= Solution
Reading the lowest point of the order-disorder boundary in the supplied experimental diagram gives approximately
$$
(\chi N)_{\min}\simeq18,\qquad f_{PI}\simeq0.45
$$
(about $0.45$–$0.47$, allowing for the graphical resolution). The ordered region just above that minimum is lamellar.
This is a reading of the supplied <polyisoprene>-<polystyrene> phase diagram. It should not be replaced by the often quoted symmetric-chain <mean-field theory> value $10.5$: that refers to a different idealization and is not the minimum drawn here.
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