polymer structure polyolefins with side chains have stereocenters on every other carbon with so many...

Polymer StructurePolyolefins with side chains have stereocenters on every other carbon

CH3n

CH3 CH3 CH3 CH3 CH3 CH3CH3

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

Isotactic configuration

Stereoisomerism

Molecular configurations continue….

R groups are situated on the same side of the chain

Syndiotactic

On alternate sides

Molecular configurations continue….

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

Geometric Isomerism

CIS-Isoprene

eg., Natural rubber

Attacked by acids/alkalis

Molecular configurations continue….

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.

Molecular Weight

Mechanical Property

Polymer MorphologyThe mechanical properties result from attractive forces between molecules

• dipole-dipole interactions,• H-bonding, • London forces, • ion-dipole interactions.

CO

O

R

CO

O

R+

+

-

-

C

HN

O

R

C

HN

O

R

+

+

-

-

H-bondingdipole-dipole

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 another2. Crystallline polymer : Consider a polymer such as nylon, which has strong intermolecular forces When the material is subjected to strong stress in onedirection, usually above Tg, so that some plastic flow can occur, the materialelongates and the crystallites are drawn together and oriented along the direction of the applied stress3. Elastomers usually are amorphous polymers.

Factors for Crystallization•Slower cooling promotes crystal formation and growth•Mechanical deformation, as in the stretching of aheated thermoplastic, tends to align the structure andincrease crystallization•Plasticizers (chemicals added to a polymer to softenit) reduce the degree of 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

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 (10-

20 nm thick) (10m long)

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The effect of temperature on the structure and behavior of thermoplastics.

Crystal Structures

Fe3C – iron carbide – orthorhombic crystal structure