Transcript Thermal Transition of Polymers

THERMAL TRANSITION
OF POLYMERS
K AU S A R AH M AD
K U L L I Y Y AH O F P H AR M AC Y , I I U M
H T T P : / / S T A F F . I I U . E D U . M Y / A K A U S A R
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CONTENTS
• Glass transition temperature
• Crystalline melting point
• Factors affecting Tg
• Factors affecting Tm
• Thermal methods of polymer analysis
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THERMAL TRANSITIONS OF POLYMERS
Completely amorphous
polymer: Tg only
Completely Crystalline: Tm
Semi-crystalline: Both Tg & Tm
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AMORPHOUS POLYMERS
Temperature
LOW
LOW MW
Glassy solid
HIGH MW
Rubbery:
1. rigid solid – rubber
HIGH
Viscous fluid
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2. rubber – liquid
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SEMICRYSTALLINE POLYMERS
Two major types of thermal transitions:
Crystalline melting point, Tm, at which the crystalline
domains melt.
• Above this temperature, polymer is liquid.
• Below this temperature, polymer forms flexible crystalline solid.
Tg, glass transition temperature.
• Below this, polymer exists as hard, rigid glassy solid.
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FACTORS AFFECTING TG
Chain
flexibility,
interactions &
structure
• Nature of polymer backbone.
• C-C and C-O bonds are flexible.
• Nature of groups directly attached
to the backbone (due to repulsion
and tendency to form H-bonding).
• Flexibility enhanced by methylene
groups.
• Flexibility reduced by ring
structures
• phenylene and amide
• substitution of H atoms by HC
groups
• eg. Tg: PTFE > PE
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…CONT’…FACTORS AFFECTING TG
Inclusion of double bond stiffens it but increases
flexibility of adjacent bonds.
Segmental rotation is affected by other chains in the
same region (mainly dipole interaction and hydrogen
bonding).
• Tg: PVC > PE (more polar).
• Tg: polyvinylidene < PVC (less polar due to symmetrical
substitution.
• Hydrogen bonding also significantly increases Tg.
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FACTORS AFFECTING TM
Same factors affect both Tg & Tm.
Usually high Tg & high Tm are found together and vice versa.
Asymmetrically structured polymers have higher Tm
• Eg PVC vs polyvinylidene
High polarity and hydrogen bonding increases Tm.
Molecular symmetry leads to high Tm.
• Eg PE has high Tm due to tightly packed crystals although Tg
is low due to flexibility of the chain.
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VALUES OF TG & TM
Polymer
Repeating Unit
Tg (˚C)
Tm (˚C)
Polyethylene
-CH2CH2-
-125
137
Natural rubber
-CH2C(CH3)=CHCH2-
-73
28
Polypropylene
-CH2CH(CH3)--
-13
176
Polyvinylchloride
-CH2CHCl-
81
273
Polystyrene
-CH2CHØ-
100
240
Polymethylmethacrylate
-CH2C(CH3)(CO2CH3)-
105
200
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THERMAL ANALYSIS OF POLYMERS
Technique
Quantity measured
Applications
Differential Scanning Heat flow
Calorimetry (DSC)
Reaction kinetics,
purity analysis,
polymer cures
Differential thermal
analysis (DTA)
Temperature
difference
Thermal stability,
phase diagram
Thermogravimetric
analysis (TGA)
mass
Thermal stability,
compositional
analysis
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DIFFERENTIAL SCANNING
CALORIMETRY (DSC)
• Polymer and reference samples are heated in such a way that they are
kept at the same (increasing) temperature.
• When a thermal transition occurs thermal energy is supplied to the
polymer or reference.
• The energy transferred is equivalent to the energy absorbed or evolved
in the transition.
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DSC PLOT
• The resulting plot of differential rate of heating Vs temperature
can be studied to measure heat capacity, melt enthalpy and
transition temperature.
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CHARACTERISTICS OF A DSC CURVE
Each substance has
a characteristic
DSC curve that can
be used for
qualitative
identification.
Endotherms
generally represent
physical changes
while exotherms
indicate
crystallization,
polymerization,
curing,
decomposition etc.
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The area under the
peak (trough) is
proportional to the
heat evolved or
absorbed during
the reaction.
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APPLICATIONS OF THE DSC CURVE
% Crystallinity
• % Crystallinity =  Hf,x/ Hf
= Heat of fusion of sample/heat of
fusion of 100% crystal
Rate of reaction
• Height of peak is proportional to rate
of reaction
• Heat of reaction = Heat absorbed or
emitted/No. of moles
Change in heat
capacity
• Manifests as a change in baseline.
• A sharp increase is typical of Tg.
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MELTING POINT FROM DSC CURVE
• Can determine the exact melting point as the
temperature at which melting is complete.
• The actual melting of the polymer takes place at
a range of temperature.
• On-set
• Mid
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THERMOGRAVIMETRIC ANALYSIS TGA
Measurement of the mass vs temperature.
Plotted as mass vs time or mass vs temperature.
Characteristics
• Unique shape for each substance.
• Temperature at which changes occur.
• Each plateau indicates stable state & any deviation or
changes shows the presence of a reaction.
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TGA CURVE
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REFERENCES
Aulton, M. E. (1988). Pharmaceutics: The Science of dosage form
design. London: Churchill Livingstone.
Wise, D. L. (2000). Handbook of Pharmaceutical Controlled
Release Technology. New York: Marcel Dekker.
Chasin, M & Langer, R (1990). Biodegradable polymers as drug
delivery systems. New York: Marcel Dekker.
Vyas, S. P & Khar, R. K. (2002). Targeted and controlled drug
delivery. New Delhi: CBS.
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