Determination of Crystallinity

Purpose

To determine the degree of crystallinity from DSC curves. 

Sample

Unvulcanized EPDM (EPG 6170) 

Conditions

Measuring cell: DSC822^e  with liquid nitrogen cooling option 

Pan: Aluminum 40 µl, with pierced lid

Sample preparation: Cube of approx. 5 mg cut from the starting material. The sample was heated to 100°C before the measuremennt and then cooled at 5 K/min to the start the temperature. 

DSC measurement: Heating from -120°C to 100° at 10K/min

Atmosphere: Nitrogen, 50 ml/min. 

Interpretation

The step in the curve at 45 °C shows the glass transition of the partially crystalline material. Only the mobile amorphous part is involved in the glass transition. The endothermic melting peak begins immediately afterward. 

Evaluation

With such broad melting peaks, it is not always so easy to reproducibly determine the beginning of the peak for the determination of the peak area. In such cases it helps if the DSC curve has been recorded over a sufficiently wide range in the melting region. The baseline of the peak should agree well with the linear extrapolation of the heat flow curve to lower temperature from the region of the melt (see Section 3.1.2. Glass transition and melting of EPDM). The degree of crystallinity, α, can then be estimated from the peak area: 

where Δh is the specific enthalpy of fusion (in J/g) of the sample determined from the peak area, and 'hc is the enthalpy of fusion of a 100% crystalline material. In the case of EPDM, the value of polyethylene of 290 J/g can be used for Δh_c. The evaluation of the measured curves yielded the following values:

Peak area: 46.0 J/g

Crystallinity: 15.9%

Conclusions

The specific enthalpy of fusion of a sample determined from the peak area is proportional to its degree of crystallinity. The proportionality factor, Δh_c, is equal to the enthalpy of fusion of a 100% crystalline material. If the value is known, the degree of crystallinity can be determined from the melting curve. 

Comments

In general, for polymers, the enthalpy of fusion of the fully crystalline material, Δh , cannot be obtained from DSC measurements - due to their molecular structure polymers have amorphous (non-crystalline) regions. The values of 'hc can often be obtained from the scientific literature or databases. One method for the determination of 'hc is the combination of wide angle X-ray scattering (WAXS) and DSC. In this method, a material is used that, because of its chain structure, can attain a relatively high degree of crystallinity. The material is thermally treated so that a high degree of crystallinity is obtained. This can be done, for example, by slow cooling from the melt (e.g. at 0.1 K/min) or through stepwise isothermal cooling. The degree of crystallinity, D, is then determined by X-ray measurements. DSC measurements yield the specific enthalpy of fusion, Δh. This data is then used to calculate Δhc: 

_{Determination of Crystallinity | Thermal Analysis Handbook No.HB404 | Application published METTLER TOLEDO TA Application Handbook Elastomers Volume 1}