Increasing crystallinity in a semicrystalline polymer typically results in ...

• A. Increases in both tensile modulus and tensile strength.
• B. Increases in modulus only.
• C. No effect on mechanical properties.
• D. Increases in ductility but lowers stiffness.

Answer: (A)

Increasing crystallinity in a semicrystalline polymer makes the material more ordered and densely packed, so the crystalline regions act as stiff, load-bearing areas that resist deformation. This raises the tensile modulus (stiffness) because the overall material becomes less able to bend or stretch under small loads. It also raises tensile strength since these hard crystalline domains can bear higher stresses and hinder easy chain slippage, improving resistance to breaking. At the same time, mobility of the chains in the amorphous regions is restricted, which reduces ductility, so you don’t gain more pliability with more crystals. So the combined effect is higher stiffness and higher strength, with reduced ductility, making the statement about increases in both modulus and tensile strength the best description. The other options conflict with how crystalline regions reinforce the material or with the typical trade-off between stiffness and ductility.