Dynamic Mechanical Analysis - Theory

1 History and introduction to DMA

As with many branches of polymer science, dynamic testing started with structural investigations on metals. Zener describes diffusion of larger molecules in solid solutions and also the Snoek effect of carbon diffusion in quenched steel samples. Both of these experiments employed torsional wire apparatus, similar to that in use in torsion pendulum experiments today. They both illustrate the two fundamental properties in dynamic mechanical property measurement, namely the sample stiffness and the anelastic effect, or ratio of energy stored to energy lost, otherwise termed the damping. Thus measured stiffness is frequently converted to a modulus to enable sample intercomparisons and damping is commonly referred to as tan d , representing the phase lag between an applied sinusoidal force and resulting displacement (this is discussed later in section x).

As with these early measurements, the most common use of dynamic mechanical measurement on polymers is for structural determination. This topic is excellently presented in McCrum, Read & Williams book, which is one of the most comprehensive references on this subject.

Many methods of polymer analysis are available now, so what does dynamic mechanical analysis have to offer over techniques such as Infra-red and NMR spectroscopy? Essentially it offers good value for money in that a single dynamic mechanical test taking a little over one hour yields a unique fingerprint of the relaxational processes for the sample and also gives the modulus and damping factor over a wide range of temperature and frequency. These data should allow positive identification of the material and may also be used in engineering calculations and specifications, all from a simply prepared sample of about 1-2g, often being cut directly from a component.

The value of a dynamic test is most significant, in that a 30 to 60 minute experiment yields a tremendous amount of information on the sample in question. The modulus value below the glass transition will tell about levels of molecular orientation and crystallinity. Transitions occurring can be related to the polymer’s structure and may be particularly useful where a multiple component blend is under investigation. Dynamic mechanical methods are the most sensitive way of measuring the glass transition itself, which is one of the key properties of a polymer from both the structural and processing viewpoint. Above the glass transition the rubbery behaviour yields important factors such as the effective cross-link density and clues to processability. Finally data can be obtained in shear mode after the melting point. This technique is complementary to infra-red or NMR results on chemical composition. These tests yield comprehensive detail on the molecular moieties present, whilst the mechanical data reveal how they are connected together.

One particularly important point to note is that dynamic mechanical data are usually directly relevant to an application. Material stiffness and damping are often required as a user specification, whereas other analytical techniques, such as molecular weight determination, are of more interest to the manufacturer than the user. Other dynamic mechanical methods are available, including ultrasonics, creep testing and a variety of resonant techniques. These all have special areas of application and are capable of extending the measurement frequency range, but none are as common in commercial use as the forced non-resonance technique.