Practical Aspects of the Flash DSC 1: Sample Preparation for Measurements of Polymers - METTLER TOLEDO

Practical Aspects of the Flash DSC 1: Sample Preparation for Measurements of Polymers

The Flash DSC 1 expands thermal analysis to scanning rates (heating and cooling rates) of several 10 000 K/s (ten thousand Kelvin per second). At such rates, the formation of structure in materials can be investigated in order to gain a better understanding of the behavior of materials in technical processes. A number of practical questions however arise when using the Flash DSC 1.


Possible Questions

Examples:

  1. How do you determine the mass of a sample?
  2. How do you measure the blank curve needed especially for normal and medium scanning rates?
  3. How do you avoid artifacts that occur at low scanning rates with some materials?
  4. How do you obtain measurement curves that you can evaluate in the first heating run?

This article proposes various solutions to answer these questions.


Introduction

Polymers consist of macromolecules with different degrees of branching. The complicated molecular structure influences molecular mobility. As a result, relatively small crystallites form in crystallizable polymers.

The melting point of the crystallites depends on their size. Small crystallites melt at lower temperatures. Furthermore, the materials contain amorphous regions that cannot crystallize.

These polymers are semicrystalline and the structure formed is metastable. This means that structural changes can occur on heating, cooling, or even during isothermal storage.

If a semicrystalline polymer is measured in a conventional DSC instrument, the DSC curves generally exhibit a glass transition whose step height, Δcp, is proportional to the content of the mobile amorphous regions. The melting process of the crystallites gives rise to the melting peak. In some materials, an exothermic event known as cold crystallization can occur between the glass transition and the melting peak.

Cold crystallization is a reorganization process in which the degree of crystallinity, that is, the content of crystallites in the material increases...



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Summary and conclusions

The Flash DSC 1 expands thermal analysis to medium to very high scanning rates. Such scanning rates play an important role in technical processes and can be used to study the kinetics of the formation of structures and phase transitions. Besides the possibility of measuring at high scanning rates, measurements can also be performed at normal and medium rates. This has the advantage that the results can be directly compared with those from conventional DSC measurements.

However, four practical questions arise in the use of the Flash DSC 1:

  • How do you determine the mass of the sample?
  • How do you determine the blank curve needed especially for normal and medium scanning rates?
  • How do you avoid artifacts that occur with some materials at low scanning rates?
  • How do you obtain measurement curves that you can evaluate in the first heating run?

This article proposes practical solutions for these four questions:

  • The sample mass can be estimated from thermal effects such as transition enthalpy and heat capacity change at the glass transition.
  • To achieve an optimum display of measurement curves, a blank curve can be subtracted that is measured at the same scanning rates. At these rates, the contribution of the sample to the heat flow can be neglected.
  • Sample-induced changes in drift can occur with polymers that are hard and in good mechanical contact with the sensor. Changes in drift occur during crystallization or below the glass transition temperature due to mechanical stress. They can be reduced or eliminated by using a contact medium that forms a thin layer with good thermal conductivity between the sample and sensor. Silicone oil has proven useful for this purpose for measurements of materials such as PA, PBT and PET.
  • The first heating run in the Flash DSC 1 can also be measured and evaluated using silicone oil.


Practical Aspects of the Flash DSC 1: Sample Preparation for Measurements of Polymers | Thermal Analysis Application No. UC364 | Application published in METTLER TOLEDO Thermal Analysis UserCom 36