How does molecular weight affect tensile modulus and tensile strength of polymers?

• A. Tensile modulus is relatively insensitive to molecular weight.
• B. Tensile modulus is relatively insensitive to molecular weight, while tensile strength increases with molecular weight.
• C. Both modulus and strength increase with molecular weight.
• D. Both decrease with molecular weight.

Answer: (B)

Molecular weight affects stiffness and strength in different ways because the underlying physics of how chains interact changes with chain length. The tensile modulus, which measures stiffness, is largely set by the stiffness of the backbone and how chains pack and interact on a short scale. Once chains are long enough to form the network and entanglements, increasing their length has little impact on the elastic response, so the modulus stays fairly constant with molecular weight. Tensile strength, on the other hand, benefits from longer chains because they create more entanglements and improve load transfer along the polymer network. These entanglements help resist chain pullout and failure, so the material can withstand higher stresses before breaking. Therefore, modulus is relatively insensitive to molecular weight, while tensile strength tends to rise with molecular weight, at least up to practical limits set by processing and crystallinity.