Rheological effects of structure formation in polymer melts

The viscosity of a number of molten polystyrenes and polypropylenes obtained in various ways from solutions is found to be very strongly dependent on the history of dissolution of the polymer (the concentration of the solution and the “goodness” of the solvent), and on the presence of certain organic oligomeric compounds in very small amounts. The viscosity of the melt may be orders of magnitude less than that of the original samples. The present discussion of these effects is based on the concept of structure formation in amorphous polymers. The structure formed on the solution appears to be very stable with respect to heating and prolonged standing at elevated temperature, although it (and hence the viscosity) strongly depends on the initial concentration of the solution and the nature of the solvent. The role played by additives, present in amounts of small fractions of a percent, is accounted for by the fact that the melt retains the microheterogeneity of the medium. The material moves without its micromosaic structure being disturbed, and the role of the additives is reduced to surface (with respect to structural elements) phenomena.