Glass transition temperature
Also searched as: Tg, glass transition, glass-rubber transition, glass transition range, second-order transition
The temperature range over which the disordered (amorphous) regions of a polymer change from a rigid, glassy state to a softer, rubbery state in which chain segments can move.
In detail
Below its glass transition, a polymer's disordered chain segments are effectively frozen and the material is stiff. Above it, segments gain enough thermal energy to rotate and slide, so the amorphous regions soften while crystalline regions stay intact until the melting point. For synthetic fibers this matters in drawing, heat setting, texturing, dyeing and ironing: a shape imposed above the glass transition and then cooled below it is locked in. Absorbed water acts as a plasticizer in hydrophilic polymers such as nylon and wool, lowering the effective transition temperature, which is why damp fabrics crease and set more easily.
The science and numbers
Below Tg the amorphous chains are locked; above it, segmental motion opens free volume, raising extensibility and letting small molecules such as dyes diffuse. PET's Tg of roughly 75 C lets as-spun PET remain a stable, supercooled melt, and slow-crystallizing polymers such as PET are drawn slightly above Tg; disperse dyeing of polyester is run well above Tg, for example at 130 C under pressure. Water is a strong plasticizer: in a study of polyamide 6,6, water-saturated samples showed a DSC glass transition more than 80 degrees lower than dry ones. For wool, DSC data fitted with the Fox equation gave a dry Tg of about 174 C, falling as water content rises. Tg is measured by differential scanning calorimetry as a step in heat capacity or by dynamic mechanical analysis as a loss peak, and the two methods give different numbers.
A practical example
A student irons two polyester swatches, one at a temperature below the fiber's glass transition and one well above it but far below melting. Only the hotter swatch keeps a sharp pleat after cooling. Repeating the test with a nylon swatch that has been dampened shows that moisture also lowers the temperature at which the fabric can be reshaped.
What to distinguish
The glass transition is not melting and is not a single sharp point. Reported values depend on method and conditions: a review of melt-spun fibers gives about 75 C for PET, while a dynamic mechanical study of electrospun PET membranes reported roughly 90 to 110 C. Heating rate, crystallinity, orientation and moisture also shift results, so a Tg figure alone does not predict safe ironing or dyeing temperatures.
Origins and history
The name comes from inorganic glasses, which soften gradually over a temperature range instead of melting at one point; polymer scientists adopted it for the comparable softening of amorphous regions in plastics and fibers. Classic mixing rules remain in use: the Fox equation, developed to estimate the Tg of blends and plasticized polymers, is still applied to keratin fibers to model how absorbed water lowers Tg. In textiles the concept became central with melt-spun synthetics, whose drawing, texturing and heat setting are planned around it.
Related terms
Sources & further reading
- Materials: Melt-spun fibers for textile applications (Hufenus et al., 2020)
- Biopolymers: The effect of water on the glass transition of human hair (Wortmann et al., 2006)
- Polymers: Hygromechanical behavior of polyamide 6.6, experiments and modeling (Wetzel et al., 2023)
- Molecules: Novel disperse dyes based on enaminones, dyeing polyester at 100 and 130 C (Abdelmoteleb et al., 2024)
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.