In Figure 2 the glass transition is represented as the intersection
of two straight lines. Above Tg the dimensional change is that
of a liquid with all its molecular degrees of freedom. Below
Tg the dimensional changes with temperature are those of an
amorphous solid whose rotational and translational degrees of
freedom have been frozen out. When a liquid is cooled rapidly
(as often occurs in plastics processing) it results in a solid
structure with more and larger voids than if the material is
cooled slowly or allowed to sit for a long time in the low end
of its Tg region. This latter case is called “physical aging”,
(as indicated by the green arrow in Figure 2). Amorphous
material which is stored in its Tg temperature range will
change mechanical properties, such as modulus, which will
affect its processing characteristics. The glass transition as
measured by TMA using the intersection of the tangents above
and below Tg indicate the extent of physical aging. If carried out
in a dilatometer the extent of volume change can be measured
(although in practice this may be a difficult measurement).
Figure 5 shows an epoxy printed circuit board heated at 5 °C/
min after cooling at 40 °C/min (top curve), 10 °C/min, 2 °C/min,
0.5 °C/min and 0.1 °C/min (bottom curve). The effect of the
physical aging is to lower the Tg. Unlike measuring the Tg by
DSC the Tg measured by TMA directly measures this physical
aging effect14. Moreover it can be directly measured on the
cooling curve (not shown in Figure 5 for simplicity) without loss
of accuracy at slow cooling rates.