Dynamic Mechanical Analysis

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Dynamic Mechanical Analysis      DMA

The DMA determines changes in sample properties resulting from changes in five experimental variables: temperature, time, frequency, force, and strain. The deformation can be applied sinusoidally, in a constant, or under a fixed rate. The DMA uses samples that can be in bulk solid, film, fiber, gel, or viscous liquid form. Interchangeable fixtures are employed to allow you to measure many properties, including: modulus, damping, creep, stress relaxation, glass transitions, and softening points. 

Dynamic Mechanical Analysis measures viscoelastic properties of materials:

Elastic modulus (storage modulus, E')

Viscous modulus (loss modulus, E'')

Damping coefficient (Tan delta) as a function of time, temperature, frequency or complex modulus. 

Transition points, alpha, beta, gamma. 

Glass Transition Temperature- the peaks below the materials’ Tg are used to determine how well a material will stand up to impact. 

Stress Relaxation

Viscoelastic materials 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  Differential Scanning Calorimetry (DSC), 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.

The storage modulus ε’ (elastic response) and loss modulus ε’’ (viscous response) of polymers are measured as a function of temperature or time as the polymer is deformed under an oscillatory load (stress) at a controlled (isothermal or programmed) temperature in a specified atmosphere. The storage modulus is related to stiffness, and the loss modulus to damping and energy dissipation. Glassy, viscoelastic, elastic, and liquid polymers can be differentiated by DMA, and some details of polymer structure can be inferred from the results. DMA is particularly useful for evaluating viscoelastic polymers that have mechanical properties, which exhibit time, frequency, and/or temperature effect. DMA is the most sensitive of all thermal analytical techniques.

Dynamic Mechanical Analysis