Molecular Theories of Polymer Viscosity
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The viscosity of polymeric liquids crucially depends on polymer concentration c and molar mass M. These dependencies can generally be predicted from relatively simple theories, because what really matters is the chain-like structure of the polymer molecule rather than its detailed chemistry. In this chapter we summarize the main concepts leading to these predictions throughout the concentration range, i.e., from dilute solutions up to polymer melts. However, only the so-called zero-shear viscosity will be considered because the nonlinear effects arising from the coupling between flow and molecular “structure” are more complex and fall outside the scope of this chapter. In dilute solutions the important concept is that of “intrinsic” viscosity which, through dependence on M, reveals structural features of the polymer (flexible chain vs. rigid rodlike, for example) as well as the solvent quality. In semidilute solutions of long polymers, the overall structure of the system becomes that of an impermanent network of entangled chains. The viscosity is then related to the elasticity of the network and to the kinetics of chain disengagement. Typically, the viscosity scaling takes the form of power laws in both M and c. The M dependencies remain the same in the particularly relevant case of polymer melts. The chapter ends with a brief description of systems with localized interactions (or sticky points) in which the viscosity is particularly sensitive to the strength of such interactions.
KeywordsFriction Coefficient Intrinsic Viscosity Complex Flow Chain Segment Molecular Theory
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