Polymer
Structure
Polyolefins with side chains have stereocenters on every other carbon
CH3
n
CH3 CH3 CH3 CH3 CH3 CH3 CH3
With so many stereocenters, the stereochemistry can be complex.
There are three main stereochemical classifications for polymers.
Atactic: random orientation
Isotactic: All stereocenters have same orientation
Syndiotactic: Alternating stereochemistry
Molecular configurations
Head-to-tail configuration
Bonded to alternate carbons
on the same side
Where, R: Alkyl radical
Molecular configurations continue….
Head-to-head configuration
Bonded to adjacent carbon
atoms
Molecular configurations continue….
Stereoisomerism
Isotactic configuration
R groups are situated
on the same side of
the chain
Molecular configurations continue….
Syndiotactic
On alternate sides
Molecular configurations continue….
Atactic
At random position
Source: William Callister 7th edition, chapter 14, page 504
Conversion from to another is only by severing branches
and through new reaction
Molecular configurations continue….
Geometric Isomerism
CIS-Isoprene
eg., Natural rubber
Attacked by acids/alkalis
TRANS-Isoprene
eg., Gutta Percha
Highly resistant to acid/alkalis
Molecular configurations continue….
Geometric Isomerism continue…
TRANS- isoprene
–Highly resistant to acids/alkalis
Why is this important?
• Tacticity affects the physical properties
• Atactic polymers will generally be amorphous, soft, flexible materials
• Isotactic and syndiotactic polymers will be more crystalline, harder and less
flexible
• Polypropylene (PP) is a good example
• Atactic P is a low melting, viscous and sticky.
• Isoatactic P is high melting (176º), crystalline, tough material. Syndiotactic P
has similar properties, but is very clear.
Polymer Morphology
All properties of any polymer
(plastic, fiber, or rubber)
result from a combination of
molecular weight and
chemical structure.
Mechanical Property
Molecular Weight
Polymer Morphology
The mechanical properties result from attractive forces between molecules
•
•
•
•
dipole-dipole interactions,
H-bonding,
London forces,
ion-dipole interactions.
+
C
dipole-dipole
-
O
+
-
O
+ C
O
C
R
O
-
R
R
H
N
O
H
N
-
R
H-bonding
+
C
O
Nylon 66
hydrogen-bonded structure for crystallites of an amide-type polymer
of hexanedioic acid and 1,6-hexanediamine.
Polymer Crystallinity
Crystallinity: Packing of chains to produce ordered atomic
array.
As crystallinity is increased in a polymer:
1. Density increases
2. Stiffness, strength, and toughness increases
3. Heat resistance increases
1. An amorphous polymer is one with no crystallites.
If the attractive forces between the chains are weak
the motions of the chain are not in some way severely restricted as by cross-linking or
large rotational barriers,
such a polymer
low tensile strength and when stressed to undergo plastic flow in which the chains slip
by one another
2. Crystallline polymer : Consider a polymer such as nylon, which has strong
intermolecular forces When the material is subjected to strong stress in one
direction, usually above Tg, so that some plastic flow can occur, the material
elongates and the crystallites are drawn together and oriented along the direction of the
applied stress
3. Elastomers usually are amorphous polymers.
Factors for Crystallization
•Slower cooling promotes crystal formation and growth
•Mechanical deformation, as in the stretching of a
heated thermoplastic, tends to align the structure and
increase crystallization
•Plasticizers (chemicals added to a polymer to soften
it) reduce the degree of crystallinity.
Polymer Crystallinity
Crystallinity characteristics
•Degree of crystallinity depends on
• rate of cooling; need sufficient time to result in ordered
configuration.
• Crystalline if chemically simple polymer. e.g.,
polyethylene, PTFE, even if rapidly cooled.
Polymer Crystallinity
•Amorphous if network polymer. Crystalline if linear
polymer (no restrictions to prevent chain alignment)
•Amorphous: Atactic stereoisomer.
•Crystalline: Isotactic or Syndiotactic stereoisomer
•Amorphous: If bulky/large side-bonded group.
Crystalline: Simple straight chain
Polymer Crystallinity continue…
•Amorphous: Most copolymers (and more irregular/
random mers)
Crystalline: Alternating or block polymers
•Amorphous: Random or graft polymers
•Crystalline: Strong, more resistant to dissolution by
softening by heat
Polymer crystals
Fringed micelle model
•Aligned small crystalline regions (crystallites or
micelles)
•Amorphous regions in-between platelets of crystals (1020 nm thick) (10m long)
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.
The effect of temperature on the structure and
behavior of thermoplastics.
Crystal Structures
Fe3C – iron carbide –
orthorhombic crystal
structure