Thermal analysis

Thermal methods are mainly used for the determination of

physical or chemical change of substances as a function of
temperature.
Based on the change in the physical property, the thermal
analytical methods named as different

These methods are mainly used in the polymer analysis

and solid state analysis.
Advantages
‰Speedy
Sensitive
Simple
Accurate
Limitations
‰Low sensitivity for the low energy transitions.
Impurities cannot be detected by the thermal analytical methods.
The results obtained are not reliable.
Interfering factors are more
Applications
Used in the polymorphs determination
Used in the moisture content estimation
Used in the powder characterisation
‰Used in the determination of crystallinity
‰Used in the protein folding and protein denaturation
THERMOGRAVIMETRY
The principle involved in the thermogravimetry is to measure the
change in weight of the substance as a function of time or
temperature. That is nothing but the weight change of the sample is
measured when the sample is heated, cooled or at a constant
temperature. The weight change is determined by the thermo
balance and the data are recorded as the thermogram by plotting
the percent weight and the temperature. This method is mainly
used in the quantitative analysis.
Factors which are affecting the thermo gravimetric curve:
‰Instrumental factors:
1. Heating rate
2. Heating temperature
3. Furnace atmosphere
4. Sample holder
‰Sample characteristics:
1. Weight
2. Particle size
3. Heat of reaction
4. Compactness of sample
5. Previous history of the sample
Types of thermogravimetry: There are mainly three types of
thermogravimetry.
1. Static thermogravimetry: The sample is maintained at
constant temperature for a period of time during which any
change in the weight of the sample is noted.
2. Dynamic thermogravimetry: The sample is subjected to the
conditions of continuous increase in the temperature usually
linear with the time.
3. Quasi-static thermogravimetry: The sample is subjected to
constant heat and constant weight of series of increasing
temperature.
Instrumentation
The instrumentation of the thermo gravimeter contains the
following components:
1. Thermo balance
2. Sample holders
3. Furnace
4. Temperature programmer
5. Recorder
1. Thermo balance: These are of two types. They are as follows:
(a) Null point type: This is of sensing unit which detects the
slightest deviation of the balance with the weight and retards to the
initial point.
b) Deflection type: This is conventional analytical beam balance
consisting of the analytical helical spring.
2. Sample holders: Generally crucibles made up of glass, quartz,
aluminium, stainless steel or platinum are used. These are of two
types. They are as follows:
(a) Shallow pan: Intended for the samples which evolve the gas
on volatilisation or vaporisation.
(b) Deep crucible: This is mainly used for the solids and liquids
samples.
3. Furnace: This is mainly used to maintain the temperature with
the temperature control which is mounted very close to the
furnace winding. The general thermocouples used are the
following:
‰Ni–Cr: 1,100 °C
‰Pt–Rd: 1,450 °C
‰Graphite tube: 1,500 °C
4. Temperature programmer: This programming is done by
increasing the voltage through the sensor-heated element by
motor driven transformer.
5. Recorders: The normally employed recorders are used with
amplifier
Working Principle
Sample is placed in the tarred TGA sample pan which is attached
to the sensitive microbalance. Then it is placed in the high
temperature furnace. The balance measures the initial sample
weight at room temperature and the change in the weight as the
sample is heated. The sample weight change causes the deflection
of beam. The resulting is fed into the coil which is placed in
between the magnets. These magnets are generated by the current
in coil which restores the beam to its original position. Then it is
transformed as data and a plot is plotted between mass and
temperature.
Advantages
‰Very reliable
‰Rugged method
‰Very accurate method
Applications
‰Used in the identification of the polymers.
‰Used in the residual solvent determination.
‰Used in the analysis of magnetic materials.
‰Used in the reaction kinetics studies.
‰Used in the moisture content determination.
‰Used in the determination of the pyrolysis of coal, petroleum and
wood.
Used in the determination of the corrosion of the metals in the
various atmospheres.
 Evaluation of the gravimetric precipitates.
Determination of the calcinations of the minerals.
Determination of the purity and stability of the compounds.
Determination of the solid state reactions.
Determination of the composition of the mixtures such as calcium
and strontium binary mixtures.
Decomposition of the inorganic and organic compounds.
Determination of the volatile compounds concentration.
‰Estimation of the plasticisers content in the samples.
DIFFERENTIAL THERMAL ANALYSIS
The basic principle involved in this method is the measurement of
the temperature inflow and out flow of the sample and the
reference materials simultaneously. This difference in the
temperature (∆T) is plotted against time or temperature. The
reference generally used in this method is the Al2O3. The
differential thermogram consists of the difference in the sample
and reference temperatures (∆T) plotted as a function of time tor
temperature T.
The endothermic peak is observed by the physical changes of the
sample with the temperature and the exothermic peak is observed by
the chemical changes of the sample with the temperature.
The following are the factors affecting the DTA curve:
‰Environmental factors: should be static atmosphere which can be
achieved by the inert gas flow.
‰Instrumental factors:
(a) Sample holders size and shape
(b) Sample holder material
(c) Heating rate
‰Sample factors
(a) Amount of the sample (b) Particle size of the sample
Instrumentation
The following are the important components of the differential
thermometer:
1. Furnace
2. Sample holder
3. Temperature controller
4. Thermocouple
5. Cooling device
1. Furnace: A tubular furnace is commonly employed. The
dimension of the furnace depends on the length of the temperature
zone desired. The choice of the resistance material depends on the
maximum temperature of the inert gas flowed through the
chamber.
2. Sample holder: The sample holders are made up of the nickel,
stainless steel, platinum, glass, sintered alumina, etc. These are of
mainly cylindrical type. The physical constant is maintained by
the use of sample cells with the thermocouple wells are used. In
the thermocouple wells, the thermocouple junctions are inserted.
3. Temperature controller and recorder: This consists of the
sensor, controller and the heating elements. The temperature
programmer transmits a certain time-based instruction to the
control unit. This achieves the linearity in the rate of heating or
cooling. Then the signals obtained from the sensor are recorded
on the paper or film by ink or electric writing or optical beam.
4. Thermocouple: These are used as the temperature sensors
which are made up of chromel or alumel wire used for 1,100 °C
and for above 1,100 °C pure platinum or Pt–rhodium wires are
used.
5. Cooling device: It is mainly used to cool the instrument to
prevent from the destruction with the high temperatures.
Interpretation of the DTA curve is the following: The DTA curve
represents the both exothermic and the endothermic reactions of
the compound. The molar enthalpy of the reaction is calculated by
the following equation:

where ∆Hm is the molar enthalpy; ∆Hr is the heat of reaction;

Mr is the relative molar mass; mis the weight of the sample.
Therefore, the heat capacity Cp is calculated from the following
equation:
where Kis the calibration factor; ∆Tis the differential temperature;
mis the weight of the sample; H is the heat of rate of reaction.
Advantages:
‰Can be used at high temperatures
‰Highly sensitive
‰Transition temperatures and reaction temperatures are accurately
determined
Disadvantages:
‰Uncertainty of heats of fusion
‰Uncertainty of the transition.
The following are the errors in DTA:
‰Buoyancy effect: The effect in the weight change when the
empty container is heated to different temperatures. To avoid,
properly calibrated sample holders are used.
‰Fluctuation of thermostats: this can be avoided by the constant
power supply.
Reaction between the sample and the container: this can be
avoided by the proper selection of the containers.
‰Convection and induction effect from the furnace: this can be
avoided by the proper design of the furnace.
Applications:
‰Used in the quality control of the glass, soil and resins.
Determination of the chemical composition of the compounds.
Degradation studies of the compounds.
Determination of the pyrolysis kinetics of the compounds.
Determination of the melting points of the different compounds.
Quantitative and qualitative identification of the minerals.
Characterisation of the polymeric materials.
Determination of the heat of the reaction.
Determination of the specific heat of the sample such as naphthalene.
Determination of the diffusivity of the samples.
DIFFERENTIAL SCANNING CALORIMETRY
Introduction
The technique was developed by E.S. Watson and M.J. O'Neill in
1962. The first adiabatic differential scanning calorimeter is used in
biochemistry was developed by P.L. Privalov and D.R. Monaselidze
in 1964. This is mainly used to study the thermal properties of the
samples.
Differential scanning calorimetry measures the temperature and
heat flow with the transition of the materials as a function of the
time and temperature. This is mainly used in the determination of
the
glass transition, crystallisation time and the temperature and the
This is helpful in the thermal characterisation of the solids and
the liquids. There are mainly two types of the differential
scanning calorimetry. They are as follows:
1. Heat flux DSC where the individual heaters are present for
sample and reference.
2. Power compensation DSC where the both sample and the
reference are placed in the same heater.
Theory
Differential scanning calorimetry is a technique used to study the
changes of polymers when they are subjected to heat. The thermal
transitions are the changes that take place in a polymer when they
are heated and can be differentiated as following:
‰First-order transition: A thermal transition that involves both a
latent heat and a change in the heat capacity of the material.
Example: Crystallisation and melting.
‰Second-order transition: A thermal transition that involves a
change in heat capacity, but does not have a latent heat.
Example: Glass transition
The DSC is mainly used to measure specific heat capacity:
Cs= CR+ (1/w) (C − CR)

where C is the specific heat of the solution; CR is the specific heat of

the solvent; W is the weight fraction of the solute.
Then the heat flow is given by the following equation:
dH/dt= Cs(dT/dt) + f(T, t)
where dH/dt is the heat flow; Cs is the specific heat capacity; dT/dt
is the heating rate; f(T,t) is the heat flow function of the time at an
absolute temperature.
Then the heat capacity is defined as the amount of heat required to
change the temperature of a specific weight of the sample:
 Heat capacity = specific heat × sample weight
Then the heat flow difference between the sample and the
reference is given by the following equation:
∆dH/dt= (dH/dt)sample– (dH/dt)reference
Instrumentation
In the most popular DSC design, two pans are placed on identically
positioned platforms connected to a furnace by a common heat flow
path. In one pan, the polymer sample is placed. The other one is
the reference pan. The reference pan is kept empty. Then turn on the
furnace to heat the two pans at a specific rate such as 10 °C per
minute. Make sure that the heating rate is constant throughout the
time.
But make sure that the two separate pans heated at the same rate as
each other.
The following are the important components of the DSC:
1. Sample holder: Aluminium or platinum pans are used as
sample holders.
2. Sensors: Platinum resistance thermocouples are used as
sensors.
3. Furnace: Two separate furnace blocks are used for the sample
and reference cells.
4. Temperature controller: The differential thermal power is
supplied to heat the sample and reference in the equal manner. For
this purpose, the temperature programmer is used to maintain
the temperature along the overall process.
Plot the temperature versus the difference in heat output of the
two heaters at a given temperature as the temperature increases.
From this, we will interpret the following parameters:
Heat capacity: The heat flow is in units of heat (q), supplied per
unit time, (t). The heating rate is temperature increase T per unit
time, t.
The amount of heat to get a certain temperature increase is called
the heat capacity, or Cp.
When the polymer heated a little more, after a certain temperature,
plot will shift upward suddenly, like the one given below:

This is known as glass transition temperature

Crystallisation: Above the glass transition, the polymers have a
lot of mobility. When they reach the right temperature, they will
form the crystals.

The temperature at the lowest point of the dip is usually

considered to be the polymer's crystallisation temperature, or Tc
Melting: Upon heating above crystallisation, temperature reaches
another thermal transition, called as melting.

The temperature at the top of the peak to be the polymer's melting

temperature, Tm. Because of the increase in the energy makes the
polymer to melt called as melting as an endothermic transition.
Putting it altogether we will get the following figure

This exothermic 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:
 ∆H= KA
where ∆His the enthalpy of transition, Kis the calorimetric constant,
and Ais the area under the curve. The calorimetric constant will vary
from instrument to instrument and can be determined by analysing a
well-characterised sample with known enthalpies of transition.
From the above plot we will get the following:
‰The crystallisation peak and the melting will only show for the
polymers that can form crystals. A completely amorphous polymer
does not show any crystallisation, or any melting either.
The glass transition temperature is a change in the heat capacity of
the polymer.This is because there is no latent heat given off, or
absorbed, by the polymer, like happens in melting and crystallisation.
Factors affecting the DSC curve are the following:
‰Furnace heating rate
Furnace atmosphere
Location of the sensor
Composition of the sample container
Amount of the sample
Shape of the sample
Size of the sample molecules
Solubility of the evolved gas in the sample
Nature of the sample
Sample packing in the sample holder
Advantages of DSC curve are the following:
‰More reliable than the other thermal methods.
‰Highly sensitive than the other thermal methods.
Requires less sample than the other thermal methods.
More accurate than the other thermal methods.
Errors in the DSC are the following:
‰Fluctuation of thermostats: this can be avoided by the constant
power supply.
‰Reaction between the sample and the container: this can be
avoided by the proper selection of the containers.
‰Convection and induction effect from the furnace: this can be
avoided by the proper design of the furnace.
Applications for the DSC are the following:
‰Used in the determination of the glass transition.
Determination of the melting and boiling points of the samples.
Determination of the crystallisation of the samples.
Thermal stability of the pharmaceutical compounds.
Determination of the reaction kinetics.
Determination of the purity of the compounds.
Determination of the specific heat capacity of the compounds.
Determination of the protein stability.
Characterisation of the lipids, nucleic acids and micellar systems.
Determination of the biocompatibility of the compounds.
‰Determination of the phase transitions of phospholipids.
Study of the human erythrocyte membrane.
Lyophilisation technique.
Study of liquid crystals: Liquid crystals are the products of a
third state obtained from the transition from solid to liquid which
shows properties of both the states. DSE is used for the study of
these transitions.
Used in the study of oxidative stability of the compounds for the
determination of optimum storage conditions.
‰Used as the initial safety screening tool to suggest the maximum
temperature for the sample degradation.
‰‰Used in the drug analysis for studying the curing
processes, monitoring of the manufacturing process, stability
studies and storage conditions.
‰Used in the chemical analysis of the compounds for the
purity and stability.
Used in the food science technology for the determination of
water dynamics.
Used in the polymer science for the determination of
composition, purity and physical properties of polymers.
Used in the study of metallic materials such as alloys.