Presentation2dsc

By
SROTA DAWN.
M.PHARM(PHARMACOLOGY)

Differential scanning
calorimetry
The technique was developed by
E.S. Watson and M.J. O'Neill in 1960,
and introduced commercially at the Pittsburgh
Conference on Analytical Chemistry and Applied
Spectroscopy in 1963

Definitions
• A calorimeter measures the heat into or out of a
sample.
• A differential calorimeter measures the heat of
sample relative to a reference.
• A differential scanning calorimeter does all of
the above and heats the sample with a
linear temperature
•Endothermic heat flows into the sample.
• Exothermic heat flows out of the sample.

This technique is used to study
1. Measures the heat loss or gain resulting from
physical or chemical change with in the sample as a
function of temperature.
2. What happens to polymers/samples upon
heating
It is used to study thermal transitions of a
polymer/sample (the changes that take place on
heating)
For example:
The melting of a crystalline polymer
The glass transition
The crystallization

Principle
• The sample and reference are maintained at the
same temperature, even during a thermal event in
the sample
• The energy required to maintain zero temperature
difference between the sample and the reference
is measured
• During a thermal event in the sample, the system
will transfer heat to or from the sample pan to
maintain the same temperature in reference and
sample pans

SIMPLESIMPLE INSRUMENTATIONINSRUMENTATION ::

 There are two pans, In sample pan, polymer is
added, while the other, reference or standard
substance is added
 Each pan sits on top of heaters which are
controlled by a computer
 The computer turns on heaters, and let them
heat the two pans at a specific rate, usually
10o
C/min.
 The computer makes absolutely sure that the
heating rate stays exactly the same throughout
the experiment
Procedure:

• Two basic types of DSC instruments:
power compensation DSC And heat-flux DSC
Differential Scanning Calorimetry Instrument
Power compensation DSC
Heat flux DSC

Power Compensation DSCPower Compensation DSC
1. Sample holder
Aluminum or Platinum pans
2. Sensors
Platinum resistance
thermocouples
Separate sensors and heaters for the sample and reference
3. Furnace
Separate blocks for sample and reference cells
4. Temperature controller
Supply the differential thermal power to the heaters to maintain the
temperature of the sample and reference at the program value
sample
pan
∆T = 0
inert gas
vacuum
inert gas
vacuum
individual
heaters controller ∆P
reference
pan
thermocouple
introduced in the early
1960s.

1. Sample and reference holders
Al or Pt pans placed on constantan disc
Sample and reference holders are
connected by a low-resistance
heat flow path
2. Sensors
• Chromel® (an alloy made of 90% nickel and 10% chromium)-constantan area
thermocouples (differential heat flow)
• Chromel®-alumel (an alloy consisting of approximately 95% nickel, 2% manganese,
2% aluminium and 1% silicon) thermocouples (sample temperature)
Thermocouple is a junction between two different metals that produces a
voltage due to a temperature difference
3. Furnace
• One block for both sample and reference cells
4. Temperature controller
• The temperature difference between the sample and reference is
converted to differential thermal power, which is supplied to the heaters to
maintain the temperature of the sample and reference at the program value
Heat Flux DSC
sample
pan
inert gas
vacuum
heating
coil
reference
pan
thermocouples Chromel
constantan
chromel/alumel
wires

What can DSC measure?
• Glass transitions
• Melting and boiling points
• Crystallisation time and temperature
• Percent crystallinity
• Heats of fusion and reactions
• Specific heat capacity
• Oxidative/thermal stability
• Reaction kinetics
• Purity

6
DSC Thermogram
Temperature
HeatFlow->exothermic
Glass
Transition
Crystallisation
Melting
Cross-Linking
(Cure)
Oxidation

 The result of a DSC experiment is a curve of
heat flux versus temperature or versus time.
 This curve can be used to calculate enthalpies
of transitions, which is done by integrating the
peak corresponding to a given transition. The
enthalpy of transition can be expressed using
equation:
 ΔH = KA

Where,
ΔH is the enthalpy of transition,
K is the calorimetric constant,
A is the area under the peak.
The calorimetric constant varies from instrument to
instrument, and can be determined by analyzing a well-
characterized material of known enthalpies of
transition.
Area under the peak is directly proportional to heat
absorbed or evolved by the reaction,
Height of the peak is directly proportional to rate of the
reaction

Two types of factors effect the DSC curve
1-Instrumental factors
a- Furnace heating rate
b- Recording or chart speed
c- Furnace atmosphere
d- Geometry of sample holder/location of sensors
e- Sensitivity of the recoding system
f-Composition of sample containers

a- Amount of sample
b- Nature of sample
c- Sample packing
d- Solubility of evolved gases in the sample
e- Particle size
f- Heat of reaction
g- Thermal conductivity

• DSC plot can be used to determine Heat Capacity.
Suppose a polymer is being heated. When heating
starts two pans, the computer will plot the difference
in heat output of the two heaters against temperature
that is plot of heat absorbed by the polymer against
temperature. The plot will look like this at first.

 The heat flow is heat (q) supplied per unit time
(t),
whereas,
 The heating rate of temperature increase (ΔT)
per unit time (t)

• By dividing heat flow (q/t) by the heating rate
(ΔT/t). It ends up with heat supplied divided
by the temperature increase, which is called
heat capacity.

When a certain amount of heat is transferred
to the sample, its temperature increases by a
certain amount, and the amount of heat it
takes to get a certain temperature
increase is called the heat capacity,
or Cp, it can be figured up from the DSC plot

On heating the polymer to a certain
temperature, plot will shift downward
suddenly, like this:

After glass transition, the polymers
have a lot of mobility. They wiggle
and squirm, and never stay in one
position for very long time. But when
they reach the right temperature,
they will give off enough energy to
move into very ordered arrangements,
which is called crystals.

The temperature at the highest point in
the peak is usually considered to be the
polymer's crystallization temperature, or
Tc

If we heat our polymer past its Tc,
eventually we'll reach another thermal
transition, called melting. When we reach
the polymer's melting temperature, Tm,
the polymer crystals begin to fall apart,
that is they melt. It comes out of their
ordered arrangements, and begin to move
around freely that can be spotted on a
DSC plot

This means that the little heater under the
sample pan has to put a lot of heat into the
polymer in order to both melt the crystals and
keep the temperature rising at the same rate as
that of the reference pan. This extra heat flow
during melting shows up as a big dip on DSC plot,
like this:

we saw a step in the plot when the polymer was
heated past its glass transition temperature. Then
we saw a big peak when the polymer reached its
crystallization temperature. Then finally we saw a
big dip when the polymer reached its melting
temperature. To put them all together, a whole
plot will often look something like this:

DSC is used in the study of liquid crystals.
Some materials go from solid to liquid, they go
through a third state, which displays
properties of both the phases. This anisotropic
liquid is known as a liquid crystalline state 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 anisotropic liquid.

To study the stability to oxidation of samples generally requires
an airtight sample chamber. Usually, such tests are done
isothermally (at constant temperature) 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.

• DSC is widely used in the pharmaceutical and
polymer industries. For polymers, 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 peak in
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 which crystallization can occur.

 Melting-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 dip that begins
at lower temperature than a pure compound.

DSC is used widely for examining polymers to check
their composition. Melting points and glass transition
temperatures for most polymers are available from
standard compilations, and the method can show up
possible polymer degradation by the lowering of the
expected melting point, which depends on the
molecular weight of the polymer, so lower grades will
have lower melting points than the expected.
Impurities in polymers can be determined by examining
thermograms for anomalous peaks, and plasticizers
can be detected at their characteristic boiling points.

In food science research, DSC is used in
conjunction with other thermal analytical
techniques to determine water dynamics.
Changes in water distribution may be
correlated with changes in texture.

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