Polymers typically display broad melting endotherms and glass transitions as major analytic
features associated with their properties. Both the glass and melting transitions are strongly
dependent on processing conditions and dispersion in structural and chemical properties of
plastics. Characterization of polymers requires a detailed analysis of these characteristic thermal
transitions using either differential scanning calorimeter (DSC) or differential thermal analysis
(DTA). Additionally, polymers are viscoelastic materials with strong time and temperature
dependencies to their mechanical properties. Temperature scans across the dynamic spectrum of
mechanical absorptions are commonly required for characterization of polymers, especially for
elastomers. These thermal/mechanical properties are characterized in dynamic mechanical/thermal
analysis (DMTA). Additionally, weight loss with heating is a common phenomena for polymers
due to degradation and loss of residual solvents and monomers. Weight loss on heating is studied
using thermal gravimetric analysis (TGA).
A complete thermal analysis of a plastic sample yields inferential information concerning the
chemical composition and structure of the material. Examples:
1. The Hoffman-Lauritzen description of the crystalline melting point associates shits in the
melting transition temperature with the thickness of lamellar crystallites in polymers. Such
structural based shifts would suggest further study using the Scherrer approach for diffraction peak
broadening and small-angle x-ray and TEM analysis.
2. Dramatic weight loss in a TGA analysis of nylon at temperatures above 100°C indicate some
association of water with the nylon chemical structure. Such an observation would suggest further
study using spectroscopic techniques.
3. In a polymer alloy (blend), the observation of two glass transition temperatures indicates a
biphasic system, a single glass transition, a miscible system following the Flory-Fox equation.
Further support for miscibility would come from microscopy and scattering (neutron, x-ray and
light can all be used to characterize miscibility).
Generally, thermal analysis is the easiest and most available of techniques to apply to a sample and
for this reason thermal analysis is often the first technique used to analytically describe a plastic
material.
Error analysis in thermal techniques usually is conducted by repetition of the measurement for at
least 3 to 10 identical samples in order to determine the standard deviation in the measurement. In
all thermal analysis techniques the instruments must be calibrated with standard samples displaying
sharp and constant transition temperatures and enthalpies of transition.
Calorimetry (Differential Scanning Calorimetry, DSC; Differential Thermal
Analysis, DTA):
Calorimetry involves the measurement of relative changes in temperature and heat or energy either
under isothermal or adiabatic conditions. Chemical calorimetry where the heats of reaction are
measured, usually involve isothermal conditions. Bomb or flame calorimeters involve adiabatic
systems where the change in temperature can be translated, using the heat capacity of the system,
into the enthalpy or energy content of a material such as in determination of the calorie content of
food. In materials characterization calorimetry usually involves an adiabatic measurement. A
calormetric measurement in materials science is carried out on a closed system where determination
of the heat, Q, associated with a change in temperature, DT, yields the heat capacity of the material,
C:
2
C
Q
T
=
D
At constant pressure:
dQ
dT
H
T
C
P N
P = ?
è ç
?? ÷
= ¶
¶ ,
The enthalpy can be calculated from CP through,
H T H T C dT P
T
T
( ) ( ) = +? 0
0
Instrumentation for Thermal Analysis, DTA and DSC:
Figure 12.1 of Campbell and White shows a schematic of a differential thermal analysis (DTA)
instrument. The instrument is composed of two identical cells in which the sample and a reference
(often an empty pan) are placed. Both cells are heated with a constant heat flux, Q, using a single
heater, and the temperatures of the two cells are measured as a function of time. If the sample
undergoes a thermal transition such as melting or glass transition a difference in temperature is
observed, DT = Tsample - Treference. Negative DT indicates an endotherm for a heating cycle.
Quantitative analysis of DTA data is complicated and the instrument is usually viewed as a fairly
crude sibling of a differential scanning calorimeter (DSC) discussed below. Recent instrumental
advancements have improved the quantitative use of DTA instruments. A DTA instrument is
generally less expensive than a DSC. Determination of transition temperatures are accurate in a
DTA. Estimates of enthalpies of transition are generally not accurate. In the DTA heat is provided
at a constant rate and temperature is a dependent parameter. In the equation above for CP, the
normal order of dependent and independent parameters in the differential is reversed, so dT/dQ is
actually measured rather than dQ/dT. This distinction is critically important in transitions where
kinetics become important such as in polymer melting and glass transition.
Figure 12.2 of Campbell and White shows a schematic of a differential scanning calorimeter
(DSC). The arrangement is similar to the DTA except that the sample and reference are provided
with separate heaters. The independent parameter is the temperature which is ramped at a
controlled rate. Feedback loops control the feed of heat to the sample and reference so the
temperature program is closely followed. The raw data from a DSC is heat flux per time or power
as a function of temperature at a fixed rate of change of temperature (typically 10C°/min). Since the
heat flux will increase with temperature ramp rate, higher heating rates lead to more sensitive
thermal spectra. On the other hand, high heating rates lead to lower resolution of the temperature
of transition and can have consequences for transitions which display kinetic features.
Campbell and White go through a useful 2 page comparison of the DSC and DTA techniques
which should be reviewed.
Data Interpretation:
The output of a DSC is a plot of heat flux (rate) versus temperature at a specified temperature ramp
rate. The heat flux can be converted to CP by dividing by the constant rate of temperature change.
The output from a DTA is temperature difference (DT) between the reference and sample cells
versus sample temperature at a specified heat flux. Qualitatively the two plots appear similar.