One of the most widely used technique to measure dimensional changes of solid or liquid materials as a function of temperature, time and applied force is the thermomechanical analysis (TMA). This method is often used to measure a material's coefficient of thermal expansion (CTE), glass transition temperature (Tg), and (compression) modulus. TMA instruments precisely apply forces and measure dimensional changes at a nanometer scale. Most TMA instruments have the ability to ramp stresses at variable rates and, therefore, can be used to run flexure tests on rubbers and plastics, for example, to determine the Youngs modulus (see ASTM E2769-16). However, a TMA can not be used to test the mechanical strength of metalic samples since the forces are to small. Another important test is creep and creep recovery of polymeric materials. In this test, samples are loaded and unloaded over periods of time to measure creep and creep recovery.
The simplest test, is the zero force themomechanical analysis, often referred to as thermodilatometric analysis (TDA). It measures the change of dimension of a test specimen when heated or cooled due to activated/deactivated atomic and molecular vibrations (also known as phonons). This measurement yields the linear expansion coefficient which is the fractional increase in length per unit rise in temperature.
Another important application is the measurement of the glass transition temperature (Tg). In this test, the dimensional change of a polymeric material as a function of temperature at constant pressure or as a function of pressure at constant temperature is measured. A discontinuity in a plot of thermal expansion (α) or compression (β) versus temperature, or alternatively, a change in slope of a plot of volume versus temperature (pressure) marks the occurence of a glass transition. For many amorphous polymers, the Tg and the CTE are related by the approximate relationship
Tg X (αr - αg) ≈ 0.115 (1)
This relation is called the Simha and Boyer rule. It states that the CTE of a rubbery or liquid polymer is always larger than that of a glassy polymer.
TMA instruments are used in many industries, including pharmaceuticals, food, agriculture, electronics, polymers, coatings and adhesives.
- R. Simha, R.F. Boyer, J. Chem. Phys., 37, 1003 (1962)