Chapter 15 - Rheological Properties of Polymer Melts

Abstract

The flow behavior of polymer melts is of great practical importance in polymer manufacturing and polymer processing. Therefore, the development of a quantitative description of flow phenomena based on a number of material properties and process parameters is highly desirable. In the same way as the mechanical behavior of solid polymers is described in terms of moduli (ratios of stress and deformation), the flow behavior of polymer melts is characterised by viscosities (ratios of stress and rate of deformation). For common liquids, the viscosity is a material constant which is only dependent on temperature and pressure but not on rate of deformation and time. For polymeric liquids, the viscosities and normal stress coefficients differ with deformation conditions. Because polymer melts are viscoelastic, their flow is accompanied by elastic effects, due to which, part of the energy exerted on the system is stored in the form of recoverable energy. For this reason, the viscosities are time and rate dependent, suggesting that polymer melts are viscoelastic. Some typical viscoelastic phenomena are the Weissenberg effect, the Barus effect, the open-siphon flow, where a liquid is sucked out of the surface, and another open-siphon effect in which the transfer of a solution of high molecular weight polymer occurs from one full container to a lower empty container without making use of a siphon pipe.