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Interpreting Unexpected Events and Transitions in DSC Results

TA039 Interpreting Unexpected Events and Transitions in DSC Results 1 TA039 Figure 1 Artificial DSC curve INTERPRETATION OF EVENTS AND TRANSITIONS

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and Transitions in DSC Results
1 TA039
Figure 1: Articial DSC curve
Event 1: Large Endothermic Start-up Hook
At the beginning of a programmed heating experiment, 
endothermic) which occurs primarily based on 
differences in the heat capacity of the sample and 
reference. Since heat capacity is directly related to 
weight, an endothermic shift indicates that the reference 
heightened by faster heating rates.
When operating subambient, the thermocouple 
is transferred from the cell cooling head. This effect 
increases as the temperature is lowered and/or the time 
at lower temperatures is increased.
Effects on Results
A large “start-up hook” or a sloping baseline make 
during the rst 2-3 minutes of the experiment, transition 
temperatures and measured heat ow (DH) may not be 
The purpose of this paper is to assist DSC users with 
results. The transitions discussed in the paper are several 
of the ones that most frequently create problems for the 
new user, and which can also fool even an experienced 
thermal analyst. 
By applying some of the recommended procedures and 
solutions, most laboratories will be able to improve the 
analysis technique which measures the temperature 
and heat ow associated with transitions in materials as 
a function of temperature and time. Such measurements 
provide quantitative and qualitative information 
about physical and chemical changes that include 
endothermic/exothermic processes or changes in heat 
capacity. Specic information that can be obtained 
Glass transition temperatures
Melting points & boiling points
Crystallization time & temperature
Percent crystallinity
Heats of fusion and reaction
Specic heat
Oxidative stability
Rate of cure
Thermal stability
Because of the wealth of information provided and 
because DSC is easy-to-use, DSC has become the 
most commonly used thermal analysis technique. 
Ease-of-use in this case refers to sample preparation 
transitions that can be the cause of less than optimum 
several of these events with causes and solutions. Figure 
1 is an articial DSC curve which was generated to 
illustrate these events/transitions. The curve is articial in 
the sense that all of these events would not occur in the 
same real world DSC curve.
2 TA039
a series of reference pans of different weight (2 mg. 
increments). When running a sample use a reference pan 
that weighs 0–10% more than sample pan. Figure 2 shows 
results with an epoxy prepreg sample. Best results are 
achieved with 1.5 lids. 2 lids results in overcompensation 
and an exothermic start-up hook. Figures 3 and 4 show 
how the glass transition results are affected by correct 
compensation. Note: these results are obtained heating 
at 20°C/minute from a 100°C isothermal hold. The impact 
of the start-up hook can also be reduced by initiating 
the heating at a temperature that is at least 2-3 minutes 
below the range of interest at the heating rate chosen 
(ie. at 20°C/minute, start experiment at least 50°C below 
the rst thermal event of interest.)
 Figure 2: Effect of reference pan weight
Figure 3: Start-up hook and T
 with no reference pan
Figure 4: Start-up hook and T
 with correct reference pan
If operating below 0°C, use 50cc/minute dry nitrogen 
purge gas through the cell base VACUUM PORT plus 
the normal purge gas. Figure 5 illustrates the typical 
improvement obtainable.
Figure 5: Proper gas purging improves subambient baseline 
Event 2: Transition(s) at 0°C
Weak transitions around 0°C indicate the presence of 
water in the sample or the purge gas. These transitions 
are usually endotherms, but may appear different than 
a melting peak. Since water can condense on both the 
sample and reference pans, the transition often appears 
as shown in Figure 6. Furthermore, the peaks may appear 
slightly lower than 0°C due to impurities dissolved by the 
moisture from the cell and pans.
Effects on Results
If water is in the sample, results may not be reproducible 
because it can act as a plasticizer and reduce transition 
temperatures. The water will also volatilize during the run, 
causing an endothermic peak and a shift in the baseline.  
3 TA039
If water is in the purge gas, it causes a perturbation in the 
near 0°C.
Keep hygroscopic samples in a dessicator and load 
them into pans in a dry box.
and after the run. A change in weight could explain an 
unexpected transition.
Figure 6: DSC transition due to moisture in the purge gas
Dry the purge gas by placing a drying tube in the line. 
Figure 7 shows an epoxy sample after loading at -100°C. 
The absence of any transitions at 0°C indicates that 
with proper precautions water condensation in the cell 
can be eliminated even under conditions which favor 
condensation. Note: Loading a sample at temperature 
below 0°C is only possible when using the liquid nitrogen 
cooling accessory (LNCA). Samples should always be 
loaded above 0°C with any other cooling accessory.
Figure 7: Quench cooling samples at subambient temperatures
Event 3: Apparent “Melting” at Glass Transition (T
Stresses built into the material as a result of processing, 
handling or thermal history are released when the 
material is heated through its glass transition. The reason 
this occurs at T
 is that the molecule goes from a rigid 
to a exible structure and thus can move to relieve the 
Effects on Results
Molecular relaxation usually appears as a weak 
endothermic transition near the end of a glass transition. 
As shown in Figure 8, this behavior can be pronounced 
enough to either shift the measured glass transition 
of the T
Figure 8: Molecular relaxation can cause T
 to appear as a melt
Relieve the internal stresses in the material by heating it 
to at least 25°C above the T
 and then quench cooling it 
to a temperature below the T
. Figure 9 shows the same 
material as in Figure 8 after curing at 200°C and then 
quench cooling to 25°C.
4 TA039
Figure 10: Effect of cooling rate on shape of T
Event 4: Exothermic Peaks Below Decomposition 
Temperature While Heating
Exothermic behavior results during curing of a 
polymer. The amount of heat associated with these 
and % crystallinity respectively provided scans of suitable 
standards are available.
When an exotherm is obtained in a polymer’s DSC prole 
at a temperature which the operator suspects is too low 
to be a decomposition, running the material in the TGA 
aids evaluation. The absence of a TGA weight loss which 
coincides with the DSC exotherm indicates that the 
exotherm is crystallization or curing.
Effects on Results
The presence or absence of exothermic crystallization 
peaks in thermoplastic materials is very dependent 
on thermal history. Therefore, DSC results will not be 
reproducible if thermal history of the sample is not tightly 
controlled. Figures 11 and 12 illustrate the different results 
cooling at 10°C/minute respectively. The quenched 
material has a well-dened T
amorphous structure which rearranges on heating to 
a crystalline structure before melting at about 235°C. 
The DH of crystallization is slightly less than the DH of 
melting which indicates that the initial structure is mostly 
amorphous. The slowly cooled material has a weak 
, indicating an initial structure that is almost entirely 
crystalline. Since it is crystalline at the start of the DSC 
experiment, no additional crystallization occurs prior to 
the melt at 235°C.
When comparing thermoplastic materials, give the 
materials a common known thermal history by either 
quench cooling or program cooling from above 
the melting temperature. ASTM D3418-82 denes 
recommended procedures for giving polymers a known 
thermal history.
Event 5: Baseline Shift After Endothermic or Exothermic 
Baseline shifts are caused by changes in sample 
weight, heating rate, or the specic heat of the sample. 
A change in specic heat often occurs after the 
sample has gone through a transition such as curing, 
crystallization, or melting. Sample weight often changes 
during volatilization or decomposition.
Effects on Results
Since H is calculated on the basis of sample weight 
change will be in error.
5 TA039
Integration of a peak which has a baseline shift is difcult 
and typically less accurate because of operator subjectivity 
weight loss has occurred.
If crystallization or melting is the cause of the transition, 
compare the DH of the transitions, using different limits and 
types of baselines. Figure 13 illustrates an example where use 
of sigmoidal baseline is required.
Event 6: Sharp Endothermic Peaks During Exothermic 
Sharp peaks similar to those in Figure 1 above 300°C, are 
usually the result of experimental phenomena rather than 
real material transitions. For example, rapid volatilization of 
gases trapped in the material can cause sharp peaks, as 
can rapid volatilization of gases trapped in a partially sealed 
Effects on Results
melting peaks associated with minor components is possible.
quantitative results since sample mass changes. If the 
DSC cell damage can occur with extended operation.
weight loss has occurred.
Reduce the temperature limit of further experiments if no 
useful information is obtained because of the volatilization.
Use a Pressure DSC cell.
By Leonard C. Thomas
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