Question

Why does polystyrene behave like a plastic rather than an elastomer at ambient temperatures? Discuss the structural features of a polymer which would make it a useful elastomer.

How would the mechanical and physical properties of thermoplastics change with (a) chain length (degree of polymerization), (b) chain stiffness, (c) side group size, and (d) degree of crystallinity.

Answer 1

Polystyrene behaves like plastic rather than elastomers as the glass transition temperature of polystyrene is above 100-degree Celsius. (The Glass Transition Temperature (Tg) is one of the most important properties of an epoxy and is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material.)

A material will be thermoplastic or elastomer depends on its glass transition temperature. If the Tg is above room temperature it will be a thermoplastic whereas if the Tg is below room temperature material will be an elastomer.

The material's properties depend on short-range van der Waals attractions between polymers chains. The polymer molecules consist of thousands of atoms so the cumulative attractive force between the molecules is large. When they are heated (or deformed at a rapid rate, due to a combination of viscoelastic and thermal insulation properties), the chains are able to 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 properties required for elastomer are

·         Viscoelasticity (both viscosity and elasticity)

·         very weak intermolecular forces,

·         Low Young's modulus and

·         high failure strain compared with other materials

Change in mechanical and physical properties with chain length and side group branching.

Chain Stiffness - The presence of the stiffening groups (such as amide, sulfone, carbonyl, p-phenylene etc.) in the polymer chain reduces the flexibility of the chain, leading to higher glass transition temperature. For example, polyethyleneterephthalete is stiffer than polyethylene adipate due to the presence of benzene ring. Therefore, Tg value is higher for polyethyleneterephthalate.

Change in mechanical and physical properties with crystallinity.