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This subject is very challenging and it is hard to decide which polymers are amorphous and which crystalline. The first thing is to say what amorphous or crystalline structures are. Second, that both can exist in the same thermoplastic material (semi-crystalline polymers) at the same time. And third, that the features depend on the structures in the polymeric material.

First: what are amorphous and crystalline structures?

Amorphous and crystalline are used to call two separate states that are particularly in polymers, in thermoplastics. They define the type of structure in the polymer matrix. I found this description and I think this shows very well the difference between them:

“The amorphous nature of polymers is analogous to a plateful of spaghetti – loose and randomly coiled.

While the crystalline state is more like the uncooked spaghetti in the box – the chains are all tightly bundled and ordered in the same direction.”

Second: Semi-crystalline polymers

Thermoplastics polymers cannot be 100% crystalline so they cannot be melt because of their structure. Most polymers are semi-crystalline materials with a maximum of 80% crystallinity.

What is happening in the structure of semi-crystalline polymers?

When a polymer is cooled to room temperature, some parts/fractions of the polymer are amorphous (un-crystallized) and others are crystalline. The amorphous part is trapped between the growing crystals of the crystalline fraction. As a result the amorphous polymer chain movements are limited by crystalline structure, and this determinates the features of the polymer.

Semicrystalline polymers are also the largest group which is used commercially.

Third: Features

Features of each plastic depends mostly on the structure. As the amorphous state and the crystalline state are very different, the features will depend on the concentration of a particular state.

High concentration of amorphous structures in polymers: the material will have a low resistance to loads but it will have an excellent elasticity.

High concentration of crystalline structures in polymers: the material will be very strong (even stronger than thermoset materials), but with a little elasticity.

Others examples:

High concentration of amorphous structures

High concentration of crystalline structures

Higher heat resistance Lower heat resistance
Sharper melting point Gradual softening / melting point
Colour is more opaque Colour is more transparent
Greater shrinkage upon cooling Lower shrinkage upon cooling
Reduced low temperature toughness Greater low temperature toughness
Higher dimensional stability Lower dimensional stability