Polymer Crystallization by Thermal Analysis

Principles and Techniques for Accurate Polymer Characterization

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Polymer Crystallization

Polymer crystallization is a complex process that occurs either when a polymer solidifies from the melt or is heating about the glass transition temperature (T_g). During this process, the polymer molecules organize into a regular, repeating pattern known as a crystal structure. This structure is highly ordered and provides the polymer with improved mechanical properties such as increased strength and stiffness.

There are several factors that can affect polymer crystallization, including the polymer's molecular weight, chemical structure, processing conditions, and the presence of additives.

Thermal analysis is a useful set of techniques that can help to understand the crystallization behavior of polymers. It can provide insight into properties of the final polymer as well as detailed information about the physical and chemical processes that drive crystal formation.

Crystallization of polymers, as well as other materials, occurs between the glass transition (T_g) and the melting temperature (T_m). In this temperature range, there is enough molecular movement for the crystalline domains to form. This process can take place during heating (cold crystallization) or during cooling from the melt. The crystal size, shape, and percentage all depend on the heating and cooling rates applied to the material.

Different techniques can be used to determine the crystallization of materials, including Differential Scanning Calorimetry (DSC), Thermomechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA), Flash Differential Scanning Calorimetry (FDSC), and Hot-stage microscopy (TOA).

TA technique	Measured properties	Comments
Differential Scanning Calorimetry (DSC)	Latent heat due to crystallization and melting events as a function of time and temperature.	The measured latent heat during crystallization is proportional to crystallinity.
Flash Differential Scanning Calorimetry (FDSC)	Similar to DSC, but can mimic processing condition due to ultrafast heating and cooling rates.	The Flash DSC measures at fast heating and cooing rates (several thousands of K/s) or fast isothermal processes (in the ms and s range).
Thermomechanical Analysis (TMA)	Dimensional changes caused by density fluctuations, contraction or expansion.	Dimensional changes are usually measured in only one direction. Therefore, the measured change is not proportional to crystallinity.
Dynamic Mechanical Analysis (DMA)	Mechanical modulus and sample stiffness.	The modulus increases during crystallization. This technique is very sensitive, but not proportional to crystallinity.
Hot-stage microscopy (TOA)	Visual observation of crystal formation or solid-solid transformation.	Crystal seeding and formation can be observed in real time to help evaluate date and draw conclusions.

For more information, take a closer look at the Thermal Analysis in Practice Handbook

Crystal Structures

Polymer Crystallization

A molecule, material, or bio-molecule (such as a protein) can exist in three different states, as shown in the image above:

Amorphous: no ordered structure
Semi-Crystalline: small or large crystalline domains that are connected by amorphous sections
Crystalline: a fully crystalline material

Polymeric materials fall into two of these categories: amorphous or semi-crystalline. It is impossible to have a polymer that is 100% crystalline due to the molecular mobility required for a polymer to reach this state.

Learn more about how thermal analysis is used for the characterization polymers.

Essential Applications for the Thermal Analysis of Polymers

If you are interested in crystallization and precipitation from a solution, we also have many applications for you,

Crystallization and Precipitation

Crystallization by DSC

Differential scanning calorimetry (DSC) is a powerful measurement technique deployed in many analytical laboratories, for both QC and R&D. It measures the heat flow produced in a sample when it is either heated, cooled, or held isothermally at a constant temperature.

DSC can measure the crystallization of a polymer during both heating and cooling measurements, as well as examine the effect of different cooling rates on crystallization.

In this example, polyethylene terephthalate (PET) goes through a heating-cooling-heating cycle. During the first heating, the polymer crystallizes before melting, indicating that it was a mostly amorphous material. Once in the melt, the PET is then cooled, and crystallization again occurs at a slightly higher temperature. Upon the second heating run, there is no crystallization peak because the PET reached maximum crystallinity during the cooling cycle.

View the complete webinar about polymer crystallization:

Polymer Crystallization Investigated by DSC and Flash DSC

Heating and Cooling of PET by DSC

Crystallization by TMA

Thermomechanical analysis (TMA) measures the dimensional changes in a material. TMA provides characteristic information on numerous types of samples, for example very thin layers, large sample cylinders, fine fibers, films, plates, soft or hard polymers, and single crystals.

Since the density of materials changes during crystallization, the TMA is a useful tool for determining the crystallization behavior of polymers. Once we can use PET as an example. When PET is heated it passed through its glass transition (Tg) at about 80 °C, an expansion is observed. Then the polymer starts to contract (110 °C), this is the onset of crystallization. In polymers, crystallization causes a shrinkage of the material and the polymer chains arrange themselves into ordered crystalline domains.

Learn more about how TMA is used to determine crystallization behavior.

Characterization of PET by TMA

Crystallization by DMA

Dynamic mechanical analysis (DMA) is an important technique used to measure the mechanical and viscoelastic properties of materials such as thermoplastics, thermosets, elastomers, ceramics, and metals. The DMA measures the modulus as a function of time or temperature and provides information on phase transitions.

Once again, we can look at PET as a model system. It can be seen that as the temperature changes the modulus also changes. The crystallization of PET is indicated by an increase modulus at 110 °C as the polymer chains to arrange themselves into a more ordered crystal structure.

Watch the DMA on demand webinar for more insight about how DMA is used to determine the crystallization behavior of materials.

Characterization of PET by DMA

Crystallization by Hot Stage Microscopy

Hot stage microscopy is a method which is widely used to visually examine all kinds of thermal transitions when the sample is heated or cooled.

Observing your sample under a microscope as it is heated or cooled can be a valuable aid to help evaluate results and understand what is actually occurring in your sample.

Read an application note about how Hot Stage can aid in curve analysis and data interpretation.

30 °C Solid crystals

89 °C During melting

95 °C Almost molten

104 °C Crystallization and evaporation

120 °C Crystallization almost finished

185 °C Beginning of the melting of the anhydride

Crystallization by Flash DSC

The Flash DSC revolutionizes rapid-scanning DSC. The instrument can analyze reorganization processes that were previously impossible to measure. Heating and cooling rates now cover a range of more than 7 decades, allowing the influence of these rates to be examined.

The ultra-high heating and cooling rates add a new dimension to the study of thermally induced physical transitions and chemical processes, for example, the crystallization and reorganization of polymers, metals, and other materials.

The above example shows the effect of different heating rates on the crystallization and reorganization of polypropylene. Real time processing conditions can be mimicked with a Flash DSC.

Fast Scanning Chip Calorimetry

Influence of the Heating Rate

Crystallization Enthalpies for Common Polymers

Different polymers have different crystallization behavior and enthalpies of crystallization. Having a list of the crystallization enthalpies of common polymers built into the STARe software makes it easy to compare and identify your samples. This type of easy-to-use information is invaluable when determining the crystallization of your materials.

Polymer	Acronym	Formula	Molecular mass (g/mol)	ΔH_m (J/g) 100%
Polyethylene, low density	PE-LD	C₂H₄	28.1	293
Polyethylene, high density	PE-HD	C₂H₄	28.1	293
Polypropylene	iPP	C₃H₆	42.1	207
Polyoxymethylene	POM	CH₂O	30.1	330
Polyvinylidene fluoride	PVDF	C₂H₂F₂	64.0	105

Discover everything STARe software, the standard in thermal analysis software, has to offer.

Tips and Hints

For crystallization determination, the measurement of good quality thermal analysis data is a challenging task. Nevertheless, these common and useful techniques can be taken advantage of with simple sample preparation, relatively short measurement time, good accuracy, and easy evaluation using commercial software.

Advice for obtaining the best possible measurement data:

Stabilize the instrument before the measurement.
Check the reproducibility using repeated measurements.
Averaging different measurements improves the accuracy.
Place the clean crucibles accurately on the sensor.
Recommendation for sample size:
- DSC: 10 to 20 mg
- TMA: < 10 mm

Download our booklet for more TA tips and hints

FAQ

What is polymer crystallization?

Polymer crystallization is the process by which polymer molecules organize into a regular, repeating pattern known as a crystal structure when they solidify from a liquid or semi-liquid state.

How do processing conditions affect polymer crystallization?

Processing conditions such as temperature, pressure, and cooling rate can all affect how the polymer molecules organize themselves as they solidify. For example, rapid cooling can often prevent the formation of crystals, while slow cooling can promote crystal growth.

What are some factors that can affect polymer crystallization?

Several factors can affect polymer crystallization, including the polymer's molecular weight, chemical structure, the presence of additives, and processing conditions.

Why is polymer crystallization important?

Polymer crystallization is important because it can significantly impact the properties and performance of polymers. By understanding how and why polymers crystallize, researchers can develop new materials with improved properties and performance.

What are nucleating agents, and how do they affect polymer crystallization?

Nucleating agents are additives that can encourage crystal formation in polymers. By providing a surface for the polymer molecules to organize around, nucleating agents can speed up the crystallization process and promote the formation of smaller, more uniform crystals.

What is the significance of polymer crystallization?

Polymer crystallization affects the physical, chemical, and mechanical properties of the polymer material. The degree of crystallinity and crystal size and shape can impact the material's thermal and mechanical properties, as well as its transparency and electrical conductivity.

What thermal analysis techniques can be used to determine crystallization?

Depending on the material property under investigation, crystallization can be determined by a wide range of thermal analysis techniques. The commonly used techniques are differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), hot-stage microscopy, and Flash DSC.

How can differential scanning calorimetry (DSC) be used to study polymer crystallization?

DSC can be used to measure the heat flow associated with polymer crystallization. By heating or cooling a polymer sample at a controlled rate, the crystallization and melting temperatures and enthalpies can be determined.

What is the significance of the melting temperature measured by DSC?

The melting temperature measured by DSC is an important parameter for characterizing the degree of crystallinity of a polymer material.

What is the effect of cooling rate on polymer crystallization?

The cooling rate can affect the degree of crystallinity and crystal size and shape of a polymer material. A faster cooling rate can result in a higher degree of crystallinity, smaller crystal size, and more homogeneous crystal distribution.

How is hot stage microscopy used to characterize the crystallization of polymers?

Hot stage microscopy is a powerful method which is widely used to visually examine physical transitions. It can be used to observe when a polymer starts to crystalize and the shape of the crystals.

How is Flash DSC used to study the crystallization behavior of polymers?

The Flash DSC uses ultra-high heating and cooling rates to examine reorganization processes of polymers. It can be used to mimic process conditions, to characterize the final properties of a material.

How can thermomechanical analysis (TMA) be used to study polymer crystallization?

TMA measures the dimensional changes of a sample as it is subjected to a controlled temperature program. During polymer crystallization, the sample undergoes changes in length, thickness, and volume due to the rearrangement of polymer chains as they form a more ordered crystalline structure.

How can dynamic mechanical Analysis (DMA) be used to study polymer crystallization?

DMA can be used to measure changes in the mechanical properties of a polymer sample as it undergoes crystallization. This technique can provide information about the kinetics and degree of crystallinity of the sample.

What is the effect of thermal history on polymer crystallization?

The thermal history of a polymer, including its processing and thermal treatment, can influence its degree of crystallinity and crystal size and shape. Thermal analysis techniques can be used to study these effects.