Strength of Polymers:           

In general, the longer the polymer chain, the stronger the polymer.  There are two reasons for this:

• longer chains are more tangled

• there are more intermolecular forces between the chains because there are more points of contact.  These forces, however, are quite weak for polyethene.

Areas in a polymer where the chains are closely packed in a regular way are said to be crystalline.  The percentage of crystallinity in a polymer is very important in determining its properties.  The more crystalline the polymer, the stronger and less flexible it becomes.

When a polymer is stretched (cold-drawn), a neck forms.  In the neck the polymer chains line up producing a more crystalline region.  Cold-drawing leads to an increase in strength.

The first polyethene which was made contained many chains which were branched.  This resulted in a relatively disorganised structure of low strength and density.  This was called low density polyethene (ldpe).

Ziegler used organometallic catalysts to produce polythene with little branching along the chain.  The chains could pack together more closely resulting in more crystalline regions.  The result was high density polyethene (hdpe) which was stronger and more dense.

Natta used Zieger's catalyst to polymerise propene.  The reaction mixture contained two forms of polypropene - a crystalline form and an amorphous (non-crystalline) form.

In the crystalline form, the methyl groups all have the same orientation along the chain.  This is called the isotactic form.  In the amorphous form, the methyl groups are randomly orientated.  This is called the atactic form.

Polymers with a regular structure are said to be stereoregular.