Question

Many linear polymers can exhibit large amounts of deformation. Explain how the structure of this type of polymer changes as it undergoes loading to failure.

The type of polymer is linear. How do linear polymers change under loading?

Answer 1

Linear polymers made up of one long continuous chain, without any excess attachments. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) are example of linear polymers. Chemical structure of polymer chain is ------CH₂-CH₂-[CH₂-CH₂-] CH₂- CH₂-----

In linear polymers, the chains are held together by weak intermolecular force of attraction. Strength increases with increase in chain length or molecular weight. Polymers of lower chain length are soft and gummy, while higher chain polymers are hard and strong .Strength of linear chain polymers depends upon the slipping power of one chain over the other. Shape of the polymer affects the resistance to slip and consequently the deformation of the polymer.

Deformation is the slipping of one chain over the other on the application of heat or pressure or both. Recoverance of original shape of the polymeric chains after the removal of stress occurs.

There are two types of deformation Plastic Deformation (Plasticity) and Elastic Deformation (Elasticity).

Plastic Deformation is the slipping of one chain over the other on the application of heat or pressure or both. It occurs only when the weak secondary intermolecular force is operating between the polymeric chains, when sufficient load is applied permanent deformation occurs as slippage. The linear polymers show the greatest degree of plastic deformation.

In heated state, linear polymer readily takes the shape of the mould, when it is injected into under pressure, called thermoplastic. At high temperature, polymers deform easily due to the weakening of secondary intermolecular force between chains so the chains can easily slip over each other. On cooling, the polymer becomes rigid in the moulded shape, because plasticity decreases with fall of temperature. So plasticity of the polymer is reversible.