Crystallization of a High-Performance Semi-Crystalline Polymer: PEEK

Introduction

In the molten state, the polymer chains of a semi-crystalline polymer are in a disordered state. During cooling, some of them rearrange to form ordered regions and crystallize. In addition to this crystalline phase, a semicrystalline polymer also contains an amorphous phase without an ordered molecular structure (see figure 1). Cooling does not lead to crystallization of this phase, but to a transitions from a soft to a hard brittle state. This transition is called the Température de Transition VitreuseThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition. 

Different methods can characterize the crystallization and Température de Transition VitreuseThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition of polymers, providing a variety of valuable information. 

A typical method for analyzing thermal transitions is Differential Scanning Calorimetry (DSC). It provides information about the glass transition, phase transformations like crystallization/melting or solid-solid Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transitions and degree of crystallinity, etc. Its ease-of-use and ability to automate measurement steps have made it a popular and widely used technique. 

CristallisationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.Crystallization and glass transition have a significant influence on the mechanical properties of a product. Another method for determining these parameters is rheology. A measurement using a rotational rheometer provides information on the rheological changes that occur as a semi-crystalline polymer cools from the melt into the glassy state. In the following, the cooling behavior of polyether ether ketone (PEEK) (see chemical structure in figure 2) is determined using the DSC 303 Caliris^® and Kinexus rotational rheometer.

Measurement Parameters

The PEEK sample was heated to above its Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature. After an isothermal phase, the polymer was cooled down at a controlled cooling rate. The standard cooling rates of the respective methods were used, i.e., 10 K/min for the DSC 300 Caliris^® and 2 K/min for the Kinexus rotational rheometer. Table 1 summarizes the measurement conditions.

Table 1: Measurement parameters

DSC 300 Caliris^®: Crystallization Behavior

Figure 3 displays the resulting curve of the DSC measurement performed on PEEK. The ExothermiqueA sample transition or a reaction is exothermic if heat is generated.exothermal peak beginning at 305°C (endset temperature) is due to the crystallization of PEEK. The step in the DSC curve with midpoint at 146°C is the glass transition.

Kinexus Rotational Rheometer: Stiffness

Figures 4 and 5 depict the typical curves resulting from the temperature sweep performed on PEEK.

The Melt State 

Providing no reaction occurs, the complex shear viscosity (figure 4) increases with decreasing temperature. This is the expected influence of temperature on stiffness in the absence of a physical or chemical process, as the mobility of polymer chains increases during heating. 

The melt state is also characterized by domination of G” over G´ (figure 5). In other words, at this temperature, the “liquid-like” properties have more influence on the deformation behavior of PEEK than the “solid-like” properties. The polymer flows for the timescale of the applied frequency, even if it still features strong elastic properties (phase angle value closer to the value 45° than to 90°).

Occurance of CristallisationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.Crystallization 

At 325°C, the slope of the complex shear viscosity curve changes (Figure 4). The complex shear viscosity increases from 7.7E+03 Pa∙s at 325°C to 9.0E+06 Pa∙s at 295°C, an increase of more than 3 decades in only 30°C! This significant increase is typical for the crystallization of a crystalline or semi-crystalline polymer. 

The process also greatly affects the elastic (G') and viscous (G") shear moduli (figure 5). Both curves increase and show the crossover at 308°C. Between crystallization and glass transition, the amorphous phase is in the rubbery plateau. The polymer chains belonging to the amorphous phase are still free to move, while the crystalline phase gives structure to the product. 

The higher the degree of crystallinity, the higher the value of the elastic shear modulus. The phase angle lies at 2° to 3°, so that the polymer is now close to a perfect elastic solid.

Glass Transition 

The glass transition is reached during further cooling. Stiffness continues to increase but not as significantly as during crystallization (3.0E+07 Pa∙s at 200°C to 1.6E+08 Pa∙s at 140°C, figure 4). 

While the glass transition temperature is usually evaluated by means of the peak temperature, which is typical for the curves of G" and δ (figure 5), cooling over the glass transition is also related with an increase in the G' curve. At temperatures lower than the glass transition temperature, the phase angle decreases again and is close to 0. The polymer is in a glassy, stiff state.

Conclusion

This application example shows how DSC and rotational rheology complement each other. Both methods provide different information describing the crystallization and glass transition of semi-crystalline polymers, thus providing a comprehensive insight into the material behavior during heating and cooling. The typical detected effects are summarized in Tables 2a and 2b.

Table 2a: Typical effects measured during crystallization and glass transition of a semi-crystalline polymer by means of the DSC 300 Caliris^®

¹ in accordance with DIN ISO 11357-5:2014
² in accordance with DIN ISO 11357-2:2014

Table 2b: Typical effects measured during crystallization and glass transition of a semi-crystalline polymer by means of the Kinexus rotational rheometer