Define Tg and explain how it changes with moisture in polymers used in composites.

Define Tg and explain how it changes with moisture in polymers used in composites.

• A. Tg is the melting temperature; moisture increases Tg.
• B. Tg is the thermo-oxidative limit.
• C. Tg is the glass transition temperature; moisture plasticizes resin, lowering Tg.
• D. Tg is the decomposition temperature; moisture raises Tg.

Answer: (C)

Glass transition temperature is the temperature at which an amorphous polymer goes from a rigid, glassy state to a softer, rubbery state as temperature increases. When moisture is absorbed by the polymer, water molecules fit between chains and disrupt intermolecular forces, increasing free volume and allowing chains to move more easily. This plasticizing effect lowers the temperature needed for the onset of segmental motion, so Tg decreases. In composites, this means the resin matrix becomes more compliant at lower temperatures when moisture is present, potentially reducing service temperature, stiffness, and dimensional stability. The basic idea is that moisture makes the polymer more pliable, shifting Tg downward, with the extent depending on the resin chemistry and crosslink density.

Glass transition temperature is the temperature at which an amorphous polymer goes from a rigid, glassy state to a softer, rubbery state as temperature increases. When moisture is absorbed by the polymer, water molecules fit between chains and disrupt intermolecular forces, increasing free volume and allowing chains to move more easily. This plasticizing effect lowers the temperature needed for the onset of segmental motion, so Tg decreases. In composites, this means the resin matrix becomes more compliant at lower temperatures when moisture is present, potentially reducing service temperature, stiffness, and dimensional stability. The basic idea is that moisture makes the polymer more pliable, shifting Tg downward, with the extent depending on the resin chemistry and crosslink density.