Effect of polymer structure on their mechanical properties

1. EFFECTS OF STRUCTURAL FACTORS
ON MECHANICAL PROPERTIES
Presented by,
Vishal K.P.
Dept. of PS&RT
CUSAT

2. Introduction
• When polymers are in to application , the
mechanical properties should be analyzed for
safety factors.
• The properties includes strength, stiffness,
ultimate tensile strength, modulus etc..
• Various structural factors also affect the
mechanical properties of a polymer.

3. Structural Properties
• Molecular weight
• Cross-linking
• Crystallinity
• Polarity
• Copolymerization
• Steric factors

4. Effect of molecular weight
• Tg-glassy to rubbery transition
• Tfl-rubber to melt flow transition
• The molecular weight has no effect on
modulus in the glassy region(below Tg)
• Tfl is dependant on molecular weight

5. Effect of Cross-linking
• Smaller the value of Mc-a measure of cross-
link density, higher the cross-link density.
• Cross-links restricts the molecular mobility
hence increases the ability to resist
deformation under load i.e., increase
modulus.
• When cross-link density increases, restriction
on molecular mobility increases and hence the
Tg.

6. Effect of Crystallinity
• Crystallinity-Molecules held together in an
orderly manner by secondary valence bonds.
• Crystallites have more modulii than
amorphous segments
• The effect on modulus below Tg is
insignificant, but has a pronounced effect
above the Tg.
• The modulus will be high for polymers with
higher percentage of crystallinity

7. Effect of crystallinity on the modulus-temperature curve

8. Effect of Polarity
• The polarity increases the Tg of the polymers
e.g.: Tg of polar PVC is 97°C higher than non-
polar polypropylene
• The modulus of polar polymer is much higher
than non-polar polymers
• Introduction of polar forces increases the
modulus because the modulus in the glassy
state is determined by the secondary bonding
forces

9. Effect of copolymerization
• Isomorphous systems
*Homogeneous copolymers
*The modulus will be in between that of two
homopolymers
• Non-Isomorphous system
*random & alternating copolymers-same effect
of isomorphous system
*graft & block copolymers-two phases as dispersion
>If dispersing medium is of high Tg,modulus will be
higher.
>If dispersing medium is of low Tg,modulus will be lower.

10. Effect of Steric factors
• Long flexible side chains decreases the Tg and
steric hinderance as a result of increasing free
volume.
• Branched side chains increases Tg and hence
increases the modulus in the amorphous
polymers.

11. Reference
• POLYMER SCIENCE and TECHNOLOGY-Robert
O.Ebewele: Chapman & Hall. Pg no.362-372

12. THANK YOU