Chapter 13 - Mechanical Properties of Solid Polymers

Abstract

The mechanical properties of polymers are of interest in all applications where polymers are used as structural materials. Mechanical behavior involves the deformation of a material under the influence of applied forces. The simplest mechanical properties are those of homogeneous isotropic and purely elastic materials; their mechanical response can be defined by only two constants, e.g., the Young modulus E and the Poisson ratio ν. For anisotropic, oriented-amorphous, crystalline, and oriented-crystalline materials, more constants are required to describe the mechanical behavior. The mechanical properties of polymers are controlled by the elastic parameters—the three moduli and the Poisson ratio; these four parameters are theoretically interrelated. The moduli are also related to the different sound velocities. Since the latter are again correlated with additive molar functions, the elastic part of the mechanical properties can be estimated or predicted by means of the additive group contribution technique. There is also an empirical relationship between the shear modulus and the transition temperatures. Since polymers are no purely elastic materials but are viscoelastic, they exhibit time and temperature dependence; these also may be estimated if the transition temperatures are known. In oriented polymers, the tensile modulus is a function of the stretch ratio.