*Article* **A Morphological Study of Dynamically Vulcanized Styrene-Ethylene-Butylene-Styrene/Styrene-Butylene-Styrene/MethylVinylSilicon Rubber Thermoplastic Elastomer**

**Chunxu Zhao, Xiaohan Chen † and Xian Chen \*,†**

Room 602, Yifu Science and Technology Building, Wangjiang Campus, Sichuan University, Chengdu 610065, China; 2020222030015@stu.scu.edu.cn (C.Z.); xc2549@columbia.edu (X.C.)

**\*** Correspondence: chen.xian@scu.edu.cn; Tel.: +86-028-85468166 † These authors contributed equally to this work.

**Abstract:** In this work, we prepared thermoplastic silicone rubber (TPSiV) by dynamically vulcanizing different relative proportions of methyl vinyl silicone rubber (MVSR), styrene ethylene butene styrene block copolymer (SEBS), and styrene butadiene styrene block copolymer (SBS). The compatibility and distribution of the MVSR phase and SEBS/SBS phase were qualitatively characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) tests on TPSiV. Subsequently, the backscattered electron signal image was analyzed using a colorimeter, and it was found that the size of the interface layer between the MVSR phase and the SEBS-SBS phase could be quantitatively characterized. This method overcomes the defect of the etching method, which cannot quantitatively analyze the size of the compatible layer between the two polymers. The final experiment proved that the two phases in TPSiV exhibited a "sea-island" structure, in which the MVSR phase acted as a dispersed phase in the SEBS-SBS phase. In addition, the addition of the silane coupling agent KH-907 (γ-isocyanatopropyltriethoxysilane) improved the mechanical properties of TPSiV, increasing the tensile strength by about 40% and the elongation at break by 30%. The permanent tensile deformation increase rate was about 15%. Through the quantitative measurement of the compatible layer, it was found that KH-907 could increase the thickness of the interface layer between the MVSR phase and the SEBS-SBS phase by more than 30%, which explained why the silane coupling agent KH-907 improved the mechanical properties of TPSiV at the micro level.

**Keywords:** thermoplastic silicone rubber; backscattered electrons; compatibility layer; scanning electron microscope; dynamic vulcanization

## **1. Introduction**

Rubber is a class of polymers that is commonly used in daily life, and vulcanization is a necessary process to impart various properties to rubber [1,2]. Dynamic vulcanization refers to the melt blending of rubber and thermoplastic polymer at a high temperature and then vulcanizing the rubber under the action of a cross-linking agent, thereby obtaining a vulcanized rubber phase with a size in the micron level and uniformly dispersed in the thermoplastic polymer. Compared with other rubber vulcanization methods, dynamic vulcanization has the advantages of greatly improving the performance of blended thermoplastic elastomers, reducing equipment investment, and improving efficiency [3]. Since Gessler first proposed the concept of dynamic vulcanization in 1962, researchers have successively discovered polypropylene (PP)/ethylene propylene diene rubber (EPDM) [4], nitrile rubber (NBR)/polylactic acid (PLA) [5], and other dynamic vulcanization systems.

Silicone rubber is a polymer material with a silicon–oxygen bond as the main chain and phenyl, vinyl, and other groups as side chains and has excellent comprehensive properties [6]. At present, researchers often improve some properties of silicone rubber by

**Citation:** Zhao, C.; Chen, X.; Chen, X. A Morphological Study of Dynamically Vulcanized Styrene-Ethylene-Butylene-Styrene/Styrene-Butylene-Styrene/MethylVinylSilicon Rubber Thermoplastic Elastomer. *Polymers* **2022**, *14*, 1654. https://doi.org/ 10.3390/polym14091654

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao and Tao Liu

Received: 16 March 2022 Accepted: 15 April 2022 Published: 20 April 2022

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blending it with different inorganic substances or polymers. In recent years, the development of special functions, such as high-voltage electrical properties, thermal conductivity, flame retardant properties, and antistatic properties of silicone rubber, has been a trend [7,8]. In addition to silicone rubber, thermoplastic elastomer is widely used because it has the physical and mechanical properties of vulcanized rubber and the processing properties of thermoplastic plastics [9,10]. As one of the most important types of thermoplastic elastomers, SBS is often used for asphalt modification [11]. Since the aging resistance of SBS is not ideal, SEBS obtained by hydrogenating SBS came into being, and it is often used in situations where high aging resistance is required [12]. In this paper, TPSiV prepared by melt blending MVSR and SEBS/SBS is also a material that can combine the biocompatibility of MVSR with the excellent mechanical properties of SEBS/SBS. This TPSiV can be used for medical elastomers and other applications. Before this, the study of this blend system was rarely reported.

Although a variety of materials with excellent properties can be obtained by melting and blending various polymers, because most polymers have poor compatibility with each other, solving the compatibility between polymers and enhancing the interaction between different phase interfaces has become the focus of researchers. Among many solutions, adding a silane coupling agent to the polymer blend system is one of the commonly used methods [13,14]. Therefore, in this paper, we chose the silane coupling agent KH-907 as the compatibilizer of TPSiV.

Scanning electron microscopy (SEM) is one of the most commonly used methods to characterize polymer compatibility and phase interface conditions [15]. The characterization of copolymer compatibility and phase interface by scanning electron microscopy can be roughly divided into two categories. One is to directly observe the surface with obvious phase separation by observing the difference in the phase separation before and after adding the compatibilizer as evidence of the enhanced interaction and improved compatibility of the two-phase interface [16–18]. The other is to dissolve the material section in a good solvent of a phase, dissolve and remove the phase, observe the section of the material, and measure the strength of the interaction between the phases by the roughness of the surface of the undissolved phase, to prove that a substance has an obvious effect on the improvement of the compatibility between the two phases [19–21]. However, this method is only applicable to the situation where one phase is completely dissolved by the solvent, and the other phase is not dissolved by the solvent at all, which is difficult in some dynamic vulcanization blend systems. In addition, SEM is often used in conjunction with Transmission Electron Microscopy (TEM) and Energy Dispersive X-Ray Spectroscopy (EDX) to characterize more information on the surface of the material [22,23]. However, TEM testing has high requirements for samples, and it is necessary to find suitable dyes.

The SEM test described above usually uses the secondary electron signal [24]. In addition to the secondary electron signal, the backscattered electron signal reflected from the sample surface can also be analyzed to obtain information about the sample surface [25,26]. This characterization method is usually used to observe the changes in alloy phases [27–29] and the phase distribution of cement cross-sections [30–32]. However, there are few reports on the characterization of dynamic vulcanization blend systems. Therefore, we applied the backscattered electron signal characterization method to the dynamic vulcanization system and quantitatively characterized the phase interface between the dispersed phase and the continuous phase by using the method combined with the colorimeter, which can avoid many shortcomings of the etching method. For example, it is impossible to effectively prove that the etched phase is completely etched, and the unetched phase collapses and easily adheres to the cross-section, which affects the observation and cannot quantitatively characterize the phase interface between the dispersed phase and the continuous phase. At the same time, this method can also observe and analyze the dynamic vulcanization blend system without obvious phase separation.
