Melting kinetics in polymers

In polymers, it is possible to obtain single chain forming single crystals. It is feasible to melt these crystals by simple consecutive detachment of chain segments from the crystalline substrate and its diffusion into the melt. However, complication in the melting process occurs when the chain in the process of detachment from the surface is shared between different crystals. Experimentally, a clear distinction in different melting processes is observed, by the differences in the activation energies required for the consecutive detachment of chain segments or of segments having topological constraints. The consecutive detachment of free chain segments starts at the melting temperature predicted from the Gibbs-Thomson equation, whereas higher temperature or time is required if the chain has to overcome the constraints

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is 4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube–nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites