Which statement explains why increasing crystallinity increases stiffness?

• A. Crystalline regions diffuse freely, increasing mobility.
• B. They form an ordered structure that is more rigid than amorphous regions.
• C. They create more free volume.
• D. They reduce molecular weight.

Answer: (B)

In polymers, stiffness comes from how much the chains can move; crystalline regions are highly ordered and tightly packed, which severely restricts segmental motion and reduces free volume. That restricted mobility makes the material resist deformation more, so the overall modulus increases as crystallinity rises. The other statements would imply more mobility or free volume, or relate to molecular weight, none of which explain the rise in stiffness with more crystalline content.