Dynamic Mechanical Analyzers
Dynamic mechanical analysis (DMA) is a technique in materials characterization that measures the response of a material to a mechanical force. An applied mechanical force provides a stress on the material, and the material’s resulting reaction, or strain, can be quantified. A simple example is the compressive strain equivalent to the relative change in length of a sample under the application of the stress. Other measures of strain apply to shear forces or torsional forces.
Stress-strain relationships are an important aspect of the material properties of a sample. They are described by various elastic moduli for a particular material, and the DMA instrument analyzes them based on its direct measurements of stress and strain. More importantly, the DMA system also provides methods for investigating these properties under many different conditions in order to more fully characterize a material.
The dynamic element of DMA is the stress applied to the sample, which is not static, but rather a force varying with time, typically sinusoidally. The variation results in a phase difference between the stress and the strain, which is also an important empirical variable. The response of a sample will vary with the frequency of the forcing, so frequency provides an additional axis for investigating the material.
The temperature of a sample will also have an effect on the response of a sample, especially when the material undergoes physical changes. For example, when an amorphous polymer approaches its glass transition temperature, its behavior changes from a brittle, glassy state to a rubbery state. Other potential variables include the magnitude of the stress and strain, and atmospheric conditions like humidity.
Mechanically, DMA instruments provide carefully calibrated forcing systems and strain-measurement technologies. Most of them have a wide variety of sample-holding mechanisms so that materials can be tested under modes including bend-ing, torsion and shear. When testing a material, a DMA instrument’s sample chamber is enclosed so that temperature and other environmental factors can be changed.
There are many different applications for DMA testing of polymers, the primary type of sample tested, from measuring glass transition to evaluating the properties of different blends of polymeric materials. Other possible samples include elastic materials, foods and ingredients, metals, adhesives, fibers, gels, films and coatings.
DMA is a subsegment of the overall thermal analysis market. Most of the major vendors in the market offer DMA in their product portfolios. TA Instruments (Waters) is the leader in thermal analysis, and the company recently added to its DMA offerings with its acquisition of the Bose ElectroForce Group (see page 2). PerkinElmer, NETZSCH and Mettler-Toledo are the next most significant vendors of DMA instrumentation. The few other market participants include ACO-EM, Hitachi High-Technologies and Psylotech. Total market demand for DMA was about $35 million in 2014.
Analyzers at a Glance:
• TA Instruments (Waters)