Molecular Forces Based Polymers

The mechanical properties of polymers are tensile strength, toughness; elasticity etc depending upon intermolecular forces like van der Waal’s forces and hydrogen bonds existing in the macromolecules. Although these intermolecular forces are present in simple molecules also, but their effect is less significant in them as compared to that in macromolecules. It is because of the fact that in polymers there is a combined effect of these forces all along the long chains. Obviously, longer the chain, more intense is the effect of intermolecular forces.

Polymers have been classified into the four categories on the basis of the magnitude of intermolecular forces present in them. These are:

1. Elastomers: these are the polymers in which the polymer chains are held up by weakest attractive forces. They are amorphous polymers having high degree of elasticity. The weal forces permit the polymer to be stretched out about 10 times their normal length but they return to their original position when the stretching force is withdrawn. In fact, these polymers consist of randomly coiled molecular chains having few cross-links. When the stress is applied, these randomly coiled chains straighten out and the polymer gets stretched. As soon as the stretching force is released, the polymer regains the original shape because weak forces do not allow the polymer to remain in the stretched form.

2. Fibers: these are the polymers which have quite strong interparticle forces such as H-bonds. They have high tensile strength and high modulus. They are thread-like polymers and can be woven into fabrics. Nylon, Dacron silk is some common examples of this types of polymers.

3. Thermoplastics: they are the polymers in which the interparticle forces of attraction are in between those of elastomers and fibers. The polymers can be easily moulded into desired shapes by heating and subsequent cooling to room-temperature. There is no cross-linking between the polymer chains. In fact, thermoplastic polymers soften on heating and become fluids but on cooling they become hard. They are capable of undergoing such reversible changes on heating and cooling repeatedly. Common examples of thermoplastics are polyethene, polystyrene, polyvinyl chloride, etc.

Plasticizers: certain plastics do not soften to workable extent on heating. For example, synthetic resins and cellulose derivatives are horny tough materials. Such plastics can be easily softened by the addition of some organic compounds which are known as plasticizers. For example, polyvinyl chloride (PVC) is extremely stiff even while hot. However, addition of di-n-butyphthalate, a plasticizer, makes it soft and workable. The plasticizing effect is due to the solubilization action and an accompanying reduction in intermolecular forces which permits free movement of molecules relative to each other.

Some important plasticizers are:

Triceryl phosphate                      Dimethyl phthalate

Triphenyl phosphate                      Camphor.

4. Thermosetting polymers: these are the polymers which become hard and infusible on heating. They are normally made from semi-fluid substances with low molecular masses, by heating in a mould. Heating results in excessive cross-linking between the chains forming three dimensional network of bonds as a consequence of which a non fusible and insoluble hard material is produced. Bakelite is a common example of thermosetting polymer.

In short, a thermoplastic material can be remelted time and again without change, while a thermosetting material undergoes a permanent change upon melting and thereafter sets to a solid which cannot be remelted.

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