DMA

1. Dynamic mechanical analysis (DMA)
Dynamic mechanical analysis (abbreviated
DMA, also known as dynamic mechanical
spectroscopy) is a technique used to study and
characterize materials. It is most useful for
studying the viscoelastic behavior of polymers.
The samples may be presented in a variety of
forms including bars, strips, discs, fibers and
films. Even powders can be tested when
suitable containment is arranged.

2. Principle of DMA
A sinusoidal stress is applied and the strain in
the material is measured, allowing one to
determine the complex modulus. The
temperature of the sample or the frequency of
the stress are often varied, leading to variations
in the complex modulus; this approach can be
used to locate the glass transition temperature
of the material, as well as to identify
transitions corresponding to other molecular
motions.

3. Material Selection
Materials having long molecules, such as
synthetic and natural polymers, are immediate
candidates, but are by no means the only ones.
Any material that forms a glass will have
viscoelastic behavior. Typically these are
referred to as ‘amorphous’, in that they have
no regular crystalline structure. There are also
large numbers of such materials that are
classified as semi-crystalline (and by
implication, semi-amorphous).

4. Viscoelastic behavior
The opposite end of the spectrum to elastic
behavior is viscous behavior. This is readily
evident in liquids where they flow in response
to an applied force. Some materials behave
between the elastic and viscous regime. Such
materials are described as viscoelastic and
many glass forming or amorphous materials
fall into this category.

5. Viscoelastic properties of materials
Polymers composed of long molecular chains
have unique viscoelastic properties, which
combine the characteristics of elastic solids
and Newtonian fluids. The solidlike and
liquidlike behavior of polymer can be modeled
mechanically with combinations of springs and
dashpots.

6. Cont…..
Some materials behave between the elastic and
viscous regime. Such materials are described
as viscoelastic and many glass forming or
amorphous materials fall into this category.
For example, the continuous increase in length
with time under the influence of a constant
applied force defines a phenomenon called
‘creep’.

7. Usage of DMA instruments
There are four main areas of DMA use:
• Molecular structure characterization
• General material analysis
• Food and biomedical testing
• Derivation of engineering data

8. Molecular structure characterization
• Molecular structure characterisation is the main
reason that DMA was developed. The ability to
explore the molecular structure via a simple
thermal scanning test, requiring a relatively small
amount of sample (0.5–2 g), gave the polymer
chemists a powerful characterisation tool.
generally the apparatus is more expensive, harder
to use, experiments take longer and specimens
frequently require special preparation. This is why
DMA has emerged as the dominant tool for the
structural evaluation of polymers.

9. General material analysis
The vast expansion of the use of DMA has
occurred in the field of polymeric material
analysis. First, DMA is one of the best
techniques for assessing the amorphous
content of a material. It is important to know
how much amorphous material is present in a
number of situations. Since DMA is sensitive
to molecular structure it is frequently used to
check one sample against another that is meant
to be the same.

10. Food and biomedical testing
An important and growing sector of DMA
application is within the food and bioscience
sector. Many samples are measured for the
reasons given above, namely the determination of
Tg, checking similarity of samples. Generally, the
techniques are similar to those required for the
analysis of polymers. However, the temperature
range is usually less (−50–200◦C) and the water
content frequently plays a pivotal part in the
sample’s properties.

11. Derivation of engineering data
A more specialized area of use of DMA is the
derivation of engineering data. DMAs are capable
of generation of modulus and damping factor (tan
δ) data over a wide range of frequency and
temperature. In such applications great care has to
be taken that meaningful data are generated.
Other mechanical testing equipment is normally
used to source static modulus or viscosity data,
but DMA is usually the only source when high
frequency or damping factor data are required.

12. Dynamic moduli of polymers
The viscoelastic property of a polymer is studied
by dynamic mechanical analysis where a
sinusoidal force (stress σ) is applied to a material
and the resulting displacement (strain) is
measured.
Viscoelastic polymers have the characteristics in between
where some phase lag will occur during DMA tests.

14. Applications
Measuring glass transition temperature
Amorphous polymers have different
glass transition temperatures, above
which the material will have rubbery
properties instead of glassy behavior
and the stiffness of the material will
drop dramatically with an increase in
viscosity. At the glass transition, the
storage modulus decreases
dramatically and the loss modulus
reaches a maximum.

15. Polymer composition
Varying the composition of monomers and
crosslinking can add or change the
functionality of a polymer that can alter the
results obtained from DMA. DMA can also be
used to effectively evaluate the miscibility of
polymers.

16. Types of analyzers
• forced resonance analyzers
force the sample to oscillate at a certain frequency
and are reliable for performing a temperature sweep.
• free resonance analyzers
measure the free oscillations of damping of the
sample being tested by suspending and swinging the
sample.

17. Test modes
• Temperature sweep
A common test method involves measuring the
complex modulus at low constant frequency while
varying the sample temperature. A prominent peak in
Tan(δ) appears at the glass transition temperature of
the polymer.
• Frequency sweep
A sample can be held to a fixed temperature and can
be tested at varying frequency. Peaks in Tan(δ) and in
E’’ with respect to frequency can be associated with
the glass transition.