Polymer Molecular
Structures
Chapter 8 presentation
By MV Mnisi
Introduction
to Polymer
Structure
• Polymers can vary
structurally, even within
the same type.
• Key aspects:
• Stereoregularity
• Branching and Crosslinking
• Copolymers
Stereoregularity
• Definition: Spatial arrangement
of atom groups in the polymer
chain.
• Example: Polypropylene
• Tacticity Types:Isotactic: All
groups on the same side.
• Syndiotactic: Alternating sides.
• Atactic: Random placement.
Impact of Tacticity
Type
Properties
Melting
Point
Isotactic
Strong, commercially
useful
175°C
(347°F)
Syndiotac
tic
Strong, different
melting behavior
131°C
(268°F)
Atactic
Soft, limited
applications
75°C
(165°F)
Polymer Structure
• Linear: Chain-like;
characteristic of
thermoplastics.
• Branched: Side chains off the
main chain.
• Cross-linked: Bonds between
chains.
Cross-Linking Effects
Lightly cross-linked: Elastomers
(elastic, resilient).
Heavily cross-linked: Thermosets
(hard, brittle).
Irreversible: Cross-linking
prevents melting; leads to burning
or degradation.
Thermoplastic vs Thermoset vs
Elastomer
PROPERTY
THERMOPLASTIC
THERMOSET
ELASTOMER
Structure
Linear/Branched
Cross-linked
Lightly crosslinked
Behavior
Meltable
Non-meltable
Stretchable
Example
Polyethylene
Epoxy
Rubber
Copolymers
Definition: Polymers with two
different mers.
Examples: Ethylene–propylene
rubber.
Benefits: Modified properties
(e.g., rubbery behavior from
stiff components).
Types of
Copolymers
Alternating: Regular pattern
(ABAB)
Random: Irregular distribution
Block: Segments of one type
Graft: Side chains of different
mer types
Crystallinity
in Polymers
•Amorphous vs Crystalline: Most
polymers are partially crystalline.
•Increased Crystallinity:
•↑ Density
•↑ Strength and stiffness
•↑ Heat resistance
•↓ Transparency
Polymer
Crystallisatio
n
• Crystallites: Ordered
regions formed by chain
folding.
• Mixed Structure:
Crystallites within
amorphous matrix.
Factors Affecting Crystallinity
Only linear polymers crystallize well.Isotactic
> Syndiotactic > Atactic (no crystals).
Copolymers resist crystallisation.
Slow cooling promotes crystals.
Stretching increases alignment.
Plasticizers reduce crystallinity.
summary
• Polymer properties are greatly influenced by
molecular structure.
• Understanding stereoregularity, branching, and
crystallinity is crucial in polymer science and
applications.