Glass transition temperature of polymers

1. 1
THE GLASS TRANSITION
TEMPERATURE
Introduction
Polymer can be crystalline, amorphous or partially crystalline
depending on whether it possesses a long range ordered
structure or not.
A crystalline polymer exist as crystalline solid with long range
order (solid phase state) or as a molten liquid (liquid phase
state) with solid to liquid transition occurring at temperature
called ‘Melting Temperature (Tm)’.
An amorphous polymer which does not possess long range
order occurs in glassy solid, rubbery or liquid state. The
temperature at which amorphous polymer passes from glassy
solid to rubbery state is called ‘Glass Transition
Temperature (Tg)’ and the one at which it passes from
rubbery state to molten liquid is the ‘Flow Temperature (Tf)’.
Partially crystalline polymer has both crystalline and
amorphous portions.
Figure 1. Change of State with Temperature in Polymeric Materials

2. 2
Glass Transition and Glass Transition
Temperature
Glass transition is an important thermal property of
amorphous polymer or amorphous portion of partially
crystalline polymer. Almost all amorphous polymers have a
‘temperature boundary’ only above which it remains soft,
flexible and rubbery (Rubbery or Viscoelastic State) and
below which it becomes hard, brittle and glassy (Glassy State)
and this temperature of transition is called Glass Transition
Temperature.
Polymers structurally consist of localized units or chain
segments. Thus they can exhibit 2 types of motions:-
1. Segmental motion or micro or internal Brownian
movement.
2. Molecular motion or macro or external Brownian
movement.
In solid (glassy) state, both motions freeze. As the temperature
increases and reaches Tg, internal Brownian movement
activates, i.e., localized mobility is activated but overall
mobility is not. This is Rubbery state. At further high
temperature (around Tm), both Brownian movements are
activated and individual polymer chains start moving apart.
This is Liquid state.
Measurement of Tg
Tg is usually measured using dilatometer, which is based on
variation in the physical property – Specific Volume. With the
increase in temperature, initially there is a gradual increase in
the volume. With further increase in temperature, a point is

3. reached where there occurs an abrupt increase in volume. This
gives the value of Tg.
Figure 2. The relationship of Specific Volume of a Polymer with Temperature
Factors Affecting Glass Tra
Temperature
The presence or absence of segmental and molecular motion,
i.e., mobility of polymer chain decides whether a polymer is
in glassy, rubbery or molten state. Thus T
the following factors:-
1. CHEMICAL STRUCTURE
This includes –
a) Molecular Geometry:
orientation leading to long range 3
i.e., stereoregular
which has irregular backbone or randomly placed side
groups is amorphous. Segmental and chain mobilities
are easier in amorphous structure, thus their T
b) Flexibility: Flexibility of polymer chain is determine
by degree of freedom with which segments rotate along
chain backbone. Higher the degree of freedom of
rotation, more flexible the chain, so more segmental
mobility, thus lower T
3
reached where there occurs an abrupt increase in volume. This
The relationship of Specific Volume of a Polymer with Temperature
Factors Affecting Glass Transition
Temperature
The presence or absence of segmental and molecular motion,
i.e., mobility of polymer chain decides whether a polymer is
molten state. Thus Tg depends mainly
-
CHEMICAL STRUCTURE
Molecular Geometry: If polymer has definite molecular
orientation leading to long range 3 - dimensional order,
i.e., stereoregular structure, it is crystalline while one
which has irregular backbone or randomly placed side
groups is amorphous. Segmental and chain mobilities
e easier in amorphous structure, thus their Tg is low.
: Flexibility of polymer chain is determine
by degree of freedom with which segments rotate along
chain backbone. Higher the degree of freedom of
rotation, more flexible the chain, so more segmental
mobility, thus lower Tg.
reached where there occurs an abrupt increase in volume. This
The relationship of Specific Volume of a Polymer with Temperature
sition
The presence or absence of segmental and molecular motion,
i.e., mobility of polymer chain decides whether a polymer is
mainly on
definite molecular
dimensional order,
structure, it is crystalline while one
which has irregular backbone or randomly placed side
groups is amorphous. Segmental and chain mobilities
is low.
: Flexibility of polymer chain is determined
by degree of freedom with which segments rotate along
chain backbone. Higher the degree of freedom of
rotation, more flexible the chain, so more segmental

4. 4
 Linear polymers with single bond have higher degree
of freedom of rotation, therefore lower Tg. For
example, polyethylene has Tg = - 120°C.
 Presence of bulky groups or aromatic or cyclic
structure in polymer backbone hinders rotation thus
increasing the value of Tg. For example, nylon has Tg
= 50°C, polystyrene has Tg = 100°C.
c) Chain Polarity and Intermolecular Force: Tg increases as
the main chain polarity increases. Due to the increase in
polarity the intermolecular forces increase leading to
formation of strong molecular aggregates. These are not
able to move easily, i.e., restricted motion occurs, and
the value of Tg increases. For example, Polypropylene
has Tg = - 20°C, Polyvinyl chloride (which has C ― Cl
polar bond) has Tg = 81°C.
2. MOLECULAR WEIGHT
Value of Tg is influenced by the molecular weight of the
polymer (Mn upto 20000) according to the Fox- Flory
Equation, i.e.,
𝑇 = 𝑇∞
−
𝐾
𝑀
where, 𝑇∞
is Tg of polymer at infinite molecular weight,
K is Fox- Flory parameter for a polymer,
Mn is number average molecular weight of polymer.

5. 5
Figure 3. Plot showing effect of Molecular Weight on Tg of Polymer
From the curve, we can see that Tg value will be lower for
low molecular weight polymers.
3. PLASTICIZERS
Plasticizers are low molecular weight, non- volatile
substances (usually liquids) which are added to polymers to
improve its flexibility and utility. For example,
Dibutylpthalate.
Plasticizer molecules penetrate into polymer matrix and
establish polar attractive forces between it and polymer
chains. These attractive forces reduce cohesive forces and
increase segmental motion, thus reducing Tg.
Importance of Glass Transition
Temperature
Glass Transition Temperature is an important parameter of a
polymeric material.
 Tg is used for evaluating the flexibility of a polymer
molecule and the type of response it would exhibit to
mechanical stress.

6. 6
 It tells about the physical state of the polymeric material at
a given temperature, thus helps in deciding its use. For
example, Hard Plastics like Polystyrene and
Polymethylmethacrylate (Tg ~ 100°C) are used in glassy
state whereas Rubbery Elastomers like Polyisoprene and
polyisobutylene (Tg ~ - 73°C) are used in soft and flexible
rubbery state.
 Tg value along with Tm value gives an indication of
temperature region at which a polymeric material
transforms from a rigid solid to viscous state. This helps in
choosing the right processing technique and temperature
for given polymeric material.
References:-
1. Principles of Polymer Chemistry; Ravve, A.
2. Polymer Science; Gowariker, Vasant R., Viswanathan,
N.V., Shreedhar, Jayadev.
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