Polymer Characterization - Laboratory Techniques and Analysis-02

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2 THERMAL ANALYSIS 
GENERAL PRINCIPLES OF OPERATION 
Thermal analysis refers to a variety of techniques in which a property of 
a sample is continuously measured as the sample is programmed through 
a predetermined temperature profile. Among the most common techni- 
ques are thermal gravimetric analysis (TA) and differential scanning 
calorimetry @SC). 
In TA the mass loss versus increasing temperature of the sample is 
recorded. The basic instrumental requirements are simple: a precision 
balance, a programmable furnace, and a recorder (Figure 1). Modern 
instruments, however, tend to be automated and include software for data 
reduction. In addition, provisions are made for surrounding the sample 
with an air, nitrogen, or an oxygen atmosphere. 
In a DSC experiment the difference in energy input to a sample and 
a reference material is measured while the sample and reference are 
subjected to a controlled temperature program. DSC requires two cells 
equipped with thermocouples in addition to a programmable furnace, 
recorder, and gas controller. Automation is even more extensive than in 
TA due to the more complicated nature of the instrumentation and 
calculations. 
A thermal analysis curve is interpreted by relating the measured 
property versus temperature data to chemical and physical events 
occurring in the sample. It is frequently a qualitative or comparative 
technique. 
In TA the mass loss can be due to such events as the volatilization 
of liquids and the decomposition and evolution of gases from solids. The 
onset of volatilization is proportional to the boiling point of the liquid. 
The residue remaining at high temperature represents the percent ash 
content of the sample. Figure 2 shows the TA spectrum of calcium 
oxalate as an example. 
17 
18 Polymer Characterization 
Figure 1. Typical components of a TA instrument. 
Figure 2. Shows the TA spectrum of calcium oxalate. 
Atmosphere 
 Control
Sample Holder
Recorder
Furnace
Balance 
Control
Recording
Balance
Furnace
Temperature
Programmer
Temperature
Sensor
joe sulton
Thermal Analysis 19 
In DSC the measured energy differential corresponds to the heat 
content (enthalpy) or the specific heat of the sample. DSC is often used 
in conjunction with TA to determine if a reaction is endothermic, such 
as melting, vaporization and sublimation, or exothermic, such as 
oxidative degradation. It is also used to determine the glass transition 
temperature of polymers. Liquids and solids can be analyzed by both 
methods of thermal analysis. The sample size is usually limited to lo- 
20 mg. 
Thermal analysis can be used to characterize the physical and 
chemical properties of a system under conditions that simulate real world 
applications. It is not simply a sample composition technique. 
Much of the data interpretation is empirical in nature and more than 
one thermal method may be required to fully understand the chemical 
and physical reactions occurring in a sample. 
Condensation of volatile reaction products on the sample support 
system of a TA can give rise to anomalous weight changes. 
THERMAL ANALYSIS OF POLYMERS 
A simple example of the relationship between “structure” and 
“properties” is the effect of increasing molecular weight of a polymer on 
its physical (mechanical) state; a progression from an oily liquid, to a 
soft viscoelastic solid, to a hard, glassy elastic solid. Even seemingly 
minor rearrangements of atomic structure can have dramatic effects as, 
for example, the atactic and syndiotactic stereoisomers of polypropylene-- 
the first being a viscoelastic amorphous polymer at room temperature 
while the second is a strong, fairly rigid plastic with a melting point 
above 160°C. At high thermal energies conformational changes via bond 
rotations are frequent on the time scale of typical processing operations 
and the polymer behaves as a liquid (melt). At lower temperatures the 
chains solidifies by either of two mechanisms: by ordered molecular 
packing in a crystal lattice, crystdization, or by a gradual freezing out 
of long range molecular motions, vitrification. These transformations, 
which define the principal rheological regimes of mechanical behavior: 
the melt, the rubbery state, and the semicrystalline and glassy amorphous 
solids, are accompanied by transitions in thermodynamic properties at the 
glass transition temperature, the crystalline melting, and the crystalli- 
zation temperatures. 
20 Polymer Characterization 
Thermal analysis techniques are designed to measure the above 
mentioned transitions both by measurements of heat capacity and 
mechanical moduhrs (stiffness). 
Differential Scanning Calorimetry @SC) 
The DSC measures the power (heat energy per unit time) differential 
between a small weighed sample of polymer (ea. 10 mg) in a sealed 
aluminum pan referenced to an empty pan in order to maintain a zero 
temperature differential between them during programmed heating and 
cooling temperature scans. The technique is most often used for 
characterizing the T,, T,, T,, and heat of fusion of polymers (Figure 3). 
The technique can also be used for studying the kinetics of chemical 
reactions, e.g., oxidation and decomposition. The conversion of a 
measured heat of fusion can be converted to a % crystallinity provided, 
of course, the heat of fusion for the 100% crystalline polymer is known. 
Thermogravimetric Analysis (TGA) 
TGA makes a continuous weighing of a small sample (ca 10 mg) in a 
controlled atmosphere (e.g., air or nitrogen) as the temperature is 
increased at a programmed linear rate. The thermogram shown in 
Figure 4 illustrates weight losses due to desorption of gases (e.g., 
moisture) or decomposition (e.g., HBr loss from halobutyl, CO, from 
calcium carbonate filler). TA is a very simple technique for quan- 
titatively analyzing for filler content of a polymer compound (e.g., 
carbon black decomposed in air but not nitrogen). While oil can be 
readily detected in the thermogram it almost always overlaps with the 
temperature range of hydrocarbon polymer degradation. The curves 
cannot be reliably deconvoluted since the actual decomposition range of 
a polymer in a polymer blend can be affected by the sample morphology. 
Thermomechanical Analysis (TMA) 
TMA consists of a quartz probe which rests on top of a flat sample (a 
few mms square) in a temperature controlled chamber. When setup in 
neutral buoyancy (with ‘flat probe’) then as the temperature is increased 
the probe rises in direct response to the expansion of the sample yielding 
I 
AH/At 
glass transition crystallization 
\ 
(onset) 
I area = 
crystalline melting 
oxidation/ 
onset 
+ 
Heating scan 
(typically 2OWmin) 
c 
exo / - 
- 
Cooling scan 
TEMPERATURE II 
+ 
i 
l Important characteristics: Ts , T,,, , heat of fusion on heating; Tc on cooling E 
2 v1’ 
Figure 3. Illustrates typical polymer DSC thermograms. z 
Thermal Analysis 23 
thermal expansion coefficient versus temperature scans. Alternatively, 
with the ‘penetration probe’ under dead loading a thermal softening 
profile is obtained (penetration distance versus temperature). Although 
this is a simple and versatile experiment, it gives only a semi-quantitative 
indication of mechanical modulus versus temperature. The DMTA, 
described below, gives an absolute modulus measurement. 
Dynamic Mechanical Thermal Analysis (DMTA) 
DMTA is a measurement of the dynamic moduli (in-phase and out-of- 
phase) in an oscillatory mechanical deformation experiment during a 
programmed temperature scan at controlled frequency. Thermograms 
are usually plotted to show elastic modulus, E, and tan 6 versus 
temperature (Figure 5). The peak of the tan 6 is a particularly 
discriminatorymeasure of T,, although this is the center of the relaxation 
whereas in the DSC experiment the onset temperature of the T, 
relaxation is usually reported. In such a case the DSC T, will be lower 
than that for DMTA by an amount that varies with the specific polymer. 
There is, in addition, a frequency effect which puts the mechanical (ca. 
1 Hz) T, about 17°C higher than that for a DSC measurement (ca. 
O.OOOlHz) for an assumed activation energy of 400 Id/mole (typical for 
polymer TJ. The DMTA has a frequency multiplexing capability which 
can be used for calculating activation energies using time-temperature 
superposition software. 
The temperature range of the DMTA is from -150°C to 300°C and 
frequencies from 0.033 to 90 Hz. The sample size for the usual flexural 
test mode is 1 mm x 10 mm x 40 mm; slightly less sample is required 
in the parallel plate shear mode. 
24 Polymer Characterization 
cJr1ve Shaft 
(selected amDlltude) 
Drive shaft 
(selected amplitude) 
clamps 
(a) Flexure mode (TV) Shear mode