If a material has very low crystallinity, its modulus relative to a material with higher crystallinity is:

• A. Much higher
• B. Much lower
• C. About the same
• D. Lower, due to less crystalline stiffening

Answer: (D)

Stiffness rises with crystallinity because crystalline regions are tightly packed and ordered, which restricts how easily the material can be elastically deformed. When crystallinity is very low, most of the material is amorphous, allowing polymer chains to move and slide past one another under small loads, so the material is more compliant and its modulus is lower. So, a material with low crystallinity indeed has a smaller modulus than one with higher crystallinity, since there’s less crystalline stiffening to resist deformation. The answer reflects this mechanism: lower modulus due to less crystalline stiffening. In practice, increasing crystallinity typically increases the Young’s modulus, all else equal, because the rigid crystalline regions bear more of the applied load.