Static Force Thermomechanometry

 

Static force thermomechanometry is the technique of measuring changes in sample dimensions with temperature (similar to thermodilatometry) but the measurements are made while the sample is under some mechanical stress. The apparatus, however, will allow the sample dimensions to be measured by changing the temperature under a constant load or conversely by changing the load at a given temperature. The apparatus that is employed is very similar to that used in thermodilatometry and is shown diagrammatically in figure 17.

 

 

Figure 17.  Apparatus for Use in Static Force Thermomechanometry

 

The sensors employed are shown to be the electromagnetic type but an optical sensor (that has already been described) would be practical as well.  The three systems shown are for sample measurement of extension, compression and flexing at different temperatures while under load. However, torque measurements are also sometimes of interest and can be measured with the same apparatus with a little modification.

 

The ranges of the different commercially available instruments, of course, vary but the average instrument has an expansion/contraction travel range of about 3mm and a maximum mass load of about 500 g. An example of the type of results that one might obtain from a polymer with some elastic properties is shown in figure 18.

 

The slope of the major part of the expansion curve will give a value for Young’s Modulus of the material. Both the expansion and the extension curves give and early inflexion indicating that there is some kind of phase transition occurring at this temperature. The curve continues normally until an apparently catastrophic change occurs where both curves fall rapidly indicating the substance has begun to undergo fusion. In this manner the temperatures of the phase change and the melting point can be accurately and unanbiguously identified. 

Figure 18. Expansion Contraction Curves for and Elastic Polymer