Many are surprised when they see that PEEK (Polyether Ether Ketone) has a glass transition temperature (Tg) of just 143°C, yet it can be used long-term at 260°C. This may seem counterintuitive, but PEEK has been widely applied in many high-temperature environments.
Today, let’s explore PEEK’s molecular structure to understand why it remains stable and reliable far beyond its Tg.
When reviewing the PEEK datasheet, many people raise the question:
“Our product operates continuously above 200°C, but PEEK’s Tg is only 143°C—won’t it have softened long ago?”
This is a valid concern, especially since amorphous materials like PC (Polycarbonate) and PMMA often rely on Tg to define the limit of their dimensional stability.
Once they exceed Tg:
Molecular chains become active.
The material retains some elasticity.
But it loses shape and mechanical strength.
However, PEEK is different—it's a semi-crystalline polymer, which fundamentally alters its high-temperature performance.
In polymers, molecular chains can arrange:
Amorphously: random, disordered.
Crystalline regions: orderly, aligned structures.
PEEK contains both amorphous and crystalline regions. When reaching its Tg of 143°C, the amorphous regions become active, but the crystalline regions still support the structure.
The crystalline zones remain stable up to PEEK’s melting point (Tm) of around 343°C, acting like steel plates within the polymer, maintaining rigidity and form.
Two key reasons:
PEEK’s crystallinity is typically 35% to 45%.
These crystalline structures form a "crystal skeleton", like cables on a suspension bridge, holding the shape and rigidity at high temperatures.
Even when the amorphous regions soften, the crystalline parts "lock" the chains in place, preventing flow or collapse.
PEEK’s backbone is composed of:
Benzene Rings (Aromatic Rings)
Ketone Groups (C=O)
These rigid structures:
Resist chain twisting or folding.
Maintain mechanical stability even above 200°C.
Prevent the material from becoming limp like common plastics.
While PEEK can endure 260°C long-term, there's a critical engineering caveat:
Avoid applying excessive mechanical loads at high temperatures.
Above Tg:
Amorphous regions are already mobile.
Modulus (stiffness) decreases.
Excessive stress can cause irreversible molecular shifts, leading to permanent deformation or creep.
Avoid constant high loads in design.
Minimize stress concentration from point contacts.
Control residual stress from heat treatment or machining to prevent deformation during service.
Tg marks the transition of the amorphous regions, but it’s the crystalline zones that give PEEK its strength and high-temperature capabilities.
When selecting high-temperature materials, don’t just look at Tg. Evaluate:
Crystallinity
Polymer backbone rigidity
Melting point (Tm)
Tg is just the surface—structural robustness is what truly determines high-temperature performance.