Structure :

In chemical terms, polystyrene is a long chain hydrocarbon wherein changing carbon centers are attached to phenyl groups (the name given to the aromatic ring benzene).

The material's properties are specified by short-range van der Waals attractions between polymers chains.
Meanwhile the molecules are long hydrocarbon chains that contain of thousands of atoms, the total attractive force between the molecules is large.
When heated (or deformed at a rapid rate, due to a combination of viscoelastic and thermal insulation properties), the chains can take on a higher degree of conformation and slide past each other.
This intermolecular weakness (versus the high intramolecular strength due to the hydrocarbon backbone) confers flexibility and elasticity.
The ability of the system to be readily deformed above its glass transition temperature allows polystyrene (and thermoplastic polymers in general) to be readily softened and molded upon heating.

Extruded polystyrene is about as strong as an unalloyed aluminium, but much more flexible and much lighter .