Which factor increases both tensile modulus and tensile strength in semicrystalline polymers?

• A. Increasing molecular weight.
• B. Increasing percent crystallinity.
• C. Increasing temperature.
• D. Increasing strain rate.

Answer: (B)

In semicrystalline polymers, the amount of crystalline material acts as a reinforcing framework that governs how stiff and strong the material is. Increasing the percent crystallinity adds more ordered, tightly packed regions that resist deformation, so the tensile modulus goes up because the material becomes harder to elasticly stretch. Those crystals also bear more load and transfer stress more effectively across the material, which raises the tensile strength as well. Of course, this usually comes with reduced ductility, since the more brittle crystalline network limits plastic deformation.

Other factors don’t produce the same clear, parallel rise in both properties. Increasing molecular weight mainly boosts entanglements and can subtly affect stiffness and strength, but it doesn’t reinforce the material as directly as crystallinity. Raising temperature softens the polymer, lowering both modulus and strength. Strain rate can change measured values due to viscoelastic effects, but it doesn’t systematically increase both properties in the way increasing crystallinity does.