Why does increasing crystallinity increase stiffness in a polymer?

• A. They diffuse more easily.
• B. They form an ordered structure that is more rigid than amorphous regions.
• C. They increase free volume.
• D. They reduce chain length.

Answer: (B)

Crystallinity creates tightly packed, highly ordered regions in a polymer. These crystalline domains restrict how much the chains can move and reduce the free volume between them, so the material becomes harder to deform. In a loaded polymer, those stiff, lattice-like regions carry more of the applied stress, boosting the overall elastic modulus and hence the stiffness. The amorphous parts are more mobile and compliant, so increasing the fraction of crystalline material makes the whole structure stiffer. The other ideas don’t fit: crystallinity doesn’t make diffusion easier (it actually hinders it), it doesn’t increase free volume (it decreases it), and it doesn’t change the chain length, which is a fixed molecular property.