Materials Science and Engineering Practice Exam

Crystallinity in polymers depends on how closely chains can pack into an ordered, repeating arrangement. Side-group size and shape strongly influence that packing: bulky pendant groups create steric clashes that disrupt regular alignment, whereas smaller side groups allow chains to come together more easily.

In this comparison, the repeating unit of PVC has a chlorine substituent, which is relatively small, while polystyrene carries a large phenyl side group. Under similar processing, the smaller chlorine groups in PVC cause less hindrance to packing, so chains can align more readily and form crystalline regions. This leads to higher crystallinity for PVC relative to polystyrene.

Other factors are less directly responsible here. Degree of polymerization affects crystal size and overall properties but not the inherent ability to crystallize; presence of double bonds would alter backbone rigidity in a way that doesn’t directly explain the packing advantage; and while chain rigidity can influence crystallization, the decisive difference between PVC and PS is the side-group bulkiness that governs how well chains can pack.