Measurement of Large Changes in Mechanical Behavior - METTLER TOLEDO

Measurement of Large Changes in Mechanical Behavior

Purpose

At the glass transition and melting of a material, the modulus changes by several decades. With shear measurements, these processes can be investigated in one experiment without changing the geometry or the conditions. The purpose of the experiment is to illustrate the wide measurement range with an example.

 

Sample

Unfilled and unvulcanized silicone oil



Conditions

Measuring cell: DMA/SDTA861e

Sample preparation: The two gaps in the shear sample holder for liquids were set to 0.3 mm and filled with sample. The diameter was 11 mm. The sample holder was then installed in the clamping assembly that had been cooled to –140 °C. 

DMA measurement: Heating from –140 °C at 2 K/min with a frequency of 1 Hz. Maximum force amplitude 5 N; maximum displacement amplitude 10 Pm; offset control zero.

 

Interpretation

The diagrams display the storage modulus (G'), the loss modulus (G") and the phase angle, g, (phase shift) as a function of temperature. The moduli are plotted logarithmically.

Below –125 °C the sample is in the glassy state. The storage modulus is relatively large – about 8*108 Pa. In this region the loss modulus G" is about 1 decade smaller than G'. At the glass transition between -125°C and -100°C, G" and G" decrease by about 3.5 decades. This decrease is coupled with a peak in the phase angle, g. The peak maximum at about -20°C can be defined as the glass transition temperature. At the maximum the angle is 1.1 radians. This corresponds to a loss factor, tan g, of 2. From -100°C onward, the modulus then increases. The sample crystallizes in this temperature range. The phase angle exhibits a shoulder. Between -110°C and -50°C the material is semicrystalline. In this range the storage modulus is up to 1.25 decades larger than the loss modulus. The phase angle, g, is about 0.05 radians. The small decrease in G" and the simultaneous increase in g are caused by the melting of small crystallites on heating. The main melting region between –40 °C and –30 °C is accompanied by a decrease of the modulus and a step-like increase in g The phase angle, g, reaches half step height at about –37 °C and the moduli intersect at a value of about 50 kPa. Afterward the sample behaves as a liquid. At higher temperatures the moduli, logarithmically displayed, decrease linearly with temperature, whereby the decrease of G" is about half that of G'. The value of G" decreases at 0.85 decades per 100 K and G' at 1.5 decades per 100 K. In the melt, g also increases slightly with temperature. At 100 °C the phase angle is 1.5 radians (tan g≈15) and almost reaches the limit of S/2, at which the material behaves like an ideal Newtonian fluid.

 

Conclusions

Besides the measurement of transitions such as the glass transition, crystallization and melting, the characteristic mechanical quantities can also be measured. With shear measurements, it is possible to measure stiff solids with a shear modulus of about 109 Pa as well as liquids in which G' is about 30 Pa without changing the geometry. In the course of the measurement G' changes by about 7.5 decades and the phase angle from 0.05 to 1.5 radians. 


 Measurement of Large Changes in Mechanical Behavior | Thermal Analysis Handbook No.HB419 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1