Scientists and engineers use the term “sub-ambient glass transition temperature” (aka “sub-Tg”) to denote the Tg of an uncured thermoset material such as resin or prepreg. This terminology comes about because a cured, engineered thermoset system has a Tg that is high (i.e., significantly above room temperature) whereas in its uncured state the Tg is below ambient temperature. The terminology is used intending to void confusion. When sub-Tg is used it is only referring to Tg of uncured thermoset materials. When the material is being cured, the glass transition increases with respect to its degree-of-cure and it is called the typical Tg instead.
Many thermoset materials are processed to thin sheets such as in resin film or prepreg. At temperature below the sub-Tg it remains a solid and the uncured resin film is brittle and easily flaked off if bent. At temperature above sub-Tg, the uncured resin film or prepreg becomes resinous and pliable. It is possible to relate sub-Tg to drapeability of prepreg or resin film for material handle ability purpose. Also the study of sub-Tg can be linked with the goal of determining the material usage time and allowable out time. In addition to drape, people have also been relating sub-Tg to prepreg tack, although tack is a material quality that has been elusive to quantify as it is found to also depend on environmental factors such as ambient humidity and processing factors such as peeling speed.
The sub-Tg can be measured by thermal analysis techniques such as DSC and DMA. Sample should be at ambient temperature during sample preparation and handling. Be sure to eliminate moisture condensation on the sample while it is equilibrating to ambient temperature. Use the same kind of care to perform sub-Tg measurement as you would with measuring Tg of a polymer.