Vicat softening temperature (VST) reflects the transformation
point of a material which is used in certain temperature and transformed
from ordinary state to softening. It is defined as the temperature
at which the tested sample is penetrated by a needle
with flat point to the depth of 1 mm under a specific load using a
selected temperature ramp. So the VST is a well-defined point in
the modulus-temperature diagram, being related to the fundamental
properties of the material