Principles of DSC and types of measurements made
1.2.1 A definition of DSC
A DSC analyser measures the energy changes that occur as a sample is heated, cooled or
held isothermally, together with the temperature at which these changes occur.
The energy changes enable the user to find and measure the transitions that occur in the
sample quantitatively, and to note the temperaturewhere they occur, and so to characterise a
material for melting processes,measurementof glass transitionsanda rangeofmore complex
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A Practical Introduction to Differential Scanning Calorimetry 3
events. One of the big advantages of DSC is that samples are very easily encapsulated, usually
with little or no preparation, ready to be placed in the DSC, so that measurements can be
quickly and easilymade. For further details of instrumental design and basic equations, refer
to Section 1.7 at the end of this chapter.
1.2.2 Heat flow measurements
The main property that is measured by DSC is heat flow, the flow of energy into or out
of the sample as a function of temperature or time, and usually shown in units of mW on
the y-axis. Since a mW is a mJ/s this is literally the flow of energy in unit time. The actual
value of heat flow measured depends upon the effect of the reference and is not absolute.
What matters is that a stable instrumental response or baseline is produced against which
any changes can be measured. The starting point of the curve on the y-axis may be chosen
as one of the starting parameters, and it should be set at or close to zero.
Two different conventions exist for the display of the heat flow curve: one shows endotherms
in the downward direction, the other upward. The operator has a choice with
most software packages. Traditionally, with heat flux systems (Section 1.7.2) endotherms
are shown as going down, since endothermic transitions result in a negative temperature
differential, whilstwith power compensation systems (Section 1.7.1) they are shown as going
up since with this principle endothermic transitions result in an increase in power supplied
to the sample. In this chapter data are shown with endotherms up.
The value of measuring energy flow is that it enables the analyst to identify the range
of different transitions that may occur in the sample as it is heated or cooled; the main
transitions are described in Section 1.5.
1.2.3 Specific heat (Cp)
The specific heat (heat capacity, Cp) of a material can be determined quantitatively using
DSC and is designated Cp since values are obtained at constant pressure. Traditionally, this
is done by subtracting a baseline from the heat flow curve in the manner described below,
but values may also be obtained using modulated temperature techniques, Section 1.6. The
subtracted curve referenced against a standard gives a quantitative value of Cp, Figure 1.1.
The accuracy that can be obtained depends upon the instrument and method in use.
In practice the traditional standard test method (see Appendix on p49) provides a fairly
rapid method for determination ofCp andmanymanufacturers provide software specifically
designed to comply with this. Three runs are required, each consisting of an isothermal
period, temperature ramp and final isotherm. This method is applied identically to the
succeeding runs:
1. First run: a baseline with uncrimped empty pans placed in the furnace.
2. Second run: as above but adding a reference (typically sapphire) to the sample pan.
3. Third run: replace the reference with your sample.
The three curves are brought up on the screen, isothermals matched, data subtracted and
referenced against the standard.Most software packageswill do this automatically, and if the
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4 Principles and Applications of Thermal Analysis
Reference, normally
sapphire
Sample
Empty pans
300
250
200
150
100
50
0
0.0 0.5 1.0 1.5 2.0
Time (min)
2.5 3.0 3.298
?50
?70
Heat flow endo up (mW)
Figure 1.1 Heat capacity of PET obtained using fast scanning techniques showing the three traces required
for subtraction. The height of the sample compared to the empty pan is divided by the scan rate and the
mass of sample to obtain a value for Cp. This is referenced against a known standard such as sapphire
for accuracy. If small heating steps of, for example, 1?C are used the area under the curve can be used to
calculate Cp. This calculation is employed as an option in stepwise heating methods.
differing weight and heat capacity of sample pans are taken into account then the baseline
and reference runs may be used for subsequent samples, provided the DSC is stable. In fact,
because the procedure is based on a subtraction technique between measurements made at
different times, any drift will cause error. The DSC must be very stable and in practice it is
best not to use an instrument at the extremes of its temperature range where stability may
be compromised. The standard most often used is sapphire, and the mass used should be
similar to the sample; in any event the sample should not be a great deal larger or errors
will be increased. This method relies on the measurement of the heat flow of the sample
compared to that of an empty pan. Whilst there may be a number of factors which dictate
the scan rate of choice it should be noted that faster scan rates result in increased values of
heat flow giving increased accuracy of measurement, and this also minimises the time of
the run and potential drift of the analyser. It has been reported that fast scan rates used by
fast scan DSC (Chapter 2) can give extremely accurate data [1].
A similar principle is employed in stepwise heating methods where the temperature may
be raised by only a fraction of a degree between a series of isotherms. This is reported to
give a very accurate value for Cp because of the series of short temperature intervals.
Specific heat data can be of value in its own right since this information is required
by chemists and chemical engineers when scaling up reactions or production processes, it
provides information for mathematical models, and is required for accurate kinetic and
other advanced calculations. It can also help with curve interpretation since the slope of the
curve is fixed and absolute, and small exothermic or endothermic events identified. Overall,
it gives more information than the heat flow trace because values are absolute, but it does
take more time, something often in short supply in industry.
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