Thermomechanical analysis is one of the thermal analysis techniques

2. 1) Introduction
2) Apparatus
3) The Parts of a TMA instrument
4) TMA PT1000
5) Dynamic and load effects
6) Applications

3. Thermomechanical Analysis or TMA is
one of the most common techniques that
measure dimensional changes of solid or
liquid materials as a function of
temperature, time and applied force.
TMA is often used to measure the
coefficients of thermal expansion (CTE),
glass transition temperature (Tg) and
compression modulus of a material by
applying constant force at varying
temperatures.

4. Many materials change their thermomechanical features during
heating and cooling. For example, phase changes may occur due to
softening thermal expansion. TMA analyzes provide valuable
information about the content, structure, production conditions and
application possibilities of different materials. The application
areas of TMA devices range from quality control to research and
development. Areas of usages are generally plastics and
elastomers, paints, adhesives, ceramics, glass, metal and composite
materials.

6. Instrumentation is arranged such that a predetermined load is applied to the sample, almost
invariably through a quartz rod. Quartz has the advantage of possessing a small
coefficient of thermal expansion and its low heat conductivity allows the a linear variable
displacement transducer (LVDT) to be spaced away from heat sources. In this
instrument, the load platform is actually a balance arm arrangement, allowing the quartz
rod and probe assembly weight to be counterbalanced. Most displacement transducers
drift significantly if their ambient temperature changes. Figure 4 shows water circulation
below the transducer to obviate the heat-rise problem. The area around the sample can
usually be both heated (electrically) and cooled (cold nitrogen gas) and the sample
temperature (sensed by a thermocouple) has to be subject to programmer control.

7. This is the most important part of the instrument. A
predetermined load is applied to the sample via probe. There
are three main types of the probe for TMA:
a) Expansion/Compression Probe:
It is used for the measurement of the deformation by the
thermal expansion and the transition of the sample under
the compressed force is applied.
b) Penetration Probe:
It is used for the measurement of the softening temperature.
c) Tension Probe:
It is used for the measurement of the thermal expansion and
the thermal shrinkage of the sample such as the film and
the fiber.
The materials of probes are quartz glass, alumina, and metals.
The choice is dependent on the temperature range and/or the
measurement purpose.

8. LVDT is a type of electrical transformer
used for measuring linear displacement
(position). LVDTs are inherently
frictionless, they have a virtually infinite
cycle life when properly used. They have
been widely used in applications such as
power turbines, nuclear reactors, aircraft
and many others. These transducers have
low hysteresis and excellent repeatability.
LVDT operation doesn’t require an
electrical contact between the moving
part and coil assembly, but instead relies
on the electromagnetic coupling.
Current is driven through the
primary coil at A, causing an
induction current to be generated
through the secondary coils at B.

9. The temperature in the system is measured by thermocouples. The position of thermocouple is
important. The thermocouple for temperature measurement is located near the sample.
For the formation of the thermocouple at least two metals should be joined together to form two
junctions. The thermocouple cannot be formed if the two junctions are not formed. Some of the
elements used commonly for thermocouple are Copper, Iron, Platinum, Rhodium, Iridium etc.

10. This system meets all TMA/DTMA
requirements and is designed with
reproducibility, accuracy and high-precision
in mind. The device is capable of
performing a range of thermo-mechanical
experiments on samples of varying shapes
and sizes and in a wide temperature
range. The integrated Force Generator
permits carrying out of both static and
dynamic measurements. This unit is
intended for use with composite materials,
glass, polymers, ceramics and metals.

11. The primary application of TMA is in the detection of changes in the modulus of a material at
major transitions, such as melting or glass transitions. The material thus becomes fluid, or at
least viscoelastic, above the transition, which leads to major complications in interpreting
the curves. The material will flow more under higher loads. This is illustrated schematically
in Figure 6 for a penetration probe being used to detect a glass transition of a polymer.
In this case, high stress (C)
activates the molecular motion
at lower temperatures and the
penetration will normally
accelerate as temperature
increases. This can be
empirically changed by choice
of compression geometry (see
Figure 5) which will produce
radically different curves.

12. This dynamic softening will not follow the material's expansion as does the static strain. Figure 7
shows, how this technique can be used to follow the softening of amorphous poly(ethylene
terephthalate) (PET) as it passes Tg and its subsequent hardening due to crystallization. For
comparison, the DSC trace for the material is also shown. The static TMA can be taken as the
locus of the lowest points of the trace throughout. It is clear that because of the complexities of
thermal expansion, viscoelastic deformation above Tg and volumetric contraction during
crystallisation, the normal TMA trace defies simple analysis.

13. Many industrial applications of TMA are comparative and used for product quality
control. This is the case for wax or fat blends where the compression geometry
automatically provides greater resistance to flow in the semi-solid materials as they
melt. A typical result is shown in Figure 9 where the melting transitions of each
wax component are defined by a sharp softening of the sample.

14. A further example of an industrial application is shown in Figure 10. TMA
with a square-ended probe is used to assess the degradation in mechanical
properties of a nylon component exposed to hot engine oil. Component B
will be close to failure in service. The changes which are assessed as a
combination of elevated Tg and reduced crystallinity, are due to oxidative
changes on the nylon and a decrease in the equilibrium level of water.

15. Figure 11 shows data for a series of poly(ester) yarns with different
degrees of frozen-in orientation. The higher the initial sample
orientation, the more complex the thermal profile. For the highest
birefringence sample (η=0.05) there are three discrete relaxations, which
allow the frozen-in strain to translate to an active stress. This would
appear to be an under-utilized technique.

16. Another interesting application of TMA is a study of the thermomechanical properties of the
polymers used as denture bases. The glass-transition temperatures of such polymers are important
because there are considerable variations in temperature in the mouth. Hugget used TMA
measurements to watch the effect of various modifications of the polymerization of methyl
methacrylate on the of the resultant material. The factors expected to increase of a polymer are:
 the presence of groups in the backbone which increase the energy required for rotation, e.g. long
side chains;
 secondary bonding between chains e.g. hydrogen bonding;
 crosslink

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