How does molecular weight affect the tensile modulus of polymers according to the material?

• A. Higher molecular weight increases modulus
• B. Higher molecular weight decreases modulus
• C. Molecular weight does not affect modulus; only tensile strength
• D. Molecular weight determines Young's modulus independently of structure

Answer: (C)

The key idea is that the stiffness of a polymer in its elastic (tensile) regime is set by bond rigidity and how the chains are constrained by structure (crystallinity, crosslinking, and chain interactions), not by how long the chains are. Increasing molecular weight mostly extends chain length and increases entanglements, which enhances properties like tensile strength and toughness, but it doesn’t significantly change the stiffness that governs the modulus. Once chains are long enough, the elastic modulus stays roughly constant with further increases in molecular weight. That’s why the statement that molecular weight does not affect the modulus, while tensile strength can be affected, is the best fit. The other ideas imply a monotonic change with weight or claim modulus is set solely by weight independent of structure, which isn’t consistent with how structure and bonding control stiffness.