Monthly Archives: March 2017

DMA Clamp Selections and Sample Geometries

A modern DMA instrument nowadays comes with several clamping fixtures to accommodate a wide variety of samples.  Modern DMA instrument also comes with sophisticated computer programming where you have a wider range of test parameter choices such as fixed frequency, frequency sweep, fixed force, fixed strain, stress relaxation, etc.  This is an evolution from early commercially available DMA instruments such as the Rheovibron, DuPont DMA 983, or Polymer Laboratories DMTA Mk II.  In my mind, it is analogous to how kitchen appliance hand-held blenders evolved: a modern hand-held blender allows for multiple rotating speed, and blade styles can be changed out for different culinary tasks.

The main advantage that comes with this evolution is versatility.  Now you have one instrument in the lab that you may use for various sample types and test modes instead of buying several machines.  The main disadvantage that comes with this evolution is, alas, also versatility.  Now you have a more complicated machine to learn, to use and to maintain. You may eventually find that some clamps or modes are never useful on this more expensive equipment you bought.

I describe DMA testing in simple terms: You hold a sample firmly, put the sample in an oven so you can heat it up, and observe how it moves when you push it this way and that way.  The said sample is usually a polymer, usually a solid, and usually cut to a rectangular shape so that the math can be applied.  The said force is usually small so that the math can be applied to calculate modulus values.

(1)  This said force is called “flexure” when you push the sample in the middle or at the end of it.  Sample is positioned either in a cantilever clamp (dual cantilever mode or single cantilever mode) or in a three-point bend clamp.

DMA flexure set up: three-point bend and cantilever

DMA flexure set up: three-point bend and cantilever

(2)  This said force is called “tension” when you pull on the sample along its length, with the sample positioned in a film clamp.

DMA tensile set up

DMA tensile set up

These two, flexure and tension, are the most commonly used set up in a DMA instrument.  The tension set up is also commonly used in a TMA instrument for film testing.

Two other clamp set up that are also included in a modern DMA instrument but are less used are compression clamp and shear clamp, which are designed for soft materials such as foam, gel, rubber, etc. and which would overlap with applications with a rheometer instrument.

DMA compression set up

DMA compression set up

DMA shear set up

DMA shear set up

In addition to flexure and tension set up as mentioned above, there is one more mode for a solid, stiff sample.  This said force is called “torsion” when you twist on the sample, with the sample held on both ends.  This set up requires a rheometer (note: not using a “DMA” instrument!) with a solid sample accessory fixture.  Historically this set up uses the Rheometric Scientific RDA instrument.

Solid torsion DMA

Solid torsion DMA