Presentation of DMA Curves - METTLER TOLEDO

Presentation of DMA Curves

Purpose

The mechanical quantities, modulus and compliance, are often displayed on a logarithmic scale. The purpose here is to show that additional information can be obtained from a logarithmic presentation compared to the use of a linear scale.

 

Sample

Unfilled and unvulcanized SBR (VSL5025-0)

 

Conditions

Measuring cell: DMA/SDTA861e with the shear sample holder

Sample preparation: The material was pressed to a 1-mm thick film. Cylinders of 4-mm diameter were punched out and mounted in the shear sample holder with 10% predeformation.

DMA measurement: The measurement was performed at 1 Hz and a heating rate of 2 K/min. In the first heating run the sample was heated to 200 °C (1st run), then cooled and measured again (2nd run). Maximum force amplitude 5 N; maximum displacement amplitude 10 Pm; offset control zer

The shear moduli are displayed on a linear scale as a function of temperature.  

 

The shear moduli are displayed on a logarithmic scale as a function of temperature.

 

Interpretation

The diagrams display the storage modulus (G') and loss modulus (G'') curves of the first (black) and second (rot) measurements of SBR. In the first diagram, the modulus is displayed on a linear scale. In the second diagram a logarithmic scale was used. 

In both diagrams, the glass transition of SBR can be seen at about –20 °C. The storage modulus decreases and the loss modulus shows a peak. In the linear presentation, the peak maximum of G'' corresponds to the point of inflection of G'. In the second run, the glass transition is somewhat broader than in the first run and the maximum of G'' is shifted by about 2 K to higher temperature. After the glass transition, all the values appear to be practically zero in linear presentation.

In the logarithmic presentation, G' also shows a step at the glass transition and G'' a peak. The temperature maximum of the log G'' peak agrees well with the onset of the step in log G'. This logarithmic presentation displays the differences between the two measurements much more clearly than the linear presentation. In the first measurement, the step height of the log G' step is about 3, i.e. 3 decades in G'. After the glass transition, the storage modulus remains almost constant at about 1 MPa (rubbery plateau) and then decreases. The decrease is coupled with a slight increase in the loss modulus. From about 40 °C onward the material begins to melt.

In the second run the step height at the glass transition is only about 1.5 decades. Afterward in the logarithmic representation G' exhibits a broad step of about 1 decade between 10 °C and 80 °C. The loss modulus also decreases in this region. The material shows no flow behavior. The reason for this is the crosslinking of the polymer during the first run.

 

Conclusions

Interpretation of the measurement curves is often easier when the modulus is displayed on a logarithmic scale rather than on a linear scale because the modulus changes by several decades at the glass transition. In the logarithmic display, the glass transition temperature can be defined as the onset of the log G' step or as the maximum temperature of the G'' peak.

 

Presentation of DMA Curves | Thermal Analysis Application No. HB601 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume  1