Novel metallic materials constructed from epoxidized styrene-butadiene copolymers and cupric oxide

Abstract

The functionalization of the insulator copolymer matrix star [styrene (30 tool%) butadiene] with epoxide groups leads to the development of a novel metallic conductor after mixing with cupric oxide (CuO), a poor electrical conductor. Conducting polymer materials are attracting research interest due to their wide range of present and possible future applications [1-6]. These polymers may be useful as microelectronic elements, for the development of photovoltaic elements, as chemical sensors, as photochromic materials and for the development of rechargeable cells [1-8]. However, making composite materials may produce polymers with improved mechanical properties. We report here, for the first time, that the epoxidation of the polymer matrix copoly(styrenebutadiene) leads to the development of a composite material with CuO that not only has good mechanical properties, but is also a metal-like electrical conductor. Epoxidation was carried out with the in situ formation of performic acid in toluene solution (5wt% solids) using an excess (110%) of the epoxidizing agent, an equimolar mixture of formic acid and hydrogen peroxide. After the addition of hydrogen peroxide at 0 °C the temperature was allowed to rise and was controlled at 20 °C for the duration of the epoxidation reaction. Other details of the procedure have been described elsewhere [9]. Epoxidized polymers were characterized using ~Hnuclear magnetic resonance (1H-NMR) spectroscopy, 13C-NMR, Fourier-transform infrared spectroscopy and volumetric analysis. The absence of 1H-NMR absorptions at 3.2-3.3 and 3.8p.p.m. indicates the absence of a diol grouping and a furan ring, respectively. Similarly, no infrared absorptions were detected for the above groups, correspondingly at 3430 and 1067 cm -1. In this work more weight was given to NMR analysis, since at higher degrees of conversion neighbouring oxirane groups may vitiate chemical analysis results through side-reactions, leading to higher-membered rings. The epoxidized copolymers were precipitated in methanol, treated in a blender, suspended in distilled water and dried at 25 °C for 48 h under vacuum. The degree of copolymer epoxidation and the polymer properties are summarized in Table I. The epoxidized polymers were dissolved in 100 ml CHC13 (Merck, pro analysis) and then added to the CuO (Merck, analytical grade) powder. Solvent was removed and the composite materials were dried to constant weight at 50 °C in a vacuum-oven, forming a thick film. The conductivity and the thermopower of these epoxidized polymers-CuO blends were measured. The star copolymer 31.5 mol% epoxidized (E2), which contained the critical quantity of 80 wt% CuO, showed an increase in conductivity by a factor of 10 s. At this concentration the CuO molecules are calculated to be in "contact". The thermopower at room temperature was about 100/xVK -~. In order to explain these results, other blends using polypyrrole (PPY), copper iridium oxide (IrO2) and titanium oxide (TiO2) were obtained. Only the first two were conductive, and their properties are summarized in Table II. An increase in conductivity was observed only in the CUE23 sample containing