In epoxy composites, Tg is defined as the temperature at which the resin transitions from glassy to rubbery.

In epoxy composites, Tg is defined as the temperature at which the resin transitions from glassy to rubbery.

• A. Temperature where resin cures
• B. Temperature where resin degrades
• C. Temperature where resin transitions from glassy to rubbery
• D. Temperature where fibers melt

Answer: (C)

The key idea is the glass transition: the temperature where a cured epoxy resin changes from a rigid, glassy state to a more flexible, rubbery state as molecular mobility increases. Below this temperature, the network is stiff with a high modulus and limited chain motion. As temperature rises and approaches Tg, segmental motion frees up, causing a drop in stiffness and a shift toward more ductile behavior. This transition is a property of the polymer network after curing and is distinct from the cure process itself, from thermal degradation, or from melting of fibers. Therefore, the temperature at which the resin transitions from glassy to rubbery correctly defines Tg.

The key idea is the glass transition: the temperature where a cured epoxy resin changes from a rigid, glassy state to a more flexible, rubbery state as molecular mobility increases. Below this temperature, the network is stiff with a high modulus and limited chain motion. As temperature rises and approaches Tg, segmental motion frees up, causing a drop in stiffness and a shift toward more ductile behavior. This transition is a property of the polymer network after curing and is distinct from the cure process itself, from thermal degradation, or from melting of fibers. Therefore, the temperature at which the resin transitions from glassy to rubbery correctly defines Tg.