Glass Transition of Compatible Polymer Blends - METTLER TOLEDO

Glass Transition of Compatible Polymer Blends

Purpose

To measure the glass transition of an elastomer based on compatible polymer blend (SBR and BR) using DSC.

 

Sample

Vulcanized elastomer based on SBR/BR blend

The polymer content was 50% and consisted of SBR and BR

The filler and ZnO content was 38%

 

Conditions

Measuring cell: DSC821with liquid nitrogen cooling option 

Pan: Aluminum standard 40 µl, pierced lid 

Sample preparation: Piece of elastomer of approx. 15 mg.  

DSC measurement: The sample was cooled from 120 °C to –140 °C at 5 K/min and then measured by heating from 140 °C to 100 °C at 10 K/min  

 

Interpretation

The glass transition is relatively broad and extends over a temperature range of 60 K between –110 °C and –50 °C. The occurrence of just one glass transition in the polymer blend shows that the polymer components are compatible. A distinct broadening of the glass transition step is noticeable in the temperature range –80 °C and –50 °C. This type of curve shape indicates that the polymer blend is not ideally homogeneous. For the amorphous unfilled polymers, the step height of the glass transition, Δcp, is expected to be about 0.5 J/gK at about –90 °C. In fact, a step height of 0.32 J/gK is measured at the glass transition temperature of –87.7 °C. This allows the proportion of filler material (which is not involved in the glass transition) to be estimated:

This gives a value of 36% for the filler content, which agrees well with the value given in the formulation. If the exact step height of the unfilled material is not known, the estimate is subject to a relatively large error. It is then better to use TGA data to determine the filler content accurately. The estimate of the filler content via the step height at the glass transition, however, yields important information on the properties of the sample or for further measurements. For example, a filler content estimated from the DSC measurement that is larger than that expected indicates that the polymer is partially crystalline - this means that the DSC curve must then exhibit a melting processes above the glass transition. Another possibility is that an elastomer exhibits two glass transitions - the second glass transition is either in a temperature range outside the measurement range or is very broad and is therefore not detected and taken into account in the first evaluation. Δcp should be evaluated according to the ASTM or Richardson methods for the estimation of αfiller. The other evaluation methods yield values for Δcp that are too high due to their definition (see Section 3.1.1).

 

Conclusions

Only one glass transition is observed with compatible polymer blends. The polymer and filler contents can be estimated from the step height of the glass transition.

Glass Transition of Compatible Polymer Blends | Thermal Analysis Application No. HB 471 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2