispersed particle size of EPDM was much larger This was due to the fact that the viscosity of EPDM is higher than that of PP, as mentioned before. When the AFM images of dynamically cured TPVs were examined, phase inversion in 70/30 EPDM/PP after vulcanization was observed. After phase inversion, the continuous EPDM phase turned into the dispersed phase and dispersed more homogeneously in the PP phase, resulting from EPDM's crosslinking during dynamic vulcanization. It is clearly seen that the phase separation observed before dynamic curing is not present in EPDM/PP/Coagent(3) systems. This was because the chain scission reactions occurring in PP were partially prevented in the presence of a coagent, and hence, the cross-linked EPDM particles were finely dispersed. The smallest dispersed particle sizes in EPDM/PP When the AFM images of dynamically cured TPVs were examined, phase inversion in 70/30 EPDM/PP after vulcanization was observed. After phase inversion, the continuous EPDM phase turned into the dispersed phase and dispersed more homogeneously in the PP phase, resulting from EPDM's crosslinking during dynamic vulcanization. It is clearly seen that the phase separation observed before dynamic curing is not present in EPDM/PP/Coagent(3) systems. This was because the chain scission reactions occurring in PP were partially prevented in the presence of a coagent, and hence, the cross-linked EPDM particles were finely dispersed. The smallest dispersed particle sizes in EPDM/PP system were obtained in the presence of OV-POSS in AFM pictures, which is another indication of the effectiveness of OV-POSS as a coagent for EPDM/PP vulcanizates.

Conclusions
In this study, the potential use of POSS nanocages having vinyl bonds was investigated as a coagent in EPDM/PP-based TPV production via dynamic vulcanization for the first time in the literature. The properties of the TPVs having POSS were compared with the TPVs produced using conventional coagents of TAC. The motivation for using POSS to prepare TPV was the potential of having increased crosslink density due to high functionality per POSS molecule and the limited segregation of the EPDM particles during dynamic vulcanization in the presence of POSS. In addition, once the OV-POSS forms the crosslink bridges between the constituents, then a self-reinforcing effect at a molecular level could be realized due to the stiff cage structure of POSS.
Rheological analyses showed that EPDM/PP TPVs, including OV-POSS as a coagent, exhibited higher complex viscosity values. The relaxation was suppressed, and the elasticity of the TPV became more dominant in the presence of OV-POSS. Mechanical test results revealed that the tensile strength and 100% modulus of EPDM/PP TPVs, including OV-POSS, was higher than that of EPDM/PP TPVs with TAC due to the higher crosslinking density in the presence of OV-POSS. SEM and AFM analysis showed that dispersed phase particle sizes decreased significantly depending on the mixing ratio and coagent type. In general, OV-POSS nanoparticles with unsaturated double bonds were found to be effective coagents for EPDM/PP systems compared to conventional coagents. Author Contributions: All authors contributed to the conception, design, material preparation, analyses, original draft preparation, writing review, and editing of the study. All authors have read and agreed to the published version of the manuscript.
Funding: This study was financially supported by The Scientific and Technological Research Council of Türkiye (TUBITAK) (Project No: 119M378).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.