Miscibility of linear and branched polyethylene blends by thermal fractionation:: use of the successive self-nucleation and annealing (SSA) technique

In the present work, the recently developed thermal fractionation technique successive self-nucleation and annealing (SSA) was applied to blends of branched and linear polyethylenes (PEs), as a method to evaluate the phenomena of miscibility and segregation in PE blends. The melting behavior of the systems, after a controlled cooling and after the application of SSA was compared. The DSC scans corresponding to cooling of blend samples from the melt or subsequent heating displayed an overlap of the exotherms and endotherms, respectively, of the blend components that could lead to interpretations of partial miscibility. These effects were interpreted as arising from: a nucleation effect of the crystals formed by the more linear PE on the crystals of branched PE, a dilution effect of the molten branched chains on the crystals formed by the more linear PE and finally possible partial miscibility effects. However, after SSA, even when the nucleation and dilution effects are still present for some of the fractions produced, the thermal fractionation procedure helps to distinguish them from co-crystallization effects. This is mainly achieved by observing how the number of thermal fractions generated by SSA in the blends varies with composition and by comparing the relative amounts of the thermal fractions produced by SSA. The results indicate that only those PE fractions that are similar in chemical structure, as regards to content and distribution of short chain branches, are probably miscible in the melt and can oppose molecular segregation during SSA and therefore produce stable co-crystals in the solid state. The application of SSA to the study of PE blends can be a quick and convenient tool for making comparisons and ascertain miscibility of different types of PE blends. (C) 2001 Elsevier Science Ltd. All rights reserved.