Page 39 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Modern Techniques 23

where E´´ is the loss modulus and is associated with the loss of energy as
heat due to the deformation of the material. The loss tangent or damping
factor, tan δ, is defined as the ratio of E´´/E.
In a DMA experiment, E´ and/or E´´ and/or tan δ are plotted as a
function of time or temperature (see Fig. 8). T , an α-transition, is usually
g
obtained from the most intense peak observed for either the E´´ or tan δ (and
significant inflection for E´) curve. Amorphous polymers have a more intense
α-peak than semi-crystalline polymers because the former are less rigid.
The glass transition temperature, determined by DMA, is depen-
dent on the heating-rate and frequency. Therefore, T values obtained by
g
this dynamic technique are generally different from that obtained by static
techniques, such as differential scanning calorimetry (DSC). Moreover, the
temperature of a polymer can also be increased by subjecting the material
to high frequency and high amplitude oscillations. Thus, when studying
dynamic mechanical properties, low frequencies and low strain amplitudes
should be used. Low strain amplitude is associated with the linear region of
a stress-strain curve, but if a large stress or strain amplitude is applied to a
viscoelastic material, high internal heat due to molecular vibration is
generated. This results in a nonlinear viscoelastic response that is quite
complex to analyze. Also, in nonlinear viscoelastic regions, the material is
permanently modified. For example, microscopic crack formation or fail-
ure due to fatigue can result.
Clamping will affect modulus results, and, therefore, absolute
modulus values are obtained with great difficulty using DMA. If care is
taken, results within a given laboratory will be reproducible, hence com-
parison amongst various materials is feasible. Although DMA is weak with
respect to the accuracy of absolute modulus, the transition temperatures can
routinely be determined with great accuracy. The method used to obtain T
g
(i.e., E´´ or tan δ peak temperature) affects the value, and, therefore, the
parameter must be specified . As long as the same parameter is used

throughout a study, the trend observed will be the same regardless of the
parameter used.

3.3 Dielectric Analysis (DEA)

Dielectric analysis (DEA) or dielectric thermal analysis (DETA)
is another important thermoanalytical technique that is rapidly evolv-
ing. This technique measures two fundamental electrical characteristics
of a material—capacitance and conductance—as a function of time,