Thermodynamics of Polymer Blends
Performance of polymer blends depends on the properties of polymeric components, as well as how they are arranged in space. The spatial arrangement is controlled by the thermodynamics and flow-imposed morphology. The word “thermodynamics” invariably bring
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THERMODYNAMICS OF POLYMER BLENDS
L. A. Utracki
National Research Council Canada, Industrial Materials Institute, Boucherville, QC, Canada
2.1
Introduction
Performance of polymer blends depends on the properties of polymeric components, as well as how they are arranged in space. The spatial arrangement is controlled by the thermodynamics and flow-imposed morphology. The word “thermodynamics” invariably brings to mind “miscibility.” However, thermodynamics has a broader use for the practitioners of polymer science and technology than predicting miscibility. The aim of this chapter is to describe how to measure, interpret, and predict the thermodynamic properties of polymer blends, as well as where to find the required information and/or the numerical values. Determination of such thermodynamic properties as the phase diagram or the Huggins-Flory binary interaction parameter, χ12, is difficult. The difficulties originate in high viscosity of macromolecular species, thus slow diffusion toward the equilibrium, heat generation when mixing and dangers of degradation. For these reasons, there is a tendency to use low molecular homologues or solutions. Furthermore, it is an accepted practice to purify the polymers before measuring their thermodynamic properties. However, the industrial polymers have high molecular weights, and are modified by incorporating low molecular weight additives. Furthermore they are processed under high flow rates and stresses that preclude the possibility of thermodynamic equilibrium. For these and other reasons, a direct application of the laboratory data to industrial systems may not always be advisable. Another difficulty originates in the lack of theories able to predict variation of thermodynamic properties for commercially attractive systems with modifiers. Different additive compositions are used by different manufacturers of the same polymer. These are being “used-up” during processing and products’ life time, their content and chemical structure change. They may significantly affect the thermodynamic properties of a polymeric mixture, by the physical, viz. that of a co-solvent, and the chemical effects. For example, additives of one polymeric component of a blend may chemically react with additives of another polymeric component, mutually neutralizing each other. In particular, these effects may be large as far as the surface and interface energies are concerned. L.A. Utracki (Ed.), Polymer Blends Handbook, 123-201. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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L. A. Utracki
2.2
Thermodynamic Principles
2.2.1
Definitions
(or intrinsic) that do not depend on the mass, and extensive that do. For example, volume, entropy and total energy of a system, are extensive variables, but the specific volume (or its reciprocity — the density), molar volume or molar free energy of mixing are intensive. It is advisable to use whenever possible the intensive variables.
For convenience, the thermodynamic systems are assumed closed, isolated from the surroundings. The laws that gove
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