Glass Transition of Incompatible Polymer Blends

Purpose

To measure the glass transition of an elastomer based on two incompatible polymers using DSC.

Sample

Elastomers based on NBR/CR and NR/SBR blends 

Composition

Conditions

Measuring cell: DSC821^e with liquid nitrogen cooling option

Pan: Aluminum standard 40 µl, pierced lid  

Sample preparation: Piece of elastomer of approx. 20 mg.  

DSC measurement: The samples were 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  

Atmosphere: Nitrogen, 50 ml/min

Interpretation

The polymers used in the polymer blends were not compatible. The DSC curve therefore exhibits two glass transitions. The NR glass transition is at –58.8 °C and that of the SBR at –44.1 °C. The ratio of the step heights is 4:1 (ratio NR to SBR). The determination of the individual step heights is relatively uncertain because the glass transitions lie very close to one another. Taking this into consideration, the ratio of the polymers estimated from the DSC curves agrees quite well with the actual ratio of 3.5:1. The equation 

gives a polymer content of 39% ('c 'cp2 are the step heights of the first and second glass transitions). The actual polymer content of the elastomer was 32.7%

With the NBR/CR blend, the glass transitions are further apart. The CR glass transition is at –49.7 °C and has a step height of 0.10 J/gK. The NBR glass transition temperature is –13.6 °C and the corresponding step height 0.11 J/gK.  

From these values, the ratio of the polymers is estimated to be about 1:1. This agrees with the value for the formulation. The polymer content, Dpolymer, is estimated to be about 42%. The actual polymer content according to the formulation is 48.8%. 

Conclusions

With incompatible polymer blends, the polymer phases are separated. Each polymer phase exhibits its own glass transition. The ratio of the two polymers can be estimated from the step height at the glass transition.

_{Glass Transition of Incompatible Polymer Blends | Thermal Analysis Application No. HB 472 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2}