Dynamic mechanical analysis (abbreviated DMA, also known as dynamic mechanical spectroscopy) is a technique used to study and characterize materials.
John Gearing has been using this DMA/DMTA technique since 1981, and helped develop the Polymer Labs DMTA and then the Triton DMA (which is now the Perkin Elmer 8000 system). Also the powder material pockets which mean that a small irregular sliver of paint, or 2 to 20 milli-grammes of powder can be run in this very sensitive DMA. (This I still find quite remarkable since 2 milligrammes of powder in 700 milligrammes of stainless steel (which is 100 times stiffer than plastic) can still show up a change in modulus through the glassy state and of course change dramatically at the glass transition, Tg. So the sample sizes are from a few milligrammes of powder, through very thin fibres of 10 micron or greater diameter – such as a human hair, up to a bar of 5 or 6mm x 10mm x 50mm – which can be as stiff as a CF composite or even a metal with a very thin coating on it.
DMA comes in two forms – the very large apparatus for measuring rubbers in shear of compression at up to 5 Mega Newtons for earthquake foundations etc. where the samples are large so equilibration at each set Temperature T is necessary before a strain or frequency sweep is performed. Then there is the much smaller systems which we use where the sample size allows for thermal scanning at up to 5°C per minute. So we can vibrate at say 3 different frequencies (multiplexing) whilst scanning at say 2°C/min and obtain very good data showing the difference between Tm and Tg for example on unknown co-polymers or even where there is just a few % resin in a composite material which some DMAs and the DSC cannot see at all.
Sensitivity of coatings on metal of 0.5 to 1mm thickness is microns of thickness to see Tg. As well as my old, but well looked-after Triton DMA we can use the TA Instruments DMA too.
A small sinusoidal force is applied to the sample in the single cantilever bending mode and the resultant small strain is compared and measured so that we record tangent delta – the angle between these two sine waves and the E’ – Young’s modulus and E” the loss modulus. For most polymers we can easily see the beta transition – which is the side chain motion of the molecules as well as the alpha transition, more commonly called Tg – the glass transition Temperature. The software automatically compares the length to thickness ration so corrects for the small shear force error for most samples. (If running a very soft foam we may well run it in shear – but can still obtain accurate data.) We occasionally use the tensile mode for fibres and very thin films, but the compressive mode is not used at all since the errors in bulk modulus corrections would be much greater than the shear corrections.
in 1999 we were asked by Roger Brown of RAPRA to write the chapter on DMA for his book – published by Elsevier – Handbook of Polymer Testing – physical methods – ISBN – 9781859573242 – and this is still used as a reference book by many labs and Universities. We used to give this book away as part of our training package when installing the ~20 Triton DMA’s that we sold for them in Europe. Over the last 20 years we have been asked for a re-print of the chapter but it was written on an old Brother word processor which has long since died – so they will have to borrow of buy the book to read it. Around this time we published application notes for many different polymers but since this technique is now so common – these are only available by special request nowadays.
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