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Morphology of Polymers

This document discusses the morphology and crystallization of polymers. It describes how polymers can crystallize from solution or melt to form thin platelets or lamellae with folded chains. When crystallized from melt, polymers often form spherulites - spherical structures made up of radial arrays of crystalline lamellae separated by amorphous tie molecules or entanglements. The polymer's chemical structure influences its ability to crystallize, with symmetrical and tactic structures promoting crystallinity over branching or atacticity. Hydrogen bonding also enhances crystallinity for some polymers.

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Younis Muhsin
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958 views7 pages

Morphology of Polymers

This document discusses the morphology and crystallization of polymers. It describes how polymers can crystallize from solution or melt to form thin platelets or lamellae with folded chains. When crystallized from melt, polymers often form spherulites - spherical structures made up of radial arrays of crystalline lamellae separated by amorphous tie molecules or entanglements. The polymer's chemical structure influences its ability to crystallize, with symmetrical and tactic structures promoting crystallinity over branching or atacticity. Hydrogen bonding also enhances crystallinity for some polymers.

Uploaded by

Younis Muhsin
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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This is followed by discussion of the morphologic features of the polymer

single crystal and those of polymers crystallized from the melt.


1. Crystal Structure of Polymers
• Crystallinity depends on the molecular structure of polymers.
• No bulk polymer is completely crystalline

• In semi-crystalline polymers, regular crystalline units are linked by


un-orientated, random conformation chains (amorphous regions)

Influence of morphology on properties


Polymers with higher crystallinity are denser, stiffer, harder, tougher and
more resistant to solvents.
Amorphous domains add flexibility and promote ease of processing below
the melting temperature.

Polymer structures
A: Linear, amorphous

B: Linear, semi-crystalline

C: Branched, amorphous
D: Slightly cross-linked

E: Cross-linked

F: Linear ladder structure


2. Morphology of Polymer Single Crystals Grown from Solution
Chain folded-model

-Many polymers crystallize as very thin platelets (or lamellae)

-Idea – the chain folds back and forth within an individual plate (chain
folded model)

Fringed-micelle structure: Suitable for natural polymers such as cellulose


and proteins that consist of fibrils. The molecules passed successively
through a number of these crystalline and intervening amorphous region
•The crystallites were pictured as sheaves of chains aligned in a parallel
fashion
Morphology of Polymers Crystallized from Melt
More commonly, many polymers that crystallize from a melt form
spherulites

One way to think of these – the chain folded lamellae have amorphous “tie
domains” between them
These plates pack into a spherical shape
• Ordinary tie-molecules bond two crystalline parts together across the
amorphous part. Two chains can also be entangled together by a physical bond
(entanglement)

Polymer structure hierarchy


Spherulites
• Following crystallization from the melt or concentrated solution,
crystallites can organize into spherical structures called spherulites

• Each spherulite contains arrays of lamellar crystallites that are typically


oriented with the chain axis perpendicular to the radial (growth) direction
of the spherulite

Spherulite nucleation and growth

• Formation of nuclei

• Accelerated crystallization: spherulites grow in radius


• Crystallization slows: spherulites begin to touch each other

• Crystallinity may still increase very slowly


What encourages crystallinity
A polymer’s chemical structure determines whether it will be crystalline
or amorphous in the solid state

Symmetrical chain structures favor crystallinity by allowing close


packing of polymer molecules in crystalline lamellae

Tacticity and geometric isomerism (i.e. trans configuration) favor


crystallinity

Branching and atacticity prevent crystallization

Crystallinity and the effect of hydrogen bonding


Specific interactions (hydrogen bonding between chains) enhance
crystallinity.
Within nylons, hydrogen bonding between;

Amide carbonyl group on one chain

Hydrogen atom of an amide group of another chain

Conformation and configuration of polymer chains in the lamellae

For many polymers, the lowest energy conformation is the extended


chain or planar zig-zag conformation (for example PE, polymers capable
of hydrogen bonding).

For polymers with larger substituent groups, the lowest energy


conformation is a helix (for example in PP, three monomer units form a
single turn in the helix).
Growth of the spherulites
At t0 the melt begins to cool

At t4 the sample is full of spherulites

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