Discussion
Crystallization is favored in polymer melts when the chain entanglement is _________________ and only polymers with _______________ molecular arrangement can crystallize.
Answer: D
The blanks are minimum, ordered.
To crystallize, chain segments must move into place and pack side by side in a regular lattice. Entanglements are knots that tie chains together and restrict this motion, so crystallization is favoured when entanglement is at its minimum. That is why slow cooling or annealing, which gives chains time to disentangle and fold, raises crystallinity.
Only chains with a regular, ordered structure can fit into a lattice: linear chains with a stereoregular (isotactic or syndiotactic) arrangement crystallize, while atactic or randomly copolymerised chains generally remain amorphous.
A random arrangement cannot pack into a crystal, and maximum entanglement hinders the chain motion that crystallization needs, so every other pair has at least one blank wrong.
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