By Associated Polymer Labs, Inc.
DMA is short for Dynamic Mechanical Thermal Analysis (DMTA), which is another name for Dynamic Mechanical Rheological Testing (DMRT).
Rheometers for mechanical rheometry are of two types:
- Linear displacement, RSA II Rheometrics Solids Analyzer
- Rotational, SR5 Rheometrics Controlled Stress Rheometer
Rheology is the science that studies the deformation and flow of materials, whether in liquid, gel, melt, solid, or composite form, in terms or their elasticity and viscosity.
Elasticity is the ability of a material to store deformational energy, and can be viewed simply as the capacity of a material to regain its original shape after being deformed.
Viscosity is a measure of a material’s resistance to flow and reflects the material’s rate of dissipation of deformational energy through flow.
Materials respond to an applied force or displacement by exhibiting either elastic, Hookean, or viscous, Newtonian, behavior, or through a combination of these, called viscoelastic behavior, in which the response of the material is time and temperature dependent. Most polymeric materials are viscoelastic.
From these measurements, properties such as the elastic modulus E’ or G’, viscous modulus E” or G”, damping factor tangent d, and the glass transition temperature Tg are easily and precisely obtained. Furthermore, if the rheometer is sufficiently sensitive, normal stress differences can also be measured.
E is the symbol for the modulus measured in tension, compression, and bending. G is the modulus measured in shear. The relationship is E(t)=3G(t). Tangent d is the ratio of the viscous modulus to the elastic modulus; E”/E’ or G”/G’.
DMA is probably the most versatile thermal analysis method available, and no other single test method provides more information about a sample is a single test. Besides material properties, DMA provides a direct link between a material’s chemical makeup and its mechanical behavior.
Materials can be characterized by DMA regardless of their kind, their physical state, or the form in which they are used. The material may be:
- Thermoplastic, a thermoset, or an elastomeric polymer
- Polymer Blend, Alloy, or Recycled
- Liquid, Gel, Hydrogel, Melt, Solid, or Composite
- Soft solids like cheese, food, toothpaste, cosmetics
- Foams, wet or dry, soft or rigid, open or closed cell
- Dispersion, emulsion, or solution
- Virgin “Neat”, or with fillers, pigments, plasticizers, fibers, nanotubes, and nano particles
Many tests are nondestructive and only small samples are needed. DMA can extent beyond the present. New and existing products are typically tested in the here and now. DMA can predict the materials performance in weeks, years, even decades from now. Using the Boltzmann superposition principle, time-temperature superposition, time-strain superposition or time-concentration superposition, rheological data generated in minutes or hours can be used to predict long term (shelf-life) material performance.
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