When discussing polymer materials, we often hear comments like “this material has excellent rigidity” or “that one has outstanding toughness.” Materials with high rigidity usually exhibit greater hardness and resistance to compression and deformation. On the other hand, tough materials are more like flexible ribbons, capable of withstanding stretching and bending with remarkable resilience.

But have you ever wondered: what performance indicators truly define a material’s rigidity or flexibility? And what fundamental factors determine whether a polymer behaves as stiff or soft? In this article, we’ll explore these questions and uncover the science behind the mechanical characteristics of polymer materials.

Understanding Rigidity and Flexibility Through Performance Indicators

Among the many mechanical properties of polymer materials, different indicators are responsible for reflecting either rigidity or flexibility.

Indicators of Rigidity:
Flexural modulus and hardness are often seen as the key representatives of rigidity. The flexural modulus measures a material’s resistance to bending deformation—the higher the value, the “stiffer” the material, making it less prone to bending. Hardness, on the other hand, directly reflects a material’s ability to resist localized surface pressure. Materials with high hardness can better maintain their shape and resist compressive deformation from external forces.

Tensile strength and compressive strength also help indicate a material’s rigidity to some extent. Tensile strength is the maximum stress a material can endure before breaking under tension. A high tensile strength means the material can withstand greater pulling forces without breaking, showcasing strong rigidity. Similarly, compressive strength reflects a material’s ability to resist compression—higher values indicate stronger rigidity.

Indicators of Flexibility:
Elongation at break and impact strength are key indicators for evaluating a material’s flexibility.

Elongation at break refers to the ratio of the material's extended length to its original length when it breaks under tension. The higher the value, the more a material can stretch before breaking, indicating better ductility and flexibility.

Impact strength measures a material’s ability to absorb energy under sudden impact. Materials with high impact strength are less likely to fracture when subjected to external forces, demonstrating excellent toughness and flexibility.

Example: PP + 40% Long Glass Fiber

Understanding Rigidity and Flexibility Through Intrinsic Factors

2. Local Degrees of Freedom
The local structure and functional groups along polymer chains also affect material rigidity and flexibility. The size, polarity, and quantity of side groups play important roles. Larger side groups hinder the movement of polymer chains, reducing flexibility and increasing stiffness. For example, bio-based polymers with long alkyl side chains show increased rigidity as the side chain lengthens. Polar side groups generate strong intermolecular interactions that further restrict chain mobility and enhance stiffness. Bio-based cellulose derivatives containing polar groups such as hydroxyl and carboxyl can have their rigidity and flexibility tuned by adjusting the number and distribution of these groups.

3. Intermolecular Forces
The strength of intermolecular forces directly impacts polymer rigidity. Stronger interactions like hydrogen bonding and van der Waals forces increase the binding between polymer chains, making it harder for chains to slide or move relative to each other, thus raising the material’s stiffness. For example, chitosan exhibits abundant hydrogen bonding between molecules, resulting in high rigidity and strength, which is why it is widely used in biomedical applications like wound dressings. Conversely, weaker intermolecular forces facilitate chain mobility, yielding more flexible materials.

4. Molecular Chain Length
Molecular chain length acts as a double-edged sword for rigidity and flexibility. Generally, longer chains increase entanglement between molecules, restricting chain movement and increasing stiffness. However, longer chains also provide more conformational freedom, offering additional modes of movement that can impart some flexibility. For bio-based polyhydroxyalkanoates (PHA), increasing the degree of polymerization (chain length) enhances tensile strength and hardness while maintaining a degree of flexibility suitable for diverse applications.

5. Crosslinking
Crosslinking refers to chemical bonds connecting polymer chains into a three-dimensional network. In lightly crosslinked materials, the chains still retain some mobility between crosslink points, preserving flexibility while increasing stiffness and strength due to the network structure. For instance, lightly crosslinked sodium alginate hydrogels have good flexibility to conform to skin and sufficient strength for wound care. Highly crosslinked materials severely restrict chain motion, making the material hard, brittle, and significantly more rigid with greatly reduced flexibility.

6. External Factors
Temperature significantly influences polymer rigidity and flexibility. As temperature rises, increased molecular thermal motion enhances chain mobility, increasing flexibility and reducing stiffness. Lower temperatures have the opposite effect. Humidity also affects some hydrophilic bio-based polymers; for example, cellulose-based materials absorb moisture in high humidity environments, which weakens intermolecular forces, softens the material, and decreases stiffness.