Mechanical and Dielectric Response

In the large majority of present day uses of polymeric materials the focus is on their mechanical performance. Properties are of a peculiar nature since polymer melts are different from low molar mass liquids and polymer solids differ from conventional crystalline solids. While the latter usually represent perfectly elastic bodies and low molar mass liquids develop viscous forces only, bulk polymers combine elastic and viscous properties in both the fluid and the solid state. Therefore, they are generally addressed as viscoelastic and, in fact, polymers are the main representatives of this special class of materials. 

Viscoelastic behavior does not just mean a superposition of independent viscous and elastic forces, but in addition it includes a new phenomenon known as anelasticity, where both become coupled. It becomes apparent in the observation that part of the deformation, although being reversible, requires a certain time to become established when a load is applied. 

Different fields are concerned and they all need their own approaches  

We shall treat the first topic in this chapter and subsequently, in Chap s. 9 and 10, large deformations, non-linear flow and the ultimate properties of yield and break. 

In electrical applications, polymers are often used as isolators. Since it is then important to be informed about possible electric losses, one needs to know their dielectric properties in dependence on frequency and temperature. As we shall see, a description of the response of dielectric materials to applied time-dependent electric fields is formally equivalent to the treatment of time-dependent mechanical responses. Therefore, we shall discuss both together in this chapter. There also exist electrically conductive polymers, namely polymers with conjugated double bonds after a doping process. Their properties will be treated separately in the next chapter. 

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Whereas for PMMA (Fig. 113) the two p peaks are quite well superposed, except in the p - a crossover region, in the case of the CMIM20 copolymer, the dielectric loss is weaker than the mechanical loss in the high-temperature part of the P transition. In contrast, in the p - a crossover region, the same behaviour is observed for the mechanical and dielectric responses, showing that, in CMIM20, the CMI units lead to a complete decoupling between the P and a transitions, which is not the case for PMMA. 

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