DSC Measurements of Elastomers

Purpose

The measurements in the previous sections were limited to the glass transition region. Measurements over a larger temperature range will now be presented. 

Sample

Different elastomers each containing two polymer constituents: 

Conditions

Measuring cell: DSC822^e with liquid nitrogen cooling option  

Pan: Aluminum 40 μl, pierced lid

Sample preparation: Cubes of approx. 20 mg cut from the starting material  

DSC measurement: Heating from –140 °C to 120 °C at 10 K/min (1 measurement), followed by cooling to –140 °C at 5 K/min and then repeating the heating program (2^nd measurement).  

Large letters are used to identify the samples. The number after the sample letter denotes whether it is the first or the second measurement. The small letters mark the thermal events. 

Interpretation

Sample A is an SBR/BR blend. The glass transition (a) of this system has already been discussed in Section 4.5.3. Glass transition of compatible polymer blends. Only a broad glass transition is observed. In the first measurement, an endothermic peak (b) with an area of 3 J/g can be seen at about 100 °C. This peak does not occur in the second measurement - it is due to the elimination of moisture. 

Sample B is an NBR/CR blend. As shown in Section 4.5.3. Glass transition of incompatible polymer blends. these two polymers are incompatible. The sample therefore exhibits two separate glass transitions, marked (a) and (b). The event marked (c) corresponds to an endothermic melting process that occurs only in the first measurement. The cooling rate (5 K/min) is still so rapid that no crystallites are formed because crystallization is so slow. This behavior and the temperature of about 40 °C indicate that the effect is due to the melting of CR (see Section 4.1.3 Measurements of the crystallization and melting of chloroprene rubber (CR)). The peak area is 0.6 J/g. This means that the enthalpy of fusion with reference to the chloroprene content is 2.5 J/g. The value indicates that chloroprene with a relatively large tendency to crystallize was used for this blend. Besides the CR melting peak, another broad melting process (d) is also observed that can be related to the NBR. This NBR peak can also be seen in the second measurement curve. The peak maximum is about 53 °C and area 2 J/g (or 8 J/g with respect to the NBR content).

Sample C is a blend of NR and SBR. As already discussed in Section 4.5.3, the incompatible polymer blend shows two glass transitions (a) and (b). The melting of NR results in a broad peak (c) of about 1 J/g. This corresponds to about 4 J/g with reference to the NR content. 

Conclusions

DSC can measure a number of other thermal events that occur, besides the actual glass transitions previously discussed. The results provide information on the type of material concerned. The thermal events involved include melting and crystallization as well as vaporization, post-vulcanization and decomposition.

^{DSC Measurements of Elastomers | Thermal Analysis Application No. HB 474 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2}