Differential scanning calorimetry or DSC is a thermoanalytical technique in which the
difference in the amount of heat required to increase the temperature of a sample and
reference is measured as a function of temperature. Both the sample and reference are maintained
at nearly the same temperature throughout the experiment. Generally, the temperature
program for a DSC analysis is designed such that the sample holder temperature increases
linearly as a function of time. The reference sample should have a well-defined heat capacity
over the range of temperatures to be scanned. The technique was developed by E.S. Watson
and M.J. O'Neill in 1962, and introduced commercially at the 1963 Pittsburgh Conference on Analytical
Chemistry and Applied Spectroscopy. The first adiabatic differential scanning calorimeter
that could be used in biochemistry was developed by P.L. Privalov and D.R. Monaselidze in 1964.
The term DSC was coined to describe this instrument which measures energy directly and allows
precise measurements of heat capacity.
Detection of phase transitions The basic principle underlying this technique
is that when the sample undergoes a physical transformation such as phase transitions,
more or less heat will need to flow to it than the reference to maintain both at the
same temperature. Whether less or more heat must flow to the sample depends on whether
the process is exothermic or endothermic. For example, as a solid sample melts to a
liquid it will require more heat flowing to the sample to increase its temperature at
the same rate as the reference. This is due to the absorption of heat by the sample as
it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample
undergoes exothermic processes less heat is required to raise the sample temperature.
By observing the difference in heat flow between the sample and reference, differential scanning
calorimeters are able to measure the amount of heat absorbed or released during such transitions.
DSC may also be used to observe more subtle physical changes, such as glass transitions.
It is widely used in industrial settings as a quality control instrument due to its applicability
in evaluating sample purity and for studying polymer curing.
DTA An alternative technique, which shares much
in common with DSC, is differential thermal analysis. In this technique it is the heat
flow to the sample and reference that remains the same rather than the temperature. When
the sample and reference are heated identically, phase changes and other thermal processes
cause a difference in temperature between the sample and reference. Both DSC and DTA
provide similar information. DSC measures the energy required to keep both the reference
and the sample at the same temperature whereas DTA measures the difference in temperature
between the sample and the reference when they are both put under the same heat.
DSC curves The result of a DSC experiment is a curve
of heat flux versus temperature or versus time. There are two different conventions:
exothermic reactions in the sample shown with a positive or negative peak, depending on
the kind of technology used in the experiment. This curve can be used to calculate enthalpies
of transitions. This is done by integrating the peak corresponding to a given transition.
It can be shown that the enthalpy of transition can be expressed using the following equation:
where is the enthalpy of transition, is the calorimetric constant, and is the area under
the curve. The calorimetric constant will vary from instrument to instrument, and can
be determined by analyzing a well-characterized sample with known enthalpies of transition.
Applications Differential scanning calorimetry can be used
to measure a number of characteristic properties of a sample. Using this technique it is possible
to observe fusion and crystallization events as well as glass transition temperatures Tg.
DSC can also be used to study oxidation, as well as other chemical reactions.
Glass transitions may occur as the temperature of an amorphous solid is increased. These
transitions appear as a step in the baseline of the recorded DSC signal. This is due to
the sample undergoing a change in heat capacity; no formal phase change occurs.
As the temperature increases, an amorphous solid will become less viscous. At some point
the molecules may obtain enough freedom of motion to spontaneously arrange themselves
into a crystalline form. This is known as the crystallization temperature. This transition
from amorphous solid to crystalline solid is an exothermic process, and results in a
peak in the DSC signal. As the temperature increases the sample eventually reaches its
melting temperature. The melting process results in an endothermic peak in the DSC curve. The
ability to determine transition temperatures and enthalpies makes DSC a valuable tool in
producing phase diagrams for various chemical systems.
Examples The technique is widely used across a range
of applications, both as a routine quality test and as a research tool. The equipment
is easy to calibrate, using low melting indium at 156.5985 °C for example, and is a rapid
and reliable method of thermal analysis. Polymers
DSC is used widely for examining polymeric materials to determine their thermal transitions.
The observed thermal transitions can be utilized to compare materials, although the transitions
do not uniquely identify composition. The composition of unknown materials may be completed
using complementary techniques such as IR spectroscopy. Melting points and glass transition
temperatures for most polymers are available from standard compilations, and the method
can show polymer degradation by the lowering of the expected melting point, Tm, for example.
Tm depends on the molecular weight of the polymer and thermal history, so lower grades
may have lower melting points than expected. The percent crystalline content of a polymer
can be estimated from the crystallization/melting peaks of the DSC graph as reference heats
of fusion can be found in the literature. DSC can also be used to study thermal degradation
of polymers using an approach such as Oxidative Onset Temperature/Time, however, the user
risks contamination of the DSC cell, which can be problematic. Thermogravimetric Analysis
may be more useful for decomposition behavior determination. Impurities in polymers can
be determined by examining thermograms for anomalous peaks, and plasticisers can be detected
at their characteristic boiling points. In addition, examination of minor events in first
heat thermal analysis data can be useful as these apparently "anomalous peaks" can in
fact also be representative of process or storage thermal history of the material or
polymer physical aging. Comparison of first and second heat data collected at consistent
heating rates can allow the analyst to learn about both polymer processing history and
material properties. Liquid crystals
DSC is used in the study of liquid crystals. As some forms of matter go from solid to liquid
they go through a third state, which displays properties of both phases. This anisotropic
liquid is known as a liquid crystalline or mesomorphous state. Using DSC, it is possible
to observe the small energy changes that occur as matter transitions from a solid to a liquid
crystal and from a liquid crystal to an isotropic liquid.
Oxidative stability Using differential scanning calorimetry to
study the stability to oxidation of samples generally requires an airtight sample chamber.
Usually, such tests are done isothermally by changing the atmosphere of the sample.
First, the sample is brought to the desired test temperature under an inert atmosphere,
usually nitrogen. Then, oxygen is added to the system. Any oxidation that occurs is observed
as a deviation in the baseline. Such analysis can be used to determine the stability and
optimum storage conditions for a material or compound.
Safety Screening DSC makes a reasonable initial safety screening
tool. In this mode the sample will be housed in a non-reactive crucible, and which will
be able to withstand pressure. The presence of an exothermic event can then be used to
assess the stability of a substance to heat. However, due to a combination of relatively
poor sensitivity, slower than normal scan rates and unknown activation energy, it is
necessary to deduct about 75-100 °C from the initial start of the observed exotherm
to suggest a maximum temperature for the material. A much more accurate data set can be obtained
from an adiabatic calorimeter, but such a test may take 2–3 days from ambient at a
rate of a 3 °C increment per half hour. Drug analysis
DSC is widely used in the pharmaceutical and polymer industries. For the polymer chemist,
DSC is a handy tool for studying curing processes, which allows the fine tuning of polymer properties.
The cross-linking of polymer molecules that occurs in the curing process is exothermic,
resulting in a positive peak in the DSC curve that usually appears soon after the glass
transition. In the pharmaceutical industry it is necessary
to have well-characterized drug compounds in order to define processing parameters.
For instance, if it is necessary to deliver a drug in the amorphous form, it is desirable
to process the drug at temperatures below those at which crystallization can occur.
General chemical analysis Freezing-point depression can be used as a
purity analysis tool when analysed by differential scanning calorimetry. This is possible because
the temperature range over which a mixture of compounds melts is dependent on their relative
amounts. Consequently, less pure compounds will exhibit a broadened melting peak that
begins at lower temperature than a pure compound. See also
References
External links www.pslc.wsdsc.htm Differential Scanning Calorimetry
information from the Polymer Science Learning Center
Bibliography Brydson, J A, Plastics Materials, Butterworth-Heinemann,
7th Ed. Ezrin, Meyer, Plastics Failure Guide: Cause
and Prevention, Hanser-SPE. Wright, D.C., Environmental Stress Cracking
of Plastics RAPRA. Lewis, Peter Rhys, and Gagg, C, Forensic Polymer
Engineering: Why polymer products fail in service, Woodhead/CRC Press.