Retarded Cross-linking in ZnO-low-density Polyethylene Nanocomposites
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ZnO nanoparticles were mixed with branched low-density polyethylene and were found to increase the resistance of the polymer to thermal degradation without changing other thermal properties. Submicron-size ZnO particles were mixed with low-density polyethylene for comparison, and it was found that the increased thermal stability of the nanocomposite was due to the surface properties of nanoparticles smaller than approximately 100 nm in diameter.
It is well known that nanoparticles of inorganic materials exhibit many properties that are unexpected from the conventional point of view.1–3 Reducing a size of the particle increases its specific surface area and modifies its surface properties.3 This, along with the important effects of spatial confinement on properties in general,4 assigns nanoparticles a unique place compared to conventional materials. To utilize these special properties of nanoparticles, they can be dispersed in polymers to make polymer nanocomposites.5 It has been expected, and verified experimentally, that nanoparticles can significantly modify important material properties when they are mixed with a polymer matrix to form a nanocomposite.6–9 The modification of the properties depends on many factors: the size of the nanoparticles used as fillers, the chemical properties at the interface between filler and matrix, the properties of the polymer matrix, and the dispersion of the filler particles.9 Therefore, the mixing process is an important factor in controlling the ultimate properties of the nanocomposite. However, polymers are susceptible to thermal degradation or crosslinking during processing and in use. Extended heating causes polymer molecules to cross-link or degrade causing the polymer properties to change with the altered molecular structure. We have found in the present work that ZnO nanoparticles increase the resistance of lowdensity polyethylene (LDPE) to thermal degradation without changing other thermal properties, such as melting temperature and viscosity. Commercial grade branched low-density polyethylene (BLDPE; DOW-681I with a melting temperature of 110 °C as determined from DSC) was used as the polymer matrix. The ZnO nanoparticles used in this work were produced by the vapor-phase condensation method by Nanophase Technologies Corporation, Romeoville, IL, and the average diameters of the three batches used were determined to be 38, 49, and 63 nm. The size 940
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 02 Sep 2014
distribution was observed with a transmission electron microscope (Philips CM-12). As seen in Fig. 1, the particle size varies approximately from 20 to 100 nm for a batch of particles with an average size of 49 nm. For comparison, ZnO particles obtained from another commercial vendor (Alfa Aesar Inc., Wards Hill, MA) were mixed with the same PE matrix. The size of these ZnO particles was on the order of a few hundred nanometers as confirmed by scanning electron microscopy. Cross-linking and degradation were observed during the m
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