Using DMTA to Analyze the Viscoelastic Behavior of Polymers

Using DMTA to Analyze the Viscoelastic Behavior of Polymers 

The classical theory of elasticity describes the mechanical properties of elastic solids, where the stress a is proportional to strain e in small deformations. Such a response of stress is independent of strain rate, with a constant E, known as Young s modulus. 

On the other hand, the classical theory of hydrodynamics describes the properties of viscous fluids, for which the resp)onse of stress is dependent on strain rate, where the constant of proportionality is known as viscosity tj. 

Polymers, when subjected to an external action (thermal, mechanical, magnetic, etc.), react by developing processes in their structures which have the property of not instantly adapting to a new equilibrium. These processes occur in characteristic times, knovm as relaxation times r. It should also be noted that the response of a polymer depends on both the time and temperature of apphcation. 

Apart from relaxation and creep experiments, two major types of test mode can be used to monitor the viscoelastic properties of polymers, namely temperature sweep and frequency sweep tests. These experiments are usually performed at short strains ( 0.5% of active length) so as to be within the linear region when a sinusoidal stress a (Eq. (12.3)) (i.e., force per area unit) is applied to a viscoelastic material the resultant strain (i.e., unitary relative geometric displacement) appears out-of-phase (Eq. (12.4)), with the angle d being a the angular frequency 

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