Comparative study of the structural, morphological and oxidative characteristics of high-density polyethylene and poly(ethylene oxide)

Abstract

A comparative study was made of the thermo-oxidative stabilities of high-density polyethylene (HDPE) and poly(ethylene oxide) (PEO) aged in air at 90 and 60°. The PEO was a commercially available grade and two types of HDPE were produced using organo-chromium catalysts supported on a porous silica substrate. Examination of the silica by scanning electron microscopy (SEM) showed it to consist of spherically shaped particles with rough, irregular particulates adhering to their surfaces. Fractured silica particles reveal a system of voids which influences the ultimate mechanical strength of the silica and hence determines the stage at which the silica particles shatter during the polymerization process. The particle size distribution of each silica support was determined by laser light scattering. It was found that the silica support which had the higher effective surface area to weight ratio increased the reactivity and productivity of the catalyst system, thus affecting the morphological characteristics of the nascent polymer particles. The SEM examination of nascent PEO showed ductile, drawn cobweb structures. Since HDPE catalysed using a bis(triphenylsilyl)chromate shows a similar cobweb morphology and is known to have an induction period proceeding steady-state polymerization, it can be inferred that PEO polymerizes after an induction period. The rate of polymer oxidation was assessed by carbonyl index measurements obtained by Fourier transform i.r. spectroscopy. The rate of oxidation correlates with the specific surface area of the polymer, which is determined by the nascent morphology. Polarized optical microscopy showed that isothermally crystallized films of the oxidized polymers display an axialitic morphology. After oxidation, it appears that the calcium oxide residue [ ca 0.8% (w/w)] in the commercial grade of PEO can act as an efficient nucleating agent for axialitic growth, when the surfaces of the residue particles are wetted by oligomeric oxidation products.