Question

Why do the Elastic modulus of polymers decrease sharply at the temperature around glass transition temperature?

Answer 1

Below their glass transition temperature, amorphous polymers are usually hard and brittle because of the low mobility of their molecules. Increasing the temperature induces molecular motion resulting in the typical rubber-elastic properties. A constant force applied to a polymer at temperatures above T_g results in a viscoelastic deformation, i.e., the polymer begins to creep. Heat resistance is thus given for amorphous polymers just below the glass transition temperature.

Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above the glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature.It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance. Another characteristic feature of semicrystalline polymers is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it.