No abstract

Chain walking polymerization provides a novel strategy to synthesize highly branched high-molecular-weight polyethylenes with controllable chain topologies ranging from linear to hyperbranched dendritic structure. In this work, we report the performance of this novel series of polyethylenes as lubricant viscosity-index improvers. By examining a range of such model polymers possessing various tailor-designed chain topologies, we report the first systematic study on the unique effects of chain topology on the polymers' viscosity thickening ability and shear stability. It is demonstrated that hyperbranched polyethylenes we prepared possess extremely high shear stability with almost zero shear degradation regardless of their high molecular weight. Our study shows that these hyperbranched polymers have great potential for formulating high-performance lubricants with superior properties.

The electrical and thermal properties of metallic nanostructures have attracted considerable fundamental and technological interests. Recent studies confirmed a dramatic decrease in the electrical and thermal conductivities when the dimension is comparable or even smaller than the electron mean free path. However, the verification of the Wiedemann-Franz law in these nanostructures remains hotly debated. The Lorenz number obtained from the two-probe measurement is found to be much larger than that from the four-probe measurement. Here, we reported the electrical and thermal properties of the individual silver nanowires measured by the two-probe and four-probe configurations. The measured electrical contact resistance is found to be nearly temperature-independent, indicating a ballistic-dominant electronic transport at the contacts. When the effect of thermal contact resistance is diminished, the Lorenz number measured by the four-probe configuration is comparable to the Sommerfeld value, verifying that the Wiedemann-Franz law holds in the monocrystalline-like silver nanowire. Comparatively, the derived electrical conductivity becomes smaller and the thermal conductivity becomes larger in the two-probe measurement, confirming that the electrical contact resistance will introduce a large error. The present study experimentally demonstrates a reasonable explanation to the discouragingly broad span in the Lorenz number obtained from different metallic nanostructures.