Which statement is true about the relationship between degree of crystallinity and tensile modulus, and about CTE values?

• A. Higher crystallinity increases modulus; polymers have the largest CTE values.
• B. Higher crystallinity decreases modulus; polymers have larger CTE values.
• C. Higher crystallinity increases modulus; polymers have smaller CTE values.
• D. Higher crystallinity decreases modulus; polymers have smaller CTE values.

Answer: (C)

Increasing crystallinity makes polymers stiffer because the ordered, tightly packed crystalline regions resist deformation and limit chain mobility. That restriction boosts the tensile modulus, so the material behaves more like a rigid solid as crystallinity rises. On thermal expansion, heating tends to make material segments move apart, but crystalline regions act like a lattice that constrains this expansion. As crystallinity increases, the overall coefficient of thermal expansion decreases. In that sense, crystalline polymers exhibit smaller CTE values than more amorphous ones, even though polymers in general often have higher CTE than metals or ceramics.