Transitions of Low Molecular Weight Components in SBR - METTLER TOLEDO

Transitions of Low Molecular Weight Components in SBR

Purpose

With polymers and elastomers there is always the possibility that crystallization and melting of low molecular weight constituents occurs. These constituents can for example be monomers, oligomers, emulsifying agents, plasticizers or other additives. The influence of the crystallization and melting of such components is shown and evaluation possibilities are discussed using a sample of an oilcontaining emulsion-polymerized SBR. 

 

Sample

Unvulcanized SBR rubber (Krylene 1721).  

 

Conditions

Measuring cell: DSC822e with liquid nitrogen cooling option.

Pan: Aluminum 40 µl, pierced lid  

Sample preparation: Cubes of approx. 5 mg cut from the starting material  

DSC measurement: Heating from –130 °C to 120 °C at 10 K/min (1st run). Afterward cooling to –130 °C at 5 K/min and repeating the first heating program (2nd run).  

Atmosphere: Nitrogen, 50 ml/min  

Interpretation

In the heating runs, a step due to the glass transition is observed at about 35 °C. Several rather weak events occur afterward. At first it is not at all clear whether the peaks are exothermic or endothermic and which areas should be measured. 

This type of thermal behavior is caused by crystallization and melting processes of oil as well by low molecular weight residues from the polymerization process such as emulsifiers. In the cooling curve, crystallization peaks can be seen above the glass transition. The glass transition and crystallization are better separated than the effects in the heating curve.

 

Evaluation

The evaluation of the glass transition in the heating curve is not a trivial matter. It is not immediately obvious whether the peak at –30 °C is an enthalpy relaxation peak or whether an exothermic event occurs at –15 °C. At temperatures above 70 °C no further thermal effects are observed. In this region, the measurement curve is determined by the heat capacity of the SBR. The extrapolation of the measurement curve to lower temperature can be used to construct a tangent for the glass transition evaluation. This gives a step height of 0.40 J/gK (2nd heating curve) at the glass transition temperature of –34.6 °C. To check this, a similar evaluation of the glass transition evaluation was performed on the cooling curve. This gave a value of 0.40 J/gK for the step height and a glass transition temperature of –39.2 °C. The good agreement of the step height in heating and cooling measurements confirms that the tangents used for the evaluation of the glass transition were correct. The difference of 4 K between the glass transition temperatures measured in the heating and cooling runs corresponds to that expected with this evaluation method.

The tangent used for the evaluation of the glass transition above the glass transition corresponds to the extrapolated DSC signal that would result if no thermal processes occurred between –20 °C and 70 °C. This tangent can therefore be used as the baseline for the evaluation of peaks due to processes occurring after the glass transition.

In the heating measurement, an exothermic process takes place after the glass transition at –5 °C. This is due to cold crystallization. The enthalpy of crystallization is 1.12 J/g.

After the cold crystallization peak, the curve exhibits an endothermic double peak with maxima at 17 °C and 37 °C. This is a melting process. The total area of the exothermic and endothermic peaks is 1.74 J/g. This means that the endothermic peaks are larger than the exothermic enthalpy of crystallization, i.e. more material melts than is crystallized. Some of the crystallites were present before the measurement. These were formed on cooling. The cooling curve does in fact show a crystallization peak between 30 °C to –20 °C. Evaluation of the cooling curve yielded a value for the enthalpy of crystallization of 1.74 J/g. This proves that the endothermic peak observed on heating is due to the melting of the crystallites that were formed beforehand on cooling (enthalpy of crystallization: 1.74 J/g) and afterward during cold crystallization on heating (enthalpy of crystallization: 1.12 J/g). 

 

Conclusions

Crystallization and melting effects of low molecular weight constituents or oligomers make the evaluation of DSC measurement curves more difficult. A quantitative evaluation is usually still possible if cooling measurements are run to aid interpretation and the right evaluation method is used.

Transitions of Low Molecular Weight Components in SBR | Thermal Analysis Application No.HB465 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2