SiO 2 -covered graphene oxide nanohybrids for in situ preparation of UHMWPE/GO(SiO 2 ) nanocomposites with superior mechanical and tribological properties Seyyed Arash Haddadi, 1 Ahmad Ramazani Saadatabadi , 1 Amanj Kheradmand, 1 Majed Amini, 1 Mohammad Ramezanzadeh 2 1 Chemical and Petroleum Engineering Department, Sharif University of Technology, P.O. Box 11365-9465, Tehran, Iran 2 Department of Surface Coatings and Corrosion, Institute for Color Science and Technology (ICST), P.O. Box 16765-654, Tehran, Iran Correspondence to: A. Ramazani Saadatabadi (E-mail: ramazani@sharif.ir) and S. A. Haddadi (E-mail: haddadi_seyyedarash@che. sharif.edu) ABSTRACT: The modified Hummer technique was used in the preparation of graphene oxide (GO) nanosheets, and then SiO 2 decorated GO [GO(SiO 2 )] nanosheets were synthesized via the sol–gel method. Then, ultrahigh-molecular-weight polyethylene (UHMWPE) nanocomposites loaded with 0.5, 1, 1.5, and 2 wt % of GO(SiO 2 ) were prepared using magnesium ethoxide/GO(SiO 2 )-supported Ziegler–Natta catalysts via the in situ polymerization. Morphological study of the prepared polymer powders was assessed using field- emission scanning electron microscopy, which showed that GO(SiO 2 ) nanohybrids have been uniformly dispersed and distributed into the UHMWPE matrix. Also, the neat UHMWPE and its nanocomposites were evaluated with different analyses, including viscosity- average molecular weight measurement, differential scanning calorimetry, thermogravimetric analysis, tensile test, scratch hardness, and pin-on-disk test. The characterization of the UHMWPE nanocomposites indicated that many characterizations, including the mechani- cal, thermal, and tribological properties of UHMWPE, were significantly improved by incorporation of these new nanosheets in spite of the molecular weight reduction of the polymeric matrix and the improved flowability and processability of the produced nanocomposite. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47796. KEYWORDS: in-situ polymerization; SiO 2 -covered graphene oxide nanohybrids; UHMWPE nanocomposites; Ziegler-Natta catalyst Received 12 November 2018; accepted 9 March 2019 DOI: 10.1002/app.47796 INTRODUCTION Karl Ziegler and Giulio Natta discovered the first catalytically active compositions for alkene polymerization and investigated the structures and properties of polymers produced by the catalysts in the early 1950s. 1,2 The first type of Ziegler–Natta catalysts, based on 3TiCl 3 .AlCl 3 and Al(C 2 H 5 ) 2 Cl, produced polypropylene (PP) with a production rate of only 5 kg g -1 of Ti. 3 The activated MgCl 2 was discovered in 1968 by Kashiwa as an appropriate sup- port for the Ziegler–Natta catalyst; 4 it improved the polymer catalyst field 5 and improved not only the concentration of active titanium species, but also their activity. 6 The overall activity of the catalysts considerably depends on the activation of the cocatalyst used. Nowadays, different cocatalysts such as triethyaluminum and tri- isobutylaluminum (TIBA) are used in petrochemical units. 3,7 An important discovery in the field of catalyst chemistry was the development of Ziegler–Natta catalysts and the successful synthesis of polyolefins, especially at the lower synthesis conditions such as process temperature and monomer pressure. 3,7 Among polyolefins, polyethylene (PE), and PP are the major synthesized polymers by Ziegler–Natta catalysts, possessing about 60% of the worldwide thermoplastics market, which is still steadily growing. 8,9 Among polyolefins, PE has the most rapid growth and is generally used as the bulk-commodity plastic material in multiple facets of our daily life, including pipes, furniture, automobile parts, laboratory equip- ment, and so forth. 7,9 Ultrahigh-molecular-weight PE (UHMWPE) is a special class of PE with desirable physicochemical, thermal, and mechanical characterizations such as remarkable wearing and friction proper- ties, high toughness, good physical stability, and chemical resis- tance and an extremely high fiber modulus. 8–12 Over the last decade, due to these notable characterizations, UHMWPE and its nanocomposites have been used in a wide variety of industrial applications, that is, conveyor lines in industries, aerospace industries, personal and vehicle armors, fabrication of bumpers and siding for harbors, boats and ships, dump truck liners, sky- diving and fishing tools and wire/cable industries, and music © 2019 Wiley Periodicals, Inc. 47796 (1 of 17) J. APPL. POLYM. SCI. 2019, DOI: 10.1002/APP.47796