MELTING POINT

The freezing point of a pure liquid is the temperature at which the liquid’s molecules lose transitional freedom and the solid’s molecules become more ordered within a definite crystalline structure. Polymers, however, are non-homogeneous and do not have a definite crystallization temperature.

When a melted polymer cools, some polymer molecules line up and form crystalline regions within the melt. The rest of the polymer remains amorphous. The temperature at which these crystallites disappear when the crystalline polymer is gradually heated is called the crystalline melting temperature, Tm. Further cooling of the polymer below Tm changes the amorphous regions into a glass-like material. The temperature at which this change occurs is termed the glass transition temperature, Tg. Elastomeric materials usually have a low Tg (low crystallinity), while highly crystalline polymers, such as polyamides, have a relatively high Tg.