[1] Thermomechanometry is the measurement of a change of a dimension or a mechanical property of the sample while it is subjected to a temperature regime.
Some characteristics of a material can be measured without disturbance, such as dimensions, mass, volume, density.
Knowledge of a structure can be gained by imposing an external stimulus and measuring the response of the material with a suitable probe.
The external stimulus can be a stress or strain, however in thermal analysis the influence is often temperature.
Changes in an amorphous polymer may involve other sub-Tg thermal transitions associated with short molecular segments, side-chains and branches.
Other relaxations may be due to release of internal stress arising from the non-equilibrium state of the glassy amorphous polymer.
If both components are semi-crystalline then the morphology will be complex since both crystal phases will likely form separately, though with influence on each other.
Cross-linking will restrict the molecular response to temperature change since degree of freedom for segmental motions are reduced as molecules become irreversibly linked.
Cross-linking chemically links molecules, while crystallinity and fillers introduce physical constraints to motion.
The sensitivity of zero stress TMA to crosslinking is low since the structure receives minimum disturbance.
The methodology must be chosen to best detect, distinguish and resolve the thermal expansion or contractions observable.
The TM instrument need only apply sufficient stress to keep the probe in contact with the specimen surface, but it must have high sensitivity to dimensional change.
The experiment must be conducted at a temperature change rate slow enough for the material to approach thermal equilibrium throughout.
While the temperature should be the same throughout the material it will not necessarily be at thermal equilibrium in the context of molecular relaxations.
Annealing at specific temperatures can be used to provide different isothermal relaxations that can be measured by a subsequent heating scan.
The limitation does not apply to sf-TM since the dimensional change of the material can be measured over any time.
The application of multiple scans is shown above to distinguish reversible from irreversible changes.
Modulated temperature TM (mt-TM) has been used as an analogous experiment to modulated-temperature DSC (mtDSC).
Some thermal processes are reversible, such as the true CTE, while others such as stress relief, orientation randomization and crystallization are irreversible within the conditions of the experiment.
The modulation conditions should be different from mt-DSC since the sample and test fixture and enclosure is larger thus requiring longer equilibration time.
mt-DSC typically uses a period of 60 s, amplitude 0.5-1.0 °C and average heating or cooling rate of 2 °C·min-1.
Temperature is the variable that can be changed to bring relaxations into measurable time ranges.
The viscoelastic recovery is exponential as the material slowly recovers some of the previously imparted creep strain.
After recovery there is a permanent unrecovered strain due to the viscous component of the properties.
[5] Analysis of the data is performed using the four component viscoelastic model where the elements are represented by combinations of springs and dashpots.
Thermomechanical instruments are distinct in that they can measure only small changes in linear dimension (typically 1 to 10 mm) so it is possible to measure yield and break properties for small specimens and those that do not change dimensions very much before exhibiting these properties.