Ppp8 2 Factors Affecting Tg

1. Physical Properties of Polymers
Muhammad Zafar Iqbal
Date: 22-05-2008

2. Sequence
Glassy State and Glass Transition Temperature
•Introduction
•States of aggregates
•States of phases
•Transitions and physical Properties
•Factors affecting Tg
•Measurement techniques for Tg from different instruments
(General Introduction)
•Detailed study of Differential Scanning Calorimeter

3. Structure- Property Relationships
•Two distinct models have been proposed for interpreting the
influence of different features on Tg in Amorphous Polymers
1- Molecular Flexibility Model
2- Free Volume Model
•Factors affecting the Tg include:
1- Chemical Structure
2- Molecular Weight
3- Plasticizers
4- Co-Polymers
5- Tg - Melting Point

4. Effect of Chemical Structure
Following facts are included in effect of chemistry on Tg:
1- Main Chain Structure
2- Side groups
3- Main- Chain Polarity
Main Chain Structure
•If the molecular geometry permits the formation of a definite
molecular orientation, leading to LRO, the polymer has high
crystallizablity.
•Polymers with stereo-regular structure are crystalline
•Polymers with irregular chain backbone or randomly placed
side groups are amorphous.

5. •The flexibility of the chain segments is determined by the
degree of freedom with which the segments rotate along the
chain backbone.
•Linear polymers with single bonds have high degree of
rotation.
•The presence of aromatics, cyclic structures in backbone
hinder this rotation..
•Higher the freedom to rotate, the more flexible are the
chains, and more is their segmental mobility, lower is its Tg.
•Due to bulky groups in backbone, intermolecular cohesive
forces are increased.
•Therefore highly crystalline polymers have high Tg
•See the examples:

6. •PETg= -125
0
C
•Nylone 6 (Polyamide)
Tg = 50
0
C

7. Effect of Side groups
•Two effects are seen normally:
1- Bulky side group effect
2- Long side group effect

8. •Flexible side group

9. •Long flexible side group

10. Chain Polarity
•As the main chain polarity is increased, Tg increases.
•Due to increase in polarity, intermolecular forces are
developed and are strengthened.
•The temperature of glass transition is depressed as the no. of
successive –CH2 or –CH3 groups in the side chain is increased.

11. Effect of Molecular Weight
•Practically, the glass transition temperature value of a
polymer is influenced by the molecular weight but upto
20000. Beyond this limit, no appreciable effect is seen.
•Fox-Flory Equation
K=Fox-Flory Parameter

12. Effect of Plasticizers (Diluents)
•Plasticizers are low mol.wt., non-volatile substances (mostly
liquids), which are added into the polymers to improve its
flexibility, processability, and utility.
•Plasticizers reduce Tg
•This is due to the reduction in cohesive forces between the
polymer chains.
•Plasticizer molecules penetrate into the polymer matrix and
establish polar attractive between it and polymer chains. These
attractive forces reduce the cohesive forces between the polar
chains and increase the segmental motion, thus reduce Tg.
•The plasticizer’s efficiency depends upon solubility parameter,
polarity, density and wt.fraction.
•Commercial selection of plasticizer is based on compatibility,
effecciency, migration effect and the cost.

13. NS- naphthyl salicylate
KP- Tri cresyl phosphate
MS- Methyl salicylate
MA- methyl acetate

14. Tg and Co-Polymers
•There are different empirical rules for predicting the average
Tg of a co-polymer or multi-component systems
•For miscible mixture: rule of mixture is applied.
T
g
= W
1
T
g,1
+ W
2
T
g,2
•If the Tg’s of the components being mixed are not too
different, then inverse rule of mixture is applied: (This is also
called Fox equation).
1/T
g
= W
1
/ T
g,1
+ W
2
/ T
g,2
•Another commonly used empirical relationship is logrithmic
rule of mixture which is given as:
ln T
g
= W
1
ln T
g,1
+ W
2
ln T
g,2

15. Pressure dependence of Tg
•Compared to the effects of molecular weight and
plasticization, Tg is relatively insensitive to pressure.
•The glass transition temperature will increase with increasing
pressure at a rate of approximately 25 K per Kbar of pressure.
•The pressure dependence of Tg can be well estimated from
compressibility and thermal expansion coefficients as:

16. Tg and Melting Point
•Many attempts have been made to study the inter-
relationship between the glass transition temperature and
melting point.
•Generally speaking, the factors which affect Tg almost affect
Tm in the same way.