Dynamic mechanical analysis
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Dynamic mechanical analysis (DMA), dynamic mechanical thermal analysis (DMTA) or dynamic thermomechanical analysis is a technique used to study and characterize materials. It is most useful for observing the viscoelastic nature of polymers. Two methods are currently used: One is the decay of free oscillations and the other is forced oscillation. Free oscillation techniques involve applying a force to a sample and allowing it to oscillate after the force is removed. Forced oscillations involve the continued application of a force to the sample. An oscillating force is applied to a sample of material and the resulting displacement of the sample is measured. This method is the most commonly used one today. Samples can be either solids or melts. Most solids are tested by linearly applied strains and melts or liquids are normally tested in shear.
The sample deforms under the load. From this the stiffness of the sample can be determined, and the sample modulus can be calculated. By measuring the time lag in the displacement compared to the applied force it is possible to determine the damping properties of the material. The time lag is reported as a phase lag, which is an angle. The damping is called tan delta, as it is reported as the tangent of the phase lag.
Viscoelastic materials such as polymers typically exist in two distinct states. They exhibit the properties of a glass (high modulus) at low temperatures and those of a rubber (low modulus) at higher temperatures. By scanning the temperature during a DMA experiment this change of state, the glass transition or alpha relaxation, can be observed. The glass transition temperature (Tg) is often measured by DSC (Differential Scanning Calorimetry), but the DMA technique is more sensitive and yields more easily interpreted data. DMA can also be used to investigate the frequency (and therefore time) dependent nature of the transition. This is usual as the degree of dependence is specific to the transition type. Tg has a strong dependence on frequency but melting is frequency independent DMA can also resolve sub- Tg transitions, like beta, gamma, and delta transitions, in many materials that the DSC technique is not sensitive enough to pick up. In addition, DMA gives modulus values.
