Influence of α-Fe₂O₃ nanorods on the thermal stability of poly(methyl methacrylate) synthesized by in situ bulk polymerisation of methyl methacrylate

Abstract

α-Fe₂O₃ nanorods were incorporated into poly(methyl methacrylate) (PMMA) by in situ radical polymerisation of methyl methacrylate initiated by 2,2′-azobisisobutyronitrile. The α-Fe₂O₃ nanorods were synthesized by forced hydrolysis of FeCl₃ and structural characterization was performed by X-ray diffraction and transmission electron microscopy. The molar mass and the polydispersity index of synthesized PMMA samples were determined by gel permeation chromatography. The content of residual monomer was determined by ¹H NMR spectroscopy. The influence of α-Fe₂O₃ nanorods on the thermal stability of the polymer was investigated using thermogravimetry and differential scanning calorimetry. The molar mass and polydispersity index of PMMA were dependent on the content of α-Fe₂O₃ nanorods. The values of the glass transition temperature of the nanocomposites were lower compared to pure PMMA. Also, the thermal stability of nanocomposites in nitrogen and air was different from that of pure PMMA.

Introduction

Incorporation of inorganic nanoparticles in a polymer matrix can significantly affect the thermal, mechanical, optical, electrical, magnetic and flammability properties [1], [2]. Additionally, nanocomposites have many advantages, such as increased strength (without compromising other mechanical properties), improved heat resistance, decreased gas permeability, and enhanced electrical conductivity over traditional polymer composites prepared with fillers in the micrometer size domain. The properties of polymer nanocomposites depend on the type of incorporated nanoparticles, their size and shape, as well as the concentration and interaction with the polymer matrix [3], [4], [5], [6], [7].

Iron oxide nanoparticles are of special significance because of their application in catalysis, as gas sensors, magnetic storage, ferrofluids, magnetic refrigeration and colour imaging. The influence of ferric oxide nanoparticles on the thermal properties of polymer nanocomposites has been widely studied [8], [9], [10], [11], [12], [13], [14], [15]. The influence of the shape of the nanoparticles on the thermal stability of the polymer matrix was investigated by incorporating β-FeOOH nanorods in a poly(methyl methacrylate) (PMMA) matrix [11]. A better thermal stability, with the same amount of inorganic phase, was achieved by β-FeOOH nanorods with a larger aspect ratio. The presence of Fe₂O₃ in a PMMA matrix improves the thermal stability and fire retardant properties of the polymer in a synergistic manner when mixed with organoclays, the most promising flame retardant additives [12], [13].

In the above-mentioned studies, the nanocomposites were obtained by dispersing the nanoparticles either in a polymer solution or in a polymer melt, using commercially available polymers. However, PMMA obtained in the shape of sheets by bulk polymerisation of methyl methacrylate (MMA) is used for many applications. Thus, in this study, PMMA/α-Fe₂O₃ nanocomposites were prepared by bulk radical polymerisation of MMA in the presence of α-Fe₂O₃ nanorods. The influence of α-Fe₂O₃ nanorods on the molar mass, polydispersity index (PDI), glass transition temperature (T_g) as well as on the thermal stability of the PMMA matrix in nitrogen and air was investigated using gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetry (TG). The structural characterization of α-Fe₂O₃ nanorods was performed by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM).