Table of Contents
Why DSC is essential in polymer characterization
When it comes to polymers to characterize polymers, the differential dynamic calorimetry (DSC) is one of the most reliable methods. It makes it possible to precisely determine thermal transitions such as glass transition, crystallization or melting point – precisely those properties that are decisive for the processing and use of a plastic.
This shows that not every plastic behaves in the same way. The structural difference between amorphous and semi-crystalline influences which thermal signals are visible in the DSC – and how they are to be interpreted. Compliance with internationally recognized standards is essential for reproducible and comparable results. ISO 11357 (Europe/international) and ASTM D3418 (USA) are two proven standards that set clear requirements for calibration, sample preparation and evaluation.
Structural differences: amorphous and semi-crystalline in comparison
The fundamental difference between amorphous and semi-crystalline polymers lies in their molecular structure – and it is precisely this structure that determines their thermal behavior.
Amorphous polymers have a disordered, confused arrangement of polymer chains. This structure does not allow a crystalline state of order. Typical representatives are polystyrene (PS)polycarbonate (PC) and polymethyl methacrylate (PMMA). Characteristic of amorphous polymers is that they do not have a melting point in the classic sense. Instead, they exhibit a glass transition temperature (Tg) – the range in which the material changes from a glassy to a rubbery state. The DSC plot shows this transition as a step or kink in the baseline.
Semi-crystalline polymers, on the other hand, consist of a combination of ordered (crystalline) and disordered (amorphous) areas. Typical representatives are polyethylene (PE), polypropylene (PP) and polyamide (PA). In addition to the glass transition (often difficult to detect), they show pronounced crystallization and melting peaks in the DSC. During cooling, the chains partially crystallize (exothermic peak), while these structures melt during reheating (endothermic peak).
The distinction is essential: while amorphous polymers are ideal for optically clear applications or fast molding, semi-crystalline grades often offer higher mechanical strength and chemical resistance.
Functionality of the DSC and typical measured variables
DSC measures the heat flow required to heat up or cool down a sample compared to an inert reference. Differences in heat flow indicate that structural or physical changes are taking place in the material.
Key performance indicators are:
- Tg (glass transition): visible in amorphous and semi-crystalline polymers
- Tc (crystallization)exothermic peak during cooling of semi-crystalline materials
- Tm (melting point)endothermic peak when heating crystalline areas
- ΔH (enthalpy)Area under the peak – used, for example, to determine the crystallinity
The shape of the thermogram provides valuable information on processability, thermal stability, ageing processes or differences due to additives.
Standard-compliant implementation: ISO 11357 and ASTM D3418 in detail
Carrying out a DSC measurement may seem simple at first glance: weigh the sample, start the temperature program, evaluate the thermogram. However, in order to obtain reproducible, comparable and interpretable results, exact compliance with the normative specifications is essential. The ISO 11357 and ASTM D3418 standards provide clear instructions for this.
- Heating and cooling rate: The standards generally recommend a linear rate of 10 K/min in order to achieve a good compromise between measurement time, sensitivity and thermal equilibrium. Too high rates can smear peaks, too low rates can unnecessarily prolong measurements.
- Calibration: Calibration is carried out using reference materials such as indium, tin or lead, whose transition temperatures and enthalpies are precisely documented. Calibration must be carried out regularly – ideally annually – and under the specific measurement conditions.
- Sample preparation: The ideal sample mass is 5-10 mg. An undried sample, such as PE, can produce artifacts, e.g. an apparent peak at ~100 °C due to evaporating moisture. This effect disappears by drying under vacuum at 80 °C.
- Atmosphere: The measurement is usually carried out under nitrogen to avoid oxidation. Oxygen can be used for certain tests (e.g. OIT). The purge gas rate is usually 50-60 ml/min.
Summary of the standard-relevant parameters
| parameters | standard specification | significance |
|---|---|---|
| Heating/cooling rate | Standard: 10 K/min | Clearly recognizable transitions, reproducible data |
| calibration | Indium, tin, etc. | Exact temperature and ΔH values |
| sample mass | 5–10 mg | Uniform heat transfer, clean signal |
| atmosphere | Nitrogen or O2 | Prevention of oxidation, defined conditions |
What can be derived from the data?
The DSC measurement provides for polyethylene (PE) a variety of revealing information. The melting temperature (Tm) shows how crystalline the material is, which in turn provides information about its homogeneity and processability. A sharp melting peak indicates a uniform material, while broad or shifted peaks indicate additives, recyclates or impurities.
The crystallinity, calculated via the enthalpy of fusion compared to the ideal value (approx. 293 J/g for PE), is decisive for the mechanical properties. A high crystallinity promotes rigidity and chemical resistance, while a lower crystallinity is associated with greater flexibility and more transparent properties.
In addition, conclusions can be drawn about previous thermal experience: Exothermic peaks during cooling or second heating indicate re-crystallization. Artifacts caused by moisture, such as a peak at around 100 °C, can be avoided by suitable sample preparation.
The method is used not least for quality assurance, for example when comparing different batches, suppliers or material qualities. This makes DSC measurement a strategic tool for material development, process optimization and long-term quality assurance.
Summary: What the DSC measurement reveals about PE
- The melting temperature indicates purity, homogeneity and suitability for processing.
- Crystallinity influences stiffness, shrinkage and flexibility.
- Re-crystallization and thermal history can be assessed on the basis of exothermic peaks.
- Moisture artifacts lead to misinterpretations and must be eliminated by drying.
- The method is ideal for batch control and quality assurance.
Conclusion
The DSC measurement is much more than a routine laboratory process – it provides crucial information about the structure, purity and thermal stability of polymers. In the case of polyethylene, for example, it can be used not only to avoid processing problems, but also to achieve targeted product improvements. Through standardized preparation and structured evaluation, sources of error can be identified, material properties optimized and product quality assured in the long term. Those who interpret thermograms correctly turn data into competitive advantages.
