THERMAL TECHNIQUES THE DEVELOPING TECHNIQUES SEEING NOWDAYS INCLUDING TGA, DSC, GLASS TRANSITION, FACTORS AFFECTING GLASS TRANSITION, TEMPERATURE

1. THERMAL TECHNIQUES
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2. INTRODUCTION
 The term thermal analysis incorporates those techniques
in which some physical parameter of the system is
determined and/or recorded as a function of temperature .
 Thermal analysis has been used to determine the
physical and chemical properties of polymers, electronic
circuit board, geological materials and coals .
 Differential scanning calorimetry (DSC) is one of the
thermo-analytical techniques. A calorimeter measures
the heat into or out of a sample. A differential calorimeter
measures the heat of sample relative to a reference.
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3.  DSC is a technique in which the difference in the amount
of heat required to increase the temperature of a sample
and reference are measured as 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.
 Only a few mg of material are required to run the
analysis.DSC is the most often used thermal analysis
method, primarily because of its speed, simplicity, and
availability .
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4. PRINCIPLE
 When a sample undergoes a physical transformation such
as a phase transition, more or less heat will need to flow
to it than to the reference , to maintain both at the same
temp. Whether more or less heat must flow to the sample
depends on whether the process is exothermic or
endothermic.
 For e.g.as a solid sample melts to a liquid it will require
more heat flowing to the sample to increase its temp. 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.
(Such as crystallization) less heat is required to raise the
sample temp.
 By observing the difference in heat flow between the
sample and reference, DSC is able to measure the
amount of heat absorbs or release during such transition .
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5. THERMAL TRANSITION OF POLYMER
 Completely amorphous polymer: Tg only
 Completely crystalline ; Tm
 Semi crystalline : both Tg and Tm
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6. AMORPHOUS POLYMER
Temperature Low molecular
weight
High MW
LOW Glassy Solid Rubbery: Rigid
solid-rubber
HIGH Viscous fluid
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7. SEMI CRYSTALLINE POLYMER
Two major types of thermal transition:
1.Crystalline mp, Tm , at which
o The crystalline domains melt.
o Above this temperature polymer is liquid.
o Below this temperature, polymer forms flexible solid.
2.Tg glass transition temperature.
 Below this, polymer exists as hard, rigid glassy solid.
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8. Specific volume–temperature curves for a semicrystalline polymer.
(A) Liquid region; (B) viscous
liquid with some elastic response; (C) rubbery region; (D) glassy
region; (E) crystallites in a rubbery matrix;
(F) crystallites in a glassy matrix
The Glass Transition
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9. Relative thermal responses of simple molecules (a) and
polymers (b).
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10. FACTORS AFFECTING THE GLASS
TRANSITION TEMPERATURE
1. Chain Flexibility
 Long-chain aliphatic groups — ether and ester
linkages — enhance chain flexibility, while rigid
groups like cyclic structures stiffen the backbone.
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11.  Bulky side groups that are stiff and close to the
backbone cause steric hindrance, decrease chain
mobility, and hence raise Tg
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12.  2. Geometric Factors
• Polymers that have symmetrical structure have lower temp
Tg than those with asymmetric structures.
• The additional groups can only be
accommodated in a conformation with a “loose” structure.
The increased free volume results in a lower Tg.
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13. Another geometric factor affecting Tg is cis–trans configuration.
Double bonds in the cis form reduce the energy barrier for rotation of
adjacent bonds, “soften” the chain, and hence reduce Tg.
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14.  3.Interchain Attractive Forces
• The presence of strong intermolecular bonds in a polymer
chain will significantly increase Tg .
• The steric effects of the pendant groups in series (CH3, –
Cl, and –CN) their polarity increases. Consequently, Tg is
increased in the order shown in the table
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15. 4.Copolymerization.
 Copolymerization is a process where in unlike
molecules join together in random sequences or
alternating sequences.
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16. 5. PLASTICIZERS
 A plasticizer is a liquid that is added to a material
(usually a elastomer)making that material softer, more
flexible (by decreasing the glass-transition temperature
Tg of the polymer), and easier to process.
 The amount of plasticizer added to a polymer varies
depending on the effect required.
 A small addition of plasticizer may be made to improve
the workability of the polymer melt.
 This contrasts with large additions made with the
specific intention of completely transforming the
properties of the product.
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17. THERMAL TRANSITIONS IN
POLYMERS
Figure shows the variation of Tm
for a homologous series of
various types of polymers. With
polyethylene as a reference,
we observe that:
• The melting points approach
that of polyethylene as the
spacing between polar groups
increases.
• For the same number of chain
atoms in the repeat unit,
polyureas, polyamides, and
polyurethanes have
higher melting points than
polyethylene, while polyesters
have lower
The Melting Temperature
1.Intermolecular bonding
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18. 2.Chain Flexibility
 Polymers with rigid chains would be expected to have
higher melting points than those with flexible molecules.
 This is because, on melting, polymers with stiff
backbones have lower conformational entropy changes
than those with flexible backbones.
 Chain flexibility is enhanced by the presence of such
groups as –O– and –(CO·O)– and by increasing the
length of (–CH2–) units in the main chain.
 Insertion of polar groups and rings restricts the rotation
of the backbone and consequently reduces
conformational changes of the backbone, as illustrated
by the following polymers
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20. INSTRUMENTATION
 There are four different types of DSC instrument
• Heat flux DSC
• Power compensated DSC
• Modulated DSC
• Hyper DSC
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21. 1.HEAT FLUX DSC
 In heat flux DSC, the difference in heat flow into the
sample and reference is measured while the sample
temperature is changed at the constant rate .
 The main assembly of the DSC cell is enclosed in a
cylindrical silver heating block, which dissipates heat to
the specimens via a chromel disc which is attached to the
silver block.
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22.  The disk has two raised platforms on which the sample
and reference pans are placed.
 A chromel disk and connecting wire are attached to the
underside of each platform, and the resulting chromel-
alumel and thermocouples are used to determine the
differential temperatures of interest.
 Alumel wires attached to the chromel discs provide the
chromel-alumel junctions for independently measuring
the sample and reference temperature.
 A separate thermocouple embedded in the silver block
serves a temperature controller for the programmed
heating cycle. And inert gas is passed through the cell at
a constant flow rate of about 40 ml/min.
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23. 2.Power compensation DSC
 In power compensation DSC, the temperatures of the
sample and reference are kept equal to each other while
both temperatures are increased or decreased linearly.
 In power compensation DSC two heating units are used.
These heating units are quite small, allowing for rapid
rates of heating, cooling and equilibrium.
 The heating units are embedded in a large temperature
controlled heat sink.
 Figure : power compensation DSC
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24.  The sample and reference holders have platinum
resistance thermometers to continuously monitor the
temperature of the materials.
 The instrument records the power difference needed to
maintain the sample and reference at the same
temperature.
 Power compensated DSC has lower sensitivity than
heat flux DSC, but its response time is more rapid.
 This makes power compensated DSC well suited for
kinetics studies in which fast equilibrations to new
temperature settings are needed.
 It is also capable of higher resolution than heat flux
DSC . 24

25.  There is direct measurements of radiation flow occur
under a light source. This way the degradation of
material can also be observed.
 Temperature range of measurement is up to 400 °C with
time constant of only 1.5 s or lower.
 Sample masses are around 20 mg.
 Crucibles of different volumes are made mostly of
aluminium.
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26. 3.Modulated DSC
 Modulated DSC uses the same heating and cell
arrangement as the heat – flux DSC method. it is a new
technique introduced in 1993 .
 The main advantage of this technique is the separation of
overlapping events in the DSC scans.
 The DSC signal is split into two components: reflecting no
reversible events and reversible events.
 MDSC is a valuable extension of conventional DSC.
 Its applicability is recognized for precise determination of
the temperature of glass transition.
 It has been applied for the determination of glass transition
of Hydroxypropylmethylcellulose films and for the study of
amorphous lactose as well as some glassy drugs . 26

27. 4. Hyper DSC
 This is high resolution of DSC or new type of power
compensating DSC provides the best results for an analysis
of melting and crystallisation of metals or detection of glass
transition temperature (Tg) in medications.
 Their ability to perform valid heat flow measurements with
fast linear controlled rates (up to 500 K/min) especially by
cooling.
 The benefits of such devices are increased sensitivity at
higher rates .
 It has a great sensitivity also at a heating rate of 500 K/min
with 1 mg of sample material.
 This technique is especially proper for the pharmaceutics
industry for testing medicaments at different temperatures
where fast heating rates are necessary to avoid other
unwanted reactions etc .
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