Which statement best captures the effect of crystallinity on modulus?

• A. Crystallinity has no impact on modulus.
• B. Crystallinity reduces modulus.
• C. Higher crystallinity raises modulus because crystalline regions are more rigid than amorphous.
• D. Modulus depends only on temperature.

Answer: (C)

Higher crystallinity raises modulus because crystalline regions are more rigid than amorphous ones. In polymers, stiffness comes from how much the chains can move under load. Amorphous areas are less constrained and allow more chain mobility, which lowers the slope of the stress–strain curve in the elastic region. When more of the material consists of crystalline, ordered domains, these stiff regions bear load more effectively and resist deformation, making the overall Young’s modulus higher. Temperature can influence modulus by softening the material, but it isn’t the only factor; the microstructure itself directly strengthens the material. So the statement that crystallinity increases modulus best captures the effect.