Cure Terms Glossary
The glass transition temperature, Tg, of a polymer is the temperature below which it is glassy and above which it is rubbery or liquid. The change into the glassy state is called vitrification. No latent heat is involved in this change, which is considered a second-order transition. It does not take place sharply at a distinct temperature, but usually occurs over a range of a few degrees. The Tg is conventionally taken as the mid point of the range, or the intersection of extrapolated linear portions of a plot such as specific volume against temperature.
No new order is introduced by the transition to a glass, which results from a major reduction in mobility at the molecular level. The change is associated with freezing out cooperative motions large chain segments (20-50 consecutive carbons) commonly called 'crankshaft rotations'.
Glasses have highly disordered molecular structures and are only associated with amorphous material. The strength of the intermolecular associations determines the stability of the glass. In the case of secondary interactions the strength increases as: van der Waals (2.5-7.5 kJ/mole) < dipolar associations (5-10 kJ/mole) < hydrogen bonds (10-30 kJ/mole). Their effect on Tg can be seen in the examples below.
The inherent rigidity of the backbone also plays a part. For example, the Tg of polyethylene terephthalate (69°C) is considerably higher than that of polyethylene adipate.
Primary bonding is much stronger than any of the above secondary interactions - hence, not surprisingly, the introduction of covalent crosslinks (e.g., ca. 350 kJ/mole for C-C) can have a profound effect on Tg. Crosslinked polymer networks can provide glasses which are stable even up to the point of thermal degradation.
For any polymer, crosslink density and Tg are inextricably linked: Tg increases with crosslink density. As cure progresses, the Tg increases until it reaches the temperature of cure - when the material will vitrify. The Tg will not readily increase any further as reaction rates drop considerably once a material becomes glassy.
The highest Tg attainable in a such a cure is that where the cure is conducted at the highest temperature without incurring degradation. This maximum glass transition temperature is designated Tg∞ .
The minimum glass
transition temperature is that for the uncured resin, and this is designated
Tg0. Thus Tg0
and Tg∞ represent the extremes of possible values for
Tg achievable in a cure, and are key points in a time-temperature-transformation