A Practical Introduction to Differential
Scanning Calorimetry
Paul Gabbott
Contents
1.1 Introduction 2
1.2 Principles of DSC and types of measurements made 2
1.2.1 A definition of DSC 2
1.2.2 Heat flow measurements 3
1.2.3 Specific heat (Cp) 3
1.2.4 Enthalpy 5
1.2.5 Derivative curves 5
1.3 Practical issues 6
1.3.1 Encapsulation 6
1.3.2 Temperature range 8
1.3.3 Scan rate 8
1.3.4 Sample size 10
1.3.5 Purge gas 10
1.3.6 Sub-ambient operation 11
1.3.7 General practical points 11
1.3.8 Preparing power compensation systems for use 11
1.4 Calibration 12
1.4.1 Why calibrate 12
1.4.2 When to calibrate 12
1.4.3 Checking performance 13
1.4.4 Parameters to be calibrated 13
1.4.5 Heat flow calibration 13
1.4.6 Temperature calibration 15
1.4.7 Temperature control (furnace) calibration 16
1.4.8 Choice of standards 16
1.4.9 Factors affecting calibration 16
1.4.10 Final comments 17
1.5 Interpretation of data 17
1.5.1 The instrumental transient 17
1.5.2 Melting 18
COPYRIGHTED MATERIAL
BLUK105-Gabbott August 2, 2007 0:22
2 Principles and Applications of Thermal Analysis
1.5.3 The glass transition 22
1.5.4 Factors affecting Tg 24
1.5.5 Calculating and assigning Tg 25
1.5.6 Enthalpic relaxation 26
1.5.7 Tg on cooling 30
1.5.8 Methods of obtaining amorphous material 31
1.5.9 Reactions 34
1.5.10 Guidelines for interpreting data 40
1.6 Oscillatory temperature profiles 42
1.6.1 Modulated temperature methods 42
1.6.2 Stepwise methods 44
1.7 DSC design 46
1.7.1 Power compensation DSC 46
1.7.2 Heat flux DSC 47
1.7.3 Differential thermal analysis DTA 48
1.7.4 Differential photocalorimetry DPC 48
1.7.5 High-pressure cells 49
Appendix: standard DSC methods 49
References 49
1.1 Introduction
Differential scanning calorimetry (DSC) is the most widely used of the thermal techniques
available to the analyst and provides a fast and easy to use method of obtaining a wealth of
information about a material, whatever the end use envisaged. It has found use inmanywideranging
applications including polymers and plastics, foods and pharmaceuticals, glasses
and ceramics, proteins and life science materials; in fact virtually any material, allowing
the analyst to quickly measure the basic properties of the material.Many of the application
areas are dealt with in greater depth within the chapters of this book, and the principles
involved extend to many other materials that may not be mentioned specifically. It is in fact
a fascinating technique and the purpose of this introduction is to provide an insight into
this method of measurement, to provide the necessary practical guidance a new user will
need to go about making measurements, and to give understanding about the information
that can be obtained and how to interpret the data.
1.2 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
BLUK105-Gabbott August 2, 2007 0:22
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
BLUK105-Gabbott August 2, 2007 0:22
4 Principles and Applications of Thermal Analysis