Category Archives: DMA

dynamic mechanical analysis

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

Using DMA Analysis for Heat Forming Applications

Many plastics come in flat sheet and one way to bend the sheet to desired shapes is to perform what is called heat forming or thermoforming process.  In thermoforming, the plastic sheet is heated until it is softened to allow bending without breaking.  Once the shape is formed the plastic is cooled and it will retain its shape without springing back to flat sheet.

So how do you know what temperature to heat up the plastic in order to thermoforming?  One way to find out is to ask the plastic manufacturer.  The plastic manufacturer typically would provide a recommended working temperature for the said plastic for thermal forming processes.  Another way to find out the process temperature is to do a little bit of scientific investigation yourself.  The rule of thumb for working temperature range is above its glass transition temperature where it softens significantly but below its melting temperature (if it has one) where it may flow too freely.  Once you find out what kind of polymer your plastic sheet is, you can look up its Tg and Tm values and come up with a working temperature for the thermoforming process.

If you are dealing with specialty polymers and you do need hard data, you can perform a simple DMA analysis to help you determine the temperature range and to verify thermoforming process ability.  A DMA thermograph will display the material’s storage modulus values with respect to temperature, as illustrated in the graph below.

DMA thermograph showing storage modulus E’ of the polymer dropped during Tg transition into the rubbery plateau region

DMA thermograph showing storage modulus E’ of the polymer dropped during Tg transition into the rubbery plateau region

The DMA thermograph shows that the polymer material softens during the Tg transition.  Typically, the modulus drop is between one to three orders of magnitude.  For example, a polymer that has a modulus drop of 2-order of magnitude during its Tg transition means its modulus value is reduced from ~ 100 MPa in the glassy state to ~ 1 MPa in the rubbery state.

The rule of thumb for working temperature range I mentioned several paragraphs before means that you should process your polymer while it is in the rubbery state, and at its rubbery plateau.

There are polymers such as high crystallinity thermoplastic polymers or highly crosslinked thermoset polymers, however, where this rule of thumb does not work well.  The reason is because the modulus drop from glassy state to the rubbery plateau region is simply not significant enough to bend the plastic.  The modulus changes in thermographs with respect to polymer structures are well documented in scientific literature and textbooks.  The classic DMA thermograph (shown below as a cartoon drawing) you find in textbooks shows how Mc (molecular weight between crosslinks) of a thermoset polymer or crystallinity of a thermoplastic polymer affect the material modulus.  In a thermoset polymer, the higher the crosslinking density (i.e., low Mc) the less modulus drop after Tg transition.  In parallel, in a thermoplastic polymer, the higher the crystallinity the less modulus drop after Tg transition.

DMA thermograph to illustrate how storage modulus of a polymer changes with increasing crosslinking density or increasing crystallinity, around Tg transition

DMA thermograph to illustrate how storage modulus of a polymer changes with increasing crosslinking density or increasing crystallinity, around Tg transition

With little changes in modulus values even by taking up the temperature to rubbery plateau region, we can explain why those high crystallinity polymers or those high thermoset polymers are difficult to form shapes using thermoforming process.

In summary, thermoforming process works well for polymers at temperature range between its Tg and Tm except for polymers that are either highly crosslinked or crystallinity.  When data is needed to assess thermoforming ability, a simple DMA analysis can be performed to obtain the needed answer.

Sub-ambient Tg of Thermoset Prepreg

Scientists and engineers use the term “sub-ambient glass transition temperature” (aka “sub-Tg”) to denote the Tg of an uncured thermoset material such as resin or prepreg. This terminology comes about because a cured, engineered thermoset system has a Tg that is high (i.e., significantly above room temperature) whereas in its uncured state the Tg is below ambient temperature. The terminology is used intending to void confusion. When sub-Tg is used it is only referring to Tg of uncured thermoset materials. When the material is being cured, the glass transition increases with respect to its degree-of-cure and it is called the typical Tg instead.

Many thermoset materials are processed to thin sheets such as in resin film or prepreg. At temperature below the sub-Tg it remains a solid and the uncured resin film is brittle and easily flaked off if bent. At temperature above sub-Tg, the uncured resin film or prepreg becomes resinous and pliable. It is possible to relate sub-Tg to drapeability of prepreg or resin film for material handle ability purpose. Also the study of sub-Tg can be linked with the goal of determining the material usage time and allowable out time. In addition to drape, people have also been relating sub-Tg to prepreg tack, although tack is a material quality that has been elusive to quantify as it is found to also depend on environmental factors such as ambient humidity and processing factors such as peeling speed.

The sub-Tg can be measured by thermal analysis techniques such as DSC and DMA. Sample should be at ambient temperature during sample preparation and handling. Be sure to eliminate moisture condensation on the sample while it is equilibrating to ambient temperature. Use the same kind of care to perform sub-Tg measurement as you would with measuring Tg of a polymer.

Obtaining Modulus Values from Dynamic Mechanical Analyzer (DMA)

Dynamic mechanical measurement of plastics is a very useful technique in viscoelastic behavior characterizations. Historically, scientists have been studying DMA thermographs to detect thermal transitions resulting from molecular motions and evaluate process-structure-property relationships. In recent years, however, there has been an increased application in DMA testing to mainly measure elastic modulus values of plastics. I wonder if this recent interest might be correlated with the increase usage of finite element analysis simulations, where people require mechanical data to be inputted into the simulation software, such as AutoCAD.

One advantage of using DMA testing is the sample size. DMA testing uses a much smaller sample size than the dog bone coupons for tensile testing or the large rectangular size for flexure testing. The smaller sample size is preferable as it is easier to fabricate a smaller piece without flaws that may affect the test results. However, the dimensional accuracy of the sample becomes more demanding in order to achieve accurate modulus values. Another advantage of using DMA testing is the ease to obtain modulus values at different temperatures. Only one sample is required in this case during a temperature ramp test.

Calculating modulus from DMA testing can be done easily as the test uses a small strain on the sample. However, strength data is not always possible to obtain. This is because DMA is designed as a tool to study polymer molecular behavior where resolution and sensitivity are important. So most commercial available DMA models have a very limited z-direction travelling distance and a small force range.

DMA thermograph is typically presented in storage modulus (E’), loss modulus (E’’), and tan delta, with respect to time or temperature. The dynamic modulus values are converted to elastic modulus value by the approximation

Elastic Modulus ~ |E*| = [E’2 + E’’2] 1/2

In an ideal elastic material, elastic modulus becomes E’ as tan delta approaches zero and E’’ becomes negligible.

Thermoset Cure Characterization Using Dynamic Mechanical Analyzer (DMA)

One person’s question on the LinkedIn discussion forum “Dynamic Mechanical Analysis (DMA / DMTA)” prompted me to write this blog entry. Do you know that DMA is an effective tool to study cure of a thermoset material? Typically, cure characterization of a thermoset is studied by DSC for kinetics information and by rheometer for viscosity information.  However, in some
instances – such as instrumentation constraint or material constraint – you can study cure with DMA. The most effective way to study cure in DMA is by supporting your thermoset material with a mesh material, such as a plain weave glass fabric. You then run it in the cantilever mode or film mode with a constant heat rate. (My personal preference is to run it in single cantilever mode.) And you should get a DMA thermograph similar to the one illustrated below.

DMA cure

DMA cure

A good reference is the book “Thermal analysis of polymers” edited by J. Menszel & B. Prime. Look into chapter 5, section 5.7.3.2 “DMA Techniques for Developing Cure Parameters”. Figure 5.44 shows a technique with wire mesh using a tensile fixture.