Why Is This Popular Engineering Plastic Both Rigid and Tough?

In the field of material science, rigidity and toughness are often seen as two ends of a balancing scale — optimizing both is a key challenge for engineering plastics.

1. Understanding the Concepts

Rigidity

Rigidity refers to a material’s ability to resist deformation under force. It is typically measured by the elastic modulus (Young’s modulus).
A higher modulus means greater rigidity. For instance, polycarbonate (PC) has a relatively high modulus, making it resistant to bending.

Toughness

Toughness is the ability of a material to absorb energy and plastically deform without breaking.
For example, polyoxymethylene (POM) exhibits good toughness. It can deform under impact and absorb energy without fracturing.

2. The Relationship Between Rigidity and Toughness

A Balancing Act

Generally, enhancing rigidity can reduce toughness, and vice versa.

• Adding glass fibers to plastics increases rigidity by tightening the internal structure. However, this makes the stress more concentrated during impact, making the material more brittle.

• Conversely, adding plasticizers increases chain mobility, improving toughness by dispersing stress, but reduces rigidity.

Achieving Both

Some advanced engineering plastics can achieve both properties through molecular design or processing techniques.

For example, high-performance nylon materials can be engineered with specific crystallinity and chain orientation to improve both rigidity and toughness. When chains are well-oriented and appropriately crystalline, the material resists deformation and absorbs energy efficiently.

3. PEEK: A High-Performance Engineering Plastic with Both Rigidity and Toughness

Molecular Structure Features

Main Chain Composition

PEEK (Polyether Ether Ketone) features a repeating structure of ether (-O-) and ketone (-CO-) groups linked with aromatic rings.

• Aromatic rings contribute to rigidity due to their stability.

• Ether and ketone groups provide limited flexibility, enabling the chain to absorb energy during deformation.

Chain Regularity

PEEK has a high degree of molecular regularity, allowing for high crystallinity, which improves rigidity.
At the same time, amorphous regions within the polymer chains offer flexibility and energy absorption — helping prevent rapid crack propagation.

4. The Advantages of PEEK Performance

• High Glass Transition Temperature (Tg): ~143°C

• High Melting Point: ~334°C

• Excellent dimensional stability at elevated temperatures

These properties mean that even under heat, PEEK retains its mechanical integrity.

• High elastic modulus = excellent rigidity

• High elongation at break + impact strength = excellent toughness

5. Applications of PEEK

Thanks to its unique combination of properties, PEEK is widely used in aerospace, medical devices, and automotive manufacturing:

• Aerospace: Long-term use under high temperatures with resistance to impact and vibration.

• Medical devices: Ideal for artificial joints, enduring movement pressure and accidental impacts without damage.