Mechanical Deformation Behavior

Abstract

The correlation between applied mechanical load and resulting deformation is described by moduli. The basic component of moduli is controlled by binding forces, originating from deformations of the electron configurations. The load acts against the binding forces. This component reacts nearly immediately and is responsible for the elastic behavior. At very low temperatures most polymers behave elastically. Below 20K, the moduli of amorphous, semicrystalline and cross-linked polymers, respectively are rather similar which suggests similar binding forces. Above 30K the asymmetry of binding potentials causes a small decrease of the moduli owing to thermal expansion of the chain distances. The response, however, is strictly elastic. In the vicinity of a glass transition temperature (dispersion region) viscoelastic processes decrease the moduli owing to unfreezing of molecular motions. Only secondary or tertiary transitions are considered here. The deformation behavior in a dispersion region depends on temperature and time. The typical deformation characteristic is plotted schematically for the Young’s modulus E in Figure 7.1 with three characteristic regions:

• (a) constant elastic modulus (symmetric potential),

• (b) temperature-dependent elastic modulus (asymmetric potential),

• (c) time- and temperature-dependent viscoelastic modulus near a glass transition temperature Tg. The latter is time-dependent, for example, a function of the strain rate $\dot \varepsilon$ or the frequency ω.