The basis of TMA is the change in the dimensions of a sample as a function of temperature. A simple way of looking at TMA is as a very sensitive micrometer. TMA is believed to have developed from hardness or penetration tests and first reported as being used on polymers in 1948. Subsequently, it has developed into a powerful tool in the analytical laboratory. TMA2 measurements record changes caused by changes in the free volume of a polymer.3 Changes in free volume, vf, can be monitored as the volumetric change in the polymer, the absorption or release of heat associated with that change, the loss of stiffness, increased flow, or the change in relaxation time. The free volume of a polymer, vf, is known to be related to viscoelasticity,4 aging,5 penetration by solvents,6 and impact properties.7 Defined as the space a molecule has

for internal movement, it is schematically shown in Figure 4.1a. As the space available for the chains to move increases, larger and larger segments may move, giving rise to thermal transitions. This is often called the crankshaft model and is shown in Figure 4.1b.